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Posttetanic Potentiation
Definition
An increase in postsynaptic response to presynaptic
release of neurotransmitter following a single stimulus
that is applied at various times after a train of stimuli.
The cause is thought to be increased release of
neurotransmitter from the presynaptic terminal.
Force Potentiation in Skeletal Muscle
Neuromuscular Junction
Posttraumatic Pain
Incisional/Postoperative Pain
Posttraumatic Stress Disorder
Definition
A disorder that develops as a consequence of exposure
to highly traumatic experiences, characterized by
inappropriate fear responses to stimuli associated with
those experiences.
Learning and Extinction
Stress
Postural Control
FAY B. HORAK
Neurological Sciences Institute, Oregon Health and
Science University, Portland, OR, USA
Introduction
To inhabit the world, in all of its unpredictable, variable
environments and situations, requires a powerful, yet
flexible, system of postural control. For example, the
ability to move from sitting to standing; to take a step; to
respond to a slip or trip; to predict and avoid obstacles;
to carry a glass of wine without spilling it, even when
walking across a rolling boat; and to orient your body to
a speeding soccer ball, all require excellent postural
control. Although neural control of postural orientation
and equilibrium involves most of the nervous system
and all body segments, the postural system is often
forgotten because it usually operates at an automatic,
non-voluntary level. Only after an injury to the nervous
system or musculo-skeletal system, when we have to
really think aboutour balance and postural alignment
or battle dizziness and spatial disorientation, do we
begin to appreciate the complex systems involved in
postural control.
Biomechanical Goals of Postural Control
Postural control involves neural control of postural
equilibrium and postural orientation [1]. Postural
equilibrium involves coordination of sensory and motor
strategies to maintain balance, that is, to stabilize the
bodyscenter of mass over its base of support. An
important goal of postural equilibrium control is to
prevent falls during both self-initiated and externally-
triggered disturbances of stability. The postural equilib-
rium system controls stability during stance posture as
well as during locomotion and performance of volun-
tary tasks. Postural orientation involves the positioning
of body alignment with respect to gravity, the support
surface, visual environment and other sensory refer-
ence frames. The goals of postural equilibrium and
postural orientation are independently controlled and
sometimes subjects give up one goal for another. For
example, an athlete may give up the goal of postural
equilibrium in order to achieve their goal to orient their
body appropriately to a ball.
Stance Posture
Although the musculoskeletal system affords some
passive stability, humans and most animals require
active postural muscle activation to maintain stance
posture against gravity and to orient their body seg-
ments appropriately to their environment. To oppose the
destabilizing effects of gravity, standing humnas are
continuously making small correction to upright body
position, called postural sway.Postural muscle tone
provides antigravity support and flexibly adjusts to
changes in support, alignment, and environmental
conditions [2]. Besides postural tone, control of
postural sway requires integration of sensory informa-
tion to detect body motion with respect to the
environment and the activation of muscles to maintain
equilibrium and alignment of segments. Postural sway
during stance can be measured with stabilometry;
quantification of forces under the feet as continuous
displacement of the center of pressure [3]. Displace-
ment of the center of pressure represents the combina-
tion of motion of the center of body mass as well as the
3212 Posttetanic Potentiation
ground reaction forces used to control the body
center of mass over the base of foot support.
Several different types of control theories have
been used to describe how the nervous system main-
tains consistent reference values for posture. Because
posture is so adaptable and flexible, depending on the
situation, models of human posture control include
optimal control and adaptive control, such as
Kalman filers, of more than one variable, such as
position of center of body mass, orientation of the trunk
and head in space, energy efficiency, etc. Postural sway
during human stance is often modeled as an inverted
pendulum biomechanical system in which the center of
mass of the body is situated at the upper end of a rigid
link that pivots about a joint at the base (i.e., the ankle),
although actual body sway includes control of multiple
segments.
Automatic Postural Responses
Automatic postural responses counteract unexpected
disturbances to equilibrium. In humans, postural re-
sponses are triggered at 100 ms in response to exter-
nal perturbations. This latency of automatic postural
responses is faster than the fastest voluntary postural
reactions but slower than the fastest stretch reflexes.
Stretch reflexes are triggered by muscle spindles
and result in activation of the stretched muscles but
these reflexes contribute little functional torque to
correct postural equilibrium. Automatic postural re-
sponses include responses in muscles that are short-
ened, as well as stretched, as well as muscles far from
the site of perturbation that can exert torque against
surfaces to correct posture [4]. The recruitment of
muscles in a postural response depends on the goal of
maintaining equilibrium and not on stereotyped reflexes.
Automatic postural responses depend on central set
so that they are specific to the conditions of support and
adapt to prior experience. Central set is the readiness of
the central nervous system for an upcoming event based
on initial conditions, prior experience and expectations.
For example, leg muscles are activated in response to
surface perturbations during free stance but arm
muscles are activated and leg muscles suppressed in
response to surface perturbations when holding onto a
stable support [5]. In addition, muscles on the back of
the legs are activated in response to forward body sway
while standing but muscles on the front of the legs and
in the arms are active when supported on the hands and
feet [6]. Postural responses change even in the first trial
after a change in body configuration but continue to
adapt with repeated trials to continue to optimize
the response for the particular conditions. For example,
a gradual adaptation of the postural response can be
observed during repeated trials of surface rotation. In
response to the first rotation, a destabilizing response
may be seen in the stretched ankle extensor muscle but
with repeated rotations this activation of the extensor is
suppressed and activation of the stabilizing ankle flexor
gradually increases.
Subjects can also influence which postural response
is selected and the magnitude of their response based on
experience, expectations, and intention [7]. For exam-
ple, the stretched ankle extensor muscle responses are
inhibited and the shortened tibialis muscles are trig-
gered when subjects are instructed to step in response to
a forward body perturbation [8]. Poor coordination of
automatic postural responses can result in failure to
return to equilibrium in response to external perturba-
tions. Automatic postural responses can be defined by
their postural strategies and postural synergies.
Postural Strategies and Postