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Influence of the Craniomandibular System on Human Postural Control with Special Consideration of Dynamic Stability

February 2025

DOI:10.5445/IR/1000177959

Thesis for: PhD

Authors:

Cagla Kettner

Karlsruhe Institute of Technology

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Cagla Kettner

Inﬂ uence of the

Craniomandibular

System on Human

Postural Control with

Special Consideration

of Dynamic Stability

C. KETTNER Effects of Craniomandibular System on Dynamic Balance

Cagla Kettner

Inﬂuence of the Craniomandibular System

on Human Postural Control with

Special Consideration of Dynamic Stability

Karlsruhe Sports Science Research

Volume 84.2025

Inﬂuence of the Craniomandibular

System on Human Postural

Control with Special Consideration

of Dynamic Stability

Cagla Kettner

This document – excluding parts marked otherwise, the cover, pictures and graphs –

is licensed under a Creative Commons Attribution 4.0 International License

(CC BY 4.0): https://creativecommons.org/licenses/by/4.0/deed.en

Impressum

Karlsruhe Institute of Technology (KIT)

Kaiserstraße 12

76131 Karlsruhe

Institute of Sports and Sports Science (IfSS)

www.ifss.kit.edu

2025

ISSN 2943-0380

DOI 10.5445/IR/1000177959

Karlsruher Institut für Technologie

Institut für Sport und Sportwissenschaft

Inﬂuence of the Craniomandibular System on Human Postural Control

with Special Consideration of Dynamic Stability

Zur Erlangung des akademischen Grades einer Doktorin der Philosophie

(Dr. phil.) von der KIT-Fakultät für Geistes- und Sozialwissenschaften des

Karlsruher Instituts für Technologie (KIT) genehmigte Dissertation

von Cagla Kettner

Tag der mündlichen Prüfung: 5. Dezember 2024

KIT-Dekan: Prof. Dr. Michael Mäs

Erster Gutachter: Prof. Dr. Thorsten Stein

Zweiter Gutachter: PD Dr. Daniel Hellmann

The cover page is licensed under a Creative Commons

Attribution-No Derivatives 4.0 International License (CC BY-ND 4.0):

https://creativecommons.org/licenses/by-nd/4.0/deed.en

Mit Ausnahme von Kapitel 5 ist dieses Werk lizenziert unter:

Acknowledgements

This dissertation was written as part of my work as an academic research assistant at the

BioMotion Center of the Institute of Sports and Sports Science at the Karlsruhe Institute

of Technology (KIT).

My heartfelt and sincere thanks go first and foremost to my doctoral supervisor Prof. Dr.

Thorsten Stein. He supervised and supported me intensively, with dedication and to an

extraordinary degree at all times. His inspiring ideas and the exciting discussions with him

helped me to develop further as a scientist. I would also like to thank my second supervi-

sor, PD Dr. Daniel Hellmann, for his competent and demanding cooperation as well as for

the contribution of his dental expertise, which was always instructive and helpful. My

heartfelt thanks also go to Lisa for the friendship that developed during the experiments.

Furthermore, I would like to thank Dr. Steffen Ringhof for the great joint work, especially

for the critical discussions in the course of the publications. I would also like to thank my

colleagues at the BioMotion Center, who felt like the second family for me in Karlsruhe.

I would also like to thank all my colleagues at the Institute of Sports and Sports Science

for the pleasant working atmosphere, and to all my friends and teammates who accom-

panied me during my academic studies.

I would especially like to thank my parents, my sister, Dila, and her family for their en-

couragement, understanding and loving support. They have always been behind me and

gave endless love in all phases of my life. For me, it is the biggest chance to have them in

my life. My last thanks go to my love, Daniel. I am always thankful for his endless trust

and devoted love.

iii

Summary

The control of posture is essential for daily living. It guarantees that movements are initi-

ated and carried out as optimally as possible in both static and dynamic settings. Postural

control entails controlling the body's position with respect to the environment for the

dual purposes of balance and orientation. To detect and correct any imbalance, the sen-

sory information from the somatosensory, visual and vestibular systems acting on the

spinal and supraspinal structures of the central nervous system (CNS) are used. The CNS

modifies the weighting and, consequently, the relative importance of sensory information

based on the type of balance task being performed. Finally, the sensory input must be

translated into motor commands to maintain the balance in a task-specific manner. Albeit

the operation of these underlying postural control mechanisms is not completely known,

impaired postural control has been linked to reduced participation in daily activities and

an increased risk of falling.

A growing body of literature suggests that the craniomandibular system can impact pos-

tural control. Particularly, submaximal jaw clenching showed stabilizing effects on balance

during upright standing. In contrast to static conditions, the effects of simultaneous jaw

clenching during dynamic balance have not yet been investigated. Furthermore, a pro-

found understanding of the potential mechanisms underlying the stabilizing effects of jaw

clenching is still lacking. On this basis, the present thesis explores the effects of simulta-

neous submaximal jaw clenching on postural control in dynamic situations. Within this

context, this thesis primarily focuses on the influences of jaw clenching on dynamic reac-

tive and dynamic steady-state balance, with an additional emphasis on the underlying

mechanisms at the kinematic and muscular levels. Furthermore, it also addresses if the

effects of jaw clenching relate generally to dual-task benefits or specifically to jaw clench-

ing.

The present thesis includes ten main chapters. In the first chapter (Chapter 1), a short

motivation together with the outline of the thesis is given. In the following chapter (Chap-

ter 2), the fundamentals of postural control as well as the craniomandibular system are

introduced, and the interrelation of these systems is reviewed. Furthermore, the current

state of the research regarding the influences of jaw clenching particularly on postural

control is provided. Chapter 3 sums up the aims and the scope of this thesis.

The subsequent chapters (Chapters 4 to 8) encompass five research articles that aimed

to investigate the effects of simultaneous submaximal jaw clenching on dynamic reactive

Summary

balance (Chapter 4-6) and dynamic steady-state balance (Chapter 7-8). Each of these re-

search articles has been published in an international peer-reviewed journal.

The study given in Chapter 4 considers the modulation of postural control during a dy-

namic reactive balance task with different simultaneous oral-motor activities, and aims

to answer the question how jaw clenching, tongue pressing and habitual stomatognatic

behavior influence dynamic reactive balance performance. In this study, dynamic reactive

balance was assessed by using an oscillating platform. A custom-made release system was

used to apply mechanical perturbations in one of the four possible directions. In addition

to the dynamic reactive balance performance, the segmental kinematics were investi-

gated for a deeper understanding of the results. The findings revealed that jaw clenching

can improve dynamic reactive balance but the favorable benefits appear to be task-spe-

cific rather than general. Tongue pressing appears not to have any effects on dynamic

reactive balancing performance. In comparison to tongue pressing and habitual stoma-

tognatic behavior conditions, the mean speeds of the analyzed segments were overall

lower for the jaw clenching condition.

Chapter 5 presents the follow-up study of the previous one, and focuses on the reflex

activities and co-contraction patterns to establish a more comprehensive view of the oc-

curring changes due to simultaneous oral-motor activities. More specifically, this study

investigated the muscle activity (i.e. iEMG) and the co-contraction behavior of the leg and

trunk muscles during the critical reflex phases under the influence of jaw clenching,

tongue clenching and habitual stomatognathic behavior. Only the direction of perturba-

tion in which an improvement in dynamic balance performance was observed with sim-

ultaneous jaw clenching was analyzed. The findings could not explain why jaw clenching

leads to better dynamic reactive balance compared with tongue pressing or habitual sto-

matognatic behavior conditions. Neither the muscle activity nor the co-contraction pat-

tern analysis revealed a general neuromechanical effect of jaw clenching on dynamic

reactive balancing performance.

The study in Chapter 6 investigates the neuromechanical effects occurring after the auto-

mation of jaw clenching task. Based on the research findings on the dual-task paradigm

during balance, it can be argued that jaw clenching is a secondary task when it is simulta-

neously performed during balance tasks, therefore, the effects of jaw clenching are not

due to specific neuromechanical effects of the jaw clenching activity but related to the

dual-task situation in general. It has not yet been investigated whether the effects are

mainly related to the dual-task benefits or the jaw clenching itself. This study addressed

this issue with an intervention study and investigated the effects of jaw clenching on dy-

namic reactive balance task performance after 1 week of jaw clenching training to

Summary

examine whether the effects were a result of a dual-task situation. The results revealed

that the automation of the jaw clenching task had no discernible effects on dynamic re-

active balance performance. Jaw clenching appeared to be associated with some modifi-

cations in reflex activities but the effects were restricted to anterior-posterior

perturbations. High learning effects of the dynamic reactive balance task were detected,

which may have masked the jaw clenching effects. More studies with different balance

tasks with lower learning effects and longer intervention periods were suggested to be

necessary.

After the previous chapters have focused on dynamic reactive balance, Chapter 7 pro-

vides the first study investigating the effects of simultaneous submaximal jaw clenching

during dynamic steady-state balance. Particularly, the steady-state phase of the dynamic

balance task after the compensation of the perturbation on the oscillating platform was

analyzed. In various postural control studies, the center of mass (CoM) is proposed as the

controlled variable albeit missing experimental verification. Nevertheless, the CoM was

shown to be an important parameter in terms of balance. This study investigated if jaw

clenching has effects on sway, control and stability of CoM during dynamic steady-state

balance. An uncontrolled manifold approach with a whole-body joint kinematics model

was utilized besides the analysis of spatial and temporal variability characteristics of the

CoM sway. The results of this comprehensive analysis provided no effects of jaw clenching

or tongue pressing on the sway, control or stability of the CoM.

Similar to the previous one, the study given in Chapter 8 focused on jaw clenching effects

on dynamic steady-state balance. In this study, another dynamic balance task was used.

More precisely, a stabilometer was used to assess the dynamic steady-state balance. A

three-armed intervention study design aimed to answer three research questions: first, if

simultaneous submaximal jaw clenching can improve dynamic steady-state balance; sec-

ond, if the effects persist after the jaw clenching task loses its novelty and potential dual-

task benefits associated with it; and third, if the improved dynamic steady-state balance

performance is related with decreased activity of posture-related muscles. The results

revealed that simultaneous submaximal jaw clenching improves dynamic steady-state

balance performance on the stabilometer. These effects persist even when the novelty of

the secondary task (i.e. jaw clenching) decreases. The secondary finding demonstrated

that the learning effects of the used dynamic steady-state balance task were high, and

might have masked the effects of balance training. Improved dynamic steady-state bal-

ance performance led to decreased activities of posture-related muscles, indicating bet-

ter movement efficiency. However, there were no jaw clenching-related alterations in

muscle activities.

Summary

Chapter 9 provides a general discussion of the findings from the presented research. The

combination of the findings from these five studies provides a more comprehensive un-

derstanding of how simultaneous submaximal jaw clenching affects balance in dynamic

situations. In essence, this thesis provides additional evidence for the impact of the cra-

niomandibular system on the postural control system, yet it does not fully identify the

underlying neuromechanical mechanisms of the effects. The partially conflicting results

may be attributed to the task-specificity of balance tasks but it can also be due to the yet

undiscovered effects of jaw clenching masked by the high learning effects of the wide-

spread balance tasks used in this thesis. Further research is recommended to fully assess

the potential of jaw clenching and to better understand the underlying neuromechanical

effects. The thesis closes with a general conclusion in Chapter 10.

vii

Zusammenfassung

Die Kontrolle der Körperhaltung ist für das tägliche Leben unerlässlich. Sie gewährleistet,

dass Bewegungen sowohl in statischen als auch in dynamischen Situationen möglichst

optimal eingeleitet und ausgeführt werden. Bei der Haltungskontrolle geht es darum, die

Orientierung und Balance des Körpers zu koordinieren, indem seine Position im Verhältnis

zur Umgebung angepasst wird. Um eine Dysbalance zu erkennen und zu korrigieren, wer-

den die sensorischen Informationen des somatosensorischen, visuellen und vestibulären

Systems genutzt, die auf die spinalen und supraspinalen Strukturen des Zentralnerven-

systems (ZNS) wirken. Das ZNS ändert die Gewichtung und damit die relative Bedeutung

der sensorischen Informationen in Abhängigkeit von der Art der zu bewältigenden Gleich-

gewichtsaufgabe. Schließlich muss der sensorische Input in motorische Befehle umge-

setzt werden, um das Gleichgewicht aufgabenspezifisch halten zu können. Obwohl die

Funktionsweise dieser zugrundeliegenden Mechanismen der Haltungskontrolle nicht voll-

ständig bekannt ist, wurde eine gestörte Haltungskontrolle mit einer verminderten Teil-

nahme an täglichen Aktivitäten und einem erhöhten Sturzrisiko in Verbindung gebracht.

Eine wachsende Zahl von Veröffentlichungen deutet darauf hin, dass das kranio-

mandibuläre System die Haltungskontrolle beeinflussen kann. Insbesondere submaxima-

les Kieferpressen zeigte stabilisierende Auswirkungen auf das Gleichgewicht beim

aufrechten Stehen. Im Gegensatz zu statischen Bedingungen sind die Auswirkungen des

gleichzeitigen Zusammenpressens der Kiefer während dynamischer Gleichgewichtsaufga-

ben noch nicht untersucht worden. Darüber hinaus fehlt noch ein tiefes Verständnis der

möglichen Mechanismen, die den stabilisierenden Effekten des Kieferpressens zugrunde

liegen. Auf dieser Grundlage untersucht die vorliegende Arbeit die Auswirkungen des

gleichzeitigen submaximalen Kieferpressens auf die menschliche Haltungskontrolle in dy-

namischen Situationen. In diesem Zusammenhang konzentriert sich diese Arbeit in erster

Linie auf die Einflüsse des Kieferpressens auf das dynamische reaktive und das dynami-

sche steady-state Gleichgewicht, mit einem zusätzlichen Schwerpunkt auf den zugrunde

liegenden Mechanismen auf kinematischer und muskulärer Ebene. Darüber hinaus wird

untersucht, ob sich die Auswirkungen des Kieferpressens allgemein auf die Dual-Task-Be-

dingungen oder speziell auf das Kieferpressen beziehen.

Die vorliegende Arbeit umfasst zehn Hauptkapitel. Im ersten Kapitel wird zunächst die

dieser Dissertationsschrift zugrunde liegende Fragestellung motiviert und die Gliederung

der Arbeit gegeben. Im darauffolgenden Kapitel 2 werden die Grundlagen der menschli-

Zusammenfassung

viii

Zusammenhänge zwischen diesen Systemen erläutert. Darüber hinaus wird der aktuelle

Stand der Forschung zu den Einflüssen des Kieferpressens insbesondere auf die mensch-

liche Haltungskontrolle dargestellt. Kapitel 3 fasst die Ziele und den Aufbau der vorliegen-

den Arbeit zusammen.

Die folgenden Kapitel 4 bis 8 umfassen fünf wissenschaftliche Artikel, die die Auswirkun-

gen des gleichzeitigen submaximalen Kieferpressens auf das dynamische reaktive Gleich-

gewicht (Kapitel 4-6) und das dynamische steady-state Gleichgewicht (Kapitel 7-8)

untersuchen. Jeder dieser wissenschaftlichen Artikel wurde in einer internationalen Fach-

zeitschrift mit Peer-Review Verfahren veröffentlicht.

Die in Kapitel 4 vorgestellte Studie untersucht die Modulation der posturalen Kontrolle

während einer dynamischen reaktiven Gleichgewichtsaufgabe mit verschiedenen gleich-

zeitigen oral-motorischen Aktivitäten und zielt darauf ab, die Frage zu beantworten, wie

Kieferpressen, Zungenpressen und habituelles stomatognatisches Verhalten die dynami-

sche reaktive Gleichgewichtsleistung beeinflussen. In dieser Studie wurde das dynami-

sche reaktive Gleichgewicht mit Hilfe einer oszillierenden Plattform untersucht. Ein

speziell angefertigtes Auslösesystem wurde verwendet, um mechanische Störungen in ei-

ner der vier möglichen Richtungen zu erzeugen. Neben der dynamischen reaktiven Gleich-

gewichtsleistung wurde auch die segmentale Kinematik untersucht, um die auf

Leistungsebene identifizierten Effekte besser erklären zu können. Die Ergebnisse zeigten,

dass das Zusammenpressen der Kiefer das dynamische reaktive Gleichgewicht verbessern

kann, aber die positiven Auswirkungen scheinen eher aufgabenspezifisch als allgemein-

gültig zu sein. Zungenpressen scheint keine Auswirkungen auf die dynamische reaktive

Gleichgewichtsleistung zu haben. Im Vergleich zu den Bedingungen des Zungenpressens

und des habituellen Kieferpressens waren die mittleren Geschwindigkeiten der unter-

suchten Segmente unter der Bedingung des Zusammenpressens der Kiefer insgesamt

niedriger.

Kapitel 5 stellt die Folgestudie der vorangegangenen Studie dar und konzentriert sich auf

die Reflexaktivitäten und Ko-Kontraktionsmuster, um einen umfassenderen Überblick

über die Veränderungen zu erhalten, die durch das gleichzeitige submaximale Kieferpres-

sen auftreten. Konkret wurden in dieser Studie die Muskelaktivität (d.h. iEMG) und das

Ko-Kontraktionsverhalten der Bein- und Rumpfmuskulatur während der kritischen Refl-

exphasen unter dem Einfluss von Kieferpressen, Zungenpressen und habituellem sto-

matognatischem Verhalten untersucht. Es wurde nur die Richtung der Störung analysiert,

in der eine Verbesserung der dynamischen Gleichgewichtsleistung bei gleichzeitigem Zu-

sammenpressen der Kiefer beobachtet wurde. Die Ergebnisse konnten nicht erklären, wa-

chen Haltungskontrolle sowie des craniomandibulären Systems vorgestellt und die

Zusammenfassung

Zungenpressen oder habituelles Kieferpressen. Weder die Muskelaktivität noch die Ana-

lyse der Ko-Kontraktionsmuster zeigten einen allgemeinen neuromechanischen Effekt

des Kieferpressens auf die dynamische reaktive Gleichgewichtsleistung.

Die Studie in Kapitel 6 untersucht die neuromechanischen Effekte, die nach der Automa-

tisierung der Aufgabe des Kieferpressens auftreten. Basierend auf den Forschungsergeb-

nissen zum Dual-Task-Paradigma im Kontext des Lösens von Gleichgewichtsaufgaben

kann argumentiert werden, dass das Kieferpressen eine sekundäre Aufgabe ist, wenn es

gleichzeitig mit Gleichgewichtsaufgaben durchgeführt wird, daher könnten die Effekte

des Kieferpressens nicht auf spezifische neuromechanische Effekte der Kieferpressaktivi-

tät zurückzuführen sein, sondern hängen mit der Dual-Task-Situation im Allgemeinen zu-

sammen. Es ist noch nicht untersucht worden, ob die Effekte hauptsächlich mit den

Vorteilen der Dual-Task-Situation oder mit dem Kieferpressen selbst zusammenhängen.

In dieser Studie wurde dieses Problem mit einer Interventionsstudie angegangen und die

Auswirkungen des Kieferpressens auf die Leistung bei dynamischen reaktiven Gleichge-

wichtsaufgaben nach einem einwöchigen Kieferpressentraining untersucht, um zu prü-

fen, ob die Auswirkungen auf eine Dual-Task-Situation zurückzuführen sind. Die

Ergebnisse zeigten, dass die Automatisierung der Aufgabe des Kieferpressens keine er-

kennbaren Auswirkungen auf die dynamische reaktive Gleichgewichtsleistung hatte. Das

Zusammenpressen des Kiefers schien mit einigen Modifikationen der Reflexaktivitäten

verbunden zu sein, aber die Auswirkungen waren auf anterior-posteriore Störungen be-

schränkt. Es wurden hohe Lerneffekte bei der dynamisch-reaktiven Gleichgewichtsauf-

gabe festgestellt, die möglicherweise die Auswirkungen des Kieferpressens überdeckten.

Weitere Studien mit anderen Gleichgewichtsaufgaben mit geringeren Lerneffekten und

längeren Interventionszeiträumen wurden als notwendig erachtet.

Nachdem sich die vorangegangenen Kapitel auf das dynamische reaktive Gleichgewicht

konzentriert haben, bietet Kapitel 7 die erste Studie, in der die Auswirkungen des gleich-

zeitigen submaximalen Kieferpressens während des dynamischen steady-state Gleichge-

wichts untersucht wurden. Insbesondere wurde die steady-state Phase der dynamischen

Gleichgewichtsaufgabe nach der Kompensation der Perturbation auf der oszillierenden

Plattform analysiert. In verschiedenen Studien zur Haltungskontrolle wird der Körper-

schwerpunkt (KSP) als Kontrollvariable vorgeschlagen, obwohl eine experimentelle Über-

prüfung fehlt. Dennoch wurde gezeigt, dass der KSP ein wichtiger Parameter für das

Gleichgewicht ist. In dieser Studie wurde untersucht, ob das Zusammenpressen der Kiefer

Auswirkungen auf das Schwanken, die Kontrolle und die Stabilität des CoM während des

dynamischen Gleichgewichts hat. Neben der Analyse der räumlichen und zeitlichen Vari-

abilität des KSPs wurde ein Uncontrolled Manifold Ansatz mit einem Ganzkörper-Gelenk-

rum Kieferpressen zu einem besseren dynamischen reaktiven Gleichgewicht führt als

Zusammenfassung

Auswirkungen des Zusammenpressens der Kiefer oder des Zungenpressens auf die

Schwankung, die Kontrolle oder die Stabilität des KSPs.

Ähnlich der vorhergehenden Studie konzentrierte sich die Studie in Kapitel 8 auf die Aus-

wirkungen des Zusammenpressens der Kiefer auf das dynamische steady-state Gleichge-

wicht. In dieser Studie wurde eine andere dynamische Gleichgwichtsaufgabe verwendet.

Genauer gesagt wurde ein Stabilometer verwendet, um das dynamische steady-state

Gleichgewicht zu bewerten. Mit Hilfe eines dreiarmigen Interventionsstudien-Designs

sollten drei Forschungsfragen beantwortet werden: erstens, ob gleichzeitiges submaxi-

males Kieferpressen das dynamische Gleichgewicht verbessern kann; zweitens, ob die Ef-

fekte anhalten, nachdem die Aufgabe des Kieferpressens ihre Neuartigkeit und die die

damit verbundenen potenziellen Dual-Task Vorteile verloren hat; und drittens, ob die ver-

besserte Leistung des dynamischen Gleichgewichts mit einer verringerten Aktivität der

haltungsbezogenen Muskeln zusammenhängt. Die Ergebnisse zeigten, dass gleichzeitiges

submaximales Kieferpressen die dynamische Gleichgewichtsleistung auf dem Stabilome-

ter verbessert. Diese Effekte bleiben auch dann bestehen, wenn die Neuartigkeit der se-

kundären Aufgabe (d. h. das Zusammenpressen der Kiefer) abnimmt. Das sekundäre

Ergebnis zeigte, dass die Lerneffekte der verwendeten dynamischen Gleichgewichtsauf-

gabe hoch waren und die Effekte des Gleichgewichtstrainings überdeckt haben könnten.

Eine verbesserte dynamische steady-state Gleichgewichtsleistung führte zu einer gerin-

geren Aktivität der haltungsrelevanten Muskeln, was auf eine gesteigerte Bewegungsef-

fizienz hindeutet. Es gab jedoch keine mit dem Kieferpressen zusammenhängenden

Veränderungen der Muskelaktivitäten.

Kapitel 9 enthält eine allgemeine Diskussion der Ergebnisse aus den vorgestellten For-

schungsarbeiten. Die Kombination der Ergebnisse dieser fünf Studien ermöglicht ein um-

fassenderes Verständnis darüber, wie gleichzeitiges submaximales Kieferpressen das

Gleichgewicht in dynamischen Situationen beeinflusst. Im Wesentlichen liefert diese Ar-

beit zusätzliche Beweise für den Einfluss des kraniomandibulären Systems auf das Hal-

tungskontrollsystem, aber sie identifiziert nicht vollständig die zugrunde liegenden

neuromechanischen Mechanismen, die diesen Einfluss ausmachen. Die teilweise wider-

sprüchlichen Ergebnisse lassen sich durch die Aufgabenspezifität der Gleichgewichtsauf-

gaben erklären, können aber auch auf die noch unentdeckten Auswirkungen des

Kieferpressens zurückzuführen sein, die durch die hohen Lerneffekte der verwendeten

dynamischen Gleichgewichtsaufgaben überdeckt werden. Weitere Untersuchungen wer-

den empfohlen, um das Potenzial des Kieferpressens vollständig zu erfassen und die zu-

grunde liegenden neuromechanischen Effekte besser zu verstehen. Die vorgelegte

Dissertationsschrift schließt in Kapitel 10 mit einer Konklusion.

kinematikmodell verwendet. Die Ergebnisse dieser umfassenden Analyse zeigten keine

 
xi 
Table of Content 
Acknowledgements ..................................................................................................... i 
Summary ................................................................................................................... iii 
Zusammenfassung ..................................................................................................... vii 
List of Figures ........................................................................................................... xiii 
List of Tables ............................................................................................................. xv 
General Introduction ............................................................................................. 1 
1  Motivation.. ........................................................................................................ .1 
2  Outline of the thesis ..............................................................................................2 
Theoretical Background ......................................................................................... 5 
1  Postural control .....................................................................................................5 
2  Craniomandibular system .................................................................................. 16 
3  Craniomandibular system and postural control ................................................ 18 
Aims and Scope of this Thesis .............................................................................. 27 
1  Influence of jaw clenching on dynamic reactive balance .................................. 28 
2  Influence of jaw clenching on dynamic steady-state balance............................ 29 
Study I – Modulation of Postural Control – Dynamic Reactive Balance ................. 31 
1  Abstract………………………………………………………………………………………………………….31 
2  Introduction…………………………………………………………………………………………………..32 
3  Methods……………………………………………………………………….………………………………. 34 
4  Results……...........................................................................................................39 
5  Discussion………………………………………………………………………………..…. .................. 41 
6  Conclusion and Outlook ..................................................................................... 46 
Study II – Modulation of Reflex Activities – Dynamic Reactive Balance……………….. 47 
1  Abstract……………………………………………………………………………………………….………... 47 
2  Introduction ....................................................................................................... 48 
3  Methods………………………………………………………………………………………………….. ..... 49 
4  Results………………………………………………………………………………………………………..….54 
5  Discussion……………………………………………………………………………………………………... 55 
6  Conclusion and Outlook ..................................................................................... 60 
   

Table of Content 
xii 
Study III – Modulation of Jaw Clenching Effects after Jaw Clenching Training……… 61 
1  Abstract………………………………………………………………………………………………………. ... 61 
2  Introduction ........................................................................................................ 62 
3  Methods………………………………………………………………………………………………………. .. 64 
4  Results…………………………………………………………………………………………………………. .. 69 
5  Discussion………………………………………………………………………………………………….. .... 76 
6  Conclusion and Outlook ...................................................................................... 80 
Study IV – Modulation of Center of Mass Movement…………………………………………. 83 
1  Abstract……………………………………………………………………………………………..…………..83 
2  Introduction ........................................................................................................ 84 
3  Methods……………………………………………………………………………………………………. ..... 88 
4  Results…………………………………………………………………………………………………………. .. 96 
5  Discussion……………………………………………………………………………………………………. .. 98 
6  Conclusion and Outlook .................................................................................... 102 
Study V –Modulation of Postural Control – Dynamic Steady-State Balance………. 103 
1  Abstract. …………………………………………………………………………………………………….. 103 
2  Introduction ...................................................................................................... 104 
3  Methods……………………………………………………………………………………………………. ... 105 
4  Results……………………………………………………………………………………………………..…. .109 
5  Discussion…………………………………………………………………………………………………... . 112 
6  Conclusion and Outlook .................................................................................... 115 
General Discussion ............................................................................................ 117 
1  Influence of jaw clenching on dynamic reactive balance ................................. 118 
2  Influence of jaw clenching on dynamic steady-state balance .......................... 124 
3  Implications and recommendations ................................................................. 131 
Conclusion ......................................................................................................... 133 
References .............................................................................................................. 135 
Appendix ................................................................................................................ 165 
Supplementary material ......................................................................................... 167 
Statutory Declaration ............................................................................................. 175 
 
 

 
xiii 
List of Figures 
Figure 2.1: a. Posturomed. b. Stabilometer. .............................................................8 
Figure 2.2: Balance strategies ................................................................................ 12 
Figure 2.3: Uncontrolled Manifold (UCM) approach ............................................. 14 
Figure 4.1:  Reflective markers used for the five anatomical regions .................... 36 
Figure 4.2:  Damping ratio calculation. ................................................................... 38 
Figure 4.3:  Mean speed of anatomical regions ...................................................... 41 
Figure 5.1:  A participant is standing ...................................................................... 51 
Figure 5.2:  The EMG signal .................................................................................... 53 
Figure 6.1:  Study design ......................................................................................... 66 
Figure 6.2:  Calculation of damping ratio ................................................................ 68 
Figure 6.3:  Damping ratio results ........................................................................... 70 
Figure 6.4: a. RoMCoM_AP, RoMCoM_ML results, b. VCoM results .................................. 71 
Figure 7.1:  Participant during single-leg stand ...................................................... 89 
Figure 8.1:  a-b. Stabilometer. c. Experimental protocol. ..................................... 107 
Figure 8.2:  Time at equilibrium for two clenching conditions ............................. 110 
Figure 8.3:  Time at equilibrium for the three groups .......................................... 110 
Figure 8.4:  Time normalized iEMGs ..................................................................... 111 

 
xv 
List of Tables 
Table 4.1:  Damping ratio (DR) for all groups and directions ................................ 40 
Table 5.1:  iEMG of M. masseter and the suprahyoid ........................................... 54 
Table 5.2:  Co-contraction ratio (CCR) of selected muscle pairs ........................... 56 
Table 6.1:  Mean muscle activities for MA ............................................................ 72 
Table 6.2:  Mean muscle activities for anterio-posterior perturbations ............... 74 
Table 6.3:  Mean muscle activities for medio-lateral perturbations. .................... 75 
Table 7.1: Stomatognathic motor conditions of the three groups ....................... 90 
Table 7.2: The UCM, the path length (PL) and the DFA scaling ............................ 97 
Table 8.1:  Time at equilibrium (TAE) increases and iEMG decreases ................. 112 

1 General Introduction

1.1 Motivation

Postural control can be defined as the control of the body's position in relation to its sur-

roundings for the dual purposes of balance and orientation (Macpherson & Horak, 2013)

and it is vital in everyday living of human. Good postural control is linked to a lower risk of

falls (Rubenstein, 2006) and injuries (Hrysomallis, 2007), whereas poor postural control

can lead to a loss of functional independence and restricted involvement in everyday ac-

tivities.

The postural control system is the consequence of a complex interplay between the nerv-

ous and musculoskeletal systems. As a result, inputs from somatosensory (e.g., joint re-

ceptors), visual and vestibular systems are used to determine the position and movement

of the body in relation to its surroundings and translated into motor commands to main-

tain the balance in a task-specific manner (Shumway-Cook & Woollacott, 2017). The un-

conscious and (semi-) automated control of posture is sensitive to several internal and

environmental factors. For example, the lack of sensory information, neuromechanical

deficiencies or external perturbations can considerably disrupt postural control processes

(Horak, 2006). The degenerative decline of the sensory and neuromechanical systems, as

particularly seen in the elderly, may substantially worsen posture and its control

(Granacher, Muehlbaue, et al., 2011). Ultimately, the number of falls may increase due to

impaired balance and orientation. Consequently, falls and their medical impacts pose a

serious risk to the quality of life of individuals, and may lead to the loss of independence

or even death (Blake et al., 1988). On this basis, the conduction of research on postural

control and its influencing factors is particularly important.

Postural control was shown to be influenced by many factors such as age (e.g., Henry &

Baudry, 2019), neurological diseases (e.g., Delafontaine et al., 2020), training (e.g.,

Sherrington et al., 2020) and mood states (e.g., Cuccia & Caradonna, 2009). Recently, a

growing body of literature indicated that the craniomandibular system can also affect the

postural control system (Julià-Sánchez et al., 2015). Particularly, jaw clenching during bal-

ance tasks was shown to contribute to the stabilization of the body, therefore improving

balance. The shown effects were, however, limited to static conditions particularly to up-

right standing (Hellmann et al., 2011a, 2015; Ringhof, Leibold, et al., 2015; Ringhof, Stein,

1 General Introduction

et al., 2015), and have not yet been investigated under dynamic conditions. In addition,

the underlying neuromechanical mechanisms remain still unknown. Furthermore, prior

studies have demonstrated that the sensory information associated with dental occlusion

is used more strongly when more challenging postural control conditions are present (e.g.,

unstable or complex balance tasks, or external perturbations (Julià-Sánchez et al., 2016,

2019; Tardieu et al., 2009)). On this basis, it can be argued that the effects of simultaneous

jaw clenching would most likely be more pronounced in challenging or dynamic balance

tasks as opposed to static ones.

Focusing on the above-mentioned research gap, this thesis aimed to investigate the ef-

fects of simultaneous jaw clenching on dynamic balance, more precisely on dynamic re-

active and dynamic steady-state balance. A thorough understanding of the notable

features and underlying mechanisms of jaw clenching on dynamic balance discovered

within this thesis could broaden the limits of the current state of knowledge on the pos-

tural control and craniomandibular systems. The gained insights may ultimately be used

for fall prevention, clinical assessments as well as training tools.

1.2 Outline of the thesis

The present thesis includes ten main chapters. In this current chapter (Chapter 1) a gen-

eral introduction is provided. Chapter 2 covers the fundamentals of postural control as

well as of the craniomandibular system. Furthermore, the interrelation of the postural

control with the craniomandibular system is reviewed. Finally, the current state of the

knowledge regarding the effects of jaw clenching particularly on postural control is pro-

vided. Chapter 3 sums up the aims and scope of the present thesis.

The subsequent chapters (Chapters 4 to 8) encompass five research articles that aimed to

answer previously deduced research questions (Chapter 3). Each research article has been

published in an international peer-reviewed journal:

• Chapter 4: Study I - Modulation of Postural Control - Dynamic Reactive Balance

Fadillioglu, C., Kanus, L., Möhler, F., Ringhof, S., Schindler, H. J., Stein, T.*, & Hellmann, D.*

(2022). Influence of controlled masticatory muscle activity on dynamic reactive balance.

Journal of Oral Rehabilitation, 49, 327–336. [*These authors have contributed equally to

this work.]

1.2 Outline of the thesis

• Chapter 5: Study II - Modulation of Reflex Activities – Dynamic Reactive Balance

Hellmann, D*., Fadillioglu, C.*, Kanus, L., Möhler, F., Schindler, H. J., Schmitter, M., Stein,

T. & Ringhof, S. (2023). Influence of oral-motor tasks on postural muscle activity during

dynamic reactive balance. Journal of Oral Rehabilitation, 00, 1-9. [*These authors have

contributed equally to this work.]

• Chapter 6: Study III - Modulation of Jaw Clenching Effects after Jaw Clenching Training

Fadillioglu, C., Kanus, L., Möhler, F., Ringhof, S., Hellmann, D.*, & Stein, T.* (2023). Effects

of jaw clenching on dynamic reactive balance task performance after 1-week of jaw

clenching training. Frontiers in Neurology, 14, 1-13. [*These authors have contributed

equally to this work.]

• Chapter 5: Study IV - Modulation of Center of Mass Movement

Fadillioglu, C., Kanus, L., Möhler, F., Ringhof, S., Hellmann, D.*, & Stein, T.* (2022). Influ-

ence of Controlled Stomatognathic Motor Activity on Sway, Control and Stability of the

Center of Mass During Dynamic Steady-State Balance—An Uncontrolled Manifold Analy-

sis. Frontiers in Human Neuroscience, 16, 1-13. [*These authors have contributed equally

to this work.]

• Chapter 8: Study V - Modulation of Postural Control - Dynamic Steady-State Balance

Fadillioglu, C., Kanus, L., Möhler, F., Ringhof, S., Hellmann, D.*, & Stein, T.* (2023). Per-

sisting effects of jaw clenching on dynamic steady-state balance. PLOS ONE, 19(2) 1-12.

[*These authors have contributed equally to this work.]

Chapter 9 summarizes the main findings of the presented research articles and provides

a general discussion as well as implications and recommendations for future research. The

thesis ends with a conclusion given in Chapter 10.

2 Theoretical Background

2.1 Postural control

Postural control is essential for daily life of human. Good postural control is associated

with a decreased risk of falls (Rubenstein, 2006) and injuries (Hrysomallis, 2007), whereas

impaired postural control may lead to the loss of functional independence as well as to

reduced participation in daily life. By definition, postural control involves the control of

the body’s position in space to establish orientation and stability (Shumway-Cook &

Woollacott, 2017). Postural orientation involves the active alignment of the body seg-

ments with respect to each other, as well as with the environment (Horak, 2006).

Whereas, postural balance refers to the ability to control the center of mass (CoM) with

respect to the base of support (BoS). The term balance is used to describe the dynamics

of body posture to prevent falling (Winter, 1995), and it is often used interchangeably with

the terms (postural) stability or (postural) equilibrium (Horak, 2006; Shumway-Cook &

Woollacott, 2017). In this thesis, the term balance is used in the sense of (postural) stabil-

ity or (postural) equilibrium.

2.1.1 Types of balance

In the older literature (e.g., Fleishman, 1964; Schnabel et al., 2016) postural balance was

often considered as a general ability. Consequently, balance has usually been assessed by

using general tests such as one-leg stance regardless of the relevant factors, e.g., type of

sports involved or training conditions. For many years, this approach has been critically

discussed, and task-specificity of postural balance has been emphasized (e.g., Giboin et

al., 2015; Kümmel et al., 2016; Ringhof & Stein, 2018).

Even though there is not yet a universal single definition, in the literature it is commonly

distinguished between static and dynamic balance. In various sources, dynamic balance is

further divided into three sub-categories: steady-state, proactive (anticipatory) and reac-

tive (compensatory) balance (Kiss, Schedler, & Muehlbauer, 2018; Lesinski et al., 2015a;

Shumway-Cook & Woollacott, 2017).

Besides the task-specific characteristics of balance, the correlations between static and

dynamic balance were shown to be extremely low (Granacher, Bridenbaugh, et al., 2011;

Kiss, Schedler, & Muehlbauer, 2018), and different mechanisms are suggested to control

2 Theoretical Background

balance under static and dynamic conditions (Shimada et al., 2003). On this basis, it is

important to consider different balance types individually.

2.1.1.1 Static steady-state balance

Static steady-state balance comprises unperturbed conditions, in which BoS does not

change, for example during quiet standing or sitting. It is often also called “static balance”.

However, this term is also misleading because in fact the body, therefore the CoM, is al-

ways in motion, even during upright standing (Macpherson & Horak, 2013). The main rea-

son for that is that the human body is mechanically unstable because it consists of many

segments that are linked by joints (Macpherson & Horak, 2013). From a mechanical per-

spective, the maintenance of static steady-state balance requires that the downward pro-

jection of the CoM stays within the BoS. Thereby, the area and location of the BoS do not

change throughout the entire process (Shumway-Cook & Woollacott, 2017).

2.1.1.2 Dynamic steady-state balance

Dynamic steady-state balance involves fundamentally the maintenance of balance after

self-initiated disturbances (e.g., swinging the lower leg to step forward) during dynamic

conditions, for example during walking or running. When walking the vertical projection

of the CoM stays continuously out of the BoS, therefore, the body is in a continuous state

of imbalance. By placing the swinging leg forward, the BoS is actively moved under the

falling CoM, ultimately a possible fall situation is prevented (Shumway-Cook & Woollacott,

2017).

2.1.1.3 Dynamic proactive balance

The main difference between reactive and proactive balance is the predictability of the

perturbations. In the case of proactive balance, the perturbation is anticipated and com-

pensated before the balance is lost (Shumway-Cook & Woollacott, 2017; Winter, 1995).

An example from real life is the landing after a counter-movement jump or lifting an object

from the ground (Shumway-Cook & Woollacott, 2017).

2.1.1.4 Dynamic reactive balance

Dynamic reactive balance can be defined as the compensation of an unpredicted postural

disturbance to maintain the balance. These postural disturbances are mostly whole-body

perturbations, caused by surface translations and rotations (Lesinski et al., 2015a;

Shumway-Cook & Woollacott, 2017). Thereby, the aim is to bring the downward projec-

tion of CoM back into the BoS, either by changing the CoM, the BoS, or both of them. An

2.1 Postural control

example from real life is the slipping due to wet floors (Gielo-Perczak et al., 2006; S. Wang

et al., 2022).

2.1.2 Assessment of balance

Balance assessment can be divided into three main categories: functional assessments,

system assessments, and quantitative assessments (Horak, 1997; Mancini & Horak, 2010).

Functional assessments are helpful to monitor the balance status and changes with inter-

vention. The usually used tests rate performance on a set of scale-based motor tasks, e.g.,

total time in a particular posture (Horak, 1997).

System assessments are helpful in identifying the disordered subcomponents or mecha-

nisms underlying balance control, and therefore, help clinicians direct specific treatments

for their patients. For example, the Balance Evaluation Systems Test (BESTest) belongs to

this category (Horak et al., 2009). BESTest comprises 6 different balance control systems

such as “Biomechanical Constraints” or “Stability in Gait” so that a specific rehabilitation

programm can be designed depending on the type of balance deficits. In this way, BESTest

allows clinicians to determine the type of balance problems to create targeted treatments

for their patients.

Objective assessments are mostly based on the quantitative assessment methods of pos-

turography, which quantifies the body sway mostly by using force recordings (Duarte et

al., 2010; Richmond et al., 2021). Posturography is further divided into static and dynamic

posturography.

Static posturography is in fact not static but aims to assess postural sway during upright

standing. It has been traditionally assessed in a laboratory by using force plates to assess

the center of pressure (CoP) (Duarte et al., 2010). Despite their accuracy and popularity,

they are also disadvantageous due to their high costs and restricted use to laboratory con-

ditions. Recent innovations resulted in alternative inexpensive and portable tools for the

quantification of CoP, such as balance plates (Richmond et al., 2021).

Dynamic posturography involves mostly the application of perturbations which are usually

mechanical and generated by using movable, computerized support surfaces (Freyler et

al., 2015; Giboin et al., 2015; Mancini & Horak, 2010). Various devices have been devel-

oped for the balance assessment (Petró et al., 2017). For example, the oscillating plat-

forms (e.g., Posturomed, Figure 2.1a) are used to apply mechanical perturbations, and

ultimately to assess dynamic reactive balance. These systems consist of a rigid platform

2 Theoretical Background

that is connected to a main frame by springs. The platform can swing freely in the plane

horizontally to the ground. Typically, an electro-magnetic (Freyler et al., 2015; D. Schmidt

et al., 2015) or motorized system (Petró et al., 2017; Ringhof & Stein, 2018) is used to

apply perturbations.

Another device that is mostly used for dynamic posturography is the balance board. These

can be categorized according to their rotation axis (Petró et al., 2017). Sagittal axis balance

board (Petró et al., 2017), also known as a stabilometer, is a commonly used device to

assess dynamic steady-state and proactive balance in various studies (Kiss, Brueckner, &

Muehlbauer, 2018; Lehmann, 2022; Muehlbauer et al., 2022; Orrell et al., 2006). The task

to be performed is mostly to keep the platform horizontal to the ground by balancing the

weight distribution in a bipedal stance (Petró et al., 2017).

Another possibility is to use sensory perturbations to specifically manipulate one or more

sensory systems used for postural control (Mancini & Horak, 2010). These types of pertur-

bations ultimately help to understand the contribution and flexible reweighing of each

sensory information that contributes to postural control in altered conditions for example

by using virtual reality technologies (Ida et al., 2022; Ketterer et al., 2022).

Figure 2.1: a. Posturomed, the oscillating platform. b. Stabilometer, the sagittal axis balance board.

2.1 Postural control

2.1.3 Sensory aspects of postural control

The postural control system is a result of the complex interaction of the neural and mus-

culoskeletal systems. Thereby, the inputs from somatosensory (proprioceptive, cutane-

ous, and joint receptors), visual and vestibular systems are used as the source of

information about the position and the movement of the body with respect to the envi-

ronment (Shumway-Cook & Woollacott, 2017). In the case of standing on a rigid and stable

support surface with sufficient light in the environment, a healthy person mostly relies on

somatosensory inputs at 70%, vestibular inputs follow them at 20%, and finally, visual in-

puts come at 10% (Horak, 2006). Depending on the environmental conditions, movement

goals and availability of the sensory information, the CNS modifies the relative sensitivity

or weighting of different sensory inputs (Peterka, 2002; Peterka & Loughlin, 2004;

Macpherson & Horak, 2013). For example, when standing on an unstable surface, the sen-

sory weighting shifts in favor of vestibular and visual inputs (Horak, 2006).

Somatosensory inputs are essential for the timing and direction of automatic postural

responses. Thereby, somatosensory fibers provide the somatosensory information, and

they have two main characteristics: they are large in diameter and respond fast. The larg-

est and fastest sensory fibers are the Ia afferents from muscle spindles and Ib afferents

from Golgi tendon organs. Fibers located in cutaneous mechanoreceptors also contribute

to somatosensory input production. Somatosensory inputs are ultimately used to build

the neural map for the position of body segments with respect to each other and the

support surface (Macpherson & Horak, 2013).

Vestibular inputs are important for the assessment of body tilt with respect to gravity as

well as the direction of the body sway. Thereby, the otolithic organs of the vestibular ap-

paratus provide information about the direction of gravity and the semicircular canals

about the velocity of head movement. Unlike somatosensory inputs, vestibular infor-

mation is not important for the timing of the postural responses but for the directional

tuning of them. Vestibular information becomes especially critical in case of reduced vis-

ual inputs (e.g., at night in a dark room) and unstable support surfaces (e.g., on a boat)

(Macpherson & Horak, 2013).

Visual inputs provide information about the orientation and motion from both near and

far. They help to reduce body sway by providing stabilizing cues and orienting the body in

the environment. They also provide information for anticipatory postural adjustments

during voluntary movements. For example, during planning where to place the feet during

stair walking with obstacles on the steps. Visual inputs and their changes can have a pow-

erful influence on postural orientation. On the other hand, processing the visual

2 Theoretical Background

information is too slow, and therefore, does not affect the postural responses significantly

during balance recovery after sudden disturbances (Macpherson & Horak, 2013).

2.1.4 Central aspects of postural control

In general, postural control has two basic modes. The first one is the “feedback mode”

which comprises compensatory postural adjustments to maintain the balance. The second

one is the “feedforward mode” which basically refers to the anticipatory postural adjust-

ments to sustain the balance. In this mode, potential postural perturbations are foreseen

and avoided by properly self-initiated counter-movements. However, neural control of

balance is not binary, e.g., not either in the feedback or feedforward mode but rather a

combination of these modes is in use (Taube & Gollhofer, 2010). To date, there is little

consensus on how the postural control mechanisms are coordinated within the CNS

(Murnaghan et al., 2014). It is suggested that posture control is distributed in all levels of

the CNS, from the spinal cord to the cerebral cortex.

The spinal cord circuits are sufficient for the antigravity support but not for the automatic

balance responses (Macpherson & Horak, 2013). The information processing in the spinal

cord is the fastest and least flexible among all neural control structures involved in pos-

tural control (Taube & Gollhofer, 2010). Postural control through the spinal cord is mostly

accomplished via stretch reflexes, reciprocal inhibition, non-reciprocal inhibition and flex-

ion reflexes. Particularly for the static steady-state balance, stretch reflexes and non-re-

ciprocal inhibition (Ib inhibition) are essential (Takakusaki, 2017).

The brain stem is connected to the spinal cord and is thought to play an important role in

postural control based on animal experiments (Taube & Gollhofer, 2010). Previous studies

revealed that mammals can compensate for perturbations by using their spinal cord and

brain stem even though the connections to higher neural centers were destroyed

(Sherrington, 1907). Furthermore, the inhibition of brain stem activity resulted in the sup-

pression of postural balance adjustments (Vinay et al., 2005).

The cerebellum, together with the brain stem, is thought to produce automatic balance

responses. The brain stem and cerebellum are highly interconnected and work together

to integrate somatosensory inputs for balance (Macpherson & Horak, 2013). Based on the

clinical observations, it was suggested that the cerebellum plays an essential role in the

selection and memorization of compensatory reactions in a situation-specific way.

The cerebral cortex is thought to be involved in both anticipatory and compensatory pos-

tural reactions but has more control over anticipatory postural adjustments than

2.1 Postural control

compensatory ones. Most voluntary movements are initiated in the cerebral cortex. Albeit

the roles of specific areas of cerebral cortex in postural control are not clearly known, it is

known that the cortex plays an important role in learning complex postural strategies (e.g.,

when dancers learn new skills requiring high-balance performance) (Macpherson &

Horak, 2013).

2.1.5 Balance strategies in postural control

One of the important parameters regarding balance is the CoM. It is defined as the point

equivalent of the total body mass with respect to the global coordinates and calculated as

the weighted average of the CoM of each body segment in the three-dimensional space.

For the maintenance of balance, the CNS must control the position and the movement of

the body’s CoM as well as the body’s rotation around its CoM (Winter, 1995; Macpherson

& Horak, 2013).

During standing, the human body is often modeled as an inverted pendulum (Hof, 2008;

Lafond et al., 2004; Mergner et al., 2003; Milton et al., 2009; Shumway-Cook & Woollacott,

2017). On this basis, two main balance recovery strategies have been proposed: (1) Fixed-

support in which CoM is maintained over the BoS (Figure 2.2a-b), and (2) change-in-sup-

port (Figure 2.2c) in which the BoS is changed to capture the CoM (Winter, 1995). Depend-

ing on the current conditions, the CNS switches between these control strategies

(Shumway-Cook & Woollacott, 2017).

Fixed-support balance recovery strategies comprise two fundamental strategies. The first

one is the ankle strategy (Figure 2.2a) in which the body is modeled as a single-segment

inverted pendulum and moves at the ankle (Runge et al., 1999). It is basically in use while

standing on a firm support surface. The second one is the hip strategy (Figure 2.2b) in

which the body is modeled as a double-segment pendulum and moves both at the ankle

and hip. The hip strategy is preferred when the effectiveness of the ankle movement is

limited (e.g., standing on a narrow beam) (Horak et al., 1990; Runge et al., 1999). Even

though it was shown that the inverted pendulum motion shows high correlations with the

human motion in quiet standing (Gage et al., 2004), when the task becomes more com-

plex, the necessity for better models emerges.

2 Theoretical Background

Figure 2.2: Balance strategies: a. Fixed-support with ankle strategy modeled as a single-segment inverted pen-

dulum; b. Fixed-support with hip strategy modeled as a double-segment pendulum; c. Change-in-

support strategy with moving base of support.

2.1.6 CNS uses synergies to execute balance strategies in

postural control

During movement control, the CNS has to coordinate a redundant musculoskeletal system

(Bernstein, 1967) consisting of more degrees of freedom than necessary to complete the

given task (Latash et al., 2002). Various approaches have been suggested to explain the

CNS deals with this redundancy, such as motor programs (R. A. Schmidt et al., 2018), op-

timal control (Todorov & Jordan, 2002) or synergies (D’Avella et al., 2003; Latash et al.,

2007). Latash et al. (2007) define “synergy” as a neural organization consisting of a multi-

element system with elemental variables (EVs) and a performance variable (PV). Two fun-

damental features of synergy are: (1) the organization of task sharing among the EVs, and

(2) the stabilization of the PV by use of the co-variation among EVs. In the redundant mus-

culoskeletal system, different combinations of EVs may result in the same PV (i.e. equiva-

lent movement solutions). This co-varied movement solution space provides flexibility for

the control of movements. In this context, redundancy is not seen as a problem but it is

an advantage for the CNS, which is also known as the “motor abundance principle”

(Gelfand & Latash, 1998). One of the possibilities to assess the equivalent movement so-

lutions is the so-called “uncontrolled manifold (UCM)” approach. Within this approach, a

model is needed in which the changes in the EVs are related to the changes in the PV.

Ultimately, the effects of the co-varied movement of EVs on the PV are analyzed (Scholz

2.1 Postural control

& Schöner, 2014). According to the UCM approach, EV space is divided into two orthogo-

nal subspaces. One subspace, that is , consists of all EV configurations that result in

the same PV (Scholz & Schöner, 1999). The term UCM refers that the elements are less

controlled as long as they lie in this parallel space (Latash et al., 2002). In other words, the

motor control system allows the EVs to show high variability as long as the desired value

of the PV is obtained. On the other hand, the solutions lying orthogonal to the UCM

() are controlled by the motor control system because the co-variation of EVs in this

subspace results in a changed PV value.

In Figure 2.3, the UCM approach is schematically explained with a three-bar linkage sys-

tem with one fixed target. Figure 2.3a shows the three EVs, these are the three bars con-

nected with the three black joints each with one degree of freedom (i.e. rotation around

the joint), and one PV, which is the red target. Thereby, the aim is to hit with the yellow

ball this red target. In Figure 2.3b and Figure 2.3c, the illustrative solutions in  and

subspaces are shown, respectively. In the solution subspace lying parallel to the

UCM (i.e. ), the co-variation of the EVs results in a non-changed PV, whereas in the

orthogonal solution subspace (i.e. ), PV changes with the co-varying movement of

the EVs.

2.1.7 Influencing factors on postural control

Balance can be influenced by many factors such as age (M. Henry & Baudry, 2019; van den

Bogaart et al., 2022), neurological diseases (Delafontaine et al., 2020), training (Keller et

al., 2012; Ringhof et al., 2018; Sherrington et al., 2008, 2020; Taube & Gollhofer, 2010),

dual-task situation (Andersson et al., 2002; Wachholz et al., 2020), mood states and anxi-

ety levels (Bolmont et al., 2002; Cuccia & Caradonna, 2009; Wada et al., 2001), head and

neck orientation (Park et al., 2012; Szczygieł et al., 2016) as well as craniomandibular sys-

tem (Julià-Sánchez et al., 2015; Sforza et al., 2006; Treffel et al., 2016). In the following

passages, these effects are briefly introduced.

2.1.7.1 Age effects

Falls and fall-related injuries are some of the most important problems, especially for

older people (Dionyssiotis, 2012). The risk of falls increases with age due to reduced reac-

tion time and impaired movement strategies (Lizama et al., 2014; Maki & Mcilroy, 1999).

Further, the variability structure was also shown to be different in older people. Particu-

larly, it was been suggested that older people have reduced flexibility in controlling multi-

ple joints during recovery from balance perturbations (Hsu et al., 2013).

2 Theoretical Background

Figure 2.3: Uncontrolled Manifold (UCM) approach. a. Schematic design showing three elementary variables

and a performance variable; b. subspace includes solutions in which desired PV is achieved;

c.  subspace includes solutions in which desired PV is not achieved.

2.1.7.2 Neurological disease effects

Different neurological diseases were shown to reduce the ability to cope with balance

perturbations (Karamanidis et al., 2020; Rubenstein, 2006; Takakusaki, 2017). For exam-

ple, it was shown that Parkinson’s patients have difficulties in increasing their BoS after

perturbations during walking (Moreno Catalá et al., 2016). Similarly, stroke patients fall

more frequently after surface perturbations compared to their age-matched healthy con-

trol group and have poorer control during recovery (Salot et al., 2016).

2.1 Postural control

2.1.7.3 Training effects

It was shown that neuromechanical adaptations occur at different sites of the CNS after

balance training, and the plasticity of the spinal, corticospinal and cortical pathways are

highly task-specific (Keller et al., 2012; Ruffieux et al., 2017; Taube & Gollhofer, 2010;

Taube et al., 2008; Zech et al., 2010). Balance training was shown to have positive effects

on motor performance, particularly on postural control, jumping ability and strength

(Gruber et al., 2007; Taube et al., 2008). On this basis, it is traditionally recommended to

improve sports performance (Zech et al., 2010), to augment the rehabilitation process

(McKeon & Hertel, 2008) as well as to decrease the risk of falls (Gauchard et al., 1999).

However, previous studies showed that balance training improves balance in a task-spe-

cific way and the transfer between different balance tasks is very limited (Giboin et al.,

2015, 2019; Kümmel et al., 2016; Ringhof & Stein, 2018). Therefore, the effectiveness and

benefits of generic balance training are questioned.

2.1.7.4 Dual-task situation effects

It was shown that simultaneously performing additional motor tasks can improve the per-

formance on the secondary balance task. Based on the general motor control and learning

literature (e.g., Schmidt et al., 2018), it is assumed that performance diminishes in one or

both tasks when these two tasks are performed simultaneously, which can be attributed

to the limited attention capacity (Woollacott & Shumway-Cook, 2002). However, previous

literature indicated that the postural control may not necessarily show this feature but

the performance of a balance task with a secondary task may actually improve perfor-

mance compared to a single-task condition (Broglio et al., 2005). This special phenome-

non in the case of postural control was suggested to be associated with the altered

attention and increased automatization of postural control processes (Andersson et al.,

2002; Wachholz et al., 2020). However, it should be noted that a secondary task may not

necessarily enhance the performance in the executed tasks but can also result in no

change (Choi et al., 2023) as well as a decrease in performance. The adverse effects can

be attributed to the parallel sharing of a limited set of resources (R. A. Schmidt et al.,

2018). Nevertheless, various studies indicated dual-task benefits regarding postural con-

trol (Andersson et al., 2002; Broglio et al., 2005; Polskaia et al., 2015; Swan et al., 2004).

2.1.7.5 Mood and anxiety effects

Anxiety can be defined as an over-aroused state that prepares the body to react mentally

and physically to potentially dangerous situations (Hoehn-Saric & McLeod, 2000). Anxiety

was shown to affect balance in various studies (Bolmont et al., 2002; Cuccia & Caradonna,

2 Theoretical Background

2009; Wada et al., 2001). Particularly, changes in sensory organization were detected in

case of an altered mood or increased anxiety.

2.1.7.6 Head and neck orientation effects

The changes in the head and neck orientation were shown to affect balance (Park et al.,

2012; Szczygieł et al., 2016). Head constitutes about 8% of the average human body

weight (Park et al., 2012). Therefore, rapid changes in the head’s orientation may disturb

balance and require compensatory movements. Further, the head movements lead to al-

terations of sensory information from visual and vestibular receptors as well as their inte-

grations (Bove et al., 2009).

2.1.7.7 Craniomandibular system effects

Among others, the effects of the craniomandibular system on balance and its control were

shown in a number of studies (e.g., Bracco et al., 2004; Cuccia & Caradonna, 2009; Hegab,

2015; Julià-Sánchez et al., 2020; Solovykh et al., 2012; Tardieu et al., 2009). In the follow-

ing sections, the craniomandibular system, its neuronal connections to the rest of the

body, its basic functions as well as its effects on postural control are explained in detail.

2.2 Craniomandibular system

The stomatognathic system comprises a complex set of orofacial structures that are linked

by the sensorimotor neural connections peripheral to the CNS, and the craniomandibular

system is a fundamental part of the stomatognathic system (Cuccia & Caradonna, 2009;

Munhoz & Marques, 2009). The craniomandibular system includes the temporomandibu-

lar joints (TMJ), masticatory muscles with the ligaments around them as well as the neural

structures (Munhoz & Marques, 2009). The basic functions of the craniomandibular sys-

tem are to change and to maintain the mandibular position. Any failure in these functions

is mostly a part of the so-called temporomandibular disorders (TMD), which can be de-

fined as the pathologies affecting the TMJ or the jaw muscles, or both (Manfredini et al.,

2011; McNeill, 1997).

2.2.1 Neuronal connections of craniomandibular system

The nervous system and the stomatognatic system are shown to be anatomically close in

proximity. Therefore, these two systems easily interact with each other (Wu et al., 2023).

For example, an abnormality of the neurological function can lead to symptoms occurring

2.2 Craniomandibular system

in the stomatognatic system, such as facial paralysis (Tischfield et al., 2010) and salivation

(Newall et al., 1996). Conversely, the symptoms in the stomatognatic system may affect

the nervous system (De Wijer et al., 1996; Wu et al., 2023).

The nervous system consists of the CNS and the peripheral nervous system (PNS). The CNS

comprises basically the brain and spinal cord, whereas the PNS comprises cranial and spi-

nal nerves (Nowinski, 2017). The 6 of the 12 cranial nerves are associated with the stoma-

tognathic system. These are (1) the trigeminal nerve which is responsible for face

sensation and mastication; (2) the facial nerve which is responsible for face movement

and taste; (3) the glossopharyngeal nerve which is responsible for taste and swallowing;

(4) vagus nerve which is responsible for movement, sensation and abdominal organs; (5)

the accessory nerve which is responsible for neck movement; and (6) hypoglossal nerve

which is responsible for the muscles of the tongue (Romano et al., 2019; Shoja et al., 2014;

Snyder & Bartoshuk, 2016; Sonne & Lopez-Ojeda, 2022; Wu et al., 2023).

2.2.2 Jaw movement and its control

The jaw is moved by the jaw muscles in a complex three-dimensional manner. The jaw

muscles can be allocated into two groups depending on whether their contraction closes

or opens to jaws. Basically, there are three jaw-closing muscles (masseter, temporalis, and

medial pterygoid) and two jaw-opening muscles (lateral pterygoid and digastric) (Murray,

2016).

The movements of the jaws can be classified as voluntary, reflexive and rhythmical jaw

movements. These are generated by the participation of many parts of the CNS (Murray,

2016). Voluntary jaw movements, such as opening, closing, protrusive and lateral jaw

movements, are initiated from the cerebral cortex and passed on to the face motor cortex.

Whereas, reflexive jaw movements demonstrate pathways that contribute to the refine-

ment of the movements, and can be used by the higher motor centers for the generation

of more complex movements. Finally, the rhythmical movements, such as mastication or

chewing, are controlled by a central pattern generator in the brainstem. During mastica-

tion, the muscles of the tongue help in maneuvering the food bolus in the mouth, whereas

the muscles of the lip and cheek help to keep the food bolus within the mouth on the

occlusal table (Sessle et al., 2013).

Jaw clenching is a task in which mainly the jaw-closing muscles, especially the masseter,

are active (D’Amico et al., 2013). Thereby, the forces are generated by a complex co-acti-

vation of muscles involved in masticatory (Schindler et al., 2007). In the normal resting

2 Theoretical Background

position of the craniomandibular system, the teeth do not have sustained contact. On this

basis, a continuous act of jaw clenching (synonymously used also as “teeth clenching”

(e.g., de Souza et al., 2021; Hellmann et al., 2011a; Iida et al., 2010)) can be seen as an

abnormal parafunctional habit. Furthermore, jaw clenching is assumed to be a risk factor

for TMD (Magnusson et al., 2005; Velly et al., 2003). For example, bruxism, which is char-

acterized by grinding and clenching of the teeth (Lobbezoo et al., 2006), was also reported

to be a risk factor for neck pain (Lavigne et al., 2001; Testa et al., 2017) as well as for the

displacement of discs (Michelotti et al., 2010).

In the case of bruxism, the jaw-closing muscles are involuntarily activated both during

sleep as well as during wakefulness (Lobbezoo et al., 2018). Even though various studies

describe it as a disorder (e.g., Lobbezoo et al., 2006; Manfredini et al., 2013), according to

the international consensus published in 2018 “in otherwise healthy individuals, bruxism

should not be considered as a disorder but rather as a behavior that can be a risk (and/or

protective) factor for certain clinical consequences.” (Lobbezoo et al., 2018). Furthermore,

observational studies reported that increased masseter activity may occur as an uncon-

scious habit for example in case of initial accelerations during track and field activities

(Nukaga et al., 2016) or landing (Nakamura et al., 2017). On this basis, the question came

up if the jaw clenching can aid in performing sports or daily activities.

Over the years, jaw clenching has gained attention due to its potential beneficial effects

during sports activities (Ringhof, Hellmann, et al., 2015), such as counter movement jump

(Ebben et al., 2008) or strength training (de Souza et al., 2021) as well as for balance

(Hellmann, 2011a, 2015). On the other hand, there are still conflicting results regarding

its impact (Forgione et al., 1992; Ringhof et al., 2016) suggesting that the effects of jaw

clenching are limited to certain conditions. Furthermore, not only the changes in the ac-

tivities (e.g., jaw clenching (Hellmann et al., 2011; Tomita et al., 2021)) but also the relative

position of the cranomandibular system elements to each other (e.g., dental occlusion

(Bracco et al., 2004; Julià-Sánchez et al., 2015; Sakaguchi et al., 2007)) were shown to be

able to affect the human postural system. In the following chapter, the effects of the cra-

niomandibular system on postural control are introduced and explained in more detail.

2.3 Craniomandibular system and postural control

The craniomandibular system is suggested to be a close component of the upper body

(Khan et al., 2013). Over the years, a lot of research was conducted to investigate the

relationship between the postural control and the craniomandibular systems as well as

the clinical impacts (e.g., Cuccia & Caradonna, 2009; Nowak et al., 2023; Sakaguchi et al.,

2.3 Craniomandibular system and postural control

2007; Solovykh et al., 2012). Various changes in the position or in the activities of the

craniomandibular system were shown to affect the body posture and its control (e.g.,

Alghadir et al., 2015a, 2015b, 2015c; Gangloff et al., 2000; Hegab, 2015; Khan et al., 2013;

Kushiro & Goto, 2011; Munhoz & Marques, 2009; Sakaguchi et al., 2007; Treffel et al.,

2016). However, it should also be noted that not all previous studies support the relation-

ship between the postural control and craniomandibular systems (e.g., Ferrario et al.,

1996; Alghadir et al., 2022).

Based on the current state of research, the effects of the change in positions and activities

of the craniomandibular system are explained in more detail in the following two sections,

respectively. Even though there is not yet a clear finding showing directly why the cranio-

mandibular system may affect the postural control, there are possible mechanisms to ex-

plain these effects, which are introduced in more detail in Section 2.3.3.

2.3.1 Change in positions of craniomandibular system

Changes in the mandibular positions may lead to changes in the body posture (Cuccia &

Caradonna, 2009; Huggare & Raustia, 1992; Sakaguchi et al., 2007; Tardieu et al., 2009).

Conversely, the changes in the body posture may result in a changed mandibular position

(Khan et al., 2013; Lund et al., 1970; Tingey et al., 2001) as well as in a changed chewing

behavior (Iizumi et al., 2017).

The studies comprising the effects of the mandibular positions on balance mostly investi-

gate the changes in the occlusion. The term “occlusion” can simply be defined as the con-

tact between the teeth (J. R. Clark & Evans, 2001; Davies & Gray, 2001; Hassan & Rahimah,

2007). The occlusion can be further categorized as static occlusion and dynamic or func-

tional occlusion. Static occlusion refers to the teeth contact when the mandible is closed

and in a static condition (J. R. Clark & Evans, 2001; Davies & Gray, 2001), whereas dynamic

occlusion indicates the contact between teeth when the mandible moves relative to the

maxilla (Davies & Gray, 2001) or during functional tasks (J. R. Clark & Evans, 2001). The

studies comprising the occlusion effects on the balance refer mostly to static occlusion

(Bracco et al., 1998; Gangloff et al., 2000; Julià-Sánchez et al., 2015, 2020; Michelotti et

al., 2011; Nowak et al., 2023; Sakaguchi et al., 2007; Tardieu et al., 2009). However, even

if it is very rare, the term “occlusion” can also be used as a synonym for “jaw clenching”

(Hosoda et al., 2007). In this thesis, the term “jaw clenching” will be preferred over “oc-

clusion” if the term “occlusion” is used to refer to “jaw clenching”.

2 Theoretical Background

In various studies, the influences of static occlusion on balance were shown (Bracco et al.,

1998; Gangloff et al., 2000; Julià-Sánchez et al., 2015, 2020; Sakaguchi et al., 2007; Tardieu

et al., 2009). In a pilot study published in 1998, it was shown that the alterations in the

mandibular positions can influence posture (Bracco et al., 1998). In another study analyz-

ing different mandibular positions imposed by interocclusal splints, it was found that den-

tal occlusion may influence the static steady-state balance (Gangloff et al., 2000). Similarly,

Sakaguchi et al. (2007) analyzed the relationship between the mandibular positions and

body posture. The authors showed that changes in mandibular position may affect the

posture and conversely, the changes in posture may affect the mandibular positions. Julià-

Sánchez et al. focused in their various studies on the effects of dental occlusion on

balance, particularly in unstable or dynamic conditions (Julià-Sánchez 2015, 2016, 2019,

2020). They concluded that (1) dental occlusion may affect balance in unstable and dy-

namic conditions but not in stable conditions (Julià-Sánchez et al., 2015, 2020), (2) when

more difficult conditions for postural control (e.g., unstable conditions, external perturba-

tions or fatigue) are present, the sensory information linked to the dental occlusion comes

more strongly into effect (Julià-Sánchez et al., 2016, 2019). Similarly, Tardieu et al. (2009)

investigated the effects of dental occlusion for static and dynamic balance with eyes open

as well as eyes closed conditions. The authors found that steady-state balance was influ-

enced by dental occlusion in dynamic conditions and in the absence of visual information.

Based on their findings, they suggested that the sensory information associated with the

dental occlusion becomes relevant when the balance task is challenging, and its im-

portance grows as the other sensory cues become scarce.

Besides the studies focusing on dental occlusion, there are also studies investigating the

malocclusion effects on balance. In a previous study, it was reported that the participants

with anterio-posterior malocclusion showed worse static steady-state balance (Nowak et

al., 2023). On the other hand, there are also studies suggesting no effect of occlusion on

balance. In an overview study focusing on the relationship between dental occlusion and

posture (Michelotti et al., 2011), the authors suggested not to perform occlusal or ortho-

dontic treatment in order to treat or prevent postural imbalances. In another study ana-

lyzing the effects of static and dynamic occlusion on balance among people with

blindness, it was suggested that the alterations in static and dynamic jaw positions do not

influence static steady-state balance (Alghadir et al., 2022). However, it should be noted

that the studies suggesting no effects of dental occlusion on balance focused on static

steady-state balance and did not investigate the effects on dynamic balance.

Besides the occlusion-related research, there are also studies that focused on the changes

in the tongue position (Alghadir et al., 2015a; Russo et al., 2020). Alghadir et al. (2015a)

investigated the effects of deliberately changed tongue position on the static steady-state

2.3 Craniomandibular system and postural control

balance during quiet standing on an unstable surface with closed eyes. Compared with

the habitual jaw position, tongue positioning against the upper incisors enhanced the bal-

ance in static steady-state conditions. On the other hand, Russo et al. (2020) compared

three different tongue positions in static steady-state conditions but with open eyes. The

authors did not detect any significant differences between different tongue positions in

terms of static steady-state balance performance. However, it should be noted that

Alghadir et al. (2015a) used the velocity of the center of gravity to operationalize the static

steady-state balance, whereas Russo et al. (2020) used the sway of the CoP.

2.3.2 Change in activities of craniomandibular system

Change in the activities of the craniomandibular system was shown to influence the pos-

tural control system, such as chewing (Alghadir et al., 2015b; Kushiro & Goto, 2011) or jaw

clenching (Alghadir et al., 2015c; Hellmann et al., 2011a; Nakamura et al., 2017; Ringhof,

Leibold, et al., 2015; Tanaka et al., 2006; Tomita et al., 2021; Treffel et al., 2016).

2.3.2.1 Chewing

Chewing is a part of daily activities for healthy people since it is used to break down food

within a series of movements. Chewing cycles are semiautomatic motor behavior and in-

volve well-trained muscles (Hellmann et al., 2011a; Lund, 1991). Kushiro and Goto (2011)

analyzed the effects of chewing gum on the static steady-state balance and found that CoP

stability increased during the mastication of chewing gum. On the other hand, in another

study analyzing the changes in body oscillations during unilateral chewing of a rubber

cube and maximum jaw clenching, no effects of these jaw motor tasks on body sway could

be detected (Hellmann et al., 2011a).

2.3.2.2 Jaw clenching

To date, there are several types of research investigating jaw clenching effects on postural

control. The first experiment addressing this issue was published at the end of the 20th

century. Ferrario et al. (1996) investigated the effects of maximum jaw clenching in centric

occlusion as well as on two cotton rolls and reported no effects of jaw clenching on static

steady-state balance. Four years later, Takada et al. (2000) showed that voluntary jaw

clenching may contribute to the facilitation of lower limb muscles through enhanced H-

reflex, which may improve the stability of the movements. However, they did not assess

the static steady-state balance performance. Sforza et al. (2006) conducted a pilot study

with male astronauts to find out if the effects of maximum jaw clenching differ when it is

performed on a splint compared to without a splint condition. They reported that a

2 Theoretical Background

functionally more symmetric mandibular position resulted in a more symmetric ster-

nocleidomastoid muscle contraction pattern as well as less body sway. In another study

by Hosoda et al. (2007), it was investigated if jaw clenching with 50% of the maximum

voluntary contraction (MVC) of the masseter muscle may affect the latency of the center

of gravity movement initiation after external perturbations (i.e. dynamic reactive bal-

ance). It was reported that the greater the external perturbation the shorter the latency

in jaw clenching conditions, whereas in non-clenching conditions, the latency increased

with increasing external perturbation. In the same year, Tanaka et al. (2006) published a

paper analyzing the influence of jaw clenching with 50% and 100% MVCs on head move-

ment and body sway after an external perturbation generated by a striking 3 kg weight.

They found that jaw clenching leads to less head movement as well as to less body sway

after the impact, and suggested that jaw clenching with a 50% MVC was more suitable

than the maximum jaw clenching condition in terms of stabilizing effects.

After 2010, the relationship between jaw clenching and postural control gained more at-

tention as a research topic (Alghadir et al., 2015c; Hellmann et al., 2011a, 2012, 2015;

Nakamura et al., 2017; Ringhof et al., 2016; Ringhof, Stein, et al., 2015; Tomita et al., 2021;

Treffel et al., 2016). Hellmann et al. (2011a) analyzed the effects of a series of jaw motor

tasks on the CoP displacements during upright standing. These jaw motor tasks included

jaw clenching at submaximal forces of 50 to 300 N, maximal jaw clenching unilateral and

bilateral as well as unilateral chewing. Compared with the mandibular rest positions, sub-

maximal jaw clenching led to significant reductions in body sway, as evidenced by a

smaller area of the CoP confidence ellipses, whereas unilateral chewing and maximal jaw

clenching tasks did not alter body sway. The subsequent studies specifically focused on

submaximal jaw clenching rather than maximal jaw clenching (Hellmann et al., 2015;

Ringhof, Leibold, et al., 2015; Ringhof, Stein, et al., 2015). Ringhof, Stein, et al. (2015)

found that CoP sway area, as well as CoP path lengths, were significantly reduced during

jaw clenching compared to the open-mouth, non-clenching control conditions for both

bipedal narrow stance and unipedal stance on dominant and non-dominant legs. In a fol-

low-up study (Hellmann et al., 2015), lower limb joint angles as well as muscle co-contrac-

tions were analyzed. Submaximal jaw clenching and non-clenching conditions did not

show any significant differences for the mean values of the lower limb joint angles,

whereas standard deviations were significantly lower during submaximal jaw clenching.

Furthermore, reductions in the joint ROMs and angular velocities as well as in the co-

contraction ratios were detected for the submaximal jaw clenching condition. The authors

concluded that submaximal jaw clenching influences muscular co-contraction patterns

which result in enhanced kinematic precision. In a further experiment by Ringhof, Leibold,

et al. (2015), it was examined if the clenching of the fist would also lead to similar

2.3 Craniomandibular system and postural control

reductions in COP sway. The findings of the study revealed that both jaw clenching and

fist clenching result in reduced COP displacements, and the two conditions did not differ

significantly from each other. It was suggested that concurrent muscle activation signifi-

cantly improves static steady-state balance possibly by facilitation of human motor excit-

ability. Michalakis et al. (2019), investigated how body weight distribution changes during

concurrent maximum jaw clenching as well as during asymmetrical jaw clenching (with 1

mm disocclusion on the right and left sides). They reported that jaw clenching and occlu-

sional stability may lead to a medio-lateral shift of the weight distribution but not to an-

terio-posterior shifts. One of the key findings of the study was that the participants shifted

their body weight opposite to the jaw clenching side. The authors suggested that this phe-

nomenon occurs to prevent falling. Based on the theory of Yoshino et al. (2003), they ar-

gumented that the participants tend to change the head position towards the jaw

clenching side, consequently, the weight distribution of the upper body and the lower

limbs shifted towards the opposing side of the jaw clenching side in order to compensate

the shifted head position.

Besides the various studies analyzing the jaw clenching effects on static steady-state bal-

ance, it was also investigated in a few studies how jaw clenching affects dynamic balance

(Nakamura et al., 2017; Ringhof et al., 2016). In a pilot study analyzing the jaw clenching

effects on landing after a jump (Nakamura et al., 2017), jaw clenching effects on dynamic

balance during landing were found to be limited. However, the jaw clenching task was not

constrained to a force level but it was reported to be at least 20% of maximum jaw clench-

ing and increased up to nearly 400%. In another study conducted by Ringhof et al. (2016),

the effects of submaximal jaw clenching on dynamic postural stability and joint kinematics

during balance recovery after forward loss of balance compared with non-clenching con-

dition were investigated. The authors found no effects of submaximal jaw clenching on

balance recovery and lower limb joint kinematics, which was in contrast to their findings

regarding the stabilizing effects of submaximal jaw clenching during upright standing

(Ringhof, Leibold, et al., 2015; Ringhof, Stein, et al., 2015). They attributed this to the dif-

ferent control strategies used for static steady-state balance and balance recovery after

simulated forward falls. Further, they argued that reductions in CoP displacement –which

leads to enhanced static steady-state balance– would not necessarily improve the balance

under dynamic conditions but even may degrade it. Finally, they suggested that future

studies should investigate the submaximal jaw clenching effects on dynamic reactive bal-

ance after unexpected perturbations as well as compared with open mouth and habitual

conditions.

2 Theoretical Background

2.3.3 Possible mechanisms of craniomandibular system effects

on postural control

Although the exact mechanisms are still unknown, the contribution of the functional sta-

tus of the stomatognathic system in the postural balance was shown to be about 2%

(Solovykh et al., 2012). There are several approaches that try to explain why and how the

stomatognathic system has an influence on posture and its control.

One essential explanation is that the trigeminal nerve plays a key role in the connection

between the stomatognathic system and the postural control system. The trigeminal

nerve is neuroanatomical connected to the several structures associated with postural

control. The mandibular nerve, which innervates the masseter muscle together with the

other masticatory muscles, is one of the three branches of the trigeminal nerve

(Buisseret-Delmas et al., 1999; Julià-Sánchez et al., 2015; Paya-Argoud et al., 2019). Fur-

thermore, in the morphological studies, it was shown that the vestibular nuclear complex

and the spinal trigeminal nuclei are connected in rats (Devoize et al., 2010; Ruggiero et

al., 1981).

Subsequent investigation regarding the influences of the craniomandibular system was

the modulation of reflexes (Boroojerdi et al., 2000; Miyahara et al., 1996; Takada et al.,

2000; Tuncer et al., 2007). In the late 1800s, Erno Jendrassik conducted groundbreaking

research on voluntary jaw clenching and its neuromechanical effects. He discovered that

in patients with neurological impairments, jaw clenching and pulling apart flexed and

hooked fingers may enhance lower limb reflexes (Jendrassik, 1885). This effect was named

after him and known as “the Jendrassik maneuver”. Various studies reported that the Jen-

drassik maneuver has potentiating effects on the reflexes as well as on the motor-evoked

potentials of the upper and lower body muscles (Bussel et al., 1978; Delwaide & Toulouse,

1981; Dowman & Wolpaw, 1988; Gregory et al., 2001; Sugawara & Kasai, 2002; Zehr &

Stein, 1999). Similar to the Jendrassik maneuver, jaw clenching is used commonly to en-

hance the Hoffmann (H)-reflex and motor-evoked potentials of both lower and upper limb

muscles (Boroojerdi et al., 2000; de Souza et al., 2021; Miyahara et al., 1996; Tuncer et al.,

2007).

The H-reflex, which is a component of the stretch reflex, was first identified by Hoffmann

(1918). Hoffman discovered a reflex reaction in the calf muscles that occurred after stim-

ulation of the posterior tibial nerve, and he showed that its latency response was equiva-

lent to the Achilles reflex (Fisher, 2012). By use of H-Reflex, the alpha motoneuron

excitability can be estimated and nervous system responses to several neuromechanical

2.3 Craniomandibular system and postural control

conditions can be assessed. It is one of the most frequently used tools for investigating

electrophysiological alterations at the spinal level and activates motor units recruited

monosynaptically via the afferent pathway (Grosprêtre & Martin, 2012; Misiaszek, 2003;

Tuncer et al., 2007). Various studies showed that the H-reflex can be facilitated through

voluntary jaw clenching (de Souza et al., 2021; Mitsuyama et al., 2017; Miyahara et al.,

1996; Takada et al., 2000; T. Takahashi et al., 2003; Tuncer et al., 2007; Yamanaka et al.,

2000). Through voluntary jaw clenching, the area representing the face in the motor cor-

tex is activated. This could spread to the other areas of the motor cortex representing the

upper and lower extremities (Boroojerdi et al., 2000). Increased excitability in the motor

system during both preparation and execution of jaw clenching motor task was shown in

various studies (Komeilipoor et al., 2017; Sugawara et al., 2005; M. Takahashi et al., 2006).

Another explanation for the jaw clenching effects on postural control may be the dual-

task paradigm. Generally, when two tasks are performed concurrently, performance in

one or both tasks diminishes (R. A. Schmidt et al., 2018). This phenomenon can be ex-

plained by the restricted attention capacity (Woollacott & Shumway-Cook, 2002). In the

case of postural control, however, past research has shown that integrating a secondary

task with a balance task may actually improve performance when compared with a single

task condition (e.g., Broglio et al., 2005). Changes in attention and greater automatization

of postural control processes can explain this effect (Andersson et al., 2002; Wachholz et

al., 2020). On this basis, it may be argued that concurrent jaw clenching, as a secondary

novel task, has enhancing effects on postural control. Therefore, it is essential to under-

stand if these effects are related to dual-task benefits or specifically to neuromechanical

effects caused by concurrent jaw clenching.

