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Reliable and effective novel home-based training
set-up for application of an evidence-based high-
loading stimulus to improve triceps surae function
Goran Radovanović, Jona Kunz, Sebastian Bohm, Adamantios Arampatzis &
To cite this article: Goran Radovanović, Jona Kunz, Sebastian Bohm, Adamantios Arampatzis &
Kirsten Legerlotz (2021): Reliable and effective novel home-based training set-up for application
of an evidence-based high-loading stimulus to improve triceps surae function, Journal of Sports
Sciences, DOI: 10.1080/02640414.2021.1959981
To link to this article: https://doi.org/10.1080/02640414.2021.1959981
© 2021 The Author(s). Published by Informa
UK Limited, trading as Taylor & Francis
Published online: 11 Aug 2021.
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SPORTS MEDICINE AND BIOMECHANICS
Reliable and eective novel home-based training set-up for application of an
evidence-based high-loading stimulus to improve triceps surae function
, Jona Kunz
, Sebastian Bohm
, Adamantios Arampatzis
and Kirsten Legerlotz
Humboldt-Universität Zu Berlin, Institute of Sport Sciences, Movement Biomechanics, Berlin, Germany;
Medical School Hamburg - University of
Applied Sciences and Medical University, Faculty of Health Sciences, Department Performance, Neuroscience, Therapy and Health, Hamburg,
Humboldt-Universität zu Berlin, Institute of Sport Sciences, Department of Training and Movement Sciences, Berlin, Germany
High-loading interventions aiming for muscle-tendon adaptations were so far implemented in on-site
facilities. To make this evidence-based stimulus more accessible, we developed an easy-to-use sling-
based training set-up for home-based Achilles tendon and triceps surae muscle strength training and
assessed its reliability and eectiveness in healthy men. To assess reliability (n=11), intra-class correlation
(ICC) and root mean square (RMS) dierences of isometric maximum voluntary contraction (MVC) of the
plantar exors were used. Eectiveness was tested in a controlled intervention trial (n=12), applying one-
legged high-loading intervention for 3 months with our mobile set-up, while the contralateral/untrained leg
served as control, and assessing plantar exor MVC, drop (DJ) and countermovement jump (CMJ) height.
Reliability was excellent between (ICC
=0.935) and within session (ICC
s=0.940–0.967). The mean RMS
dierence between and within sessions was 5.3% and 4.7%, respectively. MVCs of the trained/intervention
leg increased by 10.2±7% (P=0.004) (dynamometry) and 30.2±22.5% (mobile set-up) (P=0.012). MVC of the
untrained/control leg did not change (P>0.05). DJ height increased (P=0.025; D
=2.13) by 2.37±2.9cm. CMJ
height (P>0.05) did not change. We recommend the evidence-based high-loading application with our
novel home-based training set-up as reliable and eective improving strength and jump performance of
the plantar exor muscle-tendon unit.
Accepted 20 July 2021
Injury prevention; muscle
To improve musculoskeletal tissue function of muscles and ten-
dons, one approach is to utilise the mechanical stimulus leading
to the most pronounced adaptation. To be able to apply this
optimal stimulus, it is vital to rst know the relationship between
mechanical loading and tissue adaptation (Heinemeier & Kjaer,
2011), and second, to accurately control the applied mechanical
load to ensure optimal adaptation. Regarding tendon adapta-
tion, the characteristics of exercise stimuli and the associated
tendon tissue response in healthy adults have been identied:
High tendon strains of 4.5–6.5%, generated by muscle contrac-
tions at an intensity of ~90% of the isometric maximum volun-
tary contraction (MVC) and a stimulus duration of at least 3 s are
essential for tendon adaptation (Arampatzis et al., 2007, 2010;
Bohm et al., 2014). This high-loading exercise protocol, being
applied as ve sets of four repetitions four times per week, led to
changes in mechanical (i.e. stiness), morphological (i.e. cross-
sectional area), and material properties (i.e. elastic modulus) of
the tendon (Arampatzis et al., 2007, 2010; Bohm et al., 2014) and
signicant strength improvements of the plantar exors
(Albracht & Arampatzis, 2013; Arampatzis et al., 2010).
Aforementioned changes may potentially reduce the risk for
musculotendinous injuries (Mersmann et al., 2017). In terms of
function, increased plantar exor strength might enhance for-
ward propulsion (Hamner et al., 2010; Liu et al., 2008), improve
ankle joint motion control (i.e. during stair descent or jump
landings) (Devita & Skelly, 1992; Van Dieën et al., 2008) and
improve postural control by stabilising the ankle (Langeard
et al., 2020; Ribeiro et al., 2009). Furthermore, higher plantar
exor tendon stiness has been shown to be related to better
jump (Bojsen-Møller et al., 2005) and stability performance
(Karamanidis et al., 2008) and may improve running economy
(Albracht & Arampatzis, 2013).
High-loading interventions aiming for muscle-tendon adapta-
tions were so far implemented under laboratory conditions or in
a gym with stationary equipment (i.e. weight training machines
in on-site facilities) (Beyer et al., 2015; Bohm et al., 2014). To make
this evidence-based stimulus accessible to a wider population, it
needs to be applicable within a more practical setting, enabling
its implementation at home or in physiotherapy facilities without
stationary equipment. Providing a home exercise programme as
an alternative to a facility-based training programme is impera-
tive to physiotherapy (Kisner & Colby, 2007). It may improve
acceptance and commitment due to more convenience. For
people without access to gym facilities, e.g., in rural areas or
due to pandemic consequences (Covid-19), home-based pro-
grammes are highly advantageous (Thiebaud et al., 2014).
Indeed, therapeutic eectiveness of home-based programmes
has been demonstrated for various musculoskeletal issues
(Emery, 2005; Johansson et al., 2009; Keays et al., 2006;
CONTACT Goran Radovanović email@example.com Humboldt-Universität zu Berlin, Institute of Sport Sciences, Movement
Biomechanics, 10099 Berlin, Germany; Medical School Hamburg - University of Applied Sciences and Medical University, Faculty of Health Sciences, Department
Performance, Neuroscience, Therapy and Health, 20457 Hamburg, Germany
JOURNAL OF SPORTS SCIENCES
© 2021 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group.
This is an Open Access article distributed under the terms of the Creative Commons Attribution-NonCommercial-NoDerivatives License (http://creativecommons.org/licenses/by-nc-nd/4.0/),
which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited, and is not altered, transformed, or built upon in any way.
Littlewood et al., 2014). However, the lack of immediate control,
feedback and correction by the therapist when compared to an
on-site setting as well as instantaneous support concerning con-
trolled progression of the therapy may impose limitations to
a home-based exercise programme (Stasinopoulos et al., 2010).
To meet these limitations and enhance its eectiveness, it may
be advantageous, if a home-based exercise programme is
applied with the help of initial educational on-site session(s),
while the exact mechanical loading stimulus is easy to control
for the patient or athlete. Values describing the magnitude, or
the progression of the loading stimulus can then be reported to
the therapist, allowing comprehensive control of the therapy.
Taking the aforementioned aspects into consideration, we
developed a low-cost, simple to replicate and easy-to-use sling-
based training set-up for home-based Achilles tendon and
triceps surae muscle strength training. This training set-up
allows easy administration of the evidence-based exercise sti-
mulus at home, while enabling the users to control and adjust
the specic load via a simple biofeedback.
The aim of our study was to test the reliability of the feed-
back tted sling-based training set-up, as well as assessing the
set-up in terms of its eectiveness in improving strength and
vertical jump performance in healthy adults. Jump perfor-
mance was considered as a specic measure to assess plantar
exor muscle function. We hypothesised that the training set-
up allows reliable muscle strength assessment and that its
home-based application increases plantar exor strength and
2 Materials and methods
2.1 A priori sample size analysis
To estimate the adequate sample size for the assessment of the
intervention eectiveness, we conducted a power analysis (Faul
et al., 2007) to test the dierence between two dependent
group means, using a two-tailed test, a large eect size refer-
ring to the training eect for strength (i.e. D
= 1.15) and an
alpha of .05. The eect size was calculated as a mean of the
eect sizes of the training eects for plantar exor strength
improvements from two prior trials using the same high-
loading training protocol (Albracht & Arampatzis, 2013;
Arampatzis et al., 2007). Results showed that a total sample of
11 participants was required to achieve a power of .90.
2.2 Study design and participants
All participants provided informed written consent prior to the
experiments, and local institutional ethics approval was
obtained (Humboldt-Universität zu Berlin, Faculty of
Humanities and Social Sciences). The study was performed in
compliance with the Declaration of Helsinki.
To test the eectiveness of the mobile training set-up (see 2.4),
participants were subjected to a controlled 12-weeks long home-
based exercise intervention trial in which one leg conducted
a high-loading intervention (see 2.5), while the contralateral leg
did not receive any intervention and thus served as control. Our
intervention was conducted with 12 healthy male adults without
any leg injuries in the past 12 months. One participant did not
complete the training intervention due to an injury not related to
the study and was therefore excluded. The anthropometric data
of the remaining 11 participants were as follows: age
27.8 ± 7.1 years (range 22–44 years), body mass 75.1 ± 6.3 kg,
height 181.2 ± 6.7 cm, body mass index (BMI) 23 ± 2 kg/m
participants were physically active, but not involved in high-
performance sports. Participants were allowed to maintain their
previous individual training habits. However, no additional
strength training of the plantar exors and no implementation
of any new sort of lower body strength training was permitted.
MVCs of the plantar exors of the intervention/trained leg
and the control/untrained leg were measured at baseline (PRE)
and after completion of the intervention phase (POST) with
a stationary lab dynamometer (see 2.3.1) and the mobile train-
ing set-up (see 2.3.2). In addition, the MVCs of the intervention/
trained leg were monitored weekly with a stationary dynam-
ometer. Countermovement jump (CMJ) and drop jump (DJ)
height were assessed PRE and POST (see 2.3.3) (Figure 1).
To compare the novel mobile set-up with the dynamometer
(i.e. gold standard), we estimated the strength of the relation-
ship between the MVC values measured with the dynamometer
and the MVCs measured with the mobile training set-up apply-
ing a regression analysis (Bland & Altman, 2003).
The reliability of the MVC measurements with the training set-
up was tested in a separate experiment with ve repeated
measurement sessions within 2 weeks (Figure 1). Healthy
male amateur athletes without any leg injuries conducted
plantar exor MVCs with their dominant leg on ve dierent
days with an interval of 48 hours between MVC sessions (n = 11:
age 38.7 ± 11.2 years, body mass 82.0 ± 9.5 kg, height
181.1 ± 8.3 cm, BMI 25.0 ± 2.3 kg/m
). After the standardised
warm-up (see 2.3.2), the participants conducted ve MVCs with
a 1-min rest between repetitions in each session.
2.3 Strength and jump performance assessment
2.3.1 MVC with dynamometer
Participants were seated on the dynamometer (Biodex-System 3,
Biodex Medical Systems Inc., Shirley, NY, USA) in a standardised
position with a hip (i.e. femur-to-spine) angle of 110°, extended
knee and an ankle angle of 90°. The pelvis was xed with a rigid
belt at the dynamometer seat. Rotational axis of the ankle joint
was individually aligned with the axis of the dynamometer dur-
ing the contraction condition. Individual settings of each parti-
cipant were recorded and used for the weekly and post-
intervention measurements. After a warm-up with three sets of
ve isometric sub-maximal plantar exor contractions, partici-
pants performed ve MVCs with 1-min rest between repetitions.
2.3.2 MVC with mobile training set-up
The MVC measurements with the mobile set-up were similar to
the MVC measurements on the dynamometer referring to
warm-up and resting time. Standardised sitting position and
use of the set-up were applied according to its general settings
2G. RADOVANOVIĆ ET AL.
(see 2.4) (Figure 2). Each MVC was recorded manually by the
examiner from the strain gauge display.
2.3.3 Vertical jump performance
After a warm-up with up to 12 jumps of low to moderate intensity,
ve maximum eort CMJs and ve DJs were performed with bare
feet and 1-min rest between repetitions. Ground reaction forces
were measured with two separate force plates at a rate of 1000 Hz
(Kistler, Type 9260AA, 600 × 500 × 50 mm, Switzerland) linked to
an analogue digital converter (DAQ-System, USB 2.0, Type
5691A1). Data were recorded (BioWare Software, Type 2812A)
and jump height was calculated using a custom written Matlab
interface (version R2012a; MathWorks, Natick, MA, United States).
For the CMJ, the hands were akimbo, while no specications were
given regarding depth. For CMJ height calculation, we used the
impulse momentum method (Linthorne, 2001). DJs were per-
formed from a 15 cm box. Participants were asked to jump with
maximum eort, keeping the contact time as short as possible
Figure 1. Experimental design: Reliability assessment of the mobile training set-up and eﬀectiveness assessment of the 12-week long intervention of an isometric
high-loading home-based plantar ﬂexor muscle strength protocol. BMI = body mass index, MVC = maximum isometric voluntary contraction, PRE = baseline,
POST = after the intervention phase, DJ = drop jump, CMJ = countermovement jump.
JOURNAL OF SPORTS SCIENCES 3
while jumping as high as possible. DJ height calculation was based
on the ight-time method (Moir, 2008).
2.4 Novel mobile sling-based training set-up
The mobile set-up consisted of the following components:
a belt (Neoprene padded powerlifting belt 11.5 cm width,
RDX, US) providing counter resistance on the hip, a ratchet
strap (automatic ratchet strap, EAN 4260246942700, capa-
city of 600 kg, 1.85 m length, Iapyx) for adequately tighten-
ing the sling-belt-system with one hand, and a foot plate
(aluminium stirrup, 350 g, AMKA) for rigid force transmis-
sion. The neoprene material of the underlying powerlifting
belt provided some cushioning for the overlying ratchet
strap. In order to combine the foot plate with the strain
gauge, we used carabiners (Carabiner Micro 3, Edelrid) with
a polyamide quickdraw in between (Quickdraw Pad 19 mm,
Ocun). To allow MVC measurements and real-time feedback
and control of the applied load, the mobile set-up was
tted with a strain gauge (HS-70, Voltcraft, Germany) dis-
playing the force (kg) applied to the system (Figure 2).
Participants were advised to sit with extended knees. The
forefoot (with shoes) was placed in the foot plate with the
widest part of the shoe (i.e. ball of the foot) positioned in
the sagittal centre of the foot plate pad to ensure
a standardised contact point. The participants were advised
to always use the same shoes with a rigid sole. The ratchet
was individually set and xed as tightly as possible, to allow
for maximal isometric plantar exor contractions at
a standardised ankle angle position (90°) (Figure 2). The
straps on both sides of the pelvis were placed as close as
possible to the hip bone (i.e. os ilium) to reduce spinal load.
It was recommended to place one hand on the ground
behind the body providing a hip (i.e. femur-to-spine) angle
of >90°. For standardised alignment during the contractions,
both legs had to be parallel in the frontal plane and in
neutral rotational position in the hip joints in the transverse
The leg with the lower baseline MVC was chosen as the training
leg, while the contralateral leg served as control. The rst
training session was supervised by one examiner. The evi-
dence-based home-based training consisted of four training
sessions per week for 12 weeks. Each training session consisted
of ve sets with four repetitions each and a 1-min rest between
sets. Each repetition was a ~ 90% MVC isometric plantar exor
contraction held for 3 s followed by 3 s of rest according to
previous studies (Arampatzis et al., 2007, 2010; Bohm et al.,
2014). Based on the PRE MVC values measured with the mobile
training set-up, the initial training value (i.e. 90% of the PRE
MVC values mean) was calculated. During intervention training,
participants were instructed to observe and track the contrac-
tion-induced force magnitude on the display of their mobile
set-up (Figure 2) in order to achieve their calculated training
load. To control for delayed onset muscle soreness, any
increase in training intensity was prohibited for the rst 2
weeks. From the third week onwards, the training value was re-
calculated based on a weekly MVC test. The intervention train-
ing was monitored during the weekly MVC tests and via tele-
phone calls to ensure that the exercise programme was
conducted as planned throughout the intervention phase.
The participants used a timer app with audio signal function
on their mobile phone to help implement the contraction
frequency and contraction duration of 3 s.
Figure 2. Novel training set-up: Mobile training belt-sling-system for evidence-based plantar ﬂexor muscle-tendon unit training with an integrated strain gauge.
Seating position with knee extended and ankle angle of 90°. The strap with the ratchet was individually adjusted downwards near the hip bone to reduce spinal load
4G. RADOVANOVIĆ ET AL.
2.6 Data and statistical analysis
The a priori power analysis was conducted using G*Power 3
(Faul et al., 2007). All further statistical analyses were per-
formed using IBM SPSS Statistics software for Windows,
Version 21.0 (Armonk, NY, IBM Corp). For CMJ, DJ and
MVC analysis, the mean of the middle three out of ve
attempts was used. To assess normal distribution of the
data, a Kolmogorov-Smirno test was used. In case of nor-
mal distribution, paired T-tests were applied to establish
eectiveness in terms of Pre to POST comparisons for CMJ,
DJ and MVC measures. The eect size concerning the eect
of training on MVC, DJ and CMJ height was calculated by
and dened as D ≤ 0.2 small, D ≤ 0.5 medium,
and D ≤ 0.8 large eect (Lakens, 2013).
For assessing reliability, four dierent methods for mean
calculation were applied and compared, to identify the
most reliable approach: Mean of all ve measurements
per session (Mean), the maximum measurement
per session (Best), mean of the three maximum measure-
ments per session (Ø3Best) and mean of the middle three
measurements per session (Mid 3). A two-way mixed, sin-
gle measure, absolute agreement intra-class correlation
(ICC 3.1) was performed (Shrout & Fleiss, 1979). ICC results
were interpreted based on the classication scale: excellent
(0.90–1.00), good (0.75–0.90), moderate (0.50–0.75), and
poor (<0.50) (Rosner, 1982). Between session reliability
) and corresponding RMS dierences were calculated
based on the mean MVC values of each participant
per session over all sessions and based on four dierent
MVC mean calculation approaches. Within session reliabil-
) and corresponding RMS dierences were calcu-
lated based on every single MVC value per session and
To assess the strength of the relationship between the PRE
and POST MVC values of the trained and the untrained leg
measured with the dynamometer (i.e. independent variable)
and the PRE and POST MVC values of the trained and the
untrained leg measured with the mobile training set-up (i.e.
dependent variable), a linear regression model was used for
analysis (Bland & Altman, 2003). The interpretation of the
regression coecient (R) was: very weak (0.00–0.19), weak
(0.20–0.39), moderate (0.40–0.59), strong (0.60–0.79), very
strong (0.80–1.0) (Evans, 1996). For all statistical tests, signi-
cance was established at an alpha level of .05. For all statistical
procedures concerning PRE to POST plantar exor MVC mea-
sures due to multiple comparisons of possible intervention
eects, Bonferroni adjustments were made by the number of
tests (n = 16) performed.
3.1 Reliability of the mobile sling-based training set-up
The training set-up demonstrated excellent reliability both
between sessions (ICC
) and within session (ICC
) (TABLE 1).
Independent of the mean calculation method, the calculated
mean values showed high similarity with a range of 0.927–
0.938. Mean RMS dierence was 5.3% between sessions and
4.7% within session (TABLE 1).
3.2 Eectiveness of training
One participant (age 23 years, body mass 71.1 kg, height
174.0 cm, BMI 23.5 kg/m
) was excluded from the trial and
any data analysis due to an injury not related to the trial.
Plantar exor MVCs of the trained leg measured with the
dynamometer increased during the intervention period, being
signicantly dierent (P < .05) to the start of the intervention
from the 5
week onwards (Figure 3).
From PRE to POST, MVCs of the intervention/trained leg
increased signicantly (P = .004) by 10.2 ± 7.0% when measured
with the dynamometer (Cohen D
= 1.74) and by 30.2 ± 22.5%
(P = .012) when measured with the sling-based training set-up
= 1.43) (Figure 4). MVCs of the control/untrained leg
measured with either the dynamometer (P > .05) or the training
set-up (P > .05) did not signicantly change from PRE to POST.
Measures by the dynamometer and measures by the sling-
based training set-up demonstrated a strong linear positive
relationship between (R = 0.704) (P < .05) (Figure 5).
DJ height did signicantly change from PRE to POST (P = .0250)
indicating a large training eect (Cohen D
= 2.13) with a jump
height dierence of 2.37 ± 2.9 cm. CMJ height did not signicantly
change from PRE to POST (P > .05) (Figure 6).
Table 1. Reliability of the mobile training set-up: ICC
and RMS diﬀerences of mean values of the MVCs between measurement sessions (i.e. over all 5 time points
T1 – T5); and ICC
and RMS diﬀerences of the MVCs within one measurement session (i.e. over all MVCs per time point based on 5 MVCs per time point and participant)
using the mobile training set-up. Four diﬀerent calculation methods of the mean MVC (i.e. Mean, Best, Ø3Best and Mid 3) (n = 11) were applied to identify the best
n = 11 Mean±SD
95% CI RMS±SD (kg)
T1 59.0 ± 14.9 63.0 ± 14.9 61.0 ± 14.9 58.9 ± 14.9 0.949* 0.887–0.984 2.9 ± 1.1
T2 61.6 ± 17.6 65.3 ± 17.2 63.5 ± 17.4 61.7 ± 17.5 0.967* 0.924–0.990 3.4 ± 1.0
T3 61.9 ± 16.1 66.1 ± 18.5 63.9 ± 17 61.7 ± 15.8 0.944* 0.877–0.982 3.0 ± 1.9
T4 64.4 ± 16.1 69.5 ± 18.6 66.3 ± 16.7 63.8 ± 15.6 0.940* 0.867–0.981 3.1 ± 2.0
T5 65.6 ± 15.5 68.9 ± 15.8 67.3 ± 15.8 65.5 ± 15.6 0.967* 0.925–0.990 2.4 ± 0.9
Mean 62.5 ± 2.3 66.6 ± 2.4 64.4 ± 2.2 62.3 ± 2.2 0.953
0.935* 0.927* 0.938* 0.933*
95% CI 0.840–0.980 0.829–0.977 0.848–0.981 0.839–0.979
RMS±SD (kg) 3.3 ± 1.7 3.8 ± 1.7 3.3 ± 1.7 3.4 ± 1.7
= Intra-class correlation of mean values of the isometric maximum voluntary contractions (MVCs) over all 5 sessions (T1-T5); ICC
correlation of mean values of the MVCs within one session; kg = kilogram; Mean = average of all 5 measured MVC values; Best = highest MVC value; Ø3Best = average
of highest 3 MVC values; Mid 3 = mean of the middle 3 MVC values, excluding the highest and the lowest value; RMS = root mean square diﬀerences; SD = standard
deviation; T = time point; 95% CI = 95% conﬁdence interval. * indicates signiﬁcant diﬀerence (P < .001).
JOURNAL OF SPORTS SCIENCES 5
As hypothesised, our study showed excellent reliability of the
feedback tted sling-based training set-up. Further, the home-
based plantar exor training was eective, leading to strength
gains of 10.2 ± 7.0% (measured by dynamometry) in healthy
adults. Regarding improvements in function (i.e. vertical jump
performance), our hypothesis was conrmed as DJ height
increased from PRE to POST by 9.6 ± 19%.
While reliability and validity of the strain gauge on its own is
warranted per certicate, we investigated reliability of the
whole mobile sling-based training set-up. The excellent relia-
bility in terms of ICC within and between MVC measurement
sessions provided by the feedback mechanism (strain gauge)
(TABLE 1) enables the user to precisely control the mechanical
stimulus, and to adjust the load in a progressive manner
Figure 3. Time course of plantar ﬂexor strength increase: Isometric maximum voluntary contraction (MVC) plantar ﬂexor strength of the intervention/trained leg
during the 12-week intervention period presented as percent increase compared to baseline (i.e. PRE). POST = week 12. Data are presented as mean ± standard
deviation (except PRE showing 100% for all participants with no standard deviation). * indicates signiﬁcant diﬀerence (P < .05) compared to baseline.
Figure 4. PRE-POST comparison of plantar ﬂexor strength of trained (intervention) and untrained (control) leg: Mean torque values (Nm) measured with the
dynamometer (dynam.) and mean force values (kg) measured with the strain gauge of the mobile training set-up of the plantar ﬂexors of the trained (intervention) leg
and the untrained (control) leg before (PRE) and after (POST) the intervention phase. Data are presented as mean ± standard deviation. * indicates signiﬁcant diﬀerence
(P < .05) compared to PRE value.
6G. RADOVANOVIĆ ET AL.
depending on strength gain. Excellent reliability was conrmed
with the small within and between session RMS dierence of
4.7% and 5.3%, respectively. In this regard, coaches, therapists,
and users can rely on the values given by the strain gauge in
terms of exact application of the stimulus and adequate
planning of exercise progression. Excellent reliability was also
supported by the fact that we did not nd any dierence
between the four dierent calculation methods determining
the mean of the MVCs. This indicates a lack of outliers and
a continuous MVC performance throughout the ve MVC
Figure 5. Relationship of MVC values measured by dynamometry and mobile set-up: Linear regression analysis for all baseline (PRE) and three months after
(POST) isometric maximum voluntary contraction (MVC) values of the trained (intervention) and untrained (control) leg measured with the dynamometer in Newton
metres (Nm) to all PRE and POST MVC values of the trained (intervention) and untrained (control) leg measured with the mobile training set-up in kilogram (kg). Each
data point represents the mean for each leg, being calculated using the middle three of ﬁve measures; R
= coeﬃcient of determination; * indicates signiﬁcant
diﬀerence (P < .05).
Figure 6. PRE-POST comparison of vertical jump height: Mean jump height values of two diﬀerent jump manoeuvres (CMJ = countermovement jump; DJ = drop
jump) measured at baseline (PRE) and after the intervention (POST). Data are presented as mean ± standard deviation. * indicates signiﬁcant diﬀerence (P < .05).
JOURNAL OF SPORTS SCIENCES 7
repetitions. Further, this suggests that one MVC measurement
is sucient in determining the evidence-based training load of
90% MVC. We conclude that once the set-up is individually set
according to the initial and standardised instructions, it allows
reliable application over time.
The eectiveness of our home-based exercise protocol applied
with our novel mobile set-up was indicated by signicant
increases in plantar exor strength (Figure 4) from week 5
onwards (Figure 3). These positive eects were also reported
in studies, which applied the same plantar exor training pro-
tocol for 14 weeks of training at 90% MVC under controlled
conditions in a lab on a stationary dynamometer (Albracht &
Arampatzis, 2013; Arampatzis et al., 2007). As the home-based
application was not inferior to exercising under laboratory
conditions, it provides a suciently eective alternative train-
Statistically signicant PRE to POST dierences for the DJ
height by 9.6 ± 19% indicated an improvement in jump perfor-
mance as a specic measure of plantar exor function (Figure 6).
Considering that the training was unilateral, larger eects may
be expected with bilateral training. CMJ height did not change
with plantar exor training. However, as the plantar exors con-
tribute little to CMJ height with weak correlations between
plantar exor MVC and CMJ height being reported (Tsiokanos
et al., 2002), the detected increase in plantar exor strength may
not necessarily lead to measurable changes in CMJ height. In
sum, the application of high-loading exercise might have the
potential to improve jumping capacity in movements performed
with a larger contribution of the plantar exors.
The novel training set-up was highly feasible, if feasibility is
dened as the extent to which a planned intervention protocol
is fully realised by the participants as planned without imped-
ing incidents reported (Wang et al., 2006). Participants reported
no adverse eects or impeding incidents throughout the
course of intervention. Thus, the exercise protocol was imple-
mented as planned and safe.
Intervention-related eects on muscle strength and jump
performance varied considerably between individuals in our
study with strength improvements ranging from 2.9% (i.e.
moderate response) to 29% (i.e. strong response). High varia-
bility in response to exercise programmes is frequently
reported in the literature (Bohm et al., 2019; Hecksteden et al.,
2018) and may be explained by inter-individual variations
(Hubal et al., 2005) in training experience, anthropometric or
genetic characteristics (Atkinson & Batterham, 2015). Moreover,
the characteristics of the home-based protocol allow variation
in individual compliance rates and dierences in exercise
execution, which may be considered as another factor that
might aect outcome variability. Thus, when applying this
home-based protocol, practitioners need to be aware that the
training eect may vary considerably between individuals.
For the regression analysis, we included dependent variables
(i.e. PRE and POST). Since all participants contributed with the
same number of measurements, the eect of dependency
should not be severe. Despite a strong linear positive relation-
ship between MVC measures established with the dynam-
ometer and with the mobile set-up (Figure 5), the extent of
the PRE to POST plantar exor strength increase varies between
both methods. Thus, we do not consider the novel set-up to
represent a laboratory dynamometer substitute in terms of
a precise strength and performance assessment tool.
However, measures established by the mobile set-up con-
rmed the positive adaptations measured by the dynam-
ometer. Hence, our mobile set-up is capable of displaying and
monitoring individual strength gains.
As jumping performance was considered as a specic
assessment of function, transferability to any further functional
aspects of the plantar exor muscle-tendon unit is limited.
Moreover, as our participants were advised to execute the
exercises with extended knees, our results might not be equally
transferable to the application of this protocol with bended
knees. We decided for males only which has to be considered
when transferring our results to females. However, as strength
training interventions may lead to larger relative strength
improvements in females (Hubal et al., 2005), we would expect
similar or larger improvements in plantar exor strength and
function in females following our intervention protocol.
As we used the contralateral leg of each participant as
control, the control leg may have been aected by cross-
educational or systemic eects.
4.4 Implications for practice
This easily accessible mobile training set-up provided reliable
values to control the evidence-based stimulus and demon-
strated eectiveness for home-based strength training of the
plantar exors. As the joint angle–moment relationship has an
impact on strength development (Gordon et al., 1966; Moo
et al., 2020), users have to observe carefully that an adequate
tightness or tension of the belt and the standardised 90° of
ankle angle during the contraction are strictly adhered. Any
positional change of the shoe onto the foot plate alters the
length of the lever arm and thus has an impact on the peak
torque magnitude. Therefore, strict adherence must also be
applied regarding the standardised position of the shoe within
the foot plate. Advising the participants to perform the exer-
cises with an ankle angle of 90° and with extended knees
seemed advantageous for several reasons. Firstly, it might be
simple and easy to replicate especially when using the mobile
training set-up at home or in a clinical setting. Second, max-
imum plantar exor moments have been shown to be achieved
in close-by positions (i.e. 85° ankle angle) (Arampatzis et al.,
In healthy adults or athletes, this evidence-based protocol
may contribute to improve plantar exor strength (Albracht &
Arampatzis, 2013; Arampatzis et al., 2007). Its application may
reduce muscle strength imbalances between legs, which are
observed after injury even long after the rehabilitation process
is completed (Alfredson et al., 1996). Furthermore, the interven-
tion protocol may improve athletic performance as enhanced
running economy has been observed (Albracht & Arampatzis,
2013). Accompanying increases in plantar exor strength,
8G. RADOVANOVIĆ ET AL.
a training programme that utilises muscle contractions at an
intensity of ~90% of MVC to induce high tendon strains has
been shown to improve Achilles tendon stiness (Kubo et al.,
2012; Arampatzis et al., 2007, 2010; Bohm et al., 2014).
Regarding the patellar tendons, similar eects have been
described (Kubo et al., 2001; Kongsgaard et al., 2007). As our
home-based protocol similarly utilised muscle contractions at
~90% MVC, which led to signicant strength improvements of
the plantar exor muscles, positive alterations in tendon sti-
ness may be expected (Muraoka et al., 2005). Increased stiness
means less strain at a given load. As strain is a key predictor for
overuse tendon injury (Obst et al., 2018; Wren et al., 2003), this
approach of high-loading exercise may have the potential of
reducing muscle-tendon imbalances (Arampatzis et al., 2020),
preventing tendon overload injuries (Bohm et al., 2019;
Muraoka et al., 2005) and rehabilitating tendinopathic tendons
(Radovanović et al., 2019).
As minimal equipment (i.e. mobile set-up), minimal facilities
and sta (i.e. initial on-site supervision and subsequently on
demand remote monitoring) was required, our home-based
protocol was a low-cost programme and thus might meet
economical needs (Ribeiro et al., 2009).
Based on our results, we recommend this home-based set-up and
the evidence-based high-loading exercise protocol to be imple-
mented for training and treatment of the triceps surae muscle-
tendon unit. This set-up meets economical needs and everyday
use challenges. The mobile character provides additional value,
particularly if stationary gym facilities cannot be accessed or time
exibility is needed. The highly reliable character of the set-up
allows to control the adequate training stimulus. It may support
athletes aiming to enhance plantar exor performance of the
lower leg, or patients suering from muscular imbalances due
to an injury or post-surgery. Due to previously reported adaptive
eects in healthy tendons associated with this specic training
protocol, our approach may have the potential to contribute to
Achilles tendon injury prevention and rehabilitation.
We would like to thank Vanyo Tanchev for contributing to data collection,
Arno Schroll for statistical and Lars Janshen for methodological advice.
No potential conict of interest was reported by the author(s).
We acknowledge support by the Open Access Publication Fund of
Humboldt-Universität zu Berlin.
Goran Radovanović http://orcid.org/0000-0002-2117-8233
Sebastian Bohm http://orcid.org/0000-0002-5720-3672
Kirsten Legerlotz http://orcid.org/0000-0001-6077-5711
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