3 Aims and Scope of this Thesis

As introduced in more detail in Section 2.3, changes in the craniomandibular system were

shown to influence the postural control system, for example through dental occlusion or

concurrent clenching of the jaw. Previous studies indicated that when more difficult con-

ditions regarding postural control are present (e.g., unstable or challenging tasks, external

perturbations, or fatigue), the sensory information associated with dental occlusion

comes more strongly into play (Julià-Sánchez et al., 2016, 2019; Tardieu et al., 2009). On

this basis, it can be argued that the effects associated with the activities of the cranioman-

dibular system, such as jaw clenching, may also become more important during dynamic

or challenging balance tasks.

The influences of concurrent jaw clenching on static steady-state balance have been in-

vestigated in various studies (e.g., Hellmann et al., 2015; Ringhof, Leibold, et al., 2015;

Ringhof, Stein, et al., 2015) but its effects on balance under dynamic conditions have not

yet been considered in detail (Nakamura et al., 2017; Ringhof et al., 2016). Since the ef-

fects observed during one balance task may not always be transferable to another (Giboin

et al., 2015; Kümmel et al., 2016; Ringhof & Stein, 2018), the question arises if the influ-

ences of concurrent jaw clenching shown during static steady-state balance tasks would

also be observed during dynamic balance tasks. In addition, dynamic balance was sug-

gested to be more related to the risk of falling than static balance (Rubenstein, 2006).

Therefore, understanding balance under dynamic conditions may reveal important find-

ings regarding fall prevention as well as rehabilitation. In this thesis, it was focused on the

two sub-categories of dynamic balance, these are dynamic reactive balance and dynamic

steady-state balance, under the influence of jaw clenching.

The next five chapters (Chapter 4-8) contain five research articles. Chapter 4-6 contain

articles on jaw clenching effects on dynamic reactive balance, whereas, the following two

chapters (Chapter 7-8) focused on dynamic steady-state balance.

The research comprised within this thesis consisted of two main experiments. In the first

experiment, it was focused mainly on the influences of jaw clenching on dynamic reactive

balance, whereas in the second experiment, the jaw clenching effects on dynamic steady-

state was investigated. All of the measurements were carried out at the BioMotion Center

of the Institute of Sports and Sports Science (IfSS) at Karlsruhe Institute of Technology

(KIT) between 2019-2022. All of the papers were published in international peer-reviewed

3 Aims and Scope of this Thesis

journals between 2022-2024. The research was supported by the German Research Foun-

dation (Grant numbers: STE 2093/4-1 and HE 6961/3-1; STE 2093/4-3 and SCHM 2456/6-

3) and conducted in collaboration with the Department of Prosthodontics at the Univer-

sity of Würzburg, the Department of Sport and Sport Science at the University of Freiburg,

the Department of Diagnostic and Interventional Radiology at the University of Freiburg

and the Dental Academy for Continuing Professional Development in Karlsruhe.

3.1 Influence of jaw clenching on dynamic

reactive balance

As explained in more detail in Section 2.1.1, dynamic balance refers to postural control

either in advance of or in response to internal and external perturbations, as opposed to

static balance which deals with postural control under unperturbed conditions (Shumway-

Cook & Woollacott, 2017). Dynamic balance is essential for maintaining balance in daily

life, for example during walking or standing on a train that suddenly accelerates. Another

important point is that most falls occur under dynamic conditions, e.g., stumbling and

slipping during walking (Blake et al., 1988; Hiscock et al., 2014). On this basis, it is vital to

search for ways to improve dynamic balance. Previous studies showed that concurrent

jaw clenching may decrease the body sway and, therefore improve static steady-state bal-

ance but its effects during dynamic reactive balance have not yet been discovered.

As introduced in Section 2.1.1, dynamic reactive balance can be defined as the compen-

sation of an unpredicted perturbation. These perturbations es are mostly mechnical per-

turbations, caused by surface translations and rotations (Lesinski et al., 2015a; Shumway-

Cook & Woollacott, 2017). The first experiment of this research aimed at investigating the

effects of submaximal jaw clenching on dynamic reactive balance which was assessed by

the use of an oscillating platform with unexpected mechanical perturbations. The objec-

tives of the first experiment were first, to find out if submaximal jaw clenching has acute

enhancing effects on dynamic reactive balance (Chapter 4); second, to distinguish if the

acute effects of jaw clenching were specifically due to neuromechanical effects of this

oral-motor task or more generally due to activities of the craniomadibular system, by com-

paring jaw clenching with tongue pressing condition (Chapter 4); third, to investigate the

reflex activities and co-contraction behavior of the trunk and lower limb muscles under

the effects of three oral-motor tasks, these are jaw clenching, tongue pressing and habit-

ual stomatognatic behavior (Chapter 5); and fourth, to examine if the effects of jaw clench-

ing were associated with dual-task benefits or specifically due to neuromechanical

modulations associated with jaw clenching (Chapter 6). To reach the above-mentioned

3.2 Influence of jaw clenching on dynamic steady-state balance

objectives, a three-armed intervention experiment with 64 participants was carried out

whose details are given in the following three chapters (Chapter 4-6).

3.2 Influence of jaw clenching on dynamic

steady-state balance

Besides the dynamic reactive balance, another important dynamic balance category is the

dynamic steady-state balance, which can be defined as the maintenance of balance after

self-initiated perturbations, such as swinging the lower leg to step forward during walking

or running (Shumway-Cook & Woollacott, 2017).

The later steady-state phase of the balance task on the oscillating platform from the first

experiment was considered in this part of the thesis. Thereby, the research question to be

answered was if the sway, control and stability of the CoM during dynamic steady-state

balance were affected by the effects of submaximal jaw clenching or tongue pressing com-

pared with habitual stomatognatic behavior condition (Chapter 7).

The second experiment was carried out mainly to address the influences of concurrent

submaximal jaw clenching during dynamic steady-state balance which was assessed by

use of a stabiliometer. The objective of the second experiment was threefold: first, if con-

current submaximal jaw clenching enhances dynamic steady-state balance; second, if jaw

clenching effects persist when this secondary task loses its novelty and the increased at-

tention associated with it; and third, if the better dynamic steady-state balance perfor-

mance is associated with decreased muscle activities.

4 Study I – Modulation of Postural

Control - Dynamic Reactive Balance

Published as

Fadillioglu, C., Kanus, L., Möhler, F., Ringhof, S., Schindler, H. J., Stein, T.*, & Hellmann,

D.* (2022). Influence of controlled masticatory muscle activity on dynamic reactive

balance. Journal of Oral Rehabilitation, 49, 327–336. [*These authors have contrib-

uted equally to this work.]

4.1 Abstract

Background: The influence of the stomatognatic system on human posture control has

been investigated under static conditions, but the effects on dynamic balance have not

yet been considered. Objective: Investigating the influence of different functional stoma-

tognatic activities (jaw clenching (JAW), tongue pressing (TON) and habitual jaw position

(HAB)) on postural performance during a dynamic reactive balance task.

Methods: Forty- eight physically active and healthy adults were assigned to three groups

differing in oral- motor tasks (JAW, TON or HAB). Dynamic reactive balance was assessed

by an oscillating platform which was externally perturbed in four directions. Performance

was quantified by means of Lehr's damping ratio. Mean speeds of the selected anatomical

regions (head, trunk, pelvis, knee and foot) were analysed to determine significant per-

formance differences.

Results: The groups differed significantly in balance performance in direction F (i.e. for-

wards acceleration of the platform). Post-hoc tests revealed that the JAW group had sig-

nificantly better performance compared with both the HAB and TON groups. Better

performance was associated with a decreased mean speed of the analysed anatomical

regions.

Conclusion: JAW can improve dynamic reactive balance but the occurrence of positive

effects seems to be task- specific and not general. TON seems not to have any observable

effects on dynamic reactive balance performance, at least when evaluating it with an os-

cillating platform. JAW might be a valuable strategy which could possibly reduce the risk

of falls in elderly people; however, further investigations are still needed.

4 Study I – Modulation of Postural Control - Dynamic Reactive Balance

4.2 Introduction

Posture control has a vital role in human daily life. It ensures that movements are initiated

and executed in an optimal manner both in static and in dynamic conditions (e.g., upright

standing and locomotion, respectively) (Shumway-Cook & Woollacott, 2017). It involves

controlling the body’s position with respect to the environment for the dual purposes of

stability and orientation (Shumway-Cook & Woollacott, 2017). Multiple sensory signals

from visual, somatosensory and vestibular systems acting on the spinal and supraspinal

structures of the central nervous system (CNS) are used to detect and correct instability

in posture and to ensure balance (Takakusaki, 2017). Depending on the balance task at

hand, the CNS adapts the weighting and thereby the relative importance of sensory sig-

nals. Finally, the sensory information must be transformed into motor commands to en-

sure the body's balance in a task-specific manner. However, the functioning of these

underlying postural control mechanisms is not yet fully understood (Peterka, 2002;

Shumway-Cook & Woollacott, 2017). Impaired human postural control may lead to a re-

duced participation in daily life, an increased risk of falls and even to increased mortality

risk (Rubenstein, 2006).

The significance of the abovementioned sensory systems shows that postural control may

also be influenced by motor activity in the masticatory system (Julià-Sánchez et al., 2020).

There are a variety of studies indicating an influence of stomatognatic motor activity in

the form of chewing, tongue activity or different clenching conditions in different jaw re-

lations on human balance and posture under static conditions (Alghadir et al., 2015c;

Gangloff et al., 2000; Hellmann et al., 2011a, 2015; Julià-Sánchez et al., 2020; Ringhof et

al., 2016; Ringhof, Stein, et al., 2015; Sakaguchi et al., 2007). This means, in particular, a

reduced body sway in the anterior-posterior direction (Hellmann et al., 2011a), a reduced

variability of muscular co-contraction patterns of posture-relevant muscles of the lower

extremities and reduced trunk and head sway under the influence of controlled biting

activities (Hellmann et al., 2015). This might be interpreted as a body sway stabilizing ef-

fect. These facts in conjunction with the observations of an improved responsiveness to

auditory and visual stimuli (Garner & Miskimin, 2009), and relevant effects on force devel-

opment (Forgione et al., 1992) under the influence of biting activities, might be of clinical

relevance for the prevention of falls in elderly people. For this group there is evidence for

an increased risk of falling resulting from an insufficient dental or prosthetic status

(Mochida et al., 2018; Okubo et al., 2010).

There are several possible explanations for the measured effects of masticatory muscle

activity on posture control. First, this could be explained by the stimulation of periodontal

4.2 Introduction

receptors or by the different proprioceptive input due to different jaw relations that are

centrally integrated along with other sensory input (Boroojerdi et al., 2000). It is also con-

ceivable that the motor activity in the masticatory system facilitates the excitability of the

human motor system in a manner similar to the Jendrassik manoeuvre (Jendrassik, 1885),

which in turn increases the neural drive to the distal muscles (Ebben, 2006; Ebben et al.,

2008). A challenge in interpreting the results of these studies is the methodological het-

erogeneity and the phenomenon of interactions between postural and cognitive tasks,

shown in physiological and neurocognitive studies (Fraizer & Mitra, 2008). Therefore, an

integrative interpretation of the results appears difficult. However, a variety of neurome-

chanical interactions, for instance synchronized extension–flexion movements of the head

during jaw-opening/closing cycles (Eriksson et al., 2000), substantially increased ampli-

tudes of the human soleus H-reflex during voluntary teeth clenching (Boroojerdi et al.,

2000; Miyahara et al., 1996), neck muscle reflex responses triggered by trigeminal stimu-

lation (Abrahams et al., 1993; Alstermark et al., 1992) and co-contractions of the mastica-

tory and neck muscles (G. T. Clark et al., 1993; Giannakopoulos et al., 2018) are also

evidence for the close functional integration of the masticatory system in human motor

control processes. The neuroanatomical basis for all these phenomena was shown in ani-

mal models in the form of numerous neuroanatomical connections of the trigeminal nerve

within the brainstem, and projections to all levels of the spinal cord (Contreras et al., 1982;

Ruggiero et al., 1981).

As mentioned above, the influence of the masticatory system on human posture control

has been investigated under static conditions. The studies showed that oral-motor activi-

ties such as jaw clenching may contribute to increased postural stability, represented in

terms of decreased postural sway in upright bipedal und unipedal standing (Hellmann et

al., 2011a, 2015; Ringhof et al., 2016; Ringhof, Stein, et al., 2015). To the best of our

knowledge, the effects of motor activity of the masticatory system on dynamic balance

have not yet been investigated in depth (Ringhof et al., 2016).

Therefore, the aim of this study was to investigate the influence of different functional

stomatognatic activities on postural performance during a dynamic reactive balance task,

which was operationalized with an oscillating platform perturbed in different directions.

It was hypothesized that jaw clenching (JAW) and tongue pressing (TON) would influence

dynamic reactive balance performance. These changes in task performance were hypoth-

esized to be associated with specific adaptations in the segmental kinematics of the hu-

man body. The results of this study may contribute to the understanding of postural

control mechanisms, particularly in conjunction with the masticatory system, and might

bring up initial hypotheses as to whether masticatory muscle activity might reduce the

risk of falls.

4 Study I – Modulation of Postural Control - Dynamic Reactive Balance

4.3 Methods

4.3.1 Participants

Forty-eight physically active adults (25 female, 23 male; age: 23.8 ± 2.5 years; height: 1.73

± 0.09 m; body mass: 69.2 ± 11.4 kg) participated in the study. Their dominant legs were

determined based on self-reports or, in case of uncertainty, by means of test trials on the

oscillating platform (Ringhof & Stein, 2018). All participants gave written informed consent

prior to the study. They confirmed that they were physically active (participating in any

kind of sports regularly, at least 3 times per week), naive to the Posturomed task and had

no muscular or neurological diseases. They had also no signs and symptoms of temporo-

mandibular disorders (assessed by means of the RDC/TMD criteria (Manfredini et al.,

2011)), and presented with full dentition (except for 3rd molars) in neutral occlusion. The

study was approved by the Ethics Committee of the Karlsruhe Institute of Technology.

4.3.2 Experimental procedure

4.3.2.1 Balance tasks

Dynamic reactive balance was assessed by use of an oscillating platform, the Posturomed

(Haider-Bioswing, Weiden, Germany). This commercial device consists of a rigid platform

(12 kg, 60 cm × 60 cm) and eight 15-cm steel springs of identical strength, and can swing

along the horizontal plane in all directions. The Posturomed has previously been used in

scientific studies to systematically investigate dynamic reactive balance performance after

perturbations (Freyler et al., 2015; Kiss, 2011a). In the present study, an automatic cus-

tom-made release system was used to slowly displace the Posturomed horizontally (up to

2.5 cm) in one of the four possible directions: back (B), front (F), left (L), right (R). The

directions used here indicate, by convention, the direction to which the platform was ac-

celerated after release (e.g., B indicates that the platform was accelerated backwards after

release, which led to anterior body sway relative to the platform).

The perturbations were applied randomly in one of the four directions. The participants’

task was to compensate the perturbation as quickly as possible. Before each trial, partici-

pants were asked to stand on the platform on their dominant leg, with hands placed at

their hips, eyes focusing at a target whose height was adjusted to their eye level in ad-

vance and which was horizontally 4 m away from the center of the platform. Trials were

considered invalid if participants quitted performing their oral-motor task (JAW and TON),

4.3 Methods

had ground contact with the non-standing foot, changed the placement of their standing

foot, released one of the hands from the hip or lost their balance.

4.3.2.2 Group assignment and oral-motor tasks

For the assignment, each of the 48 participants had a familiarization period on the Pos-

turomed consisting of two static trials and two trials with perturbation. Afterwards, a

baseline measurement with perturbation and in habitual biting condition was performed

to determine the initial balance performance based on Lehr’s damping ratio (DR) (Kiss,

2011a). Based on the subjects’ baseline performance value and gender, a balanced assign-

ment to the three groups was ensured such that the initial level of performance difference

between groups is minimized. The statistical examination by means of a one-way analysis

of variance (ANOVA) revealed no baseline performance differences between the three

groups (p = 0.767). The three groups each consisting of 16 participants had to concurrently

fulfil one of the following oral-motor tasks during each trial of the experiment:

- JAW: instructed, controlled submaximal jaw clenching - activity of the mastica-

tory muscles during simultaneous occlusal loading,

- TON: instructed, controlled submaximal tongue pressing against the palate - sto-

matognatic muscle activity without occlusal loading,

- HAB: habitual stomatognatic behavior - jaw positioning without any instruction.

The respective oral-motor activity was measured by means of EMG recordings (detailed

information in the “Data collection” section). As a reference, the JAW group were trained

in submaximal jaw clenching at a force of 75 N by use of a RehaBite (Plastyle GmbH,

Uttenreuth, Germany), a medical training device consisting of liquid-filled plastic pads and

working based on hydrostatic principles (Giannakopoulos, Rauer, et al., 2018), just before

the measurements. During the training, EMG activity was monitored and training ended

once the participant achieved a consistent biting force at 75 N (resulting in a mean EMG

activity of about 5% maximum voluntary contraction, MVC). The corresponding EMG level

of biting activity was used later to determine if the submaximal jaw clenching condition

was met during the experiment. During the balance task measurements, the JAW group

performed the clenching task on an Aqualizer intraoral splint (medium volume; Dentrade

International, Cologne, Germany). The TON group also received training, which consisted

of applying a submaximal force with the tip of the tongue against the anterior hard palate.

For TON, the training ended once the participants achieved a consistent EMG activity at

5% of their MVC, measured in the region of m. digastricus venter anterior. For both groups,

training for the oral-motor task lasted approximately five minutes. The HAB group did not

receive any training or instructions.

4 Study I – Modulation of Postural Control - Dynamic Reactive Balance

4.3.2.3 Data collection

A wireless EMG system (Noraxon, Scottsdale, USA) operating at 2000 Hz was used to

measure EMG activity of the masseter for JAW and HAB; and of the suprahyoid muscles

of the floor of mouth (FoMM) for TON, measured in the region of the digastricus venter

anterior muscle. The skin over the corresponding muscles was carefully shaved, abraded

and rinsed with alcohol. Bipolar Ag/AgCl surface electrodes (diameter 14 mm, center-to-

center distance 20 mm; Noraxon Dual Electrodes, Noraxon, Scottsdale, USA) were posi-

tioned and oriented bilaterally in accordance with the European Recommendations for

Surface Electromyography.35 Afterwards MVC tests were performed.

Movements of the Posturomed platform and the participants were captured by a 3-D mo-

tion capture system (Vicon Motion Systems; Oxford Metrics Group, Oxford, UK; 10 Van-

tage V8 and 6 Vero V2.2 cameras with a recording frequency of 200 Hz). Four reflective

markers were fixed on the upper surface of the platform. Twenty reflective markers were

attached to the participants’ skin as shown in Figure 4.1.

Figure 4.1: Reflective markers used for the five anatomical regions (ARs).

After training for the oral-motor task (except for the HAB group), balance task measure-

ments began. Participants repeatedly stood on the platform, as described in the section

“Balance tasks”, and trials were recorded for 30 seconds. Between each trial, participants

had 2 minutes of resting time to prevent fatigue. Measurements ended once the partici-

pants completed 12 valid trials, three for each direction.

4.3 Methods

4.3.2.4 Data analysis

In total, 576 trials (48 participants, three valid trials for each of the four directions) were

analyzed. All data were recorded in Vicon Nexus 2.10 and exported for further processing

in MATLAB R2020a (MathWorks; Natick, USA).

Marker data were filtered by use of a fourth-order Butterworth low-pass filter with a cut-

off frequency of 10 Hz. Raw EMG data were filtered from 10 to 500 Hz by use of a fourth-

order Butterworth band-pass filter, rectified and smoothed using a sliding average with a

window frame of 30 ms and normalized to the MVC amplitudes (Hellmann et al., 2015).

To determine the respective mean EMG activities of the measured stomatognatic muscles

before and after perturbation, two time windows were used. Before: from 2500 ms before

the perturbation until the beginning of the perturbation; after: from the beginning of the

perturbation to the third maximum amplitude (Figure 4.2), which corresponds to the DR

window. The EMG activity of the measured stomatognatic muscles before and after per-

turbation for three trials for each direction and for each subject were averaged. Finally, R

and L directions were re-sorted into ipsilateral (I) and contralateral (C) according to the

standing leg of the participants.

Using the Posturomed marker data, DR (Eq. 4.1) (Kiss, 2011a) was calculated for each trial

to evaluate the dynamic reactive balance performance. DR is a parameter that relates the

actual damping to the critical damping value at which the system does not oscillate. It was

calculated for the third amplitude (Figure 4.2) as suggested by Kiss et al. (2011a) In other

words, DR in the present study quantified how well the platform was stabilized within the

first three oscillations, with larger DR values representing stronger damping and thus bet-

ter compensation of the perturbation.

4 Study I – Modulation of Postural Control - Dynamic Reactive Balance

Figure 4.2: Damping ratio calculation. Initial maximum displacement (K0) and the third positive amplitude (K3)

are shown.

In addition to DR as a measure of the performance, segmental kinematics were studied to

analyze the underlying movement patterns. Similar to Ringhof et al. (Ringhof, Stein, et al.,

2015), the body was divided into five anatomical regions (ARs; head, upper body, pelvis,

knee and foot). For each AR, the centers were calculated using the markers shown in Fig-

ure 4.1 (head = RFHD, LFHD, RBHD, LBHD; upper body = CLAV, STRN, C7, T10; pelvis = RASI,

LASI, RPSI, LPSI; knee = RKNE_med, RKNE_lat or LKNE_med, LKNE_lat; foot = RMAL_med,

RMAL_lat or LMAL_med, LMAL_lat). The resulting path lengths in 3D were calculated for

the time window defined by the DR. Since the size of this time window is trial-specific,

each path length of an AR was divided by the corresponding time window for each subject

and trial. This ultimately corresponds to the mean AR speed.

 

 ; where 



     positive amplitude

(4.1)

4.3.3 Statistics

IBM SPSS Statistics 25.0 (IBM Corporation, Armonk, NY, USA) was used to perform statis-

tical tests. Performance parameters (DR) and kinematics parameters (mean AR speed) for

three trials for each direction and for each subject were averaged. Kolmogorov-Smirnov

and Mauchly’s sphericity tests were conducted to confirm the normality and sphericity of

4.4 Results

data distribution, respectively. Greenhouse–Geisser estimates were used to correct for

violations of sphericity.

Each of the four perturbation directions were analyzed separately for performance evalu-

ation since postural response may differ depending on the perturbation direction (Akay &

Murray, 2021; C. Chen et al., 2014; Freyler et al., 2015; Kiss, 2011b; Nonnekes et al., 2013).

For each direction, a one-way ANOVA was performed to compare the groups’ balance

performances. For the segmental kinematics, a two-way ANOVA [5 ARs x 3 groups] was

calculated if significant results were present at the performance level. Tukey post-hoc tests

were performed in case of significant differences. The level of significance for all statistical

tests was set a priori to p < 0.05. Partial eta squared (small effect: 

 < 0.06; medium

effect: 0.06 < 

 < 0.14; large effect: 

 > 0.14) (Richardson, 2011) and Cohen’s d (small

effect: d < 0.50; medium effect: d = 0.5 – 0.8; large effect d > 0.8) (Cohen, 1992) were

calculated as measures of effect size for ANOVA and post-hoc tests, respectively.

4.4 Results

4.4.1 Oral-motor task

All participants in each group fulfilled their individual oral-motor task, in the sense that it

was performed before the perturbation and during their balance recovery.

- JAW: mean EMG activity of the musculus masseter was 5.59 ± 3.72% MVC before

perturbation and 4.89 ± 3.04% MVC after perturbation.

- TON: all participants showed a mean EMG activity of 3.96 ± 2.35% MVC of the

FoMM before the perturbation, and of 3.44 ± 2.06% MVC after perturbation.

- HAB: 3 of the 16 participants showed consistent habitual clenching mean EMG

activity of the musculus masseter of 6.12 ± 3.30% MVC before perturbation and

6.39 ± 2.64% MVC after perturbation. The remaining 13 participants consistently

showed a constant resting EMG activity of the musculus masseter of 0.31 ± 0.22%

MVC before perturbation and 0.34 ± 0.29% MVC after perturbation.

4.4.2 Dynamic balance performance

The mean time window of DR was 1.13 ± 0.01 s. The ANOVA results revealed that groups

had significantly different performances in direction F (forwards acceleration of the plat-

form after release) with a high effect size (p < 0.001, 

= 0.349). According to the post-

4 Study I – Modulation of Postural Control - Dynamic Reactive Balance

hoc test results, the JAW group had a significantly higher DR compared to both HAB

(p = 0.001, d = 1.03) and TON (p < 0.001, d = 1.40) groups with high effect sizes.

There were no significant differences in the remaining directions (B: p = 0.226, 

= 0.064;

I: p = 0.920, 

= 0.004; C: p = 0.607, 

= 0.022). The statistical results as well as the mean

and the standard deviation of DRs for each group and each direction are shown in Table

4.1.

Table 4.1: Damping ratio (DR) for all groups and directions and the corresponding ANOVA results.

DRs are given as mean ± standard deviation. B = back, F = forward, I = ipsilateral, C = contralateral.

4.4.3 Segmental kinematics

Segmental kinematics were analyzed for direction F because it was the only direction that

showed a significant difference between groups. The results of two-way ANOVA [5 ARs x

3 groups] revealed a significant group effect with a medium effect size (p < 0.001,



= 0.09) and a significant AR effect with a high effect size (p < 0.001, 

= 0.83). However,

there was no interaction effect between groups and ARs (p = 0.550, 

= 0.03). An over-

view of the mean AR speed data is illustrated in Figuree 4.3.

Jaw clenching

(JAW)

Tongue pressing

(TON)

Habitual

(HAB)





0.062 ± 0.003

0.055 ± 0.003

0.226

0.064

0.066 ± 0.003

0.046 ± 0.003

0.049 ± 0.003

< 0.001

0.349

0.045 ± 0.003

0.046 ± 0.008

0.048 ± 0.005

0.920

0.004

0.043 ± 0.005

0.040 ± 0.004

0.037 ± 0.004

0.607

0.022

4.5 Discussion

Figure 4.3: Mean speed of anatomical regions (ARs) for all groups. Error bars show ± SD.

The post-hoc test results for the group effect showed that the JAW group had significantly

lower speeds compared to both HAB (p < 0.001, d = 2.80) and TON (p < 0.001, d = 2.97)

groups with high effect sizes. The post-hoc test for the AR effect showed that the foot had

the highest mean speed, and that it was significantly higher than the other regions with

high effect sizes (knee: p < 0.001, d = 1.59; pelvis: p < 0.001, d = 4.57; trunk: p < 0.001,

d = 4.63; head: p < 0.001, d = 3.43). The knee had the second highest mean speed, and

this was significantly higher than the pelvis (p < 0.001, d = 3.99), trunk (p < 0.001, d = 4.04)

and head (p < 0.001, d = 2.39), each with a high effect size. The mean speeds of the trunk

and pelvis did not differ significantly from each other and were the lowest among all ARs.

The head had a significantly higher mean speed than the pelvis (p = 0.036, d = 0.69) and

trunk (p = 0.009, d = 0.74) with medium effect sizes.

4.5 Discussion

The aim of this study was to investigate the effects of motor activity of the masticatory

system in the form of jaw clenching (JAW) and tongue pressing (TON) on dynamic reactive

balance performance, and to subsequently explain significant performance effects on the

level of segmental kinematics. This study showed that JAW enhanced the dynamic reac-

tive balance performance significantly in the forward direction of perturbation, demon-

strated by an increased DR that was accompanied by a decreased mean speed of the

analyzed ARs. In the remaining three directions, no significant changes occurred. Based

on these findings, two conclusions can be drawn: (1) JAW can improve dynamic reactive

4 Study I – Modulation of Postural Control - Dynamic Reactive Balance

balance but the occurrence of the positive effects seems to be task-specific and not gen-

eral. (2) TON seems not to have any observable effects on dynamic reactive balance per-

formance, at least when evaluated on an oscillating platform.

4.5.1 Jaw clenching improves dynamic reactive balance

in a task-specific way

Dynamic reactive balance was assessed by use of an oscillating platform which was ran-

domly perturbed in four different directions. The four directions of perturbation were an-

alyzed independently, as suggested by Freyler et al. (2015) because muscle spindles

provide different information dependent on the direction as well the velocity of perturba-

tions (Akay & Murray, 2021). In addition, the direction of surface translation is an im-

portant factor for the sensation, processing and output of the postural responses (Freyler

et al., 2015; Nonnekes et al., 2013). Therefore, the four directions of perturbations were

treated as different tasks.

The participants’ task was to compensate the perturbations as quickly as possible. To be

able to assess the quality of the task solution, the DR was chosen because it is a proper

method to characterize reactive balancing capacity after sudden perturbations (Kiss,

2011a). The results for the DR parameter revealed that jaw clenching improved the dy-

namic reactive balance performance only in the F direction. This finding is in line with the

perturbation direction dependency of postural control (Akay & Murray, 2021; C. Chen et

al., 2014; Freyler et al., 2015; Kiss, 2011b; Nonnekes et al., 2013). Explicitly, F indicates

that the platform was accelerated forwards after release, which led to posterior accelera-

tion of the body with respect to the support surface. A study analyzing the effects of the

type and direction of support surface perturbation on postural responses (C. Chen et al.,

2014) showed that a forward translation is more unstable than a backward one and led to

faster muscle activation as well as to faster and larger hip and knee joint movements. In

another study comparing postural responses to backward and forward perturbations

(Nonnekes et al., 2013), it was shown that a startling auditory stimulus resulted in better

postural control but only in the backward body sway condition. Therefore, the authors

suggested that postural responses to backward and forward perturbations may be pro-

cessed by different neural circuits. In line with these findings, dynamic reactive balance

performance improvement in direction F may be attributed to a higher difficulty level of

the task compared to direction B. It may also be a reasonable explanation that JAW is

associated with adaptations in neural circuits that are recruited during forward translation

of the platform.

4.5 Discussion

Segmental kinematics were analyzed in direction F, aiming at understanding the underly-

ing postural control strategies that improved dynamic reactive balance. The two main

findings were: (1) across the three groups the foot had the highest mean speed, followed

by the knee and head. The mean speeds of the trunk and pelvis did not differ from each

other and were lower than the mean speeds of the foot, knee and head; (2) the JAW group

had a lower mean AR speed compared to both the HAB and TON groups. Consequently,

the different oral-motor tasks did not affect the relationship between regional mean

speeds (see also Figure 4.3).

The finding that the trunk and pelvis had the lowest mean speed across ARs may be ex-

plained by the stability prioritization of proximal segments over distal ones during balanc-

ing (Hughey & Fung, 2005; Munoz-Martel et al., 2019). The speeds of lower body ARs were

higher than the head, possibly because platform perturbations are compensated mainly

at the knee and ankle joints and the head remains stiller compared to lower body regions.

In addition, the participants were instructed to fix their gaze at a stationary target, which

possibly also contributed to the lower speed of the head. The second main finding, that

the JAW group had an overall lower speed in ARs than the HAB and TON groups, may be

attributed to enhanced body stiffness, similar to the study by Ringhof, Stein, et al. (2015)

However, merely based on mean AR speed results, it is difficult to draw this conclusion.

Therefore, in future studies the activity of trunk muscle groups should be analyzed.

4.5.2 Influence of stomatognatic motor behavior on dynamic

reactive balance performance

There is no consensus in existing literature about the effects of jaw clenching on motor

behavior. It can possibly be explained by the stimulation of periodontal receptors or by

the different proprioceptive inputs due to different jaw relations. Another explanation

could be the facilitation of human motor system excitability. In the present study, we hy-

pothesized that both JAW and TON would influence dynamic reactive balance perfor-

mance. This could be due to either neurophysiological coupling or an effect shown in

posture-cognition studies, showing that the release of attention away from balance con-

trol and towards a secondary task - in this case, to clench or press the tongue against the

palate - can enhance postural stability (Fraizer & Mitra, 2008). In the latter case, both JAW

and TON would enhance postural stability. Since there is no significant difference between

TON and HAB in the present study, it was concluded that dynamic reactive balance per-

formance improvement was not associated only with the stomatognatic motor activity in

general or with the dual-task paradigm. Contrarily, the significant differences between the

JAW and the HAB/TON groups indicate a specific effect of instructed jaw clenching activity

4 Study I – Modulation of Postural Control - Dynamic Reactive Balance

but in a task specific manner. It should also be noted that both the partial eta squared

(

= 0.349) and Cohen’s d (dJAW-HAB = 1.03 and dJAW-TON = 1.40) results indicated high effect

sizes for group comparisons which strengthen the explanatory power of the results con-

siderably and minimize the possibility that the findings were random effects.

A secondary finding of this study is that participants in the HAB group showed different

oral-motor behaviors. While in 13 participants the mandible was in a resting position with

no relevant muscle activity of the jaw closing muscles, three participants showed clench-

ing activity in the sense of muscle activity comparable to the JAW group. The percentage

distribution of these different habitual motor behaviors is consistent with available data

regarding the prevalence of awake bruxism (Manfredini et al., 2013). Since the clenching

activity was performed before the perturbation and during the balance recovery, in these

individuals clenching might also be part of the physiological repertoire during coping with

demanding motor tasks (Ringhof et al., 2016). However, further studies are needed to clar-

ify this hypothesis.

Another interesting finding was regarding the segmental kinematics. Mean AR speeds of

these three participants in the HAB group with clenching were larger than the mean AR

speeds of the HAB group without clenching as well as than those of the TON group and

the JAW group (See Appendix S1 Table1). This might indicate that conscious, non-habitu-

ated clenching has a different influence on balance behavior in comparison to participants

who perform clenching as a part of their physiological repertoire. However, this hypothe-

sis is vague and needs to be investigated in further studies.

4.5.3 Limitations

All the participants were physically active adults. Accordingly, statements can only be

made for this age group. Deliberate care was taken to ensure a homogeneous sample to

minimize altered postural control mechanisms due to, for example, age (M. Henry &

Baudry, 2019) or neurological disorders (Delafontaine et al., 2020). The participants were

allocated into three groups with different oral-motor tasks. On the one hand, this can be

considered as a limitation because of the possible baseline performance differences be-

tween groups. However, in order to overcome this problem, a baseline measurement was

conducted in habitual biting condition to parallelize the three different groups in terms of

both performance and gender. The statistical results revealed no baseline performance

differences between the three groups (p = 0.767). One might think that it would have been

purposeful if all subjects had performed all oral-motor tasks. However, "habitual" in this

study meant that no instruction was given regarding the status of the masticatory organ.

4.5 Discussion

Thus, an unconscious, ancestral behavioral pattern of the masticatory system during the

balancing task could be expected. By definition, an "instructed" behavioral pattern can

never correspond to an unconsciously performed behavior. An instructed “habitual” oral-

motor behavior would have potentially resulted in dual-task effects and therefore, it

would have been ultimately difficult to distinguish between cognitive and postural effects

(i.e. thinking about the instructed behavior and performing different oral-motor tasks, re-

spectively). On the other hand, building of three groups provided two main advantages.

Firstly, possible carry over effects between different oral-motor tasks were avoided. For

example, some physiological effects could have still existed after jaw clenching or tongue

pressing such as an increased excitability of the human motor system or muscles of the

masticatory system in a fatigued state. Secondly, if all the participants conducted all of the

three oral-motor tasks for each of the four directions separately, the valid trials needed

would be 36. Considering the invalid trials as well, the total trials conducted could increase

to a level at which fatigue set in and data quality decrease consequently.

In this study, the Posturomed oscillating platform was chosen to assess dynamic reactive

balance performance. The Posturomed is a widely-used device for scientific studies as well

as for training or rehabilitation (Freyler et al., 2015; Kiss, 2011a; Munoz-Martel et al.,

2019; Petró et al., 2018). However, it should be noted that stabilizing a moving platform

represents a different balance task than balancing the body on a rigid surface

(Alizadehsaravi et al., 2020). Therefore, it is worth adding that the results in this study

cannot directly be transferred to stable ground conditions (e.g., recovering from a pertur-

bation during upright standing on a rigid surface), since balance performance under vari-

ous dynamic balance conditions cannot be considered directly interchangeable (Ringhof

& Stein, 2018).

The dynamic balance performance was assessed by use of DR as suggested in other stud-

ies (Kiss, 2011a; Petró et al., 2018). Mean speed of ARs was chosen for kinematic analysis

following Ringhof, Stein, et al. (2015) as explained in detail in the “Data analysis” section.

Despite being widely used parameters, it is important to note that the calculation of these

parameters is based on linear methods, and such traditional approaches for assessing pos-

tural stability may not fully characterize the non-linear properties of postural control

(Cavanaugh et al., 2005). Therefore, it would be advisable to perform non-linear analysis

using, for example, maximum-Lyopunov exponent (Munoz-Martel et al., 2019) or entropy

measures (Cavanaugh et al., 2005) to further extend the knowledge regarding the effects

of oral-motor activity on postural control.

4 Study I – Modulation of Postural Control - Dynamic Reactive Balance

4.6 Conclusion and Outlook

The aim of this study was to investigate the influence of different functional stomatognatic

statuses (i.e. JAW, TON, HAB) on postural performance during a dynamic reactive balance

task. To the best of our knowledge, this study was the first to analyze the effects of JAW

on dynamic reactive balance performance and also the first to investigate the effects of

TON related to postural control. The results showed that JAW improves dynamic reactive

balance but the occurrence of the positive effects seems to be task-specific and not gen-

eral. Improved dynamic balance performance of the JAW group was associated with over-

all decreased speeds of ARs, but without any AR-specific changes due to functional

stomatognatic status. In addition, TON seems not to have any observable effects on dy-

namic balance performance, at least when evaluating it with an oscillating platform. The

results show that dynamic reactive balance performance improvement in this study was

not associated with stomatognatic motor activity per se or the with dual-task paradigm,

but in particular with jaw clenching activity.

Therefore, the direction-dependent improvement in dynamic reactive balance perfor-

mance due to JAW should be investigated in more detail. For this purpose, future studies

should analyze control strategies at the muscular level, such as muscular co-contractions,

to reveal if postural control in the presence of controlled oral-motor activities leads to

stiffer joints in a directionally dependent manner in the Posturomed task. Subsequently,

an in-depth analysis of adaptations in motor coordination on a kinematic as well as on a

muscular level would be useful, for example by use of matrix factorization algorithms to

extract kinematic (Federolf, 2016) or muscle synergies (Munoz-Martel et al., 2019).

Considering the initially stated potential clinical relevance of this study in terms of an in-

fluence of oral-motor training on the risk of falls, it is too early to draw final conclusions.

However, previous studies have found jaw clenching can stabilize body sway in the ante-

rior-posterior direction under static conditions (Hellmann et al., 2015; Ringhof, Stein, et

al., 2015), similar to results from the present study under dynamic conditions. This might

therefore be an aspect which should be further investigated, since it might be a valuable

strategy which could reduce the risk of falls in general or maybe especially in elderly peo-

ple.

5 Study II – Modulation of Reflex

Activities – Dynamic Reactive Balance

Accepted version of the paper published as

Hellmann, D*., Fadillioglu, C.*, Kanus, L., Möhler, F., Schindler, H. J., Schmitter, M., Stein,

T. & Ringhof, S. (2023). Influence of oral-motor tasks on postural muscle activity

during dynamic reactive balance. Journal of Oral Rehabilitation, 00: 1-9. [*These

authors have contributed equally to this work.]

5.1 Abstract

Background: Jaw clenching improves dynamic reactive balance on an oscillating platform

during forward acceleration and is associated with decreased mean sway speed of differ-

ent body regions.

Objective: It is suggested that jaw clenching as a concurrent muscle activity facilitates hu-

man motor excitability, increasing the neural drive to distal muscles. The underlying mech-

anism behind this phenomenon was studied based on leg and trunk muscle activity

(iEMG) and co-contraction ratio (CCR).

Methods: Forty-eight physically active and healthy adults were assigned to three groups,

performing three oral-motor tasks (jaw clenching, tongue pressing against the palate, or

habitual lower jaw position) during a dynamic one-legged stance reactive balance task on

an oscillating platform. The iEMG and CCR of posture-relevant muscles and muscle pairs

were analyzed during platform forward acceleration.

Results: Tongue pressing caused an adjustment of co-contraction patterns of distal muscle

groups based on changes in biomechanical coupling between the head and trunk during

static balancing at the beginning of the experiment. Neither iEMG nor CCR measurement

helped detect a general neuromuscular effect of jaw clenching on the dynamic reactive

balance.

Conclusion: The findings might indicate the existence of robust fixed patterns of rapid

postural responses during the important initial phases of balance recovery.

5 Study II – Modulation of Reflex Activities – Dynamic Reactive Balance

5.2 Introduction

Postural control is a special form of motor control, an indispensable prerequisite for hu-

man movement (Horak & Earhart, 2021). Impaired postural control is a major risk factor

for falls, and training has a central role in preventing them (Rubenstein, 2006). Multiple

sensory signals from the visual, vestibular, and somatosensory systems are used to correct

instability in balance control (Takakusaki, 2017). However, the influence of oral-motor ac-

tivities was also discussed because extensive research showed neuromuscular effects of

oral-motor activity (e.g., jaw clenching and tongue pressing) on upright posture stabiliza-

tion. Evidence indicates that jaw clenching has a reproducible stabilizing effect on human

body-sway in hip width stance (Hellmann et al. 2011a) and improves postural control dur-

ing bipedal narrow and single-leg stance [reduced center of pressure (COP) displacement

and trunk and head oscillations] (Ringhof, Stein, et al., 2015) based on increased neuro-

muscular co-contraction pattern precision (Hellmann et al., 2015). Since balance control

is task-specific rather than a general ability, it is useful to study balance tasks other than

static stance (Ringhof & Stein, 2018). Studies showed the stabilizing effects of oral-motor

tasks when standing on firm and foam surfaces (Alghadir et al., 2015c; Bracco et al., 2004;

Julià-Sánchez et al., 2020; Kushiro & Goto, 2011; Ringhof, Leibold, et al., 2015; Sakaguchi

et al., 2007; Sforza et al., 2006; Tomita et al., 2021) and during dynamic balancing tasks on

unstable platforms (Fadillioglu et al., 2022a). Fadillioglu et al. (2022a) compared the effect

of jaw clenching, tongue pressing against the palate, and habitual lower jaw position on

postural performance during a dynamic reactive balance task on an oscillating platform.

Using the Lehr's damping ratio of the first three maximum amplitudes after the perturba-

tion (Kiss, 2011a) and the mean sway speed of selected anatomical regions (head, trunk,

pelvis, knee, and foot), they showed that jaw clenching improved the dynamic reactive

balance during forward acceleration of the platform (i.e. an imminent fall backward).

However, jaw clenching showed no stabilizing effects during simulated falls forward

(Fadillioglu et al., 2022a; Ringhof et al., 2016).

As an explanation for the influence of jaw clenching on balancing behavior, it was sug-

gested that concurrent muscle activities contributed to the facilitation of human motor

excitability and increased the neural drive to distal muscles (Ebben, 2006; Ebben et al.,

2008). It was hypothesized that the jaw clenching effect on posture was induced by soma-

tosensory input modulation and facilitation of muscles such as the ankle extensor and

flexor muscles (Miyahara et al., 1996; Takada et al., 2000) and concomitant attenuation of

reciprocal Ia inhibition (Takada et al., 2000). Neuroanatomical connections and projec-

tions of the trigeminal nerve to structures associated with postural control form the basis

for these effects (Buisseret-Delmas et al., 1999; Devoize et al., 2010; Ruggiero et al., 1981).

5.3 Methods

Current evidence indicates: First, jaw clenching improves performance during dynamic re-

active balance tasks under certain conditions. Second, jaw clenching influences the excit-

ability of the motor system and thus enhances reflex responses and neurophysiological

effects, detectable based on muscle activity and/or co-contraction patterns. Therefore,

this study analyzed the reflex activities of various postural muscles in relevant reflex

phases using a dataset —first published in 2022 in this journal (Fadillioglu et al., 2022a)—

in which we found improved dynamic reactive balance during forward platform accelera-

tion under the influence of jaw clenching. We hypothesized that muscle activity and co-

contraction pattern (CCR) of relevant muscle pairs in reflexive phases would change under

the influence of jaw clenching and tongue pressing.

5.3 Methods

5.3.1 Participants

Forty-eight physically active adults (23 male, 25 female; age: 23.8 ± 2.5 years; height: 1.73

± 0.09 m; body mass: 69.2 ± 11.4 kg) participated in this study. The dominant leg of each

participant was determined by self-reports or, in case of uncertainties, by test trials on the

oscillating platform (Ringhof & Stein, 2018). All participants gave their written informed

consent before the start of the experiments. The participants confirmed they were naïve

to the balancing task, had no neurological or muscular diseases, and were physically active

(regular sporting activity, at least thrice weekly). Moreover, the participants showed no

symptoms or signs of temporomandibular disorders assessed by the research diagnostic

criteria for temporomandibular disorders (RDC/TMD (Dworkin & LeResche, 1992)) and

had full dentition, except for 3rd molars, in neutral occlusion. The study was approved by

the Ethics Committee of the Karlsruhe Institute of Technology.

5.3.2 Experimental procedure

5.3.2.1 Balance tasks

Dynamic reactive balance was evaluated using the Posturomed oscillating platform

(Haider-Bioswing, Weiden, Germany). This commercial device consists of a rigid platform

(60 x 60 cm, 12 kg) and eight 15-cm steel springs of identical strength and can swing in all

directions along the horizontal plane. The Posturomed was previously used to systemati-

cally investigate dynamic reactive balance performance after platform displacements

(Freyler et al., 2016; Keller et al., 2012; Pfusterschmied, Stöggl, et al., 2013). An automatic

5 Study II – Modulation of Reflex Activities – Dynamic Reactive Balance

custom-made release system was used to accelerate the platform horizontally (up to 2.5

cm) in one of four possible directions: back, front, left, and right (Fadillioglu et al., 2022a).

These indicate the direction to which the platform was accelerated after release (e.g.,

front indicates that the platform was accelerated forward after release, leading to poste-

rior body sway relative to the platform).

The perturbation direction was presented in a randomized order, and the participants’

task was to compensate for the perturbation as quickly as possible. Before each trial, the

participants were asked to stand with the dominant leg in the center of the Posturomed,

the non-dominant leg in the air, hands on hips, and look straight ahead at an eye-level

marker 4 m away (Figure 5.1). If a participant lost balance, touched the ground with the

non-dominant foot, released one of the hands from the hip, changed the dominant foot

position, or did not perform the oral-motor activity properly, the trial was considered in-

valid and was repeated.

5.3.2.2 Group assignment and oral-motor tasks

The participants were allowed to familiarize themselves with the Posturomed at the be-

ginning of the trial. This familiarization included two static trials and two trials with per-

turbation in the back direction. Subsequently, baseline measurements were performed,

also in the back direction, to group the participants based on their dynamic reactive bal-

ance performance, quantified by Lehr’s damping ratio (Fadillioglu et al., 2022a; Kiss,

2011a). Statistical tests found no baseline performance differences between the three

groups (ANOVA, p = 0.767). Attention was also given to ensuring equal sex distribution

within the groups. Each group had to perform one of the following oral-motor tasks while

balancing on the platform during each trial:

- JAW: instructed; controlled submaximal jaw clenching during occlusal loading

- TON: instructed; controlled submaximal tongue pressing against the palate;

stomatognathic muscle activity without occlusal loading

- HAB: without instruction; habitual stomatognathic behavior

5.3 Methods

Figure 5.1: A participant is standing with the dominant leg in the center of the Posturomed, the non-dominant

leg in the air, hands on the hips, and looking straight ahead.

Oral-motor activity based on group assignment was measured by electromyography

(EMG; detailed information in the subsection “Data collection”). Before the measure-

ments, the JAW group was trained to achieve submaximal clenching at a force of 75 N

using a RehaBite (Plastyle GmbH, Uttenreuth, Germany), a medical training device con-

sisting of liquid-filled plastic pads and working based on hydrostatic principles. Muscle

activity was displayed during training using the EMG system. The training was terminated

once the participant successfully applied a stable force of 75 N (which resulted in a mean

EMG activity of about 5% of their maximum voluntary contraction, MVC). For the training,

the TON group applied a submaximal force with the tip of the tongue against the anterior

hard palate. The training was terminated once the participants achieved a consistent EMG

activity at 5% of their MVC, measured in the region of the m. digastricus venter anterior.

The training for both groups lasted approximately five minutes. The corresponding EMG

levels of jaw clenching and tongue pressing activities were used during the measurements

to determine whether the submaximal jaw clenching or tongue pressing force was

reached. The JAW group performed their submaximal jaw clenching task on an Aqualizer

intraoral splint (Medium volume; Dentrade International, Cologne, Germany) during the

5 Study II – Modulation of Reflex Activities – Dynamic Reactive Balance

balance task measurement. The HAB group received no training or instructions regarding

oral-motor activity.

5.3.2.3 Data collection

A wireless EMG system (Noraxon, Scottsdale, USA) operating at 2,000 Hz was used to

measure EMG activity. EMG activity was derived from the M. masseter (Mass) for the JAW

and HAB groups and the suprahyoid muscles at the base of the mouth for the TON group,

measured near the digastricus venter anterior muscle. The EMG signal was also derived

from the following postural task-related skeletal muscles: M. gastrocnemius medialis

(GM), M. soleus (SOL), M. tibialis anterior (TA), M. peroneus longus (PL), M. tensor fascia

latae (FL), M. semitendinosus (SEM), M. biceps femoris (BF), M. rectus femoris (RF), M.

vastus medialis (VM), Mm. obliquus externus (OBL), Mm. rectus abdominis (ABS), and

Mm. erector spinae iliocostalis (ES) (Figure 5.2).

5.3.2.4 Data analysis

Since significant effects of JAW in dynamic reactive balancing performance were only

found for perturbation in the forward direction (Fadillioglu et al., 2022a), only the dataset

of this direction was used for further analysis.

EMG data were filtered using a fourth-order Butterworth band-pass filter (10-500 Hz) and

then rectified and normalized to the MVC amplitudes. For reflex activity analysis, four

phases were considered and defined based on the perturbation onset: (1) PRE: the time

window just before the perturbation onset (–100 to 0 ms); (2) SLR: short latency response

(30 to 60 ms); (3) MLR: medium latency response (60 to 85 ms); (4) LLR: long latency re-

sponse (85 to 120 ms). These phases were defined following previous studies (Freyler et

al., 2015; Taube et al., 2006). The integrated EMG (iEMG) was calculated for each muscle

and reflex phase. Furthermore, the following muscle pairs were selected for co-contrac-

tion analysis: GM-TA, SOL-TA, TA-BF, VM-BF, RF-BF, GM-RF, PL-SOL, VM-SEM, ABS-ES, SEM-

FL, and OBL-ES. The dominant (d) and non-dominant (nd) sides were calculated separately

for ABS-ES and OBL-ES. The co-contraction ratio (CCR) was calculated as lower/higher EMG

for each time point i, and the average CCR was calculated for each phase (Hellmann et al.,

2015).

5.3 Methods

Figure 5.2: The EMG signal was derived from the M. masseter (= Mass) and postural task-related skeletal mus-

cles: OBL = Mm. obliquus externus, ABS = Mm. rectus abdominis, FL = M. tensor fascia latae,

RF = M. rectus femoris, VM = M. vastus medialis, TA = M. tibialis anterior, ES = Mm. erector spinae

iliocostalis, BF = M. biceps femoris, SEM = M. semitendinosus, GM = M. gastrocnemius medialis,

PL = M. peroneus longus, SOL = M. soleus. d = dominant. nd = non-dominant

5.3.2.5 Statistical analysis

Statistical tests were performed in IBM SPSS Statistics for Windows, Version 29.0 (IBM

Corporation, Armonk, NY, USA). The average values over three trials were calculated for

dependent parameters (i.e. iEMG and CCR). The normality of the data distributions was

tested by Kolmogorov-Smirnov tests. A one-way ANOVA or a Kruskal-Wallis test was per-

formed for each phase (PRE, SLR, MLR, and LLR) and each dependent parameter, with the

group (INT, JAW, and HAB) as an independent parameter. The significance level was set a

priori to p < 0.05. Partial eta-squared was used to quantify the effect sizes (small effect:



 < 0.06; medium effect: 0.06 < 

 < 0.14; large effect: 

 > 0.14) (Cohen, 1988). In case

of significant differences, pairwise post-hoc tests were conducted.

5 Study II – Modulation of Reflex Activities – Dynamic Reactive Balance

5.4 Results

5.4.1 Oral-motor task

The participant performed the oral-motor tasks during all four phases (Table 5.1a). The

EMG activities of M. masseter in JAW and the suprahyoidal muscles in TON were signifi-

cantly higher than in HAB (Table 5.1b, c) in all phases. Therefore, the instructed oral-motor

tasks were fulfilled.

Table 5.1a: iEMG of M. masseter and the suprahyoid muscles in MVC % at four critical phases

Muscle

Group

PRE

SLR

MLR

LLR

M. masseter

JAW

Mean

4.94

4.75

4.92

4.91

3.01

2.99

3.12

2.86

HAB

Mean

1.36

1.20

1.11

1.38

3.04

2.62

2.32

2.97

Suprahyoidal

muscles

TON

Mean

3.35

3.52

3.19

3.34

2.32

2.57

2.05

2.20

Table 5.1b: Comparison of iEMG of M. masseter and the suprahyoid muscles at four critical phases between

the three groups using the Kruskal-Wallis test.

5.4.2 iEMG activity of the postural muscles

No significant differences were found between the JAW, TON, and HAB groups in the iEMG

activity of the investigated postural muscles during any of the four phases studied (PRE,

SLR, MLR, and LLR; Appendix S2 Table1).

PRE

SLR

MLR

LLR

















< 0.001

0.225

< 0.001

0.237

< 0.001

0.286

< 0.001

0.234

5.5 Discussion

5.4.3 Co-contraction ratio (CCR)

Before perturbation (PRE), VM-SEM and OBL-ES(d) in the TON group showed significantly

lower CCR values than in the HAB group. In the SLR and MLR phases, OBL-ES(d) in the TON

group showed significantly lower CCR values than in the JAW group and for MLR, also than

in the HAB group.

In LLR, PL-SOL showed significantly higher CCR values in the JAW and TON groups than in

the HAB group. The relevant data on significant results are listed in Tables 5.2a-c. The full

data of CCR values can be viewed in Appendix S2 Table2.

Table 5.1c: Post-hoc results for cases with significant effects in the Kruskal-Wallis test for iEMG analysis.

Phase

Groups

PRE

JAW

HAB

<0.001

HAB

TON

0.001

TON

JAW

0.240

SLR

JAW

HAB

<0.001

HAB

TON

<0.001

TON

JAW

0.384

MLR

JAW

HAB

0.002

HAB

TON

<0.001

TON

JAW

0.193

LLR

JAW

HAB

<0.001

HAB

TON

0.002

TON

JAW

0.245

5.5 Discussion

This study aimed to elucidate the neuromuscular mechanism behind the measured jaw-

clenching effects on dynamic reactive balance after forward platform acceleration (i.e. an

imminent fall backward). For this purpose, activities and co-contraction patterns of pos-

ture-relevant muscles and muscle pairs were analyzed before and during relevant reflex

phases after perturbation. Previously published data revealed that jaw clenching led to a

better dynamic reactive balance during forward platform acceleration than habitual sto-

matognathic behavior and tongue pressing (Fadillioglu et al., 2022a).

5 Study II – Modulation of Reflex Activities – Dynamic Reactive Balance

Table 5.2a: Co-contraction ratio (CCR) of selected muscle pairs at four critical phases.

Muscle pair

Group

PRE

SLR

MLR

LLR

PL-SOL

JAW

Mean

0.34

0.33

0.35

0.04

0.05

0.07

0.05

HAB

Mean

0.33

0.35

0.34

0.30

0.04

0.06

0.07

0.06

TON

Mean

0.34

0.33

0.34

0.35

0.05

0.06

0.05

VM-SEM

JAW

Mean

0.32

0.31

0.29

0.33

0.05

0.06

0.08

0.07

HAB

Mean

0.33

0.29

0.32

0.06

0.07

TON

Mean

0.27

0.28

0.27

0.08

0.09

0.10

0.08

OBL-ES

(d)

JAW

Mean

0.36

0.38

0.37

0.36

0.06

0.07

0.06

HAB

Mean

0.39

0.37

0.38

0.06

0.07

TON

Mean

0.32

0.31

0.30

0.32

0.09

0.08

0.09

Table 5.2b: One-way ANOVA results for the co-contraction ratio (CCR) at four critical phases in direction F.

PRE

SLR

MLR

LLR

















PL-SOL

0.714

0.015

0.736

0.014

0.816

0.009

0.010

0.185

VM-SEM

0.037

0.136

0.566

0.025

0.484

0.032

0.082

0.105

OBL-ES (d)

0.026

0.150

0.018

0.164

0.009

0.188

0.080

0.106

Significant results are indicated in bold.

5.5.1 Oral-motor task

The EMG activity of M. masseter and the suprahyoid muscles in the JAW and TON groups

was significantly higher than in the HAB group, indicating that the oral-motor tasks were

fulfilled during all experimental phases.

5.5 Discussion

5.5.2 Postural muscle reflexes

No differences were found between the groups at any reflex phase of the anticipatory and

compensatory postural responses (i.e. neither before nor after perturbation). Therefore,

at the iEMG level, the active groups (JAW and TON) showed no effect. Consequently, no

general effect of oral-motor activities on muscular activity in the lower trunk and legs was

evident, in agreement with the findings of Hellmann et al. (2011a) under static conditions.

We also found no significant changes in the iEMG activity of the leg muscles under the

influence of oral-motor activity. Our findings might seem somewhat unexpected, consid-

ering that earlier research emphasized that reflex responses were facilitated by voluntary

jaw clenching (Miyahara et al., 1996; Takada et al., 2000). Noteworthy, those effects were

observed in participants comfortably seated, with the knee and foot joint angles at 120°

and 100°, respectively. In our study, participants stood on one leg, with the ankle in a

neutral position and the knee and hip joints almost fully extended. This increased postural

demand (as in the transition from lying to standing or bipedal to unipedal standing) has

been shown to be accompanied by modulation of soleus and tibialis H-reflex amplitudes

(Kim et al., 2013; Unger et al., 2019; Zehr, 2002). More precisely, increases in postural

demand were correlated with decreases in reflex amplitudes. The proposed mechanism

behind this presynaptic inhibition is a shift in the central nervous system to increase vol-

untary balance control (Y. Chen & Zhou, 2011; Huang et al., 2009). However, whether jaw

clenching could counteract this presynaptic inhibition in standing positions has not been

investigated.

5.5.3 CCR of postural muscles in the various reflex phases

The balance task used in this experiment involved challenging the response to platform

displacement relative to the participant. Since the platform can oscillate, movements in

all directions in the transverse plane were possible. The forward platform acceleration in

the considered time window (100 ms pre-perturbation to 120 ms post-perturbation) re-

sulted in an anterior platform displacement. Ground displacement in an anterior-posterior

direction is mostly compensated by increasing ankle and knee joint deflections in the sag-

ittal plane and range of motion of these joints could help lower the center of gravity, lead-

ing to a fast reacquisition of a stable one (Freyler et al., 2015). Therefore, the increased

balance recovery performance found after perturbation could be achieved through

changes in the neuromuscular mechanisms that control the distal strategy mentioned

above (Freyler et al., 2015).

5 Study II – Modulation of Reflex Activities – Dynamic Reactive Balance

5.5.4 Anticipatory postural adjustments

Studies found that tongue pressing (Alghadir et al., 2015a), jaw clenching, and chewing

(Hellmann et al., 2011a) had stabilizing effects on COP displacements during quiet stance

under static conditions. Similarly, the influence of jaw clenching on the co-contraction pat-

tern of the leg muscles under such conditions has been reported (Hellmann et al., 2015).

Based on these findings, an influence of both oral-motor tasks on the CCR could be ex-

pected under the steady-state condition of the balance task in the PRE experimental

phase, particularly for the JAW group. However, tongue pressing against the palate, but

not jaw clenching, resulted in a lower CCR than the habitual stomatognathic status. There-

fore, it is unlikely that the tongue-pressing effect was based on modulating the soma-

tosensory input and/or reflex facilitation. From a biomechanical standpoint, another

hypothesis arises: a pronounced activity of the jaw-closing muscles requires no additional

cervical muscle activity to stabilize the position or relation between the lower and upper

jaws and the rest of the body. However, pressing the tongue against the palate requires

hyoid bone stabilization by increased activity of the supra- and infrahyoidal muscles and

might, therefore, influence the coordination between the head and trunk during the bal-

ancing task. Consequently, measurable adjustments in the intermuscular co-contraction

pattern might be needed to coordinate the spatial alignment between the body segments

by altering the positions of the hip and knee joints. This could proportionally explain the

measurable influence of tongue pressing on the CCR of the lower trunk and thigh muscles

during steady-state balancing in the PRE phase.

5.5.5 Compensatory postural adjustments

The neuromuscular compensation for the platform acceleration is realized by various sub-

systems and neuronal structures involved in compensatory postural responses. Postural

responses became more voluntarily controlled with time after the perturbation onset,

while muscle activity was modulated by the mono- and polysynaptic pathways during the

SLR and MLR phases, respectively (Taube et al., 2006). Determined by the anatomical

properties and function, the shank muscles — which are “near” the postural disturbance

— serve as prime movers to give a direct corrective response by varying the ankle joint

position (Freyler et al., 2015; Moore et al., 1988). During the SLR phase, this muscle activ-

ity is modulated by monosynaptic reflexes (Taube et al., 2006). Although jaw clenching

could facilitate peripheral monosynaptic reflexes of the shank muscles via the subcortical

pathway (Boroojerdi et al., 2000), the CCR of PL-SOL was unaffected during the SLR phase.

In contrast to the SLR, the MLR is modulated by supraspinal structures via polysynaptic

5.5 Discussion

pathways (Gollhofer et al., 1989). While no influence of jaw clenching on the CCR of the

shank and thigh muscles was noted, we detected a lower CCR in the TON group than in

the other groups.

The increased platform displacement with time was compensated in the LLR phase, with

most thigh muscles contributing to balance recovery (Freyler et al., 2015). The LLR is mod-

ulated under the involvement of corticospinal pathways (Taube et al., 2006). Boroojerdi

et al. showed that jaw clenching led to marked facilitation of corticospinal pathways to

the leg muscles (Boroojerdi et al., 2000). This might be the explanatory model for the ob-

served higher CCR in the JAW and TON groups than in the HAB group during the LLR phase.

5.5.6 Limitations

First, all the participants were physically active, healthy adults. Accordingly, conclusions

can only be made for this group. Secondy, this study used the oscillating Posturomed plat-

form to evaluate the dynamic reactive balance performance. The Posturomed platform is

commonly used in scientific studies (Freyler et al., 2015; Kiss, 2011a). However, it should

be noted that balance performance outcomes under different dynamic balance conditions

might not be fully comparable (Ringhof & Stein, 2018). Third, the participants were allo-

cated into three groups with different oral-motor tasks. This could be considered a limita-

tion because of possible baseline performance differences between groups. However, to

overcome this problem, a baseline measurement was conducted in the habitual biting

condition to match the three groups in terms of performance and sex. We found no base-

line performance differences between the three groups (p = 0.767). One might think it

would have been ideal if all subjects had performed all oral-motor tasks. However, “habit-

ual” in this study meant that no instructions were given regarding the status of the masti-

catory organ. Therefore, an unconscious, ancestral behavioral pattern of the masticatory

system during the balancing task could be expected. By definition, an “instructed” behav-

ioral pattern can never correspond to an unconsciously performed behavior. An instructed

“habitual” oral-motor behavior could result in dual-task effects, making it difficult to dis-

tinguish between cognitive and postural effects (i.e. thinking about the instructed behav-

ior and performing various oral-motor tasks, respectively).

The three-group approach provided two main advantages. First, possible carry-over ef-

fects between different oral-motor tasks were avoided. For example, some physiological

effects could have lingered after jaw clenching or tongue pressing, such as an increased

motor system excitability or fatigue of the masticatory muscle system. Second, we would

need 36 valid trials if all participants performed all three oral-motor tasks for each of the

5 Study II – Modulation of Reflex Activities – Dynamic Reactive Balance

four directions. Considering the number of invalid trials, the total number of trials could

result in fatigue, decreasing the data quality.

5.6 Conclusion and Outlook

We hypothesized that a neuromuscular mechanism that changes the CCR level of relevant

muscle pairs was behind the improved performance during a dynamic reactive balancing

task and concurrent jaw clenching, as shown for static conditions. Based on our results,

this hypothesis must be rejected. The results did not explain the improved dynamic reac-

tive balance under the influence of jaw clenching compared to tongue pressing or habitual

oral behavior. Neither the iEMG nor CCR analysis helped to elucidate a general neuromus-

cular effect of jaw clenching on dynamic reactive balancing performance. Other analytical

approaches seem necessary to investigate the causes of such an effect.

Based on a secondary finding, we hypothesize that tongue pressing causes an adjustment

to the CCR of distal muscle groups based on changes in the biomechanical coupling be-

tween the head and trunk. The facilitation of corticospinal pathways under the influence

of oral-motor activity seems to play a role only in later phases. This might indicate that

robust fixed patterns of rapid postural responses during the important initial phases of

balance recovery are present (Nashner, 1977).

6 Study III – Modulation of

Jaw

Clenching Effects after

Jaw

Clenching

Training

Slightly modified# version of the paper published as

Fadillioglu, C., Kanus, L., Möhler, F., Ringhof, S., Hellmann, D., & Stein, T. (2023). Effects

of jaw clenching on dynamic reactive balance task performance after 1-week of jaw

clenching training. Frontiers in Neurology, 14, 1-13. [*These authors have contrib-

uted equally to this work.]

#Table 6.1-3 were reshaped. The results were rounded to one decimal place.

6.1 Abstract

Introduction: Good balance is essential for human daily life as it may help to improve the

quality of life and reduce the risk of falls and associated injuries. The influence of jaw

clenching on balance control has been shown under static and dynamic conditions. Nev-

ertheless, it has not yet been investigated whether the effects are mainly associated with

the dual-task situation or are caused by jaw clenching itself. Therefore, this study investi-

gated the effects of jaw clenching on dynamic reactive balance task performance prior to

and after 1 week of jaw clenching training. It was hypothesized that jaw clenching has

stabilizing effects resulting in a better dynamic reactive balance performance, and these

effects are not related to dual-task benefits.

Methods: A total of 48 physically active and healthy adults (20 female, 28 male) were

distributed into three groups, one habitual control group (HAB) and two jaw clenching

groups (JAW, INT) that had to clench their jaws during the balance tasks at T1 and T2. One

of those two groups, the INT group, additionally practiced the jaw clenching task for one

week, making it familiar and implicit at T2. The HAB group did not receive any instruction

regarding jaw clenching condition. Dynamic reactive balance was assessed using an oscil-

lating platform perturbed in one of four directions in a randomized order. Kinematic and

electromyographic (EMG) data were collected by using a 3D motion capture system and a

wireless EMG system, respectively. Dynamic reactive balance was operationalized by the

6 Study III – Modulation of Jaw Clenching Effects after Jaw Clenching Training

damping ratio. Further, the range of motion of center of mass (CoM) in perturbation di-

rection (RoMCoM_AP or RoMCoM_ML) as well as the velocity of CoM (VCoM) in 3D were ana-

lyzed. The mean activity of the muscles relevant for perturbation direction were

calculated to investigate reflex activities.

Results: The results revealed that jaw clenching had no significant effects on dynamic re-

active balance performance or CoM kinematics in any of the three groups, and automa-

tion of jaw clenching in the INT group did not result in a significant change either. However,

high learning effects, as revealed by the higher damping ratio values and lower VCoM at T2,

were detected for the dynamic reactive balance task even without any deliberate balance

training in the intervention phase. In case of backwards perturbation of the platform, the

soleus activity in short latency response phase increased for the JAW group, whereas it

decreased for HAB and INT after intervention. In case of forwards acceleration of the plat-

form, JAW and INT showed a higher tibialis anterior muscle activity level in medium la-

tency response phase compared to HAB at T1.

Discussion: Based on these findings, it can be suggested that jaw clenching may lead to

some changes in reflex activities. However, the effects are limited to anterior–posterior

perturbations of the platform. Nevertheless, high learning effects may have overall over-

weighed the effects related to jaw clenching. Further studies with balance tasks leading

to less learning effects are needed to understand the altered adaptations to a dynamic

reactive balance task related to simultaneous jaw clenching. Analysis of muscle coordina-

tion (e.g., muscle synergies), instead of individual muscles, as well as other experimental

designs in which the information from other sources are reduced (e.g., closed eyes), may

also help to reveal jaw clenching effects.

6.2 Introduction

Balance is one of the essential aspects of postural control and is crucial to accomplish daily

life activities, such as unassisted standing and walking. Impaired balance control may lead

to an increased risk of falls and a reduced quality of life (Rubenstein, 2006; Shumway-Cook

& Woollacott, 2017). From a mechanical point of view, balance involves controlling the

center of mass (CoM) with respect to the base of support (Shumway-Cook & Woollacott,

2017). During standing, the CoM sways steadily within the body‘s base of support (i.e.

static steady balance), whereas during perturbations stability needs to be recovered to

bring the CoM back to allowed limits necessary for maintaining posture (i.e. dynamic re-

active balance; (Hof et al., 2005). Given the importance of balance (Shumway-Cook &

Woollacott, 2017), it is valuable to improve its control mechanisms by balance training.

6.2 Introduction

This is recommended for performance enhancement in sports (Hrysomallis, 2011), to pre-

vent injuries (Hrysomallis, 2007), and to decrease falls in at-risk groups (Sherrington et al.,

2008, 2019).

An important prerequisite for balance is the sensory input that derives from somatosen-

sory, visual and vestibular systems and provides the central nervous system (CNS) with

information regarding the state of the body and the environment. This sensory infor-

mation is weighted in a task-dependent manner (Peterka, 2002). For example, when the

support surface is rapidly displaced (i.e. the dynamic reactive balance control is chal-

lenged), the CNS mostly relies on somatosensory inputs since these enable faster reac-

tions than other systems of sensory input (Shumway-Cook & Woollacott, 2017). Given the

importance of somatosensory information for dynamic reactive balance control, any al-

teration that improves dynamic stability may be relevant for fall prevention, especially in

regards to unexpected external perturbations (Rubenstein, 2006; Winter, 1995).

A growing body of literature suggests that there is a close relationship between the sto-

matognathic system and balance (Alghadir et al., 2015a, 2015c; Allen et al., 2018;

Fadillioglu et al., 2022a; Julià-Sánchez et al., 2015; Ohlendorf et al., 2014; Ringhof, Leibold,

et al., 2015; Tomita et al., 2021; Zafar et al., 2020). The underlying mechanisms have not

yet been fully understood, however, in various studies (Boroojerdi et al., 2000; Miyahara

et al., 1996; Takada et al., 2000; Tuncer et al., 2007) it was shown that jaw clenching in a

manner similar to the Jendrassik maneuver (Jendrassik, 1885) may lead to increased mo-

tor excitability , increased H-reflex responses. In addition, co-contraction behavior of the

masticatory and neck muscles occurring as a result of complex neurophysiological inter-

actions (Giannakopoulos, Schindler, et al., 2018) may also contribute to an improved pos-

tural control, for example via a more stable head or gaze position (Abrahams, 1977;

Gangloff et al., 2000; Tanaka et al., 2006). These results are neuroanatomically supported

by findings in animal models which found neuronal links of the trigeminal nerve to nu-

merous brainstem nuclei and all levels of the spinal cord (Ruggiero et al., 1981).

Although jaw clenching has been shown to affect balance performance under both static

(Hellmann et al., 2015; Ringhof, Leibold, et al., 2015; Ringhof, Stein, et al., 2015) and dy-

namic conditions (Fadillioglu et al., 2022a; Tomita et al., 2021), it is still unknown whether

these effects are associated with the dual-task situation (i.e. influences of simultaneously-

performed additional motor tasks (Andersson et al., 2002; Broglio et al., 2005)) or those

specifically connected to jaw clenching. In general, when two tasks are performed simul-

taneously, performance decreases in one or both tasks (R. A. Schmidt et al., 2018), which

can be explained by the limited capacity of attention (Woollacott & Shumway-Cook, 2002).

However, with respect to balance control, previous studies showed that combining a

6 Study III – Modulation of Jaw Clenching Effects after Jaw Clenching Training

secondary task with a balance task may actually improve performance compared to single

task condition (Broglio et al., 2005). This phenomenon can be explained by altered atten-

tion and increased automatization of balance control processes (Andersson et al., 2002;

Wachholz et al., 2020). Therefore, one might argue that stabilizing effects on balance con-

trol could be caused by the secondary task of jaw clenching.

To sum up, the acute positive effects of jaw clenching have been shown in various studies

(Fadillioglu et al., 2022a; Hellmann et al., 2015; Ringhof, Leibold, et al., 2015; Ringhof,

Stein, et al., 2015; Tomita et al., 2021), however it has not yet been evaluated if these

effects are associated with dual-task benefits or specifically based on neurophysiological

effects caused by jaw clenching. Therefore, this study established an intervention group

(INT) that trained jaw clenching, so that it becomes an implicit task. The comparison with

a group (JAW) that was only instructed in jaw clenching shortly before T1 and T2 and with

a group without any training as well as instruction (HAB) should help to draw a firm con-

clusion about the above mentioned dual-task issue. It was hypothesized that jaw clench-

ing has an effect on dynamic reactive balance and this effect is not related to dual-task

benefits, which would be indicated by the missing differences in dynamic reactive balance

performance between the INT and JAW groups at T2.

6.3 Methods

The study design comprised two measurement times (T1 and T2, separated by one week)

and three groups (INT: intervention, JAW: jaw clenching and HAB: habitual), whose details

can be found in the following sections. The data of two groups (JAW and HAB) at T1 were

partially presented in previously published studies (Fadillioglu et al., 2022a, 2022b). An a

priori power analysis was performed based on the study by Ringhof et al. (Ringhof, Stein,

et al., 2015) that analyzed the effects of submaximal jaw clenching on postural stability.

The results revealed that 16 participants per group would be sufficient to reach a power

of > 0.8.

6.3.1 Participants

A total of 48 physically active adults (20 female, 28 male; age: 23.2 ± 2.4 years; height:

1.74 ± 0.09 m; body mass: 69.4 ± 10.4 kg) participated in this study. All participants gave

written informed consent prior to the study, confirmed that they were participating in any

kind of sports regularly at least three times per week and were naive to the balance task

instrument. They had no muscular or neurological diseases, showed no signs or symptoms

6.3 Methods

of temporomandibular disorders (based on the Research Diagnostic Criteria for Temporo-

mandibular Disorders (Dworkin & LeResche, 1992)), and presented with full dentition (ex-

cept for third molars) in neutral occlusion. The study was approved by the Ethics

Committee of the Karlsruhe Institute of Technology.

6.3.2 Study design

To investigate whether the stabilizing effects of jaw clenching are merely a result of dual-

task effects, the principal idea of our three-armed intervention study was that one of the

groups, namely INT, repeatedly practiced jaw clenching to make it a familiar and implicit

task. The details of the three different groups (INT, JAW and HAB) are shown in Figure 6.1.

Dynamic reactive balance performance was assessed by a commercially-available oscillat-

ing platform (Posturomed, Haider-Bioswing, Weiden, Germany), which has previously

been used to systematically investigate dynamic reactive balance performance after per-

turbations in many other studies (Freyler et al., 2015; Kiss, 2011a; Pfusterschmied, Stöggl,

et al., 2013). It is a rigid platform (12 kg, 60 cm × 60 cm) connected to a metal frame with

eight steel springs (15 cm) of identical strength and can swing along the horizontal plane

in all directions freely. A custom-made release system was used to apply mechanical per-

turbations in one of the four possible directions, back (B), front (F), left (L), right (R), in a

randomized order (Fadillioglu et al., 2022a). Before the trials began, the participants were

familiarized with the Posturomed by two trials without and two trials with perturbation.

Afterwards, a baseline measurement with a perturbation was conducted in the habitual

stomatognathic motor condition to determine initial balance performance (Fadillioglu et

al., 2022a). Before each trial, participants were asked to stand on the platform on their

dominant leg, hands at hips, eyes focusing on a fixed point at eye level horizontally 4 m

away from the center of the platform and to compensate the perturbation as quickly as

possible. Their dominant leg was determined based on self-reports or, in case of uncer-

tainty, by testing on the Posturomed (Fadillioglu et al., 2022a; Ringhof & Stein, 2018). In

each trial, the platform was perturbed by the release system unpredictably in one of the

four possible directions in a randomized order. The release system was used to release the

platform from its maximum displaced position along the perturbation axis. After the per-

turbation, no external resistance forces were applied and the participants had to dampen

the perturbation by bringing the platform into its central position as soon as possible.

Both INT and JAW were jaw clenching groups and were instructed to clench their jaws

during the balancing task. INT additionally trained in the jaw clenching task between T1

and T2, which were separated by one week. The purpose of this intervention was to make

6 Study III – Modulation of Jaw Clenching Effects after Jaw Clenching Training

the novel jaw clenching task more automated, such that focused attention is reduced at

T2. Groups were assigned considering the subjects’ gender as well as their initial balance

performance to ensure even distribution across the three groups. It was statistically con-

firmed that there were no baseline performance differences between the three groups

(one-way ANOVA, p = 0.920).

Figure 6.1: Study design (INT: intervention, JAW: jaw clenching, HAB: habitual, T1 and T2 are the measurement

times).

During the balance task, INT and JAW were asked to clench their jaws with a force of 75

N. To familiarize with this task, participants trained for five minutes just before the meas-

urements with a RehaBite® (Plastyle GmbH, Uttenreuth, Germany), a medical training de-

vice consisting of liquid-filled plastic pads, to get used to applying this level of force

(Fadillioglu et al., 2022a; Giannakopoulos, Rauer, et al., 2018). During the measurements,

the EMG activity of the masseter muscle corresponding to 75 N was used as reference and

the participants in these two groups bit down on an Aqualizer® intra-oral splint (medium

volume; Dentrade International, Cologne, Germany). HAB did not receive any instructions

regarding the stomatognathic motor condition or an Aqualizer®. In the one-week inter-

vention phase between T1 and T2, INT trained three times a day for 10 minutes (10 reps

of 3 sets, applying force for 10 s, stretching the jaw muscles and resting for 10 s). For this

purpose, the subjects received a RehaBite® and a diary to record the training sessions.

6.3 Methods

6.3.3 Measurements

A total of 22 anthropometric measures were manually taken from each participant and 42

reflective markers were placed on the participants’ skin in accordance with the ALASKA

modeling system (Advanced Lagrangian Solver in Kinetic Analysis, INSYS GmbH, Chemnitz,

Germany (Härtel & Hermsdorf, 2006)) to capture full body kinematics. Four reflective

markers were attached to the upper surface of the Posturomed platform (Fadillioglu et al.,

2022a) and their displacements were captured using a 3D motion capture system (Vicon

Motion Systems; Oxford Metrics Group, Oxford, UK; 10 Vantage V8 and 6 Vero V2.2 cam-

eras; 200 Hz).

The activity of nine muscles (peroneus longus (PL), soleus (SOL), tibialis anterior (TA), rec-

tus femoris (RF), semitendinosus (SM), rectus abdominis (AB), internal oblique (IO), erec-

tor spinae (ES) and masseter (MA)) were recorded using a wireless EMG system (Noraxon,

Scottsdale, USA; 2000 Hz) at the standing leg side. Before the measurements, the skin

over the relevant muscles was shaved, abraded and rinsed with alcohol. Bipolar Ag/AgCl

surface electrodes (diameter 14 mm, center-to-center distance 20mm; Noraxon Dual Elec-

trodes, Noraxon, Scottsdale, USA) were attached in accordance with the European Rec-

ommendations for Surface EMG (Hermens et al., 1999). Afterwards, maximum voluntary

contraction (MVC) tests were performed for normalization. At T1, the positions of EMG

electrodes were marked with a temporary tattoo ink, so that they could be placed on the

same positions at T2.

A total of 12 valid trials (three per each of the four perturbation directions in a randomized

order, each lasting 30 s) were recorded. Trials were invalid if participants did not apply

enough force with their jaws (for INT and JAW), touched the ground with the non-standing

foot, moved their standing foot or released their hands from the hip. The success rate was

high (i.e. only 1-2 invalid trials per participant) and did not differ between the groups. At

T1 and T2, the same measurement process was followed.

6.3.4 Data analysis

All data were recorded in Vicon Nexus 2.10 and processed with MATLAB R2021b (Math-

Works). Kinematic data were filtered by a fourth-order Butterworth low-pass filter (10 Hz),

and EMG data with a fourth-order Butterworth band-pass filter (10-500 Hz). The filtered

EMG data were rectified, and normalized to the MVC amplitudes (Hellmann et al., 2015).

R and L directions were re-sorted into ipsilateral (I) and contralateral (C) according to the

standing leg of the participants.

6 Study III – Modulation of Jaw Clenching Effects after Jaw Clenching Training

To operationalize dynamic reactive balance performance, the damping ratio (Fadillioglu et

al., 2022a; Kiss, 2011a) was calculated based on movement of the Posturomed using the

data of the markers attached on the platform (Eq. 6.1, Figure 6.2). Larger damping ratio

values represent better compensation of the perturbation, therefore better dynamic re-

active balance, and vice versa. With respect to the EMG data, three main latency re-

sponses were considered after the onset of perturbation: short (SLR, 30 to 60 ms),

medium (MLR, 60 to 85 ms), and long (LLR, 85 to 120 ms) (Freyler et al., 2015; Taube et

al., 2006). Two further time windows were considered: 100 ms before the onset of per-

turbation (PRE, -100 to 0 ms) and after the reflex phases until the end of the individual

damping ratio (DRP, 120 to 1136 ± 131 ms). Mean activities of the relevant muscles (di-

rections B: PL & SOL; F: TA & AB; I: SM & IO, and C: RF & ES (S. M. Henry et al., 1998);

additionally MA for all directions) were calculated for the five phases, that is PRE, SLR,

MLR, LLR, and DRP.

 

 ; 



    positive amplitude

(6.1)

The marker trajectories in 3D were used to estimate the CoM trajectories with the full-

body Dynamicus model (ALASKA, INSYS GmbH, Chemnitz, Germany (Härtel & Hermsdorf,

2006)). The COM displacement (Pohl et al., 2020) was calculated as the range of motion

of CoM along the perturbation axis (RoMCoM_AP for B and F, and RoMCoM_ML for I and C).

Further, the three-dimensional velocity of the CoM (VCoM) (Alizadehsaravi et al., 2020) was

calculated for each trial and averaged for the whole damping ratio time window (0 ms

until the end of the individual damping ratio).

Figure 6.2: Calculation of damping ratio. The initial maximum displacement (K0) and the third positive ampli-

tude (K3) were used for Eq. 6.1.

6.4 Results

6.3.5 Statistics

Statistical calculations were done using IBM SPSS Statistics 25.0 (IBM Corporation, Ar-

monk, NY, United States). For all dependent parameters (damping ratio, RoMCoM_AP,

RoMCoM_ML, VCoM and mean muscle activities) the three trials within each of the four per-

turbation directions were averaged. The normality of the data distributions was confirmed

by Kolmogorov- Smirnov tests. The statistical assumptions were met to perform the re-

peated measures ANOVA (rmANOVA). The four perturbation directions were analyzed

separately (Fadillioglu et al., 2022a) since it was suggested that the direction of surface

translation influences the sensation, central processing and output of the postural re-

sponses differently (C. Chen et al., 2014; Nonnekes et al., 2013). For each dependent pa-

rameter, direction and phase, a rmANOVA was calculated with the factors group (INT, JAW

and HAB) and time (T1 and T2). The significance level was set a priori to p < 0.05. In case

of significant differences, post-hoc tests or t-tests were performed for pairwise compari-

sons. Partial eta-squared and Cohen's d were calculated to quantify the effect sizes for

rmANOVA and post-hoc tests, respectively (small effect: 

 < 0.06, d < 0.50; medium ef-

fect: 0.06 < 

 < 0.14, 0.5 < d < 0.8; large effect: 

 > 0.14, d > 0.8; (Cohen, 1988). The

Bonferroni–Holm method was applied to correct the results for multiple comparisons

(Holm, 1979).

6.4 Results

6.4.1 Dynamic reactive balance performance

The results regarding damping ratio for the four directions are illustrated in Figure 6.3. For

the factor time, rmANOVA results revealed significant improvements in the directions B,

F and C with high effect sizes (B: p = 0.042, 

 = 0.168; F: p = 0.015, 

 = 0.206;

C: p < 0.001, 

 = 0.356). However, there were no significant effects for the factor group

as well as no interaction effects between the factors time and group. Accordingly, jaw

clenching had no effect on dynamic reactive balance performance. In addition, the train-

ing of jaw clenching in the INT group did not show any effects on dynamic reactive balance

performance. Independent of the groups, the dynamic reactive performance was better

at T2 compared to T1.

6 Study III – Modulation of Jaw Clenching Effects after Jaw Clenching Training

Figure 6.3: Damping ratio results. INT: intervention, JAW: jaw clenching, HAB: habitual. † signifies significant

effects for the factor time. Significance level was set at p < 0.05.

6.4.2 Center of mass kinematics

The RoMCoM_AP, RoMCoM_ML and VCoM results for the four directions are represented in Fig-

ure 6.4a-b. RoMCoM_AP, RoMCoM_ML did not show any significant effects. VCoM had significant

differences for the factor time in the directions B, F, I and C with high effect sizes

(B: p < 0.001, 

 = 0.869; F: p = 0.004, 

 = 0.289; I: p = 0.027, 

 = 0.230; C: p = 0.037,



 = 0.220). No significant effects for the factor group as well as no interaction effects

between the factors time and group were detected. The results revealed that jaw clench-

ing or its training had no significant effects on center of mass kinematics. Across the

groups, the VCoM decreased at T2.

6.4 Results

Figure 6.4: a. RoMCoM_AP, RoMCoM_ML results, b. VCoM results. † signifies significant effects for the factor time.

6 Study III – Modulation of Jaw Clenching Effects after Jaw Clenching Training

6.4.3 Jaw clenching task controlled by masseter activity

Mean activities of the muscle MA for each phase are shown in Table 6.1. MA showed

significant effects in all of the five phases for the factor time with medium effect sizes

(PRE: p < 0.001, 

 = 0.133; SLR: p < 0.001, 

 = 0.116; MLR: p < 0.001, 

 = 0.106;

LLR: p < 0.001, 

 = 0.113; DRP: p < 0.001, 

 = 0.121) and for the factor group with high

effect sizes (PRE: p < 0.001, 

 = 0.362; SLR: p < 0.001, 

 = 0.364; MLR: p < 0.001,



 = 0.356; LLR: p < 0.001, 

 = 0.351; DRP: p < 0.001, 

 = 0.340). In two of the five

phases, there were significant interaction effects of the factors group and time with me-

dium effect sizes (PRE: p = 0.002, 

 = 0.066; SLR: p = 0.003, 

 = 0.061). Post-hoc results

showed that HAB had significantly lower MA activity with high effect sizes in all of the five

phases in comparison to INT (PRE: p < 0.001, d = 1.268; SLR: p < 0.001, d = 1.260;

MLR: p < 0.001, d = 1.240; LLR: p < 0.001, d = 1.229; DRP: p < 0.001, d = 1.225) as well as

in comparison to JAW (PRE: p < 0.001, d = 1.674; SLR: p < 0.001, d = 1.681; MLR: p < 0.001,

d = 1.641; LLR: p < 0.001, d = 1.621; DRP: p < 0.001, d = 1.599).

Table 6.1: Mean muscle activities for MA in the five phases for all perturbation directions. Significant effects

are highlighted in bold. † signifies significant effects for the factor time, * for the factor group, and

# the interaction effects. The significance level was set at p < 0.05.

PRE

SLR

MLR

LLR

DRP

INT

6.4 ± 5.1*†#

5.5 ± 4.6*†#

5.2 ± 4.3*†

5.6 ± 4.7*†

6.5 ± 5.2*†

JAW

6.0 ± 4.3*†#

5.2 ± 3.7*†#

5.0 ± 3.6*†

5.3 ± 3.8*†

6.2 ± 4.4*†

HAB

1.2 ± 2.0*†#

1.0 ± 1.7*†#

1.0 ± 1.7*†

1.1 ± 1.9*†

1.4 ± 1.7*†

INT

3.6 ± 3.0*†#

3.2 ± 2.6*†#

3.1 ± 2.5*†

3.3 ± 2.7*†

4.7 ± 5.2*†

JAW

5.2 ± 3.1*†#

4.7 ± 2.9*†#

4.5 ± 2.8*†

4.8 ± 3.0*†

5.9 ± 3.6*†

HAB

0.5 ± 0.7*†#

0.4 ± 0.7*†#

0.4 ± 0.7*†

0.4 ± 0.8*†

0.4 ± 0.3*†

These results indicated, first, that the MA activity at T1 was higher than at T2 independent

of the group. Secondly, the group HAB had significantly lower MA activity compared to

both jaw clenching groups, INT and JAW, independent of the measurement time. Thirdly,

the reduction in MA activity level from T1 to T2 was partly higher for the jaw clenching

group, INT, that trained for the task between two measurement times compared to JAW

and HAB.

6.4 Results

6.4.4 Muscle activities in the critical phases for reflexes

Mean activities of the analyzed muscles for each phase are shown in Table 6.2 and Table

6.3 for anterio-posterior and medio-lateral perturbations, respectively. The significant ef-

fects are highlighted in the tables. The corresponding p-values and effect sizes are re-

ported in the following paragraphs.

For the direction B, the muscle SOL showed significant interaction effects between the

factors time and group with high effect sizes in SLR (p = 0.002, 

 = 0.240). At T2, the group

JAW had an increased level of SOL activity compared to T1, whereas the other two groups

had a decreased level. For the direction F, the muscle TA showed significant effects with

high effect sizes for the factor time in three of the five phases (PRE: p < 0.001, 

 = 0.269;

SLR: p = 0.003, 

 = 0.177; DRP: p < 0.001, 

 = 0.333) as well as for the factor group in

one phase (MLR: p < 0.001, 

 = 0.306). Across all groups the level of TA activity was de-

creased at T2 compared to T1 in PRE, SLR and DPR. Further, across the measurement times

the JAW and INT groups had a higher level of TA activity compared to HAB in MLR. The

post-hoc t-test results revealed that these differences were valid at T1, but not at T2. For

the directions C and I, no significant effects were detected.

In summary, the results showed that the reflex activity changes were limited to anterior-

posterior directions (B and F). In case of backwards acceleration of the platform, the JAW

group showed increases in SOL activity at T2, whereas the other two groups revealed de-

creases. In case of forwards acceleration of the platform, the TA activity was lower at T2

compared with T1 in three reflex phases independent of the groups. Further, the two jaw

clenching groups (JAW and INT) had higher TA activity compared to HAB in MLR phase at

T1.

6 Study III – Modulation of Jaw Clenching Effects after Jaw Clenching Training

Table 6.2: Mean muscle activities for perturbation-relevant muscles in the five phases for anterio-posterior

perturbations. Significant effects are highlighted in bold. † signifies significant effects for the factor

time, * for the factor group, and # the interaction effects. The significance level was set at p < 0.05.

Back

PRE

SLR

MLR

LLR

DRP

INT

14.4 ± 10.9

16.8 ± 16.8

17.2 ± 12.7

14.8 ± 10.7

22.6 ± 11.4

JAW

21.1 ± 11.7

22.9 ± 16.0

21.0 ± 11.9

20.1 ± 11.8

26.5 ± 10.6

HAB

37.5 ± 74.7

26.3 ± 35.1

44.5 ± 99.5

45.9 ± 103.2

37.2 ± 41.4

INT

13.7 ± 9.7

17.7 ± 17.1

18.5 ± 18.1

19.5 ± 15.7

22.6 ± 14.4

JAW

12.7 ± 6.8

16.2 ± 11.0

14.4 ± 9.1

17.2 ± 12.7

21.1 ± 10.2

HAB

19.4 ± 18.6

20.9 ± 17.0

23.2 ± 30.0

22.1 ± 23.8

25.4 ± 18.6

SOL

PRE

SLR

MLR

LLR

DRP

INT

19.6 ± 12.3

21.2 ± 11.7#

20.6 ± 24.1

21.8 ± 19.0

26.4 ± 13.6

JAW

14.6 ± 6.7

12.8 ± 6.2#

15.4 ± 12.1

18.4 ± 12.8

21.8 ± 10.1

HAB

16.2 ± 10.7

16.9 ± 13.2#

15.5 ± 16.5

13.0 ± 9.5

20.8 ± 11.6

INT

15.3 ± 5.0

13.7 ± 5.6#

14.8 ± 11.0

18.4 ± 12.3

24.3 ± 15.2

JAW

18.3 ± 7.4

18.3 ± 9.5#

14.3 ± 7.7

16.8 ± 10.1

23.6 ± 11.9

HAB

12.9 ± 8.0

11.4 ± 5.9#

10.2 ± 4.8

12.9 ± 7.1

19.6 ± 9.4

Forward

PRE

SLR

MLR

LLR

DRP

INT

12.0 ± 13.0†

12.7 ± 15.9†

11.8 ± 11.3*

9.6 ± 8.3

14.2 ± 5.8†

JAW

9.6 ± 5.8†

10.6 ± 9.5†

10.9 ± 7.1*

9.6 ± 6.9

15.4 ± 8.2†

HAB

7.1 ± 5.3†

5.5 ± 4.0†

6.4 ± 4.1*

6.2 ± 3.9

13.4 ± 7.3†

INT

5.9 ± 7.9†

5.3 ± 5.8†

4.4 ± 3.7*

6.3 ± 5.1

10.3 ± 6.4†

JAW

6.0 ± 2.9†

5.2 ± 3.0†

5.4 ± 2.9*

5.8 ± 5.2

9.7 ± 5.1†

HAB

5.8 ± 2.8†

5.8 ± 3.8†

6.5 ± 5.7*

7.0 ± 4.5

11.5 ± 5.9†

PRE

SLR

MLR

LLR

DRP

INT

1.4 ± 2.1

1.5 ± 2.5

1.4 ± 2.0

1.4 ± 1.7

1.4 ± 1.6

JAW

0.9 ± 0.6

0.9 ± 0.8

0.7 ± 0.4

1.0 ± 0.7

1.1 ± 0.7

HAB

1.1 ± 1.1

1.2 ± 1.1

1.1 ± 1.1

0.9 ± 0.8

1.4 ± 1.2

INT

0.9 ± 1.4

0.9 ± 1.5

0.8 ± 1.1

0.8 ± 1.2

1.1 ± 1.6

JAW

0.9 ± 0.7

0.8 ± 0.7

0.8 ± 0.6

0.9 ± 0.9

0.9 ± 0.7

HAB

1.3 ± 1.0

1.1 ± 1.0

1.2 ± 0.9

1.3 ± 1.0

1.3 ± 0.8

6.4 Results

Table 6.3: Mean muscle activities for perturbation-relevant muscles in the five phases for medio-lateral per-

turbations.

Ipsilateral

PRE

SLR

MLR

LLR

DRP

INT

5.0 ± 5.5

7.2 ± 9.6

6.3 ± 9.7

6.1 ± 8.2

7.9 ± 7.7

JAW

3.9 ± 3.4

3.5 ± 3.5

4.5 ± 4.1

4.1 ± 3.2

5.8 ± 4.9

HAB

5.0 ± 4.4

4.6 ± 4.0

4.7 ± 4.3

5.2 ± 4.9

6.7 ± 5.9

INT

4.1 ± 6.9

3.9 ± 5.5

4.5 ± 7.1

4.8 ± 8.8

6.5 ± 8.5

JAW

3.1 ± 2.4

3.0 ± 3.0

3.6 ± 3.1

3.8 ± 3.2

4.1 ± 2.5

HAB

3.9 ± 5.1

4.0 ± 5.5

4.6 ± 5.8

4.3 ± 5.8

5.7 ± 7.0

PRE

SLR

MLR

LLR

DRP

INT

2.8 ± 2.7

3.3 ± 3.3

2.9 ± 3.1

3.2 ± 3.5

3.7 ± 3.1

JAW

2.7 ± 1.5

2.7 ± 1.7

2.5 ± 1.3

2.6 ± 1.4

3.5 ± 1.5

HAB

2.2 ± 1.2

2.2 ± 1.3

2.1 ± 1.0

3.9 ± 3.1

INT

1.9 ± 1.8

2.6 ± 2.0

2.7 ± 2.4

2.7 ± 2.9

3.3 ± 2.3

JAW

1.9 ± 1.6

2.5 ± 2.2

2.6 ± 2.6

2.8 ± 2.5

3.3 ± 2.1

HAB

2.1 ± 1.4

1.8 ± 1.5

1.9 ± 1.2

2.0 ± 1.3

2.8 ± 2.1

Contralateral

PRE

SLR

MLR

LLR

DRP

INT

2.3 ± 4.3

3.8 ± 2.8

4.8 ± 8.3

4.5 ± 7.3

5.3 ± 3.9

JAW

3.8 ± 2.3

5.4 ± 4.4

3.6 ± 2.5

4.7 ± 2.9

6.2 ± 3.9

HAB

2.7 ± 2.7

2.5 ± 2.0

2.9 ± 2.9

2.9 ± 2.6

4.8 ± 4.9

INT

3.0 ± 3.0

3.3 ± 3.3

3.1 ± 3.1

3.3 ± 3.6

5.2 ± 5.6

JAW

3.4 ± 3.3

3.3 ± 3.1

3.6 ± 3.8

3.9 ± 4.2

5.0 ± 3.9

HAB

2.6 ± 2.0

2.6 ± 2.2

2.4 ± 1.8

2.6 ± 1.6

4.2 ± 2.7

PRE

SLR

MLR

LLR

DRP

INT

5.1 ± 3.2

5.2 ± 3.1

6.8 ± 5.6

7.2 ± 6.5

7.9 ± 5.1

JAW

4.6 ± 4.9

4.1 ± 4.3

4.7 ± 4.8

4.8 ± 3.8

8.1 ± 7.9

HAB

4.3 ± 3.4

4.7 ± 4.0

5.7 ± 7.5

4.3 ± 3.3

8.1 ± 7.1

INT

7.0 ± 6.4

5.7 ± 3.9

7.2 ± 7.7

6.8 ± 6.9

10.2 ± 8.1

JAW

4.4 ± 3.6

4.1 ± 3.1

4.5 ± 4.0

4.8 ± 4.7

6.2 ± 4.5

HAB

4.8 ± 3.4

4.6 ± 3.9

4.2 ± 3.0

4.4 ± 3.4

7.0 ± 5.2

6 Study III – Modulation of Jaw Clenching Effects after Jaw Clenching Training

6.5 Discussion

The aim of this study was to investigate the effects of jaw clenching training on a dynamic

reactive balance task performance after 1 week of jaw clenching training. It was hypoth-

esized that jaw clenching has stabilizing effects resulting in better dynamic reactive bal-

ance performance and these effects persist at T2 after intervention. This would mean that

these improvements are not a result of the dual-task effect, but are specifically associated

with jaw clenching. The results indicated that neither jaw clenching nor its automation

through training resulted in significant dynamic reactive balance performance differences.

However, independent of the groups, the dynamic reactive balance performance was bet-

ter at T2 compared to T1. As there was not any deliberate balance training in the inter-

vention phase, this result is indicative of high learning effects. Further, jaw clenching may

lead to some changes in reflex activities but they are limited to anterior-posterior pertur-

bation of the platform.

6.5.1 Effects of jaw clenching on dynamic reactive balance

performance and CoM kinematics

Dynamic reactive balance performance was operationalized by the damping ratio as in

other studies (Fadillioglu et al., 2022a; Kiss, 2011a). In addition, the RoM of CoM along

the perturbation axis as well as VCoM were calculated. In all of the directions, no significant

effects due to jaw clenching were observed. Previous studies showed that jaw clenching

may affect balance performance under static steady-state conditions (Hellmann et al.,

2011a, 2015; Ringhof, Leibold, et al., 2015; Ringhof, Stein, et al., 2015), as well as under

dynamic conditions (Fadillioglu et al., 2022a; Tomita et al., 2021). However, the nature of

these effects is still unknown and could be associated with the dual-task situation. To the

best of our knowledge, research so far has not addressed this point explicitly. This study

investigated the effects of jaw clenching on dynamic reactive balance performance after

1 week of jaw clenching training to determine if the effects are a result of the general

dual-task situation or specifically due to neurophysiological effects of jaw clenching. At T1

and T2, both INT and JAW groups were instructed to do the same dual-task. These two

groups differed only in the intervention: INT trained the jaw clenching task, whereas JAW

did not. It was assumed that after one week of training (18 training sessions á 10 minutes

of practice), the participants of INT would be able to fulfill the jaw clenching task in an

automated manner. Therefore, it was hypothesized that the INT group would have re-

duced focused attention on the secondary jaw clenching task (Andersson et al., 2002) and

therefore a worse balance performance than JAW at T2. However, the results did not

6.5 Discussion

reveal any significant performance differences between the groups. Based on this it can

be concluded that the jaw clenching task did not have any observable effects on dynamic

reactive balance performance, which was operationalized by the damping ratio and CoM

kinematics. Further, its automation also did not result in any significant changes. On the

other hand, another explanation might be that the response of the motor system to the

complexity of the present balance task possibly masked the effects of jaw clenching, which

were identified in previous experiments with static balance tasks (Hellmann et al., 2015;

Ringhof, Stein, et al., 2015). In addition, in a previous study by Tardieu et al. (Tardieu et

al., 2009), the effects of dental occlusion on postural control was investigated both in eyes

open and closed conditions. They reported that the sensory information associated with

the dental occlusion becomes more important when the other sensory cues become

scarce (e.g., eyes closed). On this basis, it can be suggested that jaw clenching task might

potentially be beneficial once sensory information from other sources reduces. Neverthe-

less, in this study the balance task was performed with open eyes since the Posturomed

task was too difficult to be handled with eyes closed.

6.5.2 High learning effects even without

training between sessions

In three of four directions (B, F and C), dynamic reactive balance performance was im-

proved at T2 even though the participants did not perform any balance training between

T1 and T2. Further, in all directions VCoM decreased significantly at T2, whereas the

RoMCoM_AP, RoMCoM_ML were not affected. It should be noted that the participants per-

formed familiarization trials before the real measurements as in similar studies (Freyler et

al., 2015; Petró et al., 2018). Further, within the individual measurement session there

were no systematic performance improvements in terms of dynamic reactive balance.

These results indicate that learning effects occurred without deliberate balance training

for this specific task. Subsequently, the question arose if the learning effects were so large

that they outweighed the possible effects of jaw clenching. With this study design, this

question cannot be answered and further studies are needed. From the findings of this

study it can be concluded that the used balance task used here shows high learning effects

and is rather unsuitable for studies in which low intervention effects on balance perfor-

mance are expected. In the present case as well as in similar cases, care should therefore

be taken to select a balance task that shows only low learning effects or a longer interven-

tion period should be scheduled between T1 and T2 to mitigate the unwanted learning

effects.

6 Study III – Modulation of Jaw Clenching Effects after Jaw Clenching Training

The results also revealed that the velocity of the CoM changed but its RoM in the pertur-

bation direction did not change at T2. This may be explained by the deceased CoM move-

ment in case of the better damping of the platform by the participants, since the CoM is

one of the controlled variables as suggested in postural studies (Richmond et al., 2021;

Winter et al., 1998). On the other hand, the RoMCoM_AP, RoMCoM_ML depended for the most

part on the initial maximum displacement of the platform, which was identical both at T1

and T2. Therefore, the RoM did not change at T2.

6.5.3 Changed muscular activity levels at T2

The results regarding muscle activities in reflex phases revealed that in case of the back-

wards perturbation of the platform (B), the SOL activity of JAW increased, whereas that

of the other two groups decreased at T2 in the SLR phase. It is important to add that SOL

is one of the most important muscles that help to restore equilibrium in response to pos-

terior translations (S. M. Henry et al., 1998). This result may be interpreted as a difference

between INT and JAW groups and it can be suggested that the jaw clenching task resulted

in an increased muscle activity in SOL at T2, but these effects were not visible when the

jaw clenching task became an implicit task and therefore lost its novelty (e.g., for the

group INT). In addition, in case of forwards acceleration of the platform, TA activity of the

both jaw clenching groups (INT and JAW) was overall higher compared to that of HAB in

the MLR phase at T1. This finding contradicts the initial hypothesis that the jaw clenching

task results in changes in reflex activities and these effects persist after 1 week of jaw

clenching training. Nevertheless, these results are only limited to this perturbation direc-

tion and to this specific reflex phase. Furthermore, these changes did not cause any effects

on dynamic reactive balance performance (i.e. damping ratio results).

In response to anterior surface translations, TA contracts to counteract the torques at the

ankle and therefore helps to restore equilibrium (S. M. Henry et al., 1998). The TA activity

decreased at T2 in three phases (PRE, SLR and DRP) across the groups, parallel to dynamic

balance performance improvements. These results indicate that in case of forwards accel-

eration of the platform, a better performance at T2 is possibly related with a decreased

TA activity. In general, significant changes were detected only for the anterior-posterior

perturbation directions. Based on these results, it can be suggested that the jaw clenching

task may result in changed muscle activity patterns, as observed with the alterations in

certain muscle activities in the reflex phases, but changes seem to be direction-dependent

as well as muscle dependent. This task specificity can be explained by the different pos-

tural responses to different perturbation directions (C. Chen et al., 2014; Kiss, 2011b;

Nonnekes et al., 2013).

6.5 Discussion

Further, it should be noted that the muscle activity changes and the dynamic balance per-

formance differences did not show a common pattern for all directions (e.g., no changes

in muscle activity levels in perturbation direction C, despite the improvements in dynamic

reactive balance performance at T2). This may also possibly have been caused by the se-

lection of the posture relevant muscles. Posture and its control are the product of inter-

muscular coordination patterns. Determining the activity of individual muscles might be

the limiting factor in the analysis presented here. In light of these aspects, the question

arises if mean muscle activities for the critical phases were sensitive enough to reveal

changes on a muscular level. Nevertheless, these parameters were used in similar studies

(e.g., iEMG in Freyler et al. (2015) and Pfusterschmied, Stöggl, et al. (2013)). In the present

study, mean muscle activity was preferred since DRP was not the same length for each

trial or participant. It was expected that increased level of reflex activities would be man-

ifested by an increased level of muscle activities (Ertuglu et al., 2018). However, poten-

tially jaw clenching effects are seen less in a changed level of individual muscle activities

and more in a changed interplay of different muscles. Therefore, in future studies the co-

ordination of different muscles should be analyzed in addition to the analysis of the activ-

ity of individual muscles. Coordination models such as muscle synergies are particularly

suitable for this purpose (Munoz-Martel et al., 2021; Ting & Macpherson, 2005).

6.5.4 Jaw clenching task controlled by masseter activity

The EMG results indicate that the activity of the MA was higher for the groups INT and

JAW compared to HAB. This suggests that the majority of the subjects in HAB, who did not

receive instructions regarding activity of the stomatognathic system, had their jaws in the

physiologically expected resting position (lips closed, teeth out of contact). It should be

noted that the participants of JAW and INT trained immediately before starting the bal-

ancing task measurements with the Rehabite® device, so that they are able to apply a

force at a level of 75 N consistently without feedback. The higher reduction in MA activity

between T1 and T2 in the INT group compared with the other groups can be attributed to

the training during the intervention phase. Similar effects were also shown in a previous

study (Hellmann et al., 2011b), in which short-term force-controlled biting on a hydrostatic

system caused long-term training effects.

A force of 75 N is easy to achieve for the stomatognathic system, as normal masticatory

activities are in the range of this force level. The RehaBite®-training in the group INT be-

tween T1 and T2 was used to turn a novel, unfamiliar task (biting on a hydrostatic system

is not part of the common functional repertoire of the stomatognathic system) into an

implicit behavior so that it would not require additional attention during the balancing

6 Study III – Modulation of Jaw Clenching Effects after Jaw Clenching Training

task. Therefore, RehaBite®-training between T1 and T2 in INT was not used to train the

masticatory muscles but to address a potential dual-task effect during the balance task. It

should also be noted that the jaw clenching task in this study is a different stomatognathic

activity than daily chewing activity occurring when eating (Hellmann et al., 2011b). During

the sub-maximum jaw clenching task, a force of 75 N was applied continuously, whereas

during chewing an alternating force is applied. On this basis, it can be assumed that the

deliberate jaw clenching task was novel to the participants at the first measurements. Fur-

ther, it was also shown that the chewing task had no significant effects on body sway re-

duction during upright standing, whereas feedback- controlled jaw clenching task had

(Hellmann et al., 2011a). This also supports that the sub-maximum jaw clenching and the

chewing tasks are not the same task and they may lead to different neurophysiological

effects.

6.5.5 Limitations

This study had some limitations: Firstly, even though the participants did not train for the

balance task, learning effects occurred in three of the four directions independent of the

group. These high learning effects may have outweighed the potential effects of jaw

clenching. For future studies, more care should be taken to minimize possible learning

effects. Secondly, all the participants were physically active and healthy adults, therefore

potentially good at balancing. The same results may not be seen in groups with compro-

mised postural control such as the elderly (M. Henry & Baudry, 2019) or people with neu-

rological disorders (Xia & Mao, 2012). In future studies, the participants with poorer

postural control might reveal effects of jaw clenching. Thirdly, the onset of the reflex

phases was defined based on Posturomed movement but not on muscle activity peaks

(Taube et al., 2006) or ankle movements, since there were no clear peaks in the EMG or

kinematics data. Finally, the group HAB did not receive any instructions regarding

stomatognathic activities. Self-administative questionnaires regarding the clenching habit

would have been useful to collect habitual status.

6.6 Conclusion and Outlook

This study investigated the effects of jaw clenching on dynamic reactive balance task per-

formance after 1-week of jaw clenching training, to examine if the effects are a result of a

dual-task situation. Both jaw clenching and automation of the jaw clenching task seemed

not to have any observable effects on dynamic reactive balance performance, but jaw

clenching seemed to be related with some changes in reflex activities. However, these

6.6 Conclusion and Outlook

effects were limited to anterior-posterior perturbations. Further studies containing other

balance tasks with less learning effects as well as with longer intervention periods are

needed. Analysis of muscle coordination as well as other experimental designs with re-

duced sensory information from other sources (e.g., closed eyes) may also help to reveal

jaw clenching effects.

7 Study IV – Modulation of Center of

Mass Movement

Published as

Fadillioglu, C., Kanus, L., Möhler, F., Ringhof, S., Hellmann, D.*, & Stein, T.* (2022). Influ-

ence of Controlled Stomatognathic Motor Activity on Sway, Control and Stability of

the Center of Mass During Dynamic Steady-State Balance—An Uncontrolled Mani-

fold Analysis. Frontiers in Human Neuroscience, 16, 1-13. [*These authors have con-

tributed equally to this work.]

7.1 Abstract

Multiple sensory signals from visual, somatosensory and vestibular systems are used for

human postural control. To maintain postural stability, the central nervous system keeps

the center of mass (CoM) within the base of support. The influence of the stomatognathic

motor system on postural control has been established under static conditions, but it has

not yet been investigated during dynamic steady-state balance. The purpose of the study

was to investigate the effects of controlled stomatognathic motor activity on the control

and stability of the CoM during dynamic steady-state balance. A total of 48 physically ac-

tive and healthy adults were assigned to three groups with different stomatognathic mo-

tor conditions: jaw clenching, tongue pressing and habitual stomatognathic behavior.

Dynamic steady-state balance was assessed using an oscillating platform and the kine-

matic data were collected with a 3D motion capturing system. The path length (PL) of the

3D CoM trajectory was used for quantifying CoM sway. Temporal dynamics of the CoM

movement was assessed with a detrended fluctuation analysis (DFA). An uncontrolled

manifold (UCM) analysis was applied to assess the stability and control of the CoM with a

subject-specific anthropometric 3D model. The statistical analysis revealed that the

groups did not differ significantly in PL, DFA scaling exponents or UCM parameters. The

results indicated that deliberate jaw clenching or tongue pressing did not seem to affect

the sway, control or stability of the CoM on an oscillating platform significantly. Because

of the task-specificity of balance, further research investigating the effects of stomatog-

nathic motor activities on dynamic steady-state balance with different movement tasks

are needed. Additionally, further analysis by use of muscle synergies or co-contractions

7 Study IV – Modulation of Center of Mass Movement

kinematics. This study can contribute to the understanding of postural control mecha-

nisms, particularly in relation to stomatognathic motor activities and under dynamic con-

ditions.

7.2 Introduction

Balance maintenance and proper body orientation in space are essential for human life.

They require a good, reliable and flexible postural control system which is capable of pro-

cessing multiple sensory feedback inputs from the visual, somatosensory and vestibular

systems in the spinal and supraspinal structures of the central nervous system (CNS) in a

task dependent manner (Takakusaki, 2017). The control of posture involves control of the

body position in space for stability and orientation. Stability is defined as the control of

the center of mass (CoM) in relation to the base of support, whereas orientation refers to

the ability to maintain an appropriate relationship between the body segments as well as

between the body and the environment (Shumway-Cook & Woollacott, 2017). A healthy

motor control system modulates the postural movements continuously as a function of

the changing tasks. The inability to modulate postural sway, but also environmental or

individual constraints may lead to poor performance, instability and falls (Haddad et al.,

2013). Furthermore, it has been shown that improved postural control is associated with

a decreased risk of falls (Horak, 2006; Rubenstein, 2006) as well as a decreased risk of

injury (Hrysomallis, 2007).

Attentional processing is required during postural tasks; therefore, they may reduce the

performance of a secondary task when performed simultaneously. On the other hand, a

secondary task may improve the postural control by an improved automaticity, an in-

creased arousal or through the utilization of reduced sway for the sake of a better supra-

postural task performance (Shumway-Cook & Woollacott, 2017). Previous studies showed

that postural control may be influenced by several factors, including motor activity in the

stomatognathic motor system (Julià-Sánchez et al., 2020). A frequently cited explanation

for this is based on the stimulation of periodontal mechano- receptors that are centrally

integrated along with other sensory input and, therefore, facilitates the excitability of the

human motor system (Boroojerdi et al., 2000) in a manner similar to the Jendrassik ma-

neuver (Jendrassik, 1885), which in turn increases the neural drive to the distal muscles

(Ebben, 2006; Ebben et al., 2008). A variety of studies indicated that stomatognathic mo-

tor activity in the form of chewing, tongue activity or different clenching conditions affects

human balance and posture under static conditions (Alghadir et al., 2015c; Gangloff et al.,

2000; Hellmann et al., 2011a, 2015; Ringhof, Leibold, et al., 2015; Ringhof, Stein, et al.,

may reveal effects on the level of muscles, which were not visible on the level of

7.2 Introduction

2015; Sakaguchi et al., 2007). Among others, a reduced body sway in the anterior- poste-

rior direction (Hellmann et al., 2011a), a reduced variability of muscular co-contraction

patterns of posture-relevant muscles of the lower extremities (Hellmann et al., 2015), and

reduced trunk and head sway under the influence of controlled biting activities were re-

ported during upright standing (Ringhof, Stein, et al., 2015). Furthermore, the review by

de Souza et al. (2021) reported that jaw clenching during activities that involve the lower

and upper limbs enhance neuromotor stimulation in terms of increased H-reflexes

(Miyahara et al., 1996) and stimulate a larger area of the brain. Specifically, a large amount

of activity was observed over the frontal, parietal, and temporal cortices and cerebellum

during hand grip combined with jaw clenching compared to without jaw clenching

(Kawakubo et al., 2014). The authors suggested that the stomatognathic motor system

may have effects on the function of remote muscles via cortical activations. Furthermore,

a higher excitability of the human motor system during voluntary jaw clenching has also

been shown (Boroojerdi et al., 2000). From an evolutionary perspective, it was hypothe-

sized that jaw clenching increases the blood flow to anterior temporal lobe structures dur-

ing acute activation of the limbic fear circuits (Bracha, Ralston, et al., 2005). Jaw clenching

may increase the blood flow to temporal lobe structures by pumping blood through the

temporal bone emissary veins, thus conferring a possible survival advantage during acti-

vation of the limbic fear-circuits in expectation of situations requiring the freeze, flight,

fight, fright acute fear response (Bracha, Bracha, et al., 2005). Stomatognathic motor ac-

tivity also seems to be part of a common physiological repertoire used to improve motor

performance during balance recovery tasks (Ringhof et al., 2016). Besides all these facts,

it should be mentioned that it stomatognathic motor activity might be of clinical relevance

for the prevention of falls. In elderly people there is evidence for an increased risk of falling

resulting from an insufficient dental or prosthetic status (Mochida et al., 2018; Okubo et

al., 2010).

In contrast to balance under static conditions (e.g., sitting or standing), the influence of

stomatognathic motor activity under dynamic conditions (e.g., standing on a balance

board or on an oscillating platform) has not yet been investigated in detail (Ringhof et al.,

2016). Since the effects found during one balance task may not necessarily be transferable

to another balance task (Giboin et al., 2015; Kümmel et al., 2016; Ringhof & Stein, 2018),

the question arises whether the effects of stomatognathic motor activity found during

static balance tasks would also be observable during dynamic ones. Accordingly, we

started to investigate the effects of stomatognathic motor activity in the form of jaw

clenching and tongue pressing on dynamic reactive balance performance (Fadillioglu et

al., 2022a). This was realized by use of an oscillating platform perturbed randomly in one

of four horizontal directions. In our previous study, the focus was on the first reactive part

7 Study IV – Modulation of Center of Mass Movement

of the task. We showed that jaw clenching improved dynamic reactive balance in a task-

specific (i.e. direction-dependent) way. The performance improvements found for jaw

clenching were associated with lower mean speeds of distinct anatomical regions com-

pared to both the tongue pressing and habitual groups. Subsequent to these findings, the

question arises as to whether this performance increase is associated with a changed

sway, stability or control of the CoM in the steady-state phase of the task.

The CoM is suggested to be the controlled variable in postural studies (Kilby et al., 2015;

Nicolai & Audiffren, 2019; Richmond et al., 2021; Winter et al., 1998), although experi-

mental verification is difficult (Shumway-Cook & Woollacott, 2017). Scholz et al. (2007)

used an uncontrolled manifold (UCM) approach to determine if the CoM is the variable

which is primarily controlled by the CNS during postural control. They showed that during

recovery from a loss of balance, the participants tend to re-establish the position of the

CoM rather than those of the joint configurations (Shumway-Cook & Woollacott, 2017),

and therefore suggested that the CoM is the key variable controlled by the CNS. In postural

control studies, CoM sway is an important parameter (Richmond et al., 2021) and its spa-

tial dynamics can be quantified among others by the total distance covered (Prieto et al.,

1996; Richmond et al., 2021). Another important aspect is the temporal dynamics of the

sway, since variations in “supra-postural” activities may lead not only to spatial but also

to temporal changes (F. C. Chen & Stoffregen, 2012). It was suggested that a detrended

fluctuation analysis (DFA) can reveal the temporal dynamics of postural data, specifically

to quantify the long-range correlations (or fractality) of the data (Duarte & Sternad, 2008;

McGrath, 2016).

When controlling the body during balance tasks, the CNS has to coordinate a redundant

musculoskeletal system (Bernstein, 1967) possessing more degrees of freedom than nec-

essary to achieve the given task (Latash et al., 2002). Different approaches have been sug-

gested to analyze how the CNS treats this redundancy, such as motor programs (R. A.

Schmidt et al., 2018), optimal control (Todorov & Jordan, 2002) or synergies (D’Avella et

al., 2003; Latash et al., 2007; Stetter et al., 2020). Latash et al. (2007) define “synergy” as

a neural organization consisting of a multi-element system that organizes sharing of a task

among a set of elemental variables (EVs), and ensures the stabilization of a performance

variable (PV) through the co-variation of EVs. The fact that different combinations of EVs

may result in the same PV indicates that the co-varied behavior provides flexibility for the

system. In this context, redundancy is considered not as a problem but as an advantage

for the motor control system. According to the motor abundance principle (Gelfand &

Latash, 1998), redundancy in the motor control system can be considered positive since

the co-variation at the level of the EVs may provide robustness against perturbations (Gera

et al., 2010).

7.2 Introduction

The UCM approach (Scholz & Schöner, 1999) is one possibility to quantify the amount of

equivalent movement solutions and the degree of stability of the PV. The UCM approach

requires a model that relates the changes in EVs to changes in the PV; and ultimately the

effects of changes in EVs on the PV are analyzed (Scholz & Schöner, 2014). Both the EVs

and PV are chosen on a physiological basis with task-specific considerations. The variabil-

ity in EVs that results in a changed PV is quantified by the  component, whereas it

is associated with the  component if the PV remains the same even if the EVs vary

over repetitions (Latash et al., 2007; Scholz & Schöner, 1999). The UCM approach has been

applied to analyze various motor tasks; for example, reaching and pointing (Domkin et al.,

2005; Tseng et al., 2002), pistol shooting (Scholz et al., 2000), sit-to-stand (Reisman et al.,

2002; Scholz et al., 2001), parkour jumps (Maldonado et al., 2018), treadmill walking (Qu,

2012; Verrel et al., 2010) and running (Möhle et al., 2019). Kinematic or kinetic data were

commonly used as EVs to investigate their effects on the PVs. There is also a number of

studies that apply UCM to postural tasks (Freitas et al., 2006; Hagio et al., 2020; Hsu et al.,

2007, 2013; Krishnamoorthy et al., 2005). When analyzing postural tasks, the CoM is typ-

ically chosen as the PV and joint angles as EVs (Freitas et al., 2006; Hagio et al., 2020; Hsu

et al., 2013, 2017; Krishnamoorthy et al., 2005; Scholz et al., 2007). By means of UCM

analysis, changes in the variability of coordinated joint movements in association with the

stability and control of the CoM have been investigated for various setups with different

research questions. In line with the previous studies, a UCM analysis was conducted in

this study with stomatognathic motor conditions as the independent variable.

The aim of this study was to investigate the influence of different stomatognathic motor

activities (jaw clenching and tongue pressing) on the sway, stability and control of the CoM

during a dynamic steady-state balance task (one-legged standing on an oscillatory plat-

form after perturbation). The path length (PL) of the 3D CoM was used to quantify the

possible effects of different stomatognathic motor activities on the spatial dynamics of

CoM sway, whereas its temporal dynamics was assessed with a DFA. A UCM approach was

applied to investigate if and how the co-variation of the joint movements led to the stabi-

lization and control of the CoM, which were quantified by  and , respec-

tively. Following the results of our above-mentioned study on the influence of jaw

clenching and tongue pressing on dynamic reactive balance performance (Fadillioglu et

al., 2022a), it was hypothesized that these activities decrease the sway and increase the

control and stability of the CoM. Therefore, a decreased PL of the CoM trajectory, an in-

creased alpha of DFA, an increased  and a decreased  for the jaw clenching

group (JAW) and the tongue pressing group (TON) compared to the group with habitual

stomatognathic behavior (HAB) were expected. The findings of this study may contribute

7 Study IV – Modulation of Center of Mass Movement

to the understanding of postural control, particularly in relation to stomatognathic motor

activities and under dynamic conditions.

7.3 Methods

This study comprised a follow-on analysis of the original data set used in Fadillioglu et al.

(2022a). In the previous study, the reactive phase of the task was analyzed, whereas in the

present one, the following steady-state phase is investigated. An a priori power analysis

was performed based on the findings of the study (Ringhof, Stein, et al., 2015) which an-

alyzed the effects of submaximal jaw clenching on postural stability and on the kinematics

of the trunk and head. The analysis revealed that 16 participants per group would be

enough to reach the sufficient power (>0.8).

7.3.1 Participants

Forty-eight healthy adults (25 female, 23 male; age: 23.8 ± 2.5 years; height: 1.73 ± 0.09

m; body mass: 69.2 ± 11.4 kg) voluntarily participated in the study after giving written

informed consent. All participants completed a questionnaire, confirmed that they were

physically active (physical activity 4.6 ± 1.5 days/week and 436 ± 247 min/week) and naive

to the tasks on an oscillating platform; had no muscular or neurological diseases; no signs

and symptoms of temporomandibular disorders (assessed by means of the RDC/TMD cri-

teria, (Dworkin & LeResche, 1992)). They presented in good oral health with full dentition

(except for 3rd molars) in neutral occlusion. The study was approved by the Ethics Com-

mittee of the Karlsruhe Institute of Technology.

7.3.2 Experimental procedure

7.3.2.1 Balance tasks

Dynamic steady-state balance was assessed by means of a Posturomed oscillating plat-

form (Haider-Bioswing, Weiden, Germany), which is a widely-used commercial device to

analyze or improve dynamic balance in scientific studies as well as in physiotherapy

(Freyler et al., 2015; Kiss, 2011a). It consists of a rigid platform (12 kg, 60 cm × 60 cm)

connected to the main frame by eight 15 cm steel springs with identical stiffness and it

can swing in the horizontal plane in all directions. In this study, an automatic custom-made

release system was used to initiate mechanical perturbations in four different directions:

back (B), front (F), left (L), right (R) (Fadillioglu et al., 2022a). By convention, these

7.3 Methods

directions indicate in which direction the platform was accelerated after release of the

platform. In each trial, a perturbation in one of the four possible directions was applied in

a randomized order. Participants stood on the platform on their dominant leg, which were

determined based on self-reports. If the participants were not sure which leg was their

dominant leg, it was determined by means of test trials on the Posturomed before the

measurements (Ringhof & Stein, 2018). During single-leg stand, they kept their hands

placed at the hips and their eyes focused on a target positioned at eye level and 4 m away

from the center of the Posturomed (Figure 7.1).

Figure 7.1: Participant during single-leg stand on the Posturomed oscillating platform.

7.3.2.2 Group assignment and masticatory system statuses

For familiarization, each participant performed two trials without and two trials with per-

turbation on the Posturomed. After that, a baseline measurement with perturbation and

in habitual stomatognathic motor condition was conducted to determine the initial bal-

ance performance quantified by Lehr’s damping ratio (DR) (Kiss, 2011a). Based on both

7 Study IV – Modulation of Center of Mass Movement

balance performance and gender, each participant was assigned to one of three groups,

such that both gender and the initial level of performance of the groups were balanced.

The statistical examination by a one-way ANOVA revealed no baseline performance differ-

ences between the three groups (p = 0.767). Each group (JAW, TON and HAB) consisted of

16 participants and performed one of the stomatognathic motor conditions simultane-

ously with the balance task during the measurements (Table 5.1).

Table 7.1: Stomatognathic motor conditions of the three groups, JAW, TON and HAB.

JAW: instructed, controlled submaximal jaw clenching with a 75 N force - activity of the

masticatory muscles during simultaneous occlusal loading

TON: instructed, controlled submaximal tongue pressing against the palate - stomatog-

nathic muscle activity without occlusal loading

HAB: habitual stomatognathic behavior - jaw positioning without any instruction

The stomatognathic motor activity was recorded by an EMG system (detailed information

in the “Data collection” section). To ensure that a force of 75 N was consistently applied,

the participants in the JAW group trained just before data acquisition with a RehaBite

(Plastyle GmbH, Uttenreuth, Germany). This medical training device with liquid-filled plas-

tic pads works based on hydrostatic principles, and can be used to control the applied

force (Giannakopoulos, Rauer, et al., 2018). As the participants trained with the RehaBite,

masseter activity was monitored to determine the corresponding muscle activity level as-

sociated with a jaw clenching force of 75 N. The determined masseter activity level was

around 5% maximum voluntary contraction (MVC) for all participants and was used to

control if the submaximal jaw clenching condition was fulfilled.

The TON group similarly trained to apply a submaximal force with the tip of the tongue

against the anterior hard palate corresponding to an EMG activity level of the suprahy-

oid muscles of the floor of the mouth of 5% MVC. Both groups trained for the stomatog-

nathic motor task for five minutes. The HAB did not receive any training or instructions.

During the measurements, the JAW group performed the jaw clenching task on an Aqual-

izer intraoral splint (medium volume; Dentrade International, Cologne, Germany).

7.3.3 Data collection

A 3D motion capture system (Vicon Motion Systems; Oxford Metrics Group, Oxford, UK;

10 Vantage V8 and 6 Vero V2.2 cameras with a recording frequency of 200 Hz) was used

7.3 Methods

to record the movements of the Posturomed platform and the participants. Four reflective

markers were attached on the upper surface of the Posturomed platform. A further 42

reflective markers were attached to the participants’ skin in accordance with the ALASKA

modelling system (Advanced Lagrangian Solver in kinetic Analysis, Insys GmbH, Chemnitz,

Germany; (Härtel & Hermsdorf, 2006)). Before data acquisition, 22 anthropometric

measures were manually taken from each participant for the ALASKA modelling.

A wireless EMG system (Noraxon, Scottsdale, USA; recording frequency of 2000 Hz) was

used to measure EMG activity of the masseter for JAW and HAB; and of the suprahy-

oid muscles of the floor of the mouth measured in the region of the digastricus venter

anterior muscle for TON. As preparation, the skin over the corresponding muscles was

carefully shaved, abraded and rinsed with alcohol. Bipolar Ag/AgCl surface electrodes (di-

ameter 14 mm, center-to-center distance 20 mm; Noraxon Dual Electrodes, Noraxon,

Scottsdale, USA) were positioned in accordance with the European Recommendations for

Surface Electromyography (Hermens et al., 1999). Afterwards, MVC tests were performed.

For each trial, participants received standardized instruction about the task to be per-

formed and were asked to compensate the perturbation as quickly as possible and to sta-

bilize their body. Between each trial, participants had two minutes of rest to prevent

fatigue. Measurements ended after completion of 12 successful balance task trials (i.e.

three trials for each direction) each lasting 20 s after initiation of the perturbation. During

the measurements, EMG activity of the masseter (for the JAW group) or the suprahy-

oid muscles of the floor of the mouth (for the TON group) was monitored and compared

with the individually determined EMG activity level corresponding to 5% of MVC. Record-

ings were stopped and the trial was considered invalid if participants stopped performing

their stomatognathic motor task (JAW and TON), had ground contact with the non-stand-

ing foot, changed the placement of their standing foot, released one of the hands from

the hip or lost their balance. All data were recorded in Vicon Nexus 2.10; where the EMG

system was connected via the Noraxon plug-in.

7.3.4 Data analysis

The collected data of 576 trials (48 participants x three valid trials x four directions) were

exported from Vicon Nexus for further analysis in MATLAB R2020a (MathWorks; Natick,

USA). For all data, the R and L directions were re-sorted as ipsilateral (I) and contralateral

7 Study IV – Modulation of Center of Mass Movement

The marker data were filtered with a Butterworth low-pass filter (fourth-order; cut-off

frequency 10 Hz); and the EMG data with a Butterworth band-pass filter (fourth-order;

cut-off frequency 10 to 500 Hz). The EMG data were then rectified and smoothed by av-

eraging with a sliding window of 30 ms and normalized to the MVC amplitudes (Hellmann

et al., 2011a).

Based on Posturomed marker data, two critical time points were separately determined

for each trial: (1) the start of the perturbation, and (2) the third highest amplitude of the

Posturomed center in the direction of perturbation (Kiss, 2011a). The time span between

(1) and (2) was assumed to be the phase in which the perturbation was maximally com-

pensated, and the time frames after (2) were considered the dynamic steady-state phase

of the movement. For all calculations, a time window of 12 s (Müller et al., 2004) was used

which started at time point (2).

7.3.5 Spatial dynamics of CoM sway

To quantify the spatial dynamics of the CoM sway, the PL was calculated. The time series

of CoM position was estimated by the subject-specific anthropometric 3D model refer-

enced and explained below (see 7.3.7 Uncontrolled manifold approach). The point-by-

point Euclidean norm of the vectors containing the 3D coordinates of the CoM was calcu-

lated to convert the three components in the x, y and z coordinates into a single value k,

where i stands for the frame number (Eq. 7.1). PL was approximated by the sum of the

distances between consecutive points of the time series k with a length of n (Prieto et al.,

1996), where n equals the total number of frames in a 12 s interval (n = 2400; Eq. 7.2).



(7.1)







(7.2)

7.3.6 Temporal dynamics of CoM sway

A detrended fluctuation analysis was performed to quantify the temporal dynamics of

CoM sway. Firstly, an integrated time series was calculated by subtracting its mean from

7.3 Methods

it (Eq. 7.3). Secondly, the data were divided into non-overlapping segments of length m

and the linear approximation was estimated by a separate least square fit in each seg-

ment. Thirdly, average fluctuation of the time series around the trend was calculated as

given in Eq. 7.4. The last two steps were repeated for all the considered m.

󰇛󰇜󰇛󰇛󰇜󰇜





(7.3)

󰇛󰇜

󰇛󰇛󰇜󰇛󰇜󰇜





(7.4)

In general F(m) increases with the increasing m and a power law is expected where the

scaling component α is a constant (Eq. 7.5). If α < 0.5 or 1 < α < 1.5, the time series inter-

preted as anti-persistent, where a smaller α indicates a more anti-persistent behavior. If

0.5 < α < 1 or 1.5 < α < 2, the time series is persistent and the larger the α, the

more persistent is the time series (Lin et al., 2008).

󰇛󰇜

(7.5)

7.3.7 Uncontrolled Manifold approach

A UCM approach was applied to investigate if and how the co-variation of the joint move-

ments led to the stabilization and control of the CoM. In accordance with the literature,

the CoM and the joint angles were selected as PV and EVs, respectively (Freitas et al.,

2006; Hagio et al., 2020; Hsu et al., 2013, 2017; Krishnamoorthy et al., 2005; Scholz et al.,

2007). To obtain joint angles, an inverse kinematics calculation was performed using a

modified version of the full-body Dynamicus (ALASKA) model (Härtel & Hermsdorf, 2006).

A subject-specific anthropometric 3D model was used to estimate the CoM as the

weighted sum of the body segments (Möhler et al., 2019).

The model was a modified version of the Hanavan model and had 50 degrees of freedom

(Hanavan, 1964). Of the 36 subject-specific anthropometric measurements needed to cal-

culate the CoM according to this model, 21 were taken manually and 15 were determined

from the reflective markers. A constant density was assumed (Ackland et al., 1988) and

the mass of each segment was estimated by volume integration. The whole-body CoM in

7 Study IV – Modulation of Center of Mass Movement

3D, , was determined by calculating the weighted sum of the body segments using

Eq. 7.6, where N is the total number of segments (N = 17;  the volume of the  seg-

ment; and  the center of gravity vector of the  segment.







  





(7.6)

The CoM, as the PV for the UCM, was defined as a function of the joint angles as the EVs

(, where j stands for the number of EVs). The mean joint configu-

ration across each trial, , was calculated as an approximation of the desired configura-

tion (Latash et al., 2007). The Jacobian matrix, 󰇛󰇜, containing all first-order partial

derivatives of the CoM coordinates with respect to the joint angles, was calculated at this

reference joint configuration. Afterwards the null space of the matrix was computed as

the linear estimate of the UCM (Eq. 7.7). The null space of the Jacobian matrix is the linear

subspace of all joint angle combinations that does not affect the position of the CoM, and

it is spanned by j-d number of basis vectors , where j and d are the number of dimensions

of EVs and PV, respectively (Scholz & Schöner, 1999).

󰇛󰇜

(7.7)

At each instant of n = 2400 trials (t = 12 s, recording frequency 200 Hz), the deviation from

the mean joint configuration 󰇛󰇜 was calculated (Hsu et al., 2013; Scholz et al., 2007)

and subsequently resolved into their projection on the null space to decompose it into

the parallel, , and orthogonal, , components (Möhler et al., 2019; Scholz & Schöner,

1999) (Eq. 7.8-9).

󰇛󰇜





(7.8)

󰇛󰇜

(7.9)

Finally, the amount of variability parallel to the UCM (, i.e. stabilizing PV) and or-

thogonal to the UCM (, i.e. changing PV) were estimated (Scholz & Schöner, 1999)

(Eq. 7.10-11). , the ratio of the two UCM components was calculated as sug-

gested by (Papi et al., 2015) to obtain a symmetrical distribution (i.e. [-1 1]. The midpoint

0 is the threshold for “synergy” and therefore appropriate for statistical calculations (Eq.

7.12).

7.3 Methods



󰇛󰇜







(7.10)











(7.11)

󰇧 

󰇨

(7.12)

The  component is a measure of the co-variation of EVs and therefore a measure for

flexibility. A higher  indicates a higher variability on the level of the EVs that does

not change the PV, and therefore a more flexible behavior of the control system, and vice

versa. The  component is a measure for control of the PV. The higher the ,

the less controlled the PV, which in this study is the CoM. Lastly,  indicates the

stability of the PV by means of kinematic synergy of the EVs. A  is inter-

preted as a synergy, whereas a  indicates no synergy (Latash et al., 2007).

In this study,  and  were used to quantify the stability and control of the

CoM (i.e. the PV), respectively.

7.3.8 Statistics

Statistical analysis was performed with IBM SPSS Statistics 25.0 (IBM Corporation, Ar-

monk, NY, USA). The PL of the CoM, DFA scaling component and three UCM parameters

(, ) for three trials for each direction and for each subject were averaged.

A Kolmogorov-Smirnov test was conducted to determine the normality of data distribu-

tion.

Each of the four directions of perturbation was analyzed separately because postural re-

sponse may differ depending on the perturbation direction (Akay & Murray, 2021; C. Chen

et al., 2014; Freyler et al., 2015; Kiss, 2011b; Nonnekes et al., 2013). For each of the four

considered parameters and for each direction, a one-way ANOVA or a Kruskal-Wallis test

was performed for the group comparisons depending on the normality of the distribution.

The level of significance was set a priori to p < 0.05. Partial eta squared (

) or Cramer‘s V

() (small effect: 

 < 0.06 or  < 0.2; medium effect: 0.06 < 

 < 0.14 or 0.2 << 0.6;

7 Study IV – Modulation of Center of Mass Movement

large effect: 

 > 0.14 or  > 0.6; (Cohen, 1988; Richardson, 2011)) were calculated to

estimate the effect sizes for normal and non-normal distribution of data, respectively.

7.4 Results

7.4.1 Sway of the center of mass

The operationalization of CoM sway in relation to the different stomatognathic motor con-

ditions was analyzed by the PL of the 3D CoM trajectory (Table 7.2). The PL results did not

show any significant changes between different stomatognathic motor conditions in the

four perturbation directions. Although B, I and C had small effect sizes, F had a medium

effect size (B: p = 0.429, 

 = 0.037; F: p = 0.182, 

 = 0.073; I: p = 0.461, 

 = 0.034;

C: p = 0.692, 

 = 0.016).

7.4.2 Detrended fluctuation analysis

Temporal dynamics of CoM sway was analyzed with a DFA (Table 7.2). The scaling compo-

nents did not differ significantly between different stomatognathic motor conditions in

the four perturbation directions (B: p = 0.103, 

 = 0.096; F: p = 0.724, 

 = 0.014;

I: p = 0.821, 

 = 0.009; C: p = 0.689, 

 = 0.016).

7.4.3 Uncontrolled manifold analysis

A UCM analysis was performed aiming at analyzing the co-variation of joint angles in re-

lation with the control as well as the stability of the CoM. The  and  com-

ponents were utilized to quantify the control and the stability of the CoM, respectively.

The results are represented in Table 7.2.

For the  component, the groups did not show any significant differences in any of

the perturbation directions and all the effect sizes were small (B: p = 0.305,  = 0.157;

F: p = 0.466, 

 = 0.033; I: p = 0.947, 

 = 0.002; C: p = 0.514, 

 = 0.029). This indicated

the control of the CoM was not affected by the stomatognathic motor conditions (i.e. JAW,

TON and HAB).

7.4 Results

Table 7.2: The UCM, the path length (PL) and the DFA scaling exponent (α) results.



(rad2/dof)

JAW

TON

HAB



 or



0.013 ± 0.012

0.012 ± 0.005

0.013 ± 0.006

0.305*

0.157*

0.011 ± 0.004

0.013 ± 0.006

0.013 ± 0.005

0.466

0.0333

0.017 ± 0.014

0.016 ± 0.009

0.016 ± 0.008

0.947

0.002

0.018 ± 0.011

0.015 ± 0.006

0.018 ± 0.006

0.514

0.029



JAW

TON

HAB





0.209 ± 0.318

0.236 ± 0.202

0.1852 ± 0.2710

0.865

0.006

0.252 ± 0.319

0.220 ± 0.317

0.1022 ± 0.2482

0.333

0.048

0.249 ± 0.266

0.297 ± 0.165

0.2095 ± 0.2653

0.585

0.024

0.179 ± 0.433

0.237 ± 0.210

0.1634 ± 0.2664

0.788

0.011

(mm)

JAW

TON

HAB





325.2 ± 174.3

408.2 ± 277.5

329.4 ± 119.3

0.429

0.037

267.1 ± 112.1

381.4 ± 253.1

304.4 ± 124.3

0.182

0.073

369.2 ± 186.2

423.3 ± 219.6

344.7 ± 125.1

0.461

0.034

366.7 ± 154.8

428.8 ± 295.3

395.6 ± 117.3

0.692

0.016

JAW

TON

HAB





1.68 ± 0.12

1.74 ± 0.09

1.76 ± 0.11

0.103

0.096

1.73 ± 0.08

1.75 ± 0.09

1.73 ± 0.08

0.724

0.014

1.73 ± 0.08

1.72 ± 0.08

1.74 ± 0.09

0.821

0.009

1.72 ± 0.09

1.70 ± 0.10

1.71 ± 0.08

0.689

0.016

Regarding the , the groups did not differ significantly in any of the perturbation

directions and all of the results showed small effect sizes (B: p = 0.805, 

 = 0.006;

F: p = 0.333, 

 = 0.048; I: p = 0.585, 

 = 0.024; C: p = 0.788, 

 = 0.011). These results

showed that the stability of the CoM was not affected by the stomatognathic motor con-

ditions (i.e. JAW, TON and HAB).

7 Study IV – Modulation of Center of Mass Movement

7.5 Discussion

The aim of this study was to investigate the effects of different stomatognathic motor

conditions on the sway, control and stability of the CoM during a dynamic steady-state

balance task. The PL of the 3D CoM, a DFA and a UCM analyses were used to quantify the

variables of interest. It was hypothesized that jaw clenching and tongue pressing decrease

the total sway, increase the persistency of CoM fluctuations, increase both the control and

stability of the CoM. Inclusion of the TON group would enable a differentiation between

the specific effects of jaw clenching with occlusal load from the effects of stomatognathic

motor activity in general, as well as from the dual-task effects. This could ultimately help

to understand if the possible modulations of posture during jaw clenching occur basically

due to a supra-postural task (Stoffregen et al., 2000) or stomatognathic activities in gen-

eral; or if any additional functional interactions such as higher excitability of the human

motor system (Boroojerdi et al., 2000), muscle co-contractions (Giannakopoulos,

Schindler, et al., 2018) or H-reflex responses (Miyahara et al., 1996) may exist. None of the

considered parameters showed significant group effects in any of the directions. Based on

these results, it can be concluded that deliberate jaw clenching or tongue pressing do not

have significant effects on the control or stability of the CoM compared to habitual sto-

matognathic motor conditions in the steady-state phase of the task. At least, the effects

could not be quantified by the used parameters. In contrast to the previously-found ef-

fects of jaw clenching on dynamic reactive balance performance (Fadillioglu et al., 2022a),

the findings in this study do not indicate any task-specific effects of stomatognathic motor

activities on dynamic steady-state balance assessed by an oscillating platform. Because of

the task-specificity of balance (Giboin et al., 2015; Kümmel et al., 2016; Ringhof & Stein,

2018), further research investigating the effects of stomatognathic motor activities on dy-

namic steady-state balance with different movement tasks are needed. To the best of our

knowledge, the present study is the first to investigate the effects of stomatognathic mo-

tor activity on dynamic steady-state balance on an oscillating platform.

7.5.1 Quantification of CoM sway

The PL of the 3D CoM position was calculated to quantify the distance covered by the CoM

during the trials. The results in this study revealed no significant differences between the

three groups for any of the directions. Nevertheless, the effect sizes for the direction F

were medium (p = 0.182; 

 = 0.073), where the JAW group had a slightly smaller PL com-

pared to TON and HAB, indicating a higher dynamic steady-state stability. It should be

noted that significant performance increases and decreased anatomical region speeds

7.5 Discussion

were seen in the F direction during the early reactive phase of the task (Fadillioglu et al.,

2022a). Although a medium effect size does not have a high statistical power, jaw clench-

ing may have effects on the steady-state stability; but these were not high enough to be

detected with the chosen methods.

The temporal dynamics of CoM sway was analyzed by a DFA, which did not show any sig-

nificant differences between groups. Overall, the scaling exponent α was larger than 1.5

for all directions and all groups, indicating a Brownian noise (McGrath, 2016). These re-

sults were slightly higher but similar to those of Liang et al. (2017), which considered the

CoM instead of center of pressure for DFA (Blázquez et al., 2010; Lin et al., 2008; Munafo

et al., 2016) as in the present study. On the other hand, even though DFA has become a

predominant method for fractal analysis, it may not provide useful results for short time

series (McGrath, 2016).

7.5.2 Analysis of control and stability of the

CoM by a UCM approach

A UCM approach was applied to investigate the co-variated movement of joints in relation

to the CoM position as the PV (Freitas et al., 2006; Hagio et al., 2020; Hsu et al., 2013,

2017; Krishnamoorthy et al., 2005; Scholz et al., 2007) under different stomatognathic

motor conditions. In this study, the  and the were directly included in the

analysis, whereas the  was only indirectly considered within the . The find-

ings indicate that jaw clenching or tongue pressing do not lead to any effects that are

quantifiable with the UCM approach.

The  component was used to investigate the stabilization of the CoM through

co-varied movements of the joint angles. The statistical analysis revealed that the three

groups did not differ in . This showed that jaw clenching or tongue pressing did

not lead to a more stable CoM compared to habitual stomatognathic motor conditions in

the steady-state phase of the task. Therefore, it contradicted our first hypothesis regarding

the stability of the CoM.

The  component was used to quantify the control of the CoM. The results showed

that the groups did not differ in terms of control of the CoM, which suggested that jaw

clenching or tongue pressing did not lead to a better control compared to habitual condi-

tions. Based on this result, the second hypothesis was rejected.

7 Study IV – Modulation of Center of Mass Movement

100

A UCM analysis was performed in the present study and a subject-specific anthropometric

3D model was used to estimate the CoM (Möhler et al., 2019). Therefore, the model cov-

ered all three movement planes and considered not only the lower body but also the up-

per body, which plays an important role in the movement of the CoM due to its high mass.

7.5.3 Effects of masticatory system on dynamic stability

As already described in the introduction, there are some phenomena described in the

literature that support the assumption of a close functional integration of the masticatory

system in the human motor control processes (Boroojerdi et al., 2000; Bracha, Bracha, et

al., 2005; Bracha, Ralston, et al., 2005; Julià-Sánchez et al., 2020; Miyahara et al., 1996).

This could be because jaw movements are proportionally driven by anterior neck muscles,

which inevitably requires co-contractions of the lateral and posterior neck muscles

(Giannakopoulos, Schindler, et al., 2018). The resulting movements of the head must in

turn be matched centrally with the further proprioceptive input of postural control. There-

fore, the integration of the stomatognathic system into postural control is not a phenom-

enon, but a basic requirement.

Jaw clenching during activities that involve the lower and upper limbs may enhance neu-

romotor stimulation by means of the H-reflex, and therefore increase the excitability of

the motor system (de Souza et al., 2021). Furthermore, high activity was observed in the

frontal, parietal, and temporal cortices and cerebellum during hand grip combined with

jaw clenching compared to without jaw clenching (Kawakubo et al., 2014). In addition,

there are also studies revealing that not only stomatognathic activity but also the use of

occlusal splints (Battaglia et al., 2018) or mouthguards (Dias et al., 2022) influence the

strength in the muscles of the other body parts. These findings indicate that there is a

close relationship between the masticatory system and muscle strength or physical exer-

tion. Although it is hard to verify the underlying mechanisms experimentally, based on

these findings it was hypothesized that jaw clenching may lead to better dynamic steady-

state stability also under dynamic conditions. However, the results in this study did not

support this hypothesis.

7.5.4 Consideration of methodical aspects

Based on their gender and baseline performance, the participants were allocated into one

of the three groups (JAW, TON and HAB). The statistical examination by ANOVA revealed

that there were no baseline performance differences between the three groups (p =

7.5 Discussion

101

0.767). The purpose of building three groups with different stomatognathic motor condi-

tions, instead of making all participants perform all the tasks, was due to three main rea-

sons. Firstly, "habitual" in this study meant that no instruction was given regarding the

stomatognathic motor activity. Therefore, an unconscious, natural behavioral pattern of

the masticatory system was ensured. An instructed “habitual” would not be physiologi-

cally possible, because an "instructed" behavioral pattern cannot lead to an unconsciously

performed behavior. Secondly, possible carry over effects between different stomatog-

nathic motor tasks were avoided. After jaw clenching or tongue pressing, there could be

sustained physiological effects such as an increased excitability of the motor system.

Thirdly, fatigue effects were avoided. If all tasks were performed for each of the four di-

rections separately, 36 valid trials would be needed. Considering the invalid trials as well,

the total number performed would be too high.

In this study, the Posturomed, an oscillating platform, was used to assess dynamic balance

performance. The platform was randomly perturbed in one of the four different direc-

tions. The perturbation directions were analyzed independently following the suggestions

of Freyler et al. (2015), because muscle spindles provide different information depending

on the direction as well the velocity of perturbations (Akay & Murray, 2021). In addition

to this, the direction of surface translation is an important factor for the sensation, pro-

cessing and output of the postural responses (Freyler et al., 2015; Nonnekes et al., 2013).

Although it was suggested that the perturbation direction may not matter during the

steady-state phase of the balancing task on an oscillating platform (Giboin et al., 2015), in

this study the directions were analyzed separately since the research question was to fur-

ther investigate the positive effects of jaw clenching, which was found only in one direc-

tion (Fadillioglu et al., 2022a).

The focus was put on the CoM in this study because it is suggested to be the controlled

variable in postural studies (Kilby et al., 2015; Nicolai & Audiffren, 2019; Richmond et al.,

2021; Winter et al., 1998). Also, in studies assessing dynamic stability by means of an os-

cillating platform, the CoM was considered as an important variable (Pfusterschmied,

Buchecker, et al., 2013; Pohl et al., 2020). Even if it is widely chosen for postural studies

and others from the field of motor control (Domkin et al., 2005; Maldonado et al., 2018;

Möhler et al., 2019; Qu, 2012; Reisman et al., 2002; Scholz et al., 2000, 2001; Tseng et al.,

2002; Verrel et al., 2010), it does not prove that it is the single right one. Another possible

PV could be the distance between the CoM and the center of the platform.

7 Study IV – Modulation of Center of Mass Movement

102

7.5.5 Limitations

All the participants in this study were physically active adults. The homogeneity of this

group helped to minimize altered postural control mechanisms due to, for example, age

(M. Henry & Baudry, 2019) or neurological disorders (Delafontaine et al., 2020). Never-

theless, it should be noted that the findings cannot be directly transferred to other groups

(e.g., elders or people with neurological disorders).

The stabilization of a moving platform and the stabilization of the body on a rigid surface

are different balance tasks (Alizadehsaravi et al., 2020). Therefore, it should be added that

the findings in this study may not be valid for balance tasks on stationary ground or for

other dynamic tasks (Giboin et al., 2015; Kümmel et al., 2016; Ringhof & Stein, 2018).

It is possible that the UCM approach and the model used in the study were not sensitive

enough to capture the possible effects due the different stomatognathic motor activities.

Therefore, further research investigating the effects of stomatognathic motor activities on

dynamic steady-state balance with other models could be useful. Additionally, further

analysis by use of muscle synergies (D’Avella et al., 2003) or co-contractions (Hellmann et

al., 2015) may reveal effects on the level of muscles, which were not visible on the level

of kinematics.

7.6 Conclusion and Outlook

To the best of our knowledge, this study investigates for the first time the effects of differ-

ent stomatognathic motor conditions (jaw clenching, tongue pressing and habitual condi-

tion) on dynamic steady-state balance. The aim was to analyze the effects particularly on

the sway, control and stability of the CoM during a dynamic steady-state task (standing

one-legged on an oscillating platform). The results revealed that deliberate jaw clenching

or tongue pressing do not seem to affect the sway, the control or the stability of the CoM

during a dynamic balance task on an oscillating platform. Due to the task-specificity of

balance, further research investigating the effects of stomatognathic motor activities on

dynamic steady-state balance with different movement tasks is needed. In addition, fur-

ther analysis by use of muscle synergies or co-contractions may reveal effects on the level

of muscles, which were not visible on the level of kinematics. This study can contribute to

the understanding of postural control mechanisms, particularly in relation to stomatog-

nathic motor activities

103

8 Study V – Modulation of Postural

Control – Dynamic Steady-State

Balance

Published as

Fadillioglu, C., Kanus, L., Möhler, F., Ringhof, S., Hellmann, D.*, & Stein, T.*

(2023). Persisting effects of jaw clenching on dynamic steady-state balance. PLOS

ONE, 19(2): e0299050. [*These authors contributed equally to this work.]

8.1 Abstract

The effects of jaw clenching on balance has been shown under static steady-state condi-

tions but the effects on dynamic steady-state balance have not yet been investigated. On

this basis, the research questions were: 1) if jaw clenching improves dynamic steady-state

balance; 2) if the effects persist when the jaw clenching task loses its novelty and the

increased attention associated with it; 3) if the improved dynamic steady-state balance

performance is associated with decreased muscle activity. A total of 48 physically active

healthy adults were assigned to three groups differing in intervention (Jaw clenching and

balance training (JBT), only balance training (OBT) or the no-training control group (CON))

and attending two measurement points separated by two weeks. A stabilometer was used

to assess the dynamic steady-state balance performance in a jaw clenching and non-

clenching condition. Dynamic steady-state balance performance was measured by the

time at equilibrium (TAE). The activities of tibialis anterior (TA), gastrocnemius medialis

(GM), rectus femoris (RF), biceps femoris (BF) and masseter (MA) muscles were recorded

by a wireless EMG system. Integrated EMG (iEMG) was calculated to quantify the muscle

activities. All groups had better dynamic steady-state balance performance in the jaw

clenching condition than non-clenching at T1, and the positive effects persisted at T2 even

though the jaw clenching task lost its novelty and attention associated with it after bal-

ance training with simultaneous jaw clenching. Independent of the intervention, all

groups had better dynamic steady-state balance performances at T2. Moreover, reduc-

tions in muscle activities were observed at T2 parallel to the dynamic steady-state balance

performance improvement. Previous studies showed that jaw clenching alters balance

8 Study V – Modulation of Postural Control – Dynamic Steady-State Balance

104

during upright standing, predictable perturbations when standing on the ground and un-

predictable perturbations when standing on an oscillating platform. This study comple-

mented the previous findings by showing positive effects of jaw clenching on dynamic

steady-state balance performance.

8.2 Introduction

The postural control system regulates the body’s position with respect to the environment

for the dual purposes of balance and orientation (Macpherson & Horak, 2013). Good bal-

ance is crucial for daily activities and is associated with decreased risk of falls (Rubenstein,

2006) and injuries (Hrysomallis, 2007). Therefore, the methods to improve postural con-

trol, such as balance training (Giboin et al., 2019), are highly appreciated. However, bal-

ance is not a general ability but task-specific (Giboin et al., 2015). Balance can generally

be classified as static steady-state, dynamic steady-state, dynamic reactive and dynamic

proactive based on the performed activity (Lesinski et al., 2015b). Static steady-state bal-

ance basically comprises unperturbed conditions, such as during quiet upright standing,

whereas dynamic steady-state balance involves the maintenance of a steady position

while moving (e.g., walking). Dynamic reactive balance can be defined as the compensa-

tion of an unpredicted postural perturbation to maintain the balance. In case of proactive

balance, a predicted perturbation is anticipated and compensated before balance is dis-

turbed (Macpherson & Horak, 2013; Shumway-Cook & Woollacott, 2017). Good balance

in one of these sub-categories does not necessarily mean good balance in the others due

to the task specificity of balance (Shumway-Cook & Woollacott, 2017). Against this back-

ground, the effects of balance must be investigated in individual sub-categories.

Postural control can be influenced by many factors including the status and activity of the

stomatognathic system. There is a growing body of literature showing the associations

between postural activities under static steady-state conditions and stomatognathic mo-

tor activities in the form of jaw clenching in different jaw relationships (e.g., maximum

intercuspation or different occlusal appliances) (Hellmann et al., 2011a, 2015; Ringhof,

Leibold, et al., 2015). Particularly regarding jaw clenching, a lower sway of body in the

anterior– posterior direction (Hellmann et al., 2011a; Ringhof, Leibold, et al., 2015), a

lower variability in muscular co-contraction patterns (Hellmann et al., 2015) and lower

sway of trunk and head during upright standing (Ringhof, Stein, et al., 2015) were previ-

ously reported. The effects of jaw clenching on dynamic and proactive balance (Fadillioglu

et al., 2022a; Tomita et al., 2021) were also shown. However, the effects of jaw clenching

on dynamic steady-state balance are not well known (Fadillioglu et al., 2022b).

8.3 Methods

105

Despite the growing evidence for a relationship between the stomatognathic system and

postural activities, the underlying mechanisms are not fully understood. Several studies

(e.g., Boroojerdi et al., 2000; Miyahara et al., 1996) suggested that jaw clenching may re-

sult in increased motor excitability similar to the Jendrassik maneuver (Gregory et al.,

2001), or an increased muscle force in association with the H-reflex mechanism (de Souza

et al., 2021). Also, the co-contraction pattern of the jaw and neck muscles may help to

improve postural control by contributing to a more stable head or gaze position (Gangloff

et al., 2000). Furthermore, neuronal links of the trigeminal nerve to the rest of the nervous

system were shown in animal models (Ruggiero et al., 1981). Another possible explanation

might be that the instruction of jaw clenching during the simultaneous performance of a

balancing task might create a dual-task scenario. In this case, the attention increases due

to the secondary task, and consequently automatization of postural control is enhanced

(Wachholz et al., 2020). Based on these findings, it may be hypothesized that simultane-

ous execution of the jaw clenching task improves balance performance due to its novelty

and increased requirement of attention, but not specifically due to neurophysiological ef-

fects.

Previous studies showed various effects of jaw clenching during upright standing

(Hellmann et al., 2011a, 2015; Ringhof, Leibold, et al., 2015; Ringhof, Stein, et al., 2015),

during predictable perturbations applied when standing on the ground (Tomita et al.,

2021) and during unpredictable perturbations when standing on an oscillating platform

(Fadillioglu et al., 2022a). However, the effects of jaw clenching during a dynamic steady-

state balance task have not been fully investigated. In this study, this research gap was

addressed. Using two measurement times (T1 and T2) two weeks apart, it was evaluated

whether the stabilizing effects of jaw clenching persist at T2, despite the diminished nov-

elty and competing influence of a secondary task (and therefore decreased attention). It

was hypothesized that (1) jaw clenching improves dynamic steady-state balance at T1; (2)

the effects persist at T2; and (3) better dynamic steady-state balance performance is as-

sociated with decreased muscle activity due to movement efficiency (Brueckner et al.,

2019).

8.3 Methods

8.3.1 Participants

An a priori power analysis was conducted based on a study analyzing the effects of jaw

clenching on postural stability during upright standing (Ringhof, Stein, et al., 2015). That

8 Study V – Modulation of Postural Control – Dynamic Steady-State Balance

106

analysis revealed that 16 participants per group would be enough to reach sufficient

power (>0.8). On this basis, 48 healthy adults (21 female, 27 male; age: 22.9 ± 2.5 years;

height: 1.74 ± 0.09 m; body mass: 70.0 ± 12.2 kg) voluntarily participated after giving writ-

ten informed consent. They were physically active (active 4.2 ± 1.2 days/week and 368 ±

153 min/week), naive to the stabilometer task and had no muscular or neurological dis-

eases. They had no signs and symptoms of temporomandibular disorders (assessed by

means of the research diagnostic criteria for temporomandibular disorders (Dworkin &

LeResche, 1992)) and presented with full dentition (except for 3rd molars) in neutral oc-

clusion. The recruitment period for this study was between 13.09.2021-27.07.2022. The

study was approved by the Ethics Committee of the Karlsruhe Institute of Technology.

8.3.2 Instrumentation

Dynamic steady-state balance was assessed using a stabilometer (Stability Platform,

Model 16030, Lafayette Instrument Company, Lafayette, IN, USA) containing a 65×107 cm

wooden platform with a maximum deviation of ± 15° (Figure 8.1a-b). EMG data of the

tibialis anterior (TA), gastrocnemius medialis (GM), rectus femoris (RF), biceps femoris (BF)

and masseter (MA) of the right side were recorded by a wireless EMG system (Noraxon,

Scottsdale, USA; 2000 Hz). As preparation, the skin over the muscles was carefully shaved,

abraded, and rinsed with alcohol. Bipolar Ag/AgCl surface electrodes (diameter 14 mm,

center-to-center distance 20 mm; Noraxon Dual Electrodes, Noraxon, Scottsdale, USA)

were positioned in accordance with the European Recommendations for Surface EMG

(Hermens et al., 1999). The positions of the EMG electrodes were marked with temporary

tattoo ink (MyJagua, Greven, Germany) at T1 to allow identical positioning at T2.

8.3.3 Protocol

The experimental protocol is illustrated in Fig 8.1c. First, the participants were familiarized

to the stabilometer by standing on it for 1 min with rubber bands under it (the easier form

of the task), then for 1 min without the rubber bands (the task to be performed during

the measurements). Afterwards, a baseline measurement of 30 s was performed to de-

termine the initial dynamic steady-state balance performance operationalized by the time

at equilibrium (TAE; for details see the “Data analysis” section). Both baseline measure-

ment result and gender were considered to assign the participants to one of three groups:

jaw clenching and balance training (JBT), only balance training (OBT) or the no-training

control group (CON). Statistical examination by one-way ANOVA revealed no baseline

8.3 Methods

107

performance differences between the three groups (p = 0.982). All groups had 7 female

and 9 male participants.

Figure 8.1: a. Stabilometer. b. Degrees of freedom and maximum deviation of the platform. c. Experimental

protocol.

After warming up on a treadmill (h/p/cosmos Saturn, Nussdorf-Traunstein, Germany) for

5 min at 6 km/h, maximum voluntary contraction (MVC) tests were performed for each

muscle. Just before the measurements, each participant trained with a RehaBite® (Plastyle

GmbH, Uttenreuth, Germany) to become familiar with applying a submaximal force of 75

N (Hellmann et al., 2011a). The EMG data of MA were monitored during training to deter-

mine the corresponding muscle activity for later use as reference during the measure-

ments (Fadillioglu et al., 2022a, 2022b). During the subsequent balancing task,

participants clenched on an Aqualizer intraoral splint (medium volume; Dentrade Inter-

national, Cologne, Germany).

Regarding the balance task, participants were asked to keep the stabilometer platform in

the horizontal position as long as possible and to focus on a target positioned at eye level

8 Study V – Modulation of Postural Control – Dynamic Steady-State Balance

108

and 3 m away from the center of the platform. For the jaw clenching trials, participants

were asked to simultaneously clench their jaws. Five valid trials, each 30 s, were collected

for each condition (clenching/non-clenching). There was a break of 30 s between each

trial to avoid fatigue. The order of clenching conditions was counterbalanced within the

groups and each participant was randomly assigned to an order. At T2, the same protocol

as during T1 was executed except for the baseline measurement.

8.3.4 Intervention

Between T1 and T2, the participants of JBT and OBT followed a two-week training program

comprising six training sessions at least two days apart from each other, whereas CON did

not train. Each training session was performed in the BioMotion Center under the super-

vision of experienced staff and lasted about 15 min. As in the measurements, participants

were asked to keep the platform in the horizontal position as long as possible. In total, 10

trials (2 sets of 5 trials) of 30 s were performed in each training session. There was a break

of 30 s between each trial and 2 min between each set. The participants of JBT trained in

the jaw clenching condition and OBT in the non-clenching condition. In each training ses-

sion, JBT additionally trained with the Rehabite® for five minutes before balance training

to get used to the jaw clenching task.

8.3.5 Data analysis

All data were recorded in Vicon Nexus 2.12 (Vicon Motion Systems; Oxford Metrics Group,

Oxford, UK) and exported for further processing in MATLAB R2022a (MathWorks, Natick,

USA). The analog output signal of the platform was filtered with a Butterworth low-pass

filter (fourth-order; cut-off frequency 10 Hz); and EMG data with a Butterworth band-pass

filter (fourth-order; cut-off frequency 10-500 Hz). After filtering, EMG data were rectified

and smoothed by averaging with a sliding window of 30 ms and finally normalized to the

MVC references (Hellmann et al., 2011a). For each trial, time at equilibrium (TAE, ± 3°

deviation from the horizontal position (Brueckner et al., 2019; Kiss, Brueckner, &

Muehlbauer., 2018) for at least 500 ms (Giboin et al., 2015)) as well as time normalized

iEMG for each muscle were calculated. A higher TAE was considered as better dynamic

steady-state balance performance.

8.4 Results

109

8.3.6 Statistics

Statistical analysis was done with IBM SPSS Statistics 29.0 (IBM Corporation, Armonk, NY,

USA). Kolmogorov-Smirnov tests were performed to determine the normality of data dis-

tribution. For each measurement time and condition, the trial with the highest TAE was

used for statistical tests.

For TAE at T1, a paired t-test was performed to analyze the effects of jaw clenching on

dynamic steady-state balance performance (Hypothesis 1). Additionally, for each depend-

ent parameter (i.e. TAE and iEMG), a three-factorial mixed ANOVA (3 groups x 2 clenching

conditions x 2 measurement times) was conducted to test the remaining hypotheses.

Post-hoc t-tests for pairwise group comparisons were run with Bonferroni-Holm correc-

tions in the case of interaction effects. The correlation between the changes in dynamic

steady-state balance performance (i.e. ∆TAE as TAE(T2)-TAE(T1)) and muscle activities (i.e.

∆iEMG as iEMG(T1)-iEMG(T2)) was quantified by Spearman correlation tests. By conven-

tion, a positive ∆TAE indicated an increased TAE at T2, whereas a positive ∆iEMG indicated

a decreased iEMG at T2. The differences were normalized to the values at T1. The level of

significance was set a priori to p < 0.05. Cohen’s d and partial eta squared (ƞ2p) were cal-

culated to estimate effect sizes (small ƞ2p< 0.06; medium: 0.06 < ƞ2p < 0.14; large: ƞ2p >

0.14) (Cohen, 1988).

8.4 Results

The activity of MA was 7.9 ± 6.00% of MVC at T1 and 7.5 ± 5.4% of MVC at T2 for the jaw

clenching condition, and for the non-clenching condition it was 0.4 ± 0.2% of MVC and 0.3

± 0.2% of MVC at T1 and T2, respectively. This indicated that the participants performed

the clenching tasks successfully.

The descriptive data of the TAE can be found in Appendix S3 Table1. The TAE results at T1

are presented in Fig 8.2. The TAE was significantly higher in the jaw clenching condition

than the non-clenching condition at T1 with high effect sizes (p = 0.006, d = 3.95). This

showed that all participants had a better dynamic steady-state balance performance in

jaw clenching condition than the non-biting condition at T1, which was in line with the

hypothesis 1.

The balance and jaw clenching training effects are depicted in Fig 8.3. The ANOVA results

revealed statistically significant effects for the factor time (p < 0.001, ƞ2p = 0.616) and the

factor clenching condition (p = 0.008, ƞ2p = 0.146) with high effect sizes. Although there

8 Study V – Modulation of Postural Control – Dynamic Steady-State Balance

110

were no significant interaction effects between the factors time and group, the effect size

was medium (p = 0.174, ƞ2p = 0.075). There were no significant differences between the

groups over two clenching conditions, but the effects sizes were high (OBT vs. CON:

p = 0.207, d = 5.48; JBT vs. OBT: p = 0.356, d = 3.66, JBT vs. CON: p = 0.214, d = 5.50). These

results indicated that the effects of jaw-clenching on dynamic steady-state balance per-

formance persisted at T2, which supported the hypothesis 2.

Figure 8.2: Time at equilibrium for two clenching conditions at T1.

Figure 8.3: Time at equilibrium for the three groups at two measurement times. Significant differences for the

factor time are indicated with * and for the factor clenching condition with †.

The time normalized iEMGs are represented in Fig 8.4 and the descriptive data can be

found in S3 Table. The ANOVA results showed that all muscle activity was significantly

8.4 Results

111

decreased at T2 with high effect sizes (TA: p < 0.001, ƞ2p = 0.321; GM: p < 0.001,

ƞ2p = 0.289; RF: p < 0.001, ƞ2p = 0.327; and BF: p < 0.001, ƞ2p = 0.425). Further, GM showed

significant interaction effects between the factors time and clenching with a medium ef-

fect size (GM: p = 0.034, ƞ2p = 0.097). These finding partly supported the hypothesis 3,

since at T2 all the muscle activities decreased parallel to the dynamic steady-state balance

performance improvement. However, in case of jaw clenching condition there was not

any decrease in muscle activities although the dynamic steady-state balance performance

was better.

Figure 8.4: Time normalized iEMGs of four muscles: tibialis anterior (TA), gastrocnemius medialis (GM), rectus

femoris (RF) and biceps femoris (BF). JBT= jaw clenching and balance training, OBT= only balance

training and CON = no-training control group. Significant differences for the factor time are indi-

cated with * and interaction effects between the factors time and clenching condition with #.

The TAE increases and iEMG decreases between two measurement points are represented

as the medians and 25th-75th percentiles in Table 1 (Sainani, 2012). The correlations be-

tween the increases in TAE and the decreases in iEMG for all muscles are also shown in

Table 1. The results showed that the dynamic steady-state balance performance

8 Study V – Modulation of Postural Control – Dynamic Steady-State Balance

112

improvements significantly correlated with the decreases in RF activity with a moderate

correlation coefficient. The rest of the muscles did not show any significant correlations.

Table 8.1: Time at equilibrium (TAE) increases and iEMG decreases of tibialis anterior (TA), gastrocnemius

medialis (GM), rectus femoris (RF) and biceps femoris (BF) between T1 and T2, together with their

correlations. The results of the clenching and non-clenching conditions were averaged for both T1

and T2. Significant changes are shown in bold.

Median

25th-75th per-

centile

Correlation with

TAE

rho

TAE increase in %

36.4

[12.1 84.1]

iEMG decrease in

49.8

[29.8 76.6]

0.088

0.249

47.0

[23.0 65.2]

0.054

0.280

44.9

[14.3 59.5]

0.011

0.366

41.1

[ 9.1 59.0]

0.222

0.179

8.5 Discussion

This study investigated the effects of jaw clenching on dynamic steady-state balance task

performance and investigated if the stabilizing effects of jaw clenching persist when the

novelty of the task and the focused attention associated with it diminish. Further, activity

of the selected task-relevant muscles was analyzed to better understand improvements in

dynamic steady-state balance performance.

8.5.1 Persistence of jaw clenching effects

The results showed that dynamic steady-state balance performance was better in the jaw

clenching condition compared with the non-clenching condition at both T1 and T2, which

was consistent with previously-shown effects during static steady-state balance (Hellmann

et al., 2011a, 2015; Ringhof, Leibold, et al., 2015; Ringhof, Stein, et al., 2015). As the effects

persist at T2, it can be suggested that the performance improvements are related specifi-

cally to the jaw clenching task, but not to the novelty of the secondary task and the ac-

companying automatization of the balance task. Various studies have shown that jaw

clenching alters postural control during upright standing (Hellmann et al., 2011a, 2015;

Ringhof, Leibold, et al., 2015; Ringhof, Stein, et al., 2015), predictable perturbations,

8.5 Discussion

113

standing on the ground (Tomita et al., 2021) and during unpredictable perturbations ap-

plied when standing on an oscillating platform (Fadillioglu et al., 2022a). This study com-

plemented the previous findings by showing positive effects of jaw clenching on dynamic

steady-state balance performance.

8.5.2 No effects of balance training

In previous studies, training improved balance in a task-specific way (Giboin et al., 2019),

reduced the incidence of falls (Sherrington et al., 2020) and enhanced motor performance

(Gruber & Gollhofer, 2004). The current three-armed study design aimed to investigate

the effects of simultaneous jaw clenching during balance training. Pairwise comparisons

of the groups provide information on (1) if balance training alone improved the dynamic

steady-state balance performance more than the no training condition (OBT vs. CON), and

(2) if simultaneous jaw clenching during balance training altered the balance training ef-

fects (JBT vs. OBT) which can be explained by the automatization of the dual-task. All of

the groups improved at T2 independent of their training situation. Interestingly, no signif-

icant interaction effects between the factors time and group were detected. This indicated

that all groups improved their dynamic steady-state balance performance with no signifi-

cant group differences. However, it should be noted that there was a medium interaction

effect for TAE. Further, the post-hoc pairwise comparisons at T2 showed high effect sizes.

The dynamic steady-state balance performance improvement, as the difference of TAE

between T1 and T2 over two clenching conditions, were lower in CON by more than 2 s

compared with the other two groups (JBT = 6.4 s, OBT = 6.3 s and CON = 3.7 s). Neverthe-

less, none of the differences reached the level of significance. Ultimately, the learning ef-

fects of the balance task were seemingly higher than the balance training effect, therefore

the former outweighed the latter in terms of significance level. This finding is interesting

since previous studies showed that dynamic steady-state balance performance improves

after balance training comprising the same task used for testing (e.g., Giboin et al., 2015).

On the other hand, learning effects within a measurement session were also reported in

previous studies in which the stabiliometer was used to quantify the dynamic steady-state

balance performance (Brueckner et al., 2019; Steiner et al., 2016). In this study, high learn-

ing effects of the balance task in the initial phase may have masked the effects of the

balance training. For future studies, it is advisable to take more care to minimize possible

learning effects when designing the study.

8 Study V – Modulation of Postural Control – Dynamic Steady-State Balance

114

8.5.3 Limited effects of jaw clenching on muscle activity

The iEMG results revealed that all muscle activity decreased at T2. Considering that dy-

namic steady-state balance performance was better at T2, it can be suggested that better

performance is associated with decreased muscle activities. However, the dynamic

steady-state balance performance improvements and the muscle activity reductions from

T1 to T2 correlated significantly only for one of the analyzed muscles, that is RF, with a

moderate correlation coefficient. The reason for the non-significant correlations may be

the linear approach used both for the calculation of the changes between the two meas-

urement sessions and for the correlations. For example, in a previous study comparing the

muscle activation during back squats with different loads showed that the correlation be-

tween the changes in the loads and the muscle activations are not linear (van den Tillaar

et al., 2019). Based on this finding, it can be suggested that the non-linear approaches for

the correlation between the dynamic steady-state balance performance improvements

and the iEMG reductions might reveal significant and stronger correlations. Nevertheless,

all of the muscles showed reduced activities at T2 parallel to the dynamic steady-state

balance performance improvement. These findings are in line with previous studies (e.g.,

Brueckner et al., 2019) reporting practice-related reductions in muscle activations, which

could relate to improved movement efficiency. On the other hand, the iEMG results in this

study did not show any decrease in the jaw clenching condition, although the dynamic

steady-state balance performance in the jaw clenching condition was significantly better

than in the non-clenching. Further, the activity of GM decreased less at T2 in the jaw

clenching condition compared with the non-clenching condition. Based on these findings,

it can be suggested that dynamic steady-state balance performance improvement due to

jaw clenching was not associated solely with movement efficiency, but could be explained

by other mechanisms that are currently undiscovered.

8.5.4 Limitations

Certain limitations of this study should be considered (1) since the participants were phys-

ically active adults, the results are not necessarily valid for other groups. (2) The best trial

was taken instead of the average of five trials, since previous studies reported that the

participants improved their dynamic steady- state balance performances on the stabi-

lometer during trials on the first measurement day (Brueckner et al., 2019; Steiner et al.,

2016). Taking the best trials aimed to eliminate the additional effects due to different

learning curves at T1 and T2. (3) Significant time effects were found even for the CON

group, who did not train between two measurement times. These high learning effects

8.6 Conclusion and Outlook

115

may have outweighed the other effects. (4) Considering the task-specific characteristics

of balance (Giboin et al., 2015), it is important to add that the results are not generalizable

to other static or dynamic balance tasks.

8.6 Conclusion and Outlook

This study investigated the effects of jaw clenching on dynamic steady-state balance per-

formance across two measurement times separated by two weeks. The findings indicated

that jaw clenching was associated with a better dynamic steady-state balance perfor-

mance and the effects persisted even when the jaw clenching task lost its novelty and

competing influence. Independent of the intervention, all groups had better dynamic

steady-state balance performances at T2, which indicated high learning effects of the dy-

namic steady-state balance task. Moreover, learning-related reductions in muscle activity

were observed at T2.

117

9 General Discussion

Several studies reported that stomatognatic motor activities in the form of chewing as

well as jaw clenching may affect postural control under static conditions (e.g., Hellmann

et al., 2011a, 2015; Kushiro & Goto, 2011; Ringhof, Leibold, et al., 2015; Ringhof, Stein, et

al., 2015). This entails, in particular, reduced anterior-posterior body sway (Hellmann et

al., 2011a), reduced variability of muscular co-contraction patterns of posture-relevant

muscles of the lower extremities, and reduced trunk and head sway in response to con-

trolled stomatognatic motor activities (Hellmann et al., 2015). These neuromechnical ef-

fects can be interpreted as stabilizing effects concerning the body sway. However, good

balance during a balance task does not necessarily mean good balance in the others due

to the task-specific characteristics of balance (Giboin et al., 2015; Ringhof & Stein, 2018;

Shumway-Cook & Woollacott, 2017).

In contrast to balance in more static conditions (e.g., sitting or standing), the influence of

stomatognathic motor activities in dynamic situations (e.g., standing on an oscillating plat-

form with perturbations) has not yet been thoroughly studied. In this thesis, this research

gap was addressed. Furthermore, previous research showed that when more difficult pos-

tural control conditions are present (e.g., unstable or complex balance tasks, or external

perturbations), the sensory information related to dental occlusion is more strongly used

(Julià-Sánchez et al., 2016, 2019; Tardieu et al., 2009). On this basis, the impacts of crani-

omandibular system activities, such as jaw clenching, may become more significant during

dynamic or difficult balancing tasks compared with static ones.

The current thesis aims to investigate the effects of concurrent activities of the cranio-

mandibular system on postural control in dynamic conditions, particularly during dynamic

reactive balance and dynamic steady-state balance. Dynamic reactive balance can be de-

scribed as compensation for unpredictable perturbations to preserve balance. These pos-

tural perturbations are primarily whole-body perturbations generated by surface

translations and rotations. Dynamic steady-state balance involves fundamentally the

maintenance of balance after self-initiated disturbances, such as swinging the leg forward

during walking or running (Shumway-Cook & Woollacott, 2017).

The following two main topics were examined within this thesis: (1) the influence of jaw

clenching on dynamic reactive balance, and (2) the influence of jaw clenching on dynamic

steady-state balance. Five studies encompassing the methods of dynamic posturography

together with the biomechanical analysis were carried out. The current chapter (Chapter

9 General Discussion

118

9) summarizes the main findings of these five studies described in the previous chapters

(Chapter 4-8) and discusses the potential mechanisms of the detected effects of jaw

clenching. In the last part (Section 9.3), some implications and recommendations for fu-

ture research are provided.

9.1 Influence of jaw clenching on

dynamic

reactive balance

The studies provided in Chapters 4-6 focused on the influences of submaximal jaw clench-

ing on dynamic reactive balance. In Chapter 4, the following research questions are issued:

(1) if submaximal jaw clenching has acute enhancing effects on dynamic reactive balance

compared with the habitual stomatognatic behavior condition (discussed in Section 9.1.1)

; (2) if the acute effects of jaw clenching were specifically due to neuromechanical effects

of this oral-motor activity or more generally due to activities of the craniomandibular sys-

tem (discussed in Section 9.1.2).

Chapter 5 deals with the research questions if the reflex activities and co-contraction be-

havior of the trunk and lower limb muscles change under the effects of different oral-

motor tasks (discussed in Section 9.1.3), whereas Chapter 6 focused on whether the ef-

fects of jaw clenching were associated with dual-task benefits or specifically due to neu-

romechanical modulations associated with jaw clenching (discussed in Section 9.1.4).

9.1.1 Jaw clenching improves dynamic reactive balance

performance in a direction-specific manner

The study explained in Chapter 1 revealed that submaximal jaw clenching improved dy-

namic reactive balance performance in one of the four possible perturbation directions

compared with the tongue pressing and habitual stomatognatic behavior conditions. In

the remaining three perturbation directions, no significant effects of simultaneous jaw

clenching were detected. On this basis, it was concluded that submaximal jaw clenching

can improve dynamic reactive balance but the occurrence of these enhancing effects was

direction-specific.

In this experiment, dynamic reactive balance was assessed by using an oscillating platform

that was perturbed unpredictably and randomly in one of the possible directions, these

are backward, forward, ipsilateral and contralateral. The four perturbation directions were

examined separately as suggested by Freyler et al. (2015) and were considered distinct

9.1 Influence of jaw clenching on dynamic reactive balance

119

tasks. The reason for this approach is that muscle spindles convey different information

depending on the direction and velocity of perturbations (Akay & Murray, 2021). Further-

more, the perception, processing, and outcome of the postural responses are strongly

influenced by the direction of surface translations (Freyler et al., 2015; Nonnekes et al.,

2013).

The dynamic reactive balance task in this experiment was to compensate for the pertur-

bation as fast as possible and thereby bring the oscillating platform into its neutral posi-

tion. The operationalization of dynamic reactive balance performance was done by use of

DR as suggested by Kiss (2011a). DR revealed how quickly the damping occurred and,

therefore, how good the dynamic reactive balance performance was. The experiment re-

sults indicated that simultaneous submaximal jaw clenching led to better compensation

of the perturbations, and therefore, improved the dynamic reactive balance performance

only in case of forward acceleration of the oscillating platform during the initiation of the

perturbation. Forward acceleration of the platform in turn led to backward sway of the

body. As shown in a study investigating the effects of type and direction of perturbations

(C. Chen et al., 2014), forward surface translations are less stable than backward ones.

They cause faster muscle activations as well as faster and larger lower limb joint move-

ments. Furthermore, in another study comparing backward and forward perturbations in

terms of postural outputs, it was suggested that postural responses to backward and for-

ward perturbations may be processed by different neural circuits (Nonnekes et al., 2013).

The direction-specific characteristics of the results in this experiment can possibly be at-

tributed to the neural circuits recruited during jaw clenching which are relevant for for-

ward perturbations but not for backward ones.

For a better understanding of the improved dynamic reactive balance performance, the

segmental kinematics were analyzed in the forward perturbation direction, in which sig-

nificant effects of submaximal jaw clenching were detected. More precisely, the body was

divided into five anatomical segments, these are head, trunk, pelvis, knee and foot, and

the mean speed of these regions was calculated during the dynamic reactive balance task.

The results revealed that (1) the foot had the highest mean speed among the three oral-

motor task conditions, followed by the knee and the head. The mean speeds of the foot,

knee, and head were higher than those of the trunk and pelvis, whereas the speeds of the

trunk and pelvis did not differ from each other; (2) the mean speeds of the analyzed seg-

ments were overall lower during simultaneous submaximal jaw clenching compared with

the other two oral-motor tasks.

The results showed that the trunk and pelvis exhibited the lowest mean speed across the

analyzed regions. This could be attributed to proximal segments being prioritized over

9 General Discussion

120

distal ones for stability during balance tasks (Hughey & Fung, 2005; Munoz-Martel et al.,

2019). The second main finding of the segmental kinematics analysis was that jaw clench-

ing led to lower mean speeds of the analyzed segments.

9.1.2 Tongue pressing does not lead to identical effects

as jaw clenching

The tongue has a highly organized and extensive representation at various levels of the

brain (Bordoni et al., 2018). Changes in tongue position were shown to have effects on

the whole body, for example on cardiac function (J. E. Schmidt et al., 2009) and postural

control (Alghadir et al., 2015a). Activities of the tongue were also indirectly investigated

in studies focusing on chewing (Kushiro & Goto, 2011), in which the tongue supports the

movement (Greet & De Vree, 1984; Lund, 1991). However, to the best of our knowledge,

no study investigated the effects of tongue pressing, during which the tongue is primarily

active in executing the oral-motor task.

To date, there is no consensus or clear explanation regarding the effects of jaw clenching

on motor behavior. Possible explanations could be the stimulation of periodontal recep-

tors, different proprioceptive inputs due to different jaw relations, or the facilitation of

human motor system excitability (Komeilipoor et al., 2017; M. Takahashi et al., 2006). The

secondary goal of the second experiment comprised in the current thesis was to investi-

gate the effects of an instructed, controlled submaximal tongue pressing against the pal-

ate, which can also be described as stomatognatic muscle activity without occlusal

loading. It was hypothesized that both submaximal jaw clenching and tongue pressing

would enhance dynamic reactive balance performance. The expected enhancing effects

could be explained by the neuromechanical coupling of the stomatognatic system and the

postural control system (Cuccia & Caradonna, 2009; Hegab, 2015) or by the dual-task par-

adigm (Fraizer & Mitra, 2008). The results of the experiment revealed that submaximal

pressing with the tongue seems not to have any observable effects on dynamic reactive

balance performance whereas simultaneous submaximal jaw clenching improved dy-

namic reactive balance performance in the case of forward acceleration of the platform

as explained in the previous section (Section 9.1.1). Based on these results, it was sug-

gested that dynamic reactive balance performance improvement was not due to stoma-

tognatic motor activity per se or the dual-task paradigm, but in particular to the

submaximal clenching of jaws.

9.1 Influence of jaw clenching on dynamic reactive balance

121

9.1.3 Co-contraction behavior does not change during

simultaneous jaw clenching

Chapter 1 showed that simultaneous submaximal jaw clenching improves dynamic reac-

tive balance in a direction-dependent manner. Particularly in the case of forward acceler-

ation of the platform, jaw clenching was found to result in a better compensation of

external perturbations. Furthermore, the segmental kinematics analysis revealed that

simultaneous jaw clenching led to lower mean speeds of the analyzed body segments but

did not affect the relationship between regional mean speeds.

Various studies reported some changes in reflex activities due to simultaneous jaw clench-

ing (Miyahara et al., 1996; Takada et al., 2000; Tomita et al., 2021). Furthermore, a previ-

ous study showing the stabilizing effects of jaw clenching during static steady-state

balance showed that jaw clenching may lead to changes in co-contraction patterns

(Hellmann et al., 2015). On this basis, a follow-up analysis was conducted which was pre-

sented in Chapter 5. With this follow-up study, it was aimed to investigate the neurome-

chanical mechanisms underlying the observed jaw clenching effects on dynamic reactive

balance. To accomplish this, the activity and co-contraction patterns of posture-related

muscles and muscle pairs were examined before and throughout important reflex phases

following forward perturbations. It was hypothesized that muscle activity and co-contrac-

tion patterns of relevant muscles and muscle pairs in reflexive phases change under the

influence of simultaneous submaximal jaw clenching. The findings might ultimately help

to understand the neuromechanical changes occurring under the effects of submaximal

jaw clenching.

The results revealed neither before nor after perturbations significant differences in mus-

cle activities in critical reflexive phases between the groups. This meant that simultaneous

jaw clenching and tongue pressing did not seem to result in any changes in anticipatory

or compensatory postural adjustments. This finding was in conflict with the hypothesis as

well as with previous studies showing the facilitation of reflex responses due to jaw

clenching (Miyahara et al., 1996; Takada et al., 2000; Tomita et al., 2021). Particularly,

Tomita et al. (2021) showed that jaw clenching leads to an earlier onset of anticipatory

postural adjustments as well as larger peaks in trunk and lower limb muscle activities both

before and after perturbations. On the other hand, they did not detect any changes in

CoM or CoP displacements due to jaw clenching. However, in their experiment, the par-

ticipants stood bipedal on a rigid support surface and external perturbations were applied

by a pendulum device (Kanekar & Aruin, 2015; Santos & Aruin, 2008), whereas, in the

current study presented in this thesis the participant stood on one leg on an oscillating

9 General Discussion

122

platform that was perturbed externally. The different results from earlier research and the

current study may be explained by the task-specificity of balance (Giboin et al., 2015;

Ringhof & Stein, 2018).

The results regarding the co-contraction ratios revealed some reductions under the effects

of the tongue pressing. Particularly, before the perturbation and in short to medium la-

tency phases, significant differences were detected for two muscle pairs from the upper

leg and trunk. Furthermore, in the phase of late latency response, both jaw clenching and

tongue pressing led to increased co-contraction for the muscle pair of calves compared

with the habitual stomatognatic behavior conditions. These effects may be explained by

the facilitation of corticospinal pathways under the influence of stomatognathic activities.

Since, this reflex phase is modulated by the involvement of corticospinal pathways (Taube

et al., 2006), and jaw clenching was shown to lead to increased facilitation of corticospinal

pathways to the leg muscles (Boroojerdi et al., 2000). Nevertheless, none of the results

indicated effects that were specific to the jaw clenching condition, which would have

helped to explain the neuromechanical mechanisms underlying the observed jaw clench-

ing effects on dynamic reactive balance. Based on the findings, it can be concluded that

neither muscle activities nor co-contraction patterns of posture-related muscles and mus-

cle pairs helped to elucidate the neuromechanical effects of jaw clenching which were

visible in dynamic reactive balance performance (Fadillioglu et al., 2022a).

9.1.4 Automation of the jaw clenching does not have any

observable effects on dynamic reactive balance

The study presented in Chapter 6 investigated the effects of submaximal jaw clenching on

dynamic reactive balance task performance after 1-week of jaw clenching training. Vari-

ous studies showed that jaw clenching influences balance performance under certain con-

ditions (e.g., upright standing (Hellmann et al., 2011a; Ringhof, Leibold, et al., 2015;

Tanaka et al., 2006)). However, it is not known whether these effects are related to the

dual-task situation (i.e. the effects of concurrently performed additional motor tasks

(Broglio et al., 2005; Fraizer & Mitra, 2008; Ghai et al., 2017) or those particular to jaw

clenching.

Previous studies reported that the dual-task paradigm can be utilized to improve perfor-

mance in primary motor tasks (e.g., balance tasks (Andersson et al., 2002; Broglio et al.,

2005)). Thereby, a simultaneously performed secondary task sensitively directs the indi-

viduals’ attention to an external source. This attentional shift allows the motor system to

work automatically, resulting in improved performance in the primary motor task (Ghai et

9.1 Influence of jaw clenching on dynamic reactive balance

123

al., 2017; Polskaia et al., 2015; Swan et al., 2004; Wulf et al., 2001). However, it should be

noted that a secondary task may not necessarily improve the performance in the executed

tasks but can also result in no change (Choi et al., 2023) as well as performance decre-

ments. The adverse effects can be explained by the parallel sharing of a limited set of

resources (R. A. Schmidt et al., 2018). Nevertheless, a growing body of literature suggests

that postural control can benefit from a dual-task situation (Andersson et al., 2002; Broglio

et al., 2005; Polskaia et al., 2015; Swan et al., 2004). On this basis, it can be argued that

the effects of jaw clenching on postural control are related to the dual-task paradigm but

not the specific effects of the jaw clenching task.

In this study, given in Chapter 6, the aim was to evaluate whether the effects of submaxi-

mal jaw clenching are associated with general dual-task benefits or specifically due to

neuromechanical connections of the stomatognathic system to the postural control sys-

tem. It was hypothesized that submaximal jaw clenching specifically affects dynamic re-

active balance performance, and this effect is not due to dual-task benefit. With the study

design establishing an intervention group that trained the submaximal jaw clenching task

three times a day during one week, it was aimed that for this group the jaw clenching task

becomes an explicit task and loses its novelty. Ultimately, increased focused attention as-

sociated with a secondary novel task, therefore, the dual-task benefits would also disap-

pear.

9.1.4.1 Effects of jaw clenching and its automation

The results revealed that neither jaw clenching nor its automation through jaw clenching

training resulted in significant dynamic reactive balance performance changes as assessed

by the compensation of perturbation (DR, (Kiss, 2011a)) and COM sway. These findings

did not support the hypothesis of this study. On the other hand, muscle activities in reflex

phases revealed some changes in jaw clenching conditions in lower leg muscles and par-

ticularly for the anterior-posterior perturbation directions but not for the medio-lateral

ones. Based on these findings, it can be suggested that the submaximal jaw clenching task

may result in changes in reflex activities but changes are direction-specific as well as mus-

cle-specific. On the other hand, the postural control process is a product of complex inter-

actions of the postural control system and comprises inter-muscular coordination

patterns. Determination of individual muscle activities in certain important reflex phases

may have been a limiting factor in this study. Therefore, in future studies, the coordination

of multiple muscles should be investigated alongside the activity of individual muscles.

For example, coordination models, such as muscular synergies, can be used to do this

(D’Avella et al., 2003; Munoz-Martel et al., 2021; Ting & Macpherson, 2005).

9 General Discussion

124

9.1.4.2 High learning effects without balance training between sessions

A secondary important finding of this study was the high learning effects of the balance

task even though there was not any balance training performed between the two meas-

urement sessions 1-week apart. In three of the four perturbation directions, the pertur-

bation was better compensated at the posttest compared with the pretest regardless of

the groups. Furthermore, for all perturbation directions, the velocity of CoM decreased at

the posttest. Even though, the participants performed familiarization trials before the

measurements and no systematical dynamic reactive balance performance improvements

within the measurement sessions were observed, the learning of balance task was appar-

ently inevitable. The debate emerged as to whether the learning effects of balance task

were so high that they overweighed the potential beneficial effects of jaw clenching. This

question cannot be answered using the current study design and additional research is

required.

9.2 Influence of jaw clenching on

dynamic

steady

-state

balance

The research presented in Chapters 7-8 concentrated on the influences of submaximal jaw

clenching on dynamic steady-state balance. Chapter 7 focused on the research question

whether the sway, control and stability of the CoM during dynamic steady-state balance

was affected by the three different oral-motor tasks, which is discussed in Section 9.2.1.

The research questions issued in Chapter 8 were threefold: first, if concurrent submaximal

jaw clenching improves dynamic steady-state balance (discussed in Section 9.2.2); second,

if jaw clenching effects persist when the novelty of the secondary jaw clenching task, and

potential dual-task benefits, decrease (discussed in Section 9.2.3); and third, if a better

dynamic steady-state balance performance is associated with decreased muscle activities

(discussed in Section 9.2.4).

9.2.1 Oral-motor activities do not change sway, control

and stability of center of mass during dynamic

steady-state balance

Controlling posture entails regulating the body's position in space in order to maintain

stability and orientation. Stability is described as the ability to control the CoM with re-

spect to the BoS, whereas orientation is the ability to maintain an adequate relationship

9.2 Influence of jaw clenching on dynamic steady-state balance

125

between body segments as well as the body and its surroundings (Shumway-Cook &

Woollacott, 2017). In various postural control studies, the CoM is proposed as the con-

trolled variable, although experimental verification is difficult (Kilby et al., 2015; Nicolai &

Audiffren, 2019; Richmond et al., 2021; Winter et al., 1998). For example, Scholz et al.

(2007) implemented the UCM technique to see whether the CoM is the primary variable

controlled by the CNS for postural control. Their findings revealed that after recovery from

a loss of balance, participants tend to re-establish the position of the CoM rather than the

joint configurations, implying that the CoM is the primary variable controlled by the CNS.

Furthermore, the study presented in Chapter 4 revealed that the participants had less

movement in their pelvis, which can be used as an approximation of the CoM (Yang & Pai,

2014), compared with the distal segments. This might be interpreted as a hint that the

CNS primarily control the CoM.

In the first experiment comprised in this thesis, the participants stood one-legged on an

oscillating platform, which was unexpectedly perturbed. The first three swings of the plat-

form were considered as the phase in which the participants tried to compensate for this

external perturbation (Kiss, 2011a), therefore, the dynamic-reactive balance was of great

importance. The main aim in the second phase of this balance task (i.e. after the compen-

sation of the perturbation) was to stand one-legged on the oscillating platform, therefore,

dynamic steady-state balance was challenged in this phase. The study given in Chapter 1

concentrated on the dynamic steady-state phase of the balance task. This study aimed to

investigate how different stomatognathic motor activities (i.e. submaximal jaw clenching,

submaximal tongue pressing and habitual stomatognatic behavior) affect the sway, stabil-

ity and control of the CoM during a dynamic steady-state balance task. Particularly, the

sway, temporal dynamics, stability and control of CoM were investigated, which are dis-

cussed in the following sub-chapters.

9.2.1.1 Sway of CoM

The swaying behavior of the CoM is an important feature for postural control studies

(Asslände et al., 2020; Richmond et al., 2021). The spatial dynamics of the CoM can be

assessed among others by the total distance covered in a certain period of time (Prieto et

al., 1996; Richmond et al., 2021). To assess the spatial dynamics of the CoM sway, the total

distance covered in three-dimensional vector space was calculated for the following inter-

val of 12 s after the compensation for the perturbation. The results showed no significant

effects between the stomatognathic motor activities independent of the direction of the

prior perturbation. This indicated that the swaying behavior of the CoM was not affected

by the stomatognathic motor activities. However, it should be noted that the effect size

was medium (p = 0.182; η2 p = 0.073) in the direction of perturbation in which the dynamic

9 General Discussion

126

reactive balance performance was shown to be better (discussed in 9.1.1). Eventhough a

medium effect size does not have a high statistical power, jaw clenching may have effects

on the dynamic steady-state balance performance but these were not high enough to be

detected by using the chosen methods.

9.2.1.2 Temporal dynamics of CoM

Temporal dynamics of the CoM sway is another crucial feature regarding postural control,

since fluctuations in supra-postural activity can result in both temporal and spatial

changes (F. C. Chen & Stoffregen, 2012). Previous studies suggested that a DFA can meas-

ure the long-range correlations, or fractality, of the data (McGrath, 2016), therefore, DFA

can be used to investigate the temporal dynamics of postural movements (Duarte &

Sternad, 2008; Fink et al., 2019; Lobo da Costa et al., 2019; C. C. Wang & Yang, 2012). In

this study, DFA was applied to assess the temporal dynamics of CoM sway. The scaling

components, α , were used as a measure for the persistency of the sway (Lin et al., 2008).

The findings revealed that the stomatognathic motor activities did not differ in terms of

the temporal dynamics of the CoM sway. The scaling exponents α were larger than 1.5,

suggesting a Brownian noise (McGrath, 2016), across both perturbation directions and

simultaneously performed stomatognathic motor tasks. These findings indicated that the

CoM sway was persistent in all conditions and the level of persistency was not affected by

the stomatognathic motor activities.

9.2.1.3 Stability and control of CoM estimated by a UCM approach

The CNS must coordinate a redundant musculoskeletal system having more degrees of

freedom than necessary to complete the movement tasks (Bernstein, 1967; Latash et al.,

2002). Synergies are one of the different approaches used to model how CNS deals with

this redundancy (D’Avella et al., 2003; Latash et al., 2007). According to Latash et al.

(2007), "synergy" is a neural organization composed of a multi-element system that coor-

dinates the distribution of a task among a group of so-called EVs. The co-variation of EVs

are used to stabilize a targeted variable, the so-called PV. Thereby, there are a number of

equivalent movement solutions in which the true value of PV is achieved. One option to

estimate the quantity of equivalent movement solutions and the level of stability of the

PV is the UCM approach (Scholz & Schöner, 1999). In this study, the UCM method was

used to analyze how joint movement co-variation affects the stability and control of CoM,

as measured by  and ,respectively (Freitas et al., 2006; Hagio et al., 2020;

Hsu et al., 2013, 2017; Krishnamoorthy et al., 2005; Scholz et al., 2007).

The findings of UCM analysis in this study revealed that the three simultaneously per-

formed stomatognathic motor tasks did not lead to any differences in , This

9.2 Influence of jaw clenching on dynamic steady-state balance

127

demonstrated that submaximal jaw clenching or tongue pressing did not result in a more

stable CoM than habitual stomatognatic behavior conditions during dynamic steady-state

balance. The results of  also showed no significant differences between the stoma-

tognathic motor tasks, which indicated that simultaneously performed submaximal jaw

clenching or tongue pressing did not lead to different control compared to habitual sto-

matognatic behavior conditions during dynamic steady-state balance.

9.2.2 Dynamic steady-state balance performance improves

during simultaneous jaw clenching

The last study of this thesis, presented in Chapter 8, focused on the submaximal jaw

clenching effects on dynamic steady-state balance. Dynamic steady-state balance assess-

ment was done by use of a stabilometer. Thereby, the dynamic steady-state balance task

was to keep the stabilometer platform in the horizontal position as long as possible. As a

performance measure, the total time at the horizontal position was calculated, where the

horizontal position was defined as ±3° deviation from the horizontal neutral position

(Brueckner et al., 2019; Kiss, Brueckner, & Muehlbauer, 2018) for at least 500 ms (Giboin

et al., 2015).

In the first research question of this study, the acute effects, therefore the results of the

pretest, were addressed. The statistical analysis revealed that the participants could keep

the platform in the horizontal position longer during simultaneous jaw clenching com-

pared with the non-clenching condition. This demonstrated that at the pretest, all partic-

ipants performed better in the dynamic steady-state balance task in the jaw clenching

condition than in the non-clenching one. This finding supported the first hypothesis that

dynamic steady-state balance performance improves during simultaneous jaw clenching.

In previous studies, it was shown that jaw clenching may alter static steady-state balance

(Hellmann et al., 2011a, 2015; Ringhof, Leibold, et al., 2015; Ringhof, Stein, et al., 2015)

and dynamic proactive balance (Tomita et al., 2021). Furthermore, the study explained in

Chapter 4 revealed that jaw clenching alters balance during dynamic reactive balance in a

direction-specific manner. The findings of the current study complemented the previous

ones by showing the enhancing effects of simultaneous submaximal jaw clenching on dy-

namic steady-state balance performance.

In Section 9.2.1, the dynamic steady-state part of the balance task on the oscillating plat-

form was analyzed. Thereby, the sway, control and stability of the CoM were investigated

under the effects of different stomatognathic motor activities. The findings revealed that

9 General Discussion

128

simultaneous submaximal jaw clenching does not influence the performance variable

CoM during dynamic steady-state balance. However, the balance task was not the same

but it was standing one-legged on an oscillating platform after the compensation of the

perturbation, whereas in this study the participants stood bipedal on a stabilometer and

tried to keep the platform parallel to the ground. Partially conflicting results from these

two studies may be explained by the task-specificity of balance (Giboin et al., 2015;

Ringhof & Stein, 2018). The results of one balance task are not necessarily transferable to

another one but the findings should be interpreted very carefully.

Another explanation for why there were jaw clenching effects in this study and not in the

previous one might be the different difficulty levels of the dynamic steady-state balance

tasks. Previous studies indicated that when more difficult balance tasks are present, the

sensory information related to dental occlusion is more strongly used (Julià-Sánchez et

al., 2016, 2019; Tardieu et al., 2009). On this basis, it can be argued that the effects of jaw

clenching may become more significant during more difficult balance tasks. Possibly the

most difficult part of the balance task on the oscillating platform was the compensation

of the perturbation. Afterward, the balance task was just one-legged standing on an un-

stable support surface, which might have been not challenging enough for healthy physi-

cally active adults. Whereas, the balance task on the stabilometer was to keep an easily

tiltable platform parallel to the ground as long as possible, which was probably more dif-

ficult to execute.

9.2.3 Jaw clenching effects persist despite decreased novelty

of jaw clenching task

Similar to Section 9.1.4, this part of this study aimed to understand whether the effects of

jaw clenching on postural control are related to the dual-task paradigm (Andersson et al.,

2002; Broglio et al., 2005; Polskaia et al., 2015; Swan et al., 2004). The study designs dif-

fered in several points, particularly in the type of balance task and intervention. In the

former study, the participants executed a dynamic reactive balance task and the interven-

tion group trained solely for jaw clenching between the pretest and the posttest. Whereas

the current study comprised a dynamic steady-state balance task, and secondly, the group

in which a decrease in the novelty of the jaw clenching task was expected at the posttest,

trained for dynamic steady-state balance task with simultaneous jaw clenching. In other

words, the participants in the current study trained for jaw clenching together with the

balance task, in the form of a dual-task.

9.2 Influence of jaw clenching on dynamic steady-state balance

129

In the current study, it was hypothesized that the effects of jaw clenching on dynamic

steady-state balance performance persist even if the novelty of the jaw clenching task and

the increased requirement of attention due to its novelty decrease (the second hypothe-

sis). The findings revealed that the participants could keep the stabilometer in a horizontal

position longer during simultaneous submaximal jaw clenching compared to non-clench-

ing conditions at both measurement times. This supported the second hypothesis by in-

dicating that the dynamic steady-state balance performance was better during the jaw

clenching condition compared with the non-clenching condition at the posttest further

on.

A secondary finding of this study was the high learning effects of the balance task as in

the previous study discussed in Section 9.1.4. In the current study, a three-armed experi-

mental design was used, where two groups trained for the dynamic steady-state balance

task in the intervention phase. One of the groups trained the dynamic steady-state bal-

ance task in classical single-task form, whereas the other one trained in the form of a dual-

task, together with simultaneous jaw clenching. The third control group had no training

between the pretest and the posttest. The results revealed that regardless of their training

status, all of the groups improved their dynamic steady-state performance at the posttest.

Interestingly, all groups increased their dynamic steady-state balance performance, with

no significant group differences. This indicated that the learning effects of the balance

task were high and an additonal balance training seemed not to lead any further improve-

ment in dynamic steady-state balance performance at the posttest. This finding is note-

worthy because prior research has shown that dynamic steady-state balance performance

improves following balance training with the same balance task used for testing (e.g.,

Giboin et al., 2015; Kiss, Brueckner, & Muehlbauer, 2018). In this study, the effects of bal-

ance training may have been masked by the high learning effects of the balance task.

9.2.4 Dynamic steady-state balance performance improvement

is accompanied by decreased muscle activities

The stabiliometer used in this study was also used in various studies to assess dynamic

steady-state balance performance (Brueckner et al., 2019; Steib et al., 2018; Steiner et al.,

2016; Taubert et al., 2010). Thereby, continuous postural adjustments are required to

maintain an unstable platform in the horizontal position. In most of the studies, the dy-

namic steady-state balance performance was considered but the EMG data were rarely

analyzed (Brueckner et al., 2019; Taubert et al., 2010). Brueckner et al. (2019) analyzed

the effects of dynamic balance task training for two days and showed reduced muscle

9 General Discussion

130

activity as well as a reduced EMG intensity in calf muscles. They explained these altera-

tions with movement efficiency.

The third aim of the current study given in Chapter 8 was to investigate the changes in

muscle activities for a deeper understanding of the adaptations occurring during a dy-

namic steady-state balance task with simultaneous jaw clenching. Based on the findings

of Brueckner et al. (2019), it was hypothesized that better dynamic steady-state balance

performance is associated with decreased muscle activity due to movement efficiency.

The results revealed that all muscle activities were less at the posttest. Given that dynamic

steady-state balance performance was higher at the posttest, it may be argued that im-

proved dynamic steady-state balance performance is related to lower muscle activities.

However, only one of the examined muscles, which was a quadriceps muscle, showed a

significant moderate correlation between dynamic steady-state balance performance im-

provement and muscle activity reduction from the pretest to the posttest. The non-signif-

icant correlations in the remaining muscles could be attributed to the linear technique

used for both the calculation of changes between the two measurement times and the

correlations between the parameters. For example, a previous study examining the acti-

vation of the muscles during back squats with varying weights revealed a nonlinear rela-

tionship between the changes in loads and the activations of the muscles (van den Tillaar

et al., 2019). This finding suggests that the correlation between the dynamic steady-state

balance performance improvements and the muscle activity reductions may be significant

and larger when evaluated by using non-linear methods. Nevertheless, the dynamic

steady-state balance performance increase was accompanied by decreased activity of all

of the analyzed muscles. These findings were consistent with previous studies (e.g.,

Brueckner et al., 2019) that observed training-related reductions in muscle activities,

which may be explained by improved movement efficiency. On the other hand, even

though the dynamic steady-state balance performance was significantly better during sim-

ultaneous jaw clenching than in the non-clenching condition over two measurement

times, the muscle activities did not demonstrate any systematic changes between the sto-

matognathic motor activities. Only the activity reduction of one of the analyzed calf mus-

cles was less in the jaw clenching condition compared with the non-clenching condition

between the pretest and the posttest. These findings implied that the improvement in

dynamic steady-state balance performance during simultaneous jaw clenching may have

been caused by additional, as of yet unidentified mechanisms in addition to movement

efficiency.

9.3 Implications and recommendations

131

9.3 Implications and recommendations

The findings of this thesis revealed the effects of submaximal jaw clenching on dynamic

balance only to a limited extent. Particularly, simultaneous submaximal jaw clenching re-

sulted in a better dynamic reactive balance, thereby, reducing the speed of the body seg-

ments but reflexive activities remained unchanged. Furthermore, through jaw clenching,

dynamic steady-state could be improved but not for all dynamic steady-state balance task

conditions. Importantly, the occurrence of the effects seemed not to be associated with

dual-task benefits but specifically with the jaw clenching task. Considering the yet remain-

ing contradictions and research gaps, further studies are necessary to enhance the find-

ings reported here, ideally with different dynamic balance tasks that would have less

learning effect. The current state of the research could not yet fully eliminate the uncer-

tainty regarding the potential benefits of jaw clenching concerning balance performance

under dynamic conditions.

In addition to investigating jaw clenching effects on a balance performance level, the cur-

rent thesis undertook detailed biomechanical investigations to clarify the underlying neu-

romechanical mechanisms. The studies provided some clues but could not completely

clarify these mechanisms. Future studies should aim to identify the processes behind the

effects of concurrent submaximal jaw clenching. In addition, the following issues should

be considered in future research:

• Experimental designs that would minimize the unwanted learning effects of

balance tasks which may potentially mask the effects of jaw clenching;

• Assessment of jaw clenching effects for different groups of participants that

have impaired balance (e.g., elderly, neurological patients);

• Investigation of momentaneous, but not sustained, jaw clenching to in-

crease its real-life scenario compatibility;

• Investigation of the effects of jaw clenching in the longer term (e.g., 10-12

weeks) to ultimately examine its feasibility as a supportive tool for balance

improvement in dynamic situations;

• Consideration of experimental designs in which sensory information from

primary sources is reduced (e.g., eyes closed);

• Examination of synergies of posture-related muscles under the effects of jaw

clenching.

133

10 Conclusion

The control of human posture is essential for daily life. Good balance is associated with a

lower incidence of falls and accidents (Hrysomallis, 2007; Rubenstein, 2006), whereas

poor balance can result in a loss of functional independence and limited participation in

daily activities.

The postural control system comprises a complex interaction of multiple systems of the

body. Thereby, a continuous flow of sensory inputs is used to determine the position and

movement of the body in relation to its surroundings and ultimately translated into motor

commands to maintain the balance in sustainedly changing situations (Shumway-Cook &

Woollacott, 2017). Various factors have been shown to influence balance, including the

state of the craniomandibular system. Both the positions and activities of the cranioman-

dibular system may modulate balance. Particularly, jaw clenching has gained attention in

various studies, probably because it facilitates the excitability of the human motor system

like the Jendrassik manoeuvre (Boroojerdi et al., 2000; Gregory et al., 2001). In contrast

to balance in more static conditions (e.g., upright standing), the influence of jaw clenching

in dynamic situations (e.g., standing on unstable support surfaces) has not yet been thor-

oughly studied.

The present thesis addressed the above-mentioned research gap and aimed to gain a

more detailed insight into the neuromechanical mechanisms of concurrent jaw clenching

during dynamic balance. Thereby, the biomechanical assessment methods comprising dy-

namic posturography, three-dimensional motion capturing, inverse kinematics as well as

EMG analysis were applied. Five research articles that were published in international

peer-reviewed journals aimed to close this research gap. The results of these studies were

given in Chapter 4-8 and discussed in Chapter 9, whereas in Chapter 1-3 theoretical back-

ground and important terms were introduced.

The five studies presented in this thesis revealed basically the following findings:

i. Simultaneous submaximal jaw clenching can lead to a better compensation of

external perturbations and therefore, improve dynamic reactive balance but the

occurrence of these effects is direction-specific. Furthermore, better perturba-

tion compensation due to jaw clenching was accompanied by lower mean speeds

of the body segments.

10. Conclusion

134

ii. Simultaneous submaximal tongue pressing does not seem to result in a better

dynamic reactive balance performance. The detected dynamic reactive balance

performance improvement during simultaneous jaw clenching was not due to

stomatognatic motor activity per se or the dual-task paradigm, but in particular

to the submaximal jaw clenching task.

iii. The neuromechanical impact of jaw clenching which was evident in dynamic re-

active balance performance in the case of forward acceleration of the platform

can not be explained by the changes in muscle activity or co-contraction patterns

of posture-related muscles and muscle pairs.

iv. The automation of jaw clenching seems not to have any effects on dynamic re-

active balance performance, whereas the jaw clenching task was associated with

some modulations of reflex activities in lower leg muscles in a direction-specific

manner. The learning effects of the dynamic reactive balance task were high, and

may potentially have masked the effects of jaw clenching.

v. Neither submaximal jaw clenching nor tongue pressing influence the sway, con-

trol or stability of the CoM during dynamic steady-state balance after the com-

pensation of perturbations on the oscillating platform.

vi. Dynamic steady-state balance performance improves with simultaneous sub-

maximal jaw clenching during a balance task on the stabilometer, and the en-

hancing effects of jaw clenching persist despite the decreased novelty of the jaw

clenching task. The secondary finding revealed high learning effects of the bal-

ance task which may possible have masked the effects of balance training.

vii. The dynamic steady-state balance performance improvement was accompanied

by decreased activity of posture-related muscles, which can be explained by the

increased movement efficiency. However, there were not any jaw clenching-re-

lated changes in muscle activities.

To sum up, from the standpoints of basic and applied research, postural control is a crucial

and multidisciplinary area of study. Research from various fields, including sports science,

biomechanics and neuroscience, has contributed significantly to the understanding of

postural control processes. The present thesis added another puzzle piece to the whole

picture by discovering the effects of jaw clenching on balance in dynamic situations, albeit

not yet completely. More research is needed to fully assess the potential of the jaw clench-

ing task and to better understand the underlying neuromechanical impacts.

135

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Appendix

167

Supplementary material

S1 Table1: Mean speed of the five anatomical regions given as mean and standard deviation (SD) for different

groups. JAW: jaw clenching, TON: tongue pressing, HAB: habitual jaw position.

Head

Trunk

Pelvis

Foot

Knee

in mm/s

JAW

Mean

33.82

27.68

29.41

107.40

78.88

9.72

7.02

7.54

19.02

12.20

TON

Mean

46.12

32.64

34.06

129.71

92.21

23.22

9.43

7.72

27.44

18.38

HAB

all

Mean

47.79

33.75

35.03

124.70

87.91

19.50

9.53

9.38

27.41

18.43

HAB

w/o clen-

ching

Mean

42.01

31.66

33.11

118.99

84.88

12.87

7.85

6.75

22.67

17.74

HAB

with clen-

ching

Mean

72.81

42.80

43.32

149.47

101.07

26.47

12.64

16.11

37.67

18.44

Supplementary material

168

S2 Table 1: iEMG of all muscles in MVC% ∙ ms for four critical phases.

Muscle

pair

Group

PRE

SLR

MLR

LLR

JAW

Mean

18.64

17.46

19.92

18.90

12.15

10.36

13.25

13.27

HAB

Mean

35.57

25.31

37.62

43.69

82.52

50.16

74.04

90.84

TON

Mean

17.61

25.79

16.44

14.90

27.81

50.98

11.64

11.72

SOL

JAW

Mean

16.39

14.67

13.30

15.55

7.10

8.20

6.01

7.46

HAB

Mean

15.42

18.88

14.67

12.04

12.14

23.70

13.49

7.47

TON

Mean

14.44

12.84

14.17

12.24

5.66

7.39

5.38

JAW

Mean

16.42

13.42

13.10

15.44

10.48

7.98

7.25

8.33

HAB

Mean

15.75

21.49

17.47

13.96

10.48

18.89

14.30

9.74

TON

Mean

18.82

18.20

16.47

16.81

10.79

12.71

9.74

10.76

JAW

Mean

9.63

9.99

10.86

9.60

5.85

6.45

7.11

6.94

HAB

Mean

7.09

5.77

6.39

6.19

5.28

4.44

4.15

3.92

TON

Mean

8.70

9.33

9.61

8.60

6.70

6.12

6.61

6.02

JAW

Mean

3.23

3.22

3.58

3.52

2.50

2.42

3.07

2.74

HAB

Mean

7.21

7.00

6.17

5.04

13.50

12.54

10.89

6.21

TON

Mean

2.73

2.86

2.54

2.76

2.86

2.85

2.88

3.14

Supplementary material

169

JAW

Mean

3.44

3.62

3.07

3.26

2.55

2.59

2.00

2.20

HAB

Mean

2.02

1.52

2.22

1.81

1.71

1.27

1.61

1.43

TON

Mean

3.74

3.20

3.60

3.68

2.89

2.66

2.80

3.18

JAW

Mean

2.11

2.10

1.95

2.09

2.33

2.29

2.37

2.33

HAB

Mean

1.89

1.97

2.16

1.99

1.80

1.56

1.98

1.83

TON

Mean

2.12

2.14

2.01

2.06

1.92

1.87

1.51

1.49

SEM

JAW

Mean

4.05

3.17

4.07

4.28

3.34

3.64

3.91

3.20

HAB

Mean

3.79

3.76

3.39

3.96

3.86

4.19

2.93

4.29

TON

Mean

3.88

4.12

4.69

3.81

4.56

5.11

6.41

4.63

JAW

Mean

5.49

5.01

5.83

5.70

8.97

6.98

10.15

9.05

HAB

Mean

4.27

4.21

4.35

3.47

3.54

3.90

3.60

2.50

TON

Mean

5.31

4.82

5.56

4.66

4.15

3.92

4.66

2.79

ABS (d)

JAW

Mean

0.86

0.78

0.75

0.96

0.57

0.62

0.43

0.75

HAB

Mean

1.14

1.02

1.08

0.90

1.12

0.74

1.07

0.76

TON

Mean

1.81

1.70

1.78

1.75

1.86

1.60

1.72

1.86

ABS

(nd)

JAW

Mean

0.98

0.97

1.18

1.01

0.71

0.78

1.08

0.70

HAB

Mean

1.20

1.29

1.26

1.27

1.03

1.51

1.14

1.15

TON

Mean

1.86

1.89

1.65

1.50

2.10

2.56

1.92

1.55

Supplementary material

170

OBL (d)

JAW

Mean

2.67

2.37

2.61

2.60

1.29

1.36

1.53

1.43

HAB

Mean

2.18

2.22

2.16

2.00

1.01

1.32

1.38

1.20

TON

Mean

3.51

4.00

3.70

4.08

3.13

3.55

3.51

3.83

OBL

(nd)

JAW

Mean

3.06

2.66

3.25

3.17

1.88

1.87

2.78

2.12

HAB

Mean

2.91

2.76

3.09

2.91

1.52

1.75

2.69

1.68

TON

Mean

4.42

4.24

4.12

4.06

5.62

4.37

4.22

4.56

ES (d)

JAW

Mean

3.94

3.36

3.86

4.02

3.42

2.86

3.81

3.87

HAB

Mean

4.04

4.39

3.93

3.71

2.99

3.59

3.27

2.73

TON

Mean

7.76

6.51

7.18

6.04

11.57

8.82

9.45

7.01

ES (nd)

JAW

Mean

3.21

3.30

3.02

3.24

2.93

3.03

2.18

3.12

HAB

Mean

3.12

3.04

3.39

3.18

2.09

2.03

2.36

2.58

TON

Mean

6.95

6.66

6.35

5.11

11.33

10.10

10.02

6.35

Mass

JAW

Mean

4.94

4.75

4.92

4.91

3.01

2.99

3.12

2.86

HAB

Mean

1.36

1.20

1.11

1.38

3.04

2.62

2.32

2.97

TON

Mean

3.35

3.52

3.19

3.34

2.32

2.57

2.05

2.20

PRE: the time window just before the perturbation onset (–100 to 0 ms); SLR: short latency response (30 to 60

ms); MLR: medium latency response (60 to 85 ms); LLR: long latency response (85 to 120 ms). SD = standard

deviation. GM = M. gastrocnemius medialis, SOL = M. soleus, TA = M. tibialis anterior, PL = M. peroneus longus,

FL = M. tensor fascia latae, SEM = M. semitendinosus, BF = M. biceps femoris, RF = M. rectus femoris, VM = M.

vastus medialis, OBL = Mm. obliquus externus, ABS = Mm. rectus abdominis, ES = Mm. erector spinae iliocostalis.

d = dominant. nd = non-dominant.

Supplementary material

171

S2 Table 2: Co-contraction ratio (CCR) of selected muscle pairs for four critical.

Muscle

pair

Group

PRE

SLR

MLR

LLR

GM-TA

JAW

Mean

0.32

0.33

0.32

0.06

0.07

0.06

HAB

Mean

0.30

0.28

0.31

0.30

0.06

0.09

0.06

0.07

TON

Mean

0.31

0.32

0.31

0.08

0.09

0.10

SOL-TA

JAW

Mean

0.33

0.32

0.05

0.06

0.05

HAB

Mean

0.31

0.30

0.31

0.33

0.07

0.08

0.07

0.08

TON

Mean

0.33

0.32

0.33

0.08

0.09

TA-BF

JAW

Mean

0.26

0.24

0.25

0.08

0.10

0.09

HAB

Mean

0.26

0.30

0.27

0.26

0.07

0.08

TON

Mean

0.26

0.27

0.26

0.27

0.10

0.11

0.09

VM-BF

JAW

Mean

0.32

0.33

0.08

0.10

0.08

HAB

Mean

0.30

0.32

0.30

0.31

0.07

0.09

0.07

TON

Mean

0.32

0.33

0.32

0.04

0.05

0.06

RF-BF

JAW

Mean

0.30

0.28

0.29

0.30

0.10

0.09

0.10

HAB

Mean

0.33

0.31

0.33

0.05

0.06

0.05

TON

Mean

0.31

0.30

0.31

0.08

Supplementary material

172

GM-RF

JAW

Mean

0.25

0.27

0.26

0.09

0.11

HAB

Mean

0.22

0.19

0.24

0.23

0.09

0.11

0.08

TON

Mean

0.26

0.25

0.26

0.11

0.13

PL-SOL

JAW

Mean

0.34

0.33

0.35

0.04

0.05

0.07

0.05

HAB

Mean

0.33

0.35

0.34

0.30

0.04

0.06

0.07

0.06

TON

Mean

0.34

0.33

0.34

0.35

0.05

0.06

0.05

VM-

SEM

JAW

Mean

0.32

0.31

0.29

0.33

0.05

0.06

0.08

0.07

HAB

Mean

0.33

0.29

0.32

0.06

0.07

TON

Mean

0.27

0.28

0.27

0.08

0.09

0.10

0.08

ABS-ES

(d)

JAW

Mean

0.30

0.28

0.29

0.13

0.12

HAB

Mean

0.29

0.30

0.29

0.27

0.11

0.10

0.09

TON

Mean

0.30

0.29

0.11

0.10

0.11

0.10

ABS-ES

(nd)

JAW

Mean

0.30

0.28

0.31

0.11

0.10

0.09

0.10

HAB

Mean

0.30

0.29

0.28

0.11

0.12

0.11

0.10

TON

Mean

0.31

0.32

0.12

0.11

0.12

0.10

SEM-

JAW

Mean

0.28

0.27

0.31

0.08

0.10

0.09

0.07

HAB

Mean

0.32

0.31

0.05

0.07

0.06

TON

Mean

0.29

0.28

0.29

0.07

0.08

0.10

0.08

Supplementary material

173

OBL-ES

(d)

JAW

Mean

0.36

0.38

0.37

0.36

0.06

0.07

0.06

HAB

Mean

0.39

0.37

0.38

0.06

0.07

TON

Mean

0.32

0.31

0.30

0.32

0.09

0.08

0.09

OBL-ES

(nd)

JAW

Mean

0.34

0.33

0.34

0.33

0.10

0.09

0.08

HAB

Mean

0.36

0.37

0.06

0.07

0.08

0.07

TON

Mean

0.33

0.32

0.35

0.08

0.07

0.11

0.09

PRE: the time window just before the perturbation onset (–100 to 0 ms); SLR: short latency response (30 to 60

ms); MLR: medium latency response (60 to 85 ms); LLR: long latency response (85 to 120 ms). SD = standard

deviation. GM = M. gastrocnemius medialis, SOL = M. soleus, TA = M. tibialis anterior, PL = M. peroneus longus,

FL = M. tensor fascia latae, SEM = M. semitendinosus, BF = M. biceps femoris, RF = M. rectus femoris, VM = M.

vastus medialis, OBL = Mm. obliquus externus, ABS = Mm. rectus abdominis, ES = Mm. erector spinae iliocostalis.

d = dominant. nd = non-dominant.

Supplementary material

174

S3 Table1: Descriptive data used in this study represented as mean ± standard deviation. TAE: Time at equi-

librium; TA: tibialis anterior, GM: gastrocnemius medialis, RF: rectus femoris, BF: biceps femoris.

Jaw clen-

ching

Non-clen-

ching

Jaw clen-

ching

Non-clen-

ching

TAE in s

JBT

15.8 ± 6.1

14.0 ± 5.8

21.7 ± 4.2

20.8 ± 3.6

OBT

16.0 ± 6.0

14.9 ± 5.2

21.9 ± 3.9

21.5 ± 4.0

CON

15.8 ± 5.9

14.3 ± 8.1

19.0 ± 6.8

18.5 ± 7.1

iEMG of TA in

JBT

10.8 ± 5.9

11.2 ± 7.9

5.2 ± 4.0

3.9 ± 3.3

OBT

10.6 ± 8.5

9.3 ± 7.9

4.0 ± 4.4

3.2 ± 3.1

CON

12.0 ± 15.9

10.4 ± 11.3

6.2 ± 5.6

6.9 ± 5.5

iEMG of GM

in %

JBT

6.7 ± 6.5

5.8 ± 6.1

4.2 ± 4.7

4.2 ± 5.5

OBT

7.9 ± 8.7

7.3 ± 6.8

2.9 ± 2.8

3.2 ± 2.6

CON

8.6 ± 6.4

8.3 ± 6.7

4.7 ± 3.3

5.5 ± 4.6

iEMG of RF in

JBT

7.4 ± 6.4

6.9 ± 5.5

4.0 ± 3.0

4.2 ± 3.3

OBT

7.1 ± 6.6

6.6 ± 6.7

3.1 ± 3.0

2.8 ± 3.0

CON

6.6 ± 5.4

5.3 ± 3.5

4.1 ± 3.1

4.3 ± 3.5

iEMG of BF in

JBT

3.7 ± 1.9

3.6 ± 2.3

2.1 ± 2.1

2.1 ± 2.3

OBT

4.7 ± 3.8

4.0 ± 3.2

2.4 ± 2.3

2.4 ± 2.1

CON

3.6 ± 2.0

4.4 ± 3.2

2.5 ± 1.6

2.7 ± 1.3

175

Statutory Declaration

Hiermit erkläre ich, dass ich die vorliegende Dissertation mit dem Titel

„Influence of the Craniomandibular System on Human Postural Control with Spe-

cial Consideration of Dynamic Stability”

selbständig angerfertigt wurde und keine anderen als die angegebenen Hilfsmittel benutzt

sowie die wörtlich oder inhaltlich übernommenen Stellen als solche kenntlich gemacht

und die Satzung des Karlsruher Instituts für Technologie (KIT) zur Sicherung guter wissen-

schaftlicher Praxis beachtet habe. Diese Arbeit wurde nicht bereits anderweitig als Prü-

fungsarbeit verwendet.

Karlsruhe, den 15.12.2024

Cagla Kettner (née Fadilli-

oglu) has a background in

mechanical engineering and

pursued her master’s degree

in sports sciences at the

Karlsruhe Institute of Tech-

nology, where she later ear-

ned her PhD. Her research

focuses on human postural control, running biome-

chanics, and performance diagnostics using state-

of-the-art methods and technologies. Her work

spans fundamental research and applied studies,

bridging the gap between engineering and sports

science. She has contributed to various research pro-

jects, including the inﬂ uence of the craniomandibu-

lar system on dynamic balance, the biomechanics of

running with different shoe conﬁ gurations, and per-

formance diagnostics in recurve archery. In addition

to her research, Cagla is actively involved in teaching

courses on biomechanics, training science, and data

analysis. With a lifelong passion for sports, including

basketball, track & ﬁ eld, and windsurﬁ ng, she brings

a dynamic perspective to her scientiﬁ c endeavors.

C. KETTNER Effects of Craniomandibular System on Dynamic Balance

ISSN 2943-0380

KARLSRUHE SPORTS SCIENCE RESEARCH | BAND 84

ResearchGate has not been able to resolve any citations for this publication.

The interaction between the nervous system and the stomatognathic system: from development to diseases

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The crosstalk between the nerve and stomatognathic systems plays a more important role in organismal health than previously appreciated with the presence of emerging concept of the "brain-oral axis". A deeper understanding of the intricate interaction between the nervous system and the stomatognathic system is warranted, considering their significant developmental homology and anatomical proximity, and the more complex innervation of the jawbone compared to other skeletons. In this review, we provide an in-depth look at studies concerning neurodevelopment, craniofacial development, and congenital anomalies that occur when the two systems develop abnormally. It summarizes the cross-regulation between nerves and jawbones and the effects of various states of the jawbone on intrabony nerve distribution. Diseases closely related to both the nervous system and the stomatognathic system are divided into craniofacial diseases caused by neurological illnesses, and neurological diseases caused by an aberrant stomatognathic system. The two-way relationships between common diseases, such as periodontitis and neurodegenerative disorders, and depression and oral diseases were also discussed. This review provides valuable insights into novel strategies for neuro-skeletal tissue engineering and early prevention and treatment of orofacial and neurological diseases.

Is There a Correlation between Dental Occlusion, Postural Stability and Selected Gait Parameters in Adults?

Article

Full-text available

Jan 2023
Int J Environ Res Publ Health

Background: There is still an ongoing debate about the role of the craniomandibular system, including occlusal conditions, on postural stability. This study aims to assess the role of antero-posterior malocclusion on postural control and plantar pressure distribution during standing and walking. Methods: 90 healthy volunteers (aged 19 to 35) were qualified for the study. The subjects were assigned to three groups, depending on the occlusion type. Each group (Angle Class I, II and III) consisted of 30 people. The research procedure included a clinical occlusal assessment performed by a dentist. Postural control measurements were carried out using a force platform by measuring plantar pressure distribution during standing (six trials with and without visual control) and walking test conditions. Results: The tendency to shift the CoP forward is demonstrated by Angle Class II subjects and backwards by Class I and III subjects (p < 0.001). Individuals with a malocclusion demonstrated significantly higher selected stabilographic parameters while standing on both feet (with eyes open and closed) and during the single-leg test with eyes open (p < 0.05). The analysis of the dynamic test results showed no significant correlations between Angle Classes and the selected gait parameters. Conclusions: Analyses conducted among individuals with malocclusions showed the impact of occlusion on static postural stability. In order to diagnose and effectively treat malocclusion, a multidisciplinary approach with the participation of dentists and physiotherapy specialists is necessary, with the use of stabilometric and kinematic posture assessments.

Effect of practice on learning a balance task in children, adolescents, and young adults

Article

Full-text available

Oct 2022

Background A lower developmental stage of the postural control system in childhood compared to adolescence and adulthood was reported in numerous studies and suggests differences (i.e., less improvements in children than in adolescents and young adults due to the immature postural control system) during learning a balance task. Therefore, the present study examined the effect practice on learning (i.e., retention and transfer) a balance task in healthy children, adolescents, and young adults. Methods Healthy children (n = 32, 8.5 ± 0.5 years), adolescents (n = 30, 14.6 ± 0.6 years), and young adults (n = 28, 24.3 ± 3.3 years) practiced balancing on a stabilometer (i.e., to keep the platform as close to horizontal as possible) for 2 days. On the third day, learning was assessed using a retention (i.e., balance task only) and a transfer (i.e., balance task plus concurrent motor interference task) test. The root-mean-square-error (RMSE) was calculated and used as outcome measures. Results Over the course of practice, significant improvements (p < 0.001) were detected in favor of children and young adults. However, neither the retention nor the transfer test showed significant group differences. Conclusion Our findings indicate that learning a balance task did not seem to be influenced by the developmental stage of the postural control system.

Effect of static and dynamic jaw positions on postural stability among people with blindness

Article

Full-text available

Aug 2022

Background: In comparison with the people with normal sight, the mean center of gravity (COG) velocity is significantly higher among blind people. A strong relationship has been shown between jaw and neck sensorimotor and postural control. The purpose of this study was to determine the effect of different static and dynamic jaw positions on postural stability among subjects with blindness. Methods: Postural stability was measured as COG velocity in 39 blind subjects under the following five conditions: resting jaw (natural jaw position with no instructions, control), open jaw (teeth of both jaws slightly apart), clenched jaw (teeth tightly closed across each other), chewing (a standard bolus of gum at the natural palace), and tongue position (positioned behind the upper incisors) while standing on firm and foam surfaces. Results: The mean COG velocity while standing on the firm surface during resting, open jaw position, clenched jaw position, chewing, and tongue positions were 0.54, 0.50, 0.44, 0.59, and 0.46 deg/s, respectively. The mean COG velocity while standing on the foam surface during resting, open jaw position, clenched jaw position, chewing, and tongue positions were 1.42, 1.23, 1.10, 1.14, and 1.06 deg/s, respectively. Compared to the firm surface, the COG velocity was significantly higher on the foam surface in all five conditions (p < .001). In the comparison between the conditions, there were no significant differences in either the firm or foam surface in all five conditions (p > .05). Conclusion: People with blindness behave in the same way as sighted subjects on firm and foam surfaces. However, changes in static and dynamic jaw positions do not affect postural stability among them.

Sinusoidal Optic Flow Perturbations Reduce Transient but Not Continuous Postural Stability: A Virtual Reality-Based Study

Article

Full-text available

May 2022

Optic flow perturbations induced by virtual reality (VR) are increasingly used in the rehabilitation of postural control and gait. Here, VR offers the possibility to decouple the visual from the somatosensory and vestibular system. By this means, it enables training under conflicting sensorimotor stimulation that creates additional demands on sensory reweighting and balance control. Even though current VR-interventions still lack a well-defined standardized metric to generate optic flow perturbations that can challenge balance in a repeatable manner, continuous oscillations of the VR are typically used as a rehabilitation tool. We therefore investigated if continuous sensory conflicts induced by optic flow perturbations can challenge the postural system sustainably. Eighteen young adults (m = 8, f = 10, age = 24.1 ± 2.0 yrs) were recruited for the study. The VR was provided using a state-of-the-art head-mounted display including the virtual replica of the real environment. After familiarization in quiet stance without and with VR, bipedal balance was perturbed by sinusoidal rotations of the visual scenery in the sagittal plane with an amplitude of 8° and a frequency of 0.2 Hz. Postural stability was quantified by mean center of mass speed derived from 3D-kinematics. A rmANOVA found increased postural instability only during the first perturbation cycle, i.e., the first 5 s. Succeeding the first perturbation cycle, visual afferents were downregulated to reduce the destabilizing influence of the sensory conflicts. In essence, only the transient beginning of sinusoidal oscillation alters balance compared to quiet standing. Therefore, continuous sinusoidal optic flow perturbations appear to be not suitable for balance training as they cannot trigger persisting sensory conflicts and hence challenge the postural system sustainably. Our study provides rationale for using unexpected and discrete optic flow perturbation paradigms to induce sustainable sensory conflicts.

Influence of Controlled Stomatognathic Motor Activity on Sway, Control and Stability of the Center of Mass During Dynamic Steady-State Balance-An Uncontrolled Manifold Analysis

Article

Full-text available

Mar 2022
FRONT HUM NEUROSCI

Multiple sensory signals from visual, somatosensory and vestibular systems are used for human postural control. To maintain postural stability, the central nervous system keeps the center of mass (CoM) within the base of support. The influence of the stomatognathic motor system on postural control has been established under static conditions, but it has not yet been investigated during dynamic steady-state balance. The purpose of the study was to investigate the effects of controlled stomatognathic motor activity on the control and stability of the CoM during dynamic steady-state balance. A total of 48 physically active and healthy adults were assigned to three groups with different stomatognathic motor conditions: jaw clenching, tongue pressing and habitual stomatognathic behavior. Dynamic steady-state balance was assessed using an oscillating platform and the kinematic data were collected with a 3D motion capturing system. The path length (PL) of the 3D CoM trajectory was used for quantifying CoM sway. Temporal dynamics of the CoM movement was assessed with a detrended fluctuation analysis (DFA). An uncontrolled manifold (UCM) analysis was applied to assess the stability and control of the CoM with a subject-specific anthropometric 3D model. The statistical analysis revealed that the groups did not differ significantly in PL, DFA scaling exponents or UCM parameters. The results indicated that deliberate jaw clenching or tongue pressing did not seem to affect the sway, control or stability of the CoM on an oscillating platform significantly. Because of the task-specificity of balance, further research investigating the effects of stomatognathic motor activities on dynamic steady-state balance with different movement tasks are needed. Additionally, further analysis by use of muscle synergies or co-contractions may reveal effects on the level of muscles, which were not visible on the level of kinematics. This study can contribute to the understanding of postural control mechanisms, particularly in relation to stomatognathic motor activities and under dynamic conditions.

Effects of age and surface instability on the control of the center of mass

Article

Full-text available

Apr 2022
HUM MOVEMENT SCI

During standing, posture can be controlled by accelerating the Center of Mass (CoM) through shifting the center of pressure (CoP) within the base of support by applying ankle moments (“CoP mechanism”), or through the “counter-rotation mechanism”, i.e., changing the angular momentum of segments around the CoM to change the direction of the ground reaction force. Postural control develops over the lifespan; at both the beginning and the end of the lifespan adequate postural control appears more challenging. In this study, we aimed to assess mediolateral balance performance and the related use of the postural control mechanisms in children, older adults and younger adults when standing on different (unstable) surfaces. Sixteen pre-pubertal children (6-9y), 17 younger adults (18-24y) and eight older adults (65-80y) performed bipedal upright standing trials of 16 s on a rigid surface and on three balance boards that could freely move in the frontal plane, varying in height (15–19 cm) of the surface of the board above the point of contact with the floor. Full body kinematics (16 segments, 48 markers, using SIMI 3D-motion analysis system (GmbH) and DeepLabCut and Anipose) were retrieved. Performance related outcome measures, i.e., the number of trials with balance loss and the Root Mean Square (RMS) of the time series of the CoM acceleration, the contributions of the CoP mechanism and the counter-rotation mechanism to the CoM acceleration in the frontal plane and selected kinematic measures, i.e. the orientation of the board and the head and the Mean Power Frequency (MPF) of the balance board orientation and the CoM acceleration were determined. Balance loss only occurred when standing on the highest balance board, twice in one older adult once in one younger adult. In children and older adults, the RMS of the CoM accelerations were larger, corresponding to poorer balance performance. Across age groups and conditions, the contribution of the CoP mechanism to the total CoM acceleration was much larger than that of the counter-rotation mechanisms, ranging from 94% to 113% vs 23% to 38% (with totals higher than 100% indicating opposite effects of both mechanisms). Deviations in head orientation were small compared to deviations in balance board orientation. We suggest that the CoP mechanism is dominant, since the counter-rotation mechanism would conflict with stabilizing the orientation of the head in space.

Priming cardiovascular exercise improves complex motor skill learning by affecting the trajectory of learning-related brain plasticity

Article

Full-text available

Jan 2022

In recent years, mounting evidence from animal models and studies in humans has accumulated for the role of cardiovascular exercise (CE) in improving motor performance and learning. Both CE and motor learning may induce highly dynamic structural and functional brain changes, but how both processes interact to boost learning is presently unclear. Here, we hypothesized that subjects receiving CE would show a different pattern of learning-related brain plasticity compared to non-CE controls, which in turn associates with improved motor learning. To address this issue, we paired CE and motor learning sequentially in a randomized controlled trial with healthy human participants. Specifically, we compared the effects of a 2-week CE intervention against a non-CE control group on subsequent learning of a challenging dynamic balancing task (DBT) over 6 consecutive weeks. Structural and functional MRI measurements were conducted at regular 2-week time intervals to investigate dynamic brain changes during the experiment. The trajectory of learning-related changes in white matter microstructure beneath parieto-occipital and primary sensorimotor areas of the right hemisphere differed between the CE vs. non-CE groups, and these changes correlated with improved learning of the CE group. While group differences in sensorimotor white matter were already present immediately after CE and persisted during DBT learning, parieto-occipital effects gradually emerged during motor learning. Finally, we found that spontaneous neural activity at rest in gray matter spatially adjacent to white matter findings was also altered, therefore indicating a meaningful link between structural and functional plasticity. Collectively, these findings may lead to a better understanding of the neural mechanisms mediating the CE-learning link within the brain.

Dual-Task Differences in Individuals with Chronic Ankle Instability: A Systematic Review with Meta-Analysis

Article

Aug 2023
GAIT POSTURE

Background: Individuals with Chronic Ankle Instability (CAI) demonstrate altered gait mechanics, impaired proprioception, and decreased postural control. In addition, individuals with CAI have been found to have complex neurophysiological changes, including during dual-task perturbations. However, the results of studies on whether cognitive tasks affect postural control are inconclusive. Research question: Do individuals with CAI have worse dual-task performance compared to healthy controls? Methods: We searched 4 electronic databases (PubMed, MEDLINE, Cumulative Index of Nursing and Allied Health Literature, and SPORTDiscus) from inception to October 2022. Search terms consisted of: ("dual-task*" OR "dual task*" OR dual-task OR Multitask* OR Multi-task* OR attention OR cognit*) AND (balance OR "postural control" OR "postural sway" OR kinetics OR kinematics OR gait) NOT (concussion OR "traumatic brain injury") combined using the operator "AND" ("ankle sprain" OR "ankle instability" OR CAI). Studies were included if the physical task was postural control or gait, and if they compared control and CAI groups. Results: A total of 9 studies were included in the systematic-review portion, 5 of which were included in the meta-analysis. Due to assessing multiple types of dual-tasks, 10 effects were assessed for meta-analysis across postural control studies. A random-effects model for the control group in the mediolateral direction indicated a significant overall Fisher's Z mean effect size (Δ = 0.732, p = 0.029) with high heterogeneity between studies (Q=76.61; I2 = 88.25% P < 0.001). There were no significant differences between dual-tasking in the CAI group individually or when comparing control to CAI groups. Significance: The results of our study indicate that cognitive loading did not affect the postural control except for the control group in the mediolateral direction. Variations in dual-task results may be due to the difficulty of the task as well as the heterogeneity of CAI groups.

Gait Slip-Induced Fall-Type Assessment Based on Regular Gait Characteristics in Older Adults

Article

Apr 2022
J APPL BIOMECH

Older adults could experience split falls or feet-forward falls following an unexpected slip in gait due to different neuromuscular vulnerabilities, and different intervention strategies would be required for each type of faller. Thus, this study aimed to investigate the key factors affecting the fall types based on regular gait pattern. A total of 105 healthy older adults who experienced a laboratory-induced slip and fall were included. Their natural walking trial immediately prior to the novel slip trial was analyzed. To identify the factors related to fall type, gait characteristics and demographic factors were determined using univariate logistic regression, and then stepwise logistic regression was conducted to assess the slip-induced fall type based on these factors. The best fall-type prediction model involves gait speed and recovery foot angular velocity, which could predict 70.5% of feet-forward falls and 86.9% of split falls. Body mass index was also a crucial fall-type prediction with an overall prediction accuracy of 70.5%. Along with gait parameters, 84.1% of feet-forward falls and 78.7% of split falls could be predicted. The findings in this study revealed the determinators related to fall types, which enhances our knowledge of the mechanism associated to slip-induced fall and would be helpful for the development of tailored interventions for slip-induced fall prevention.

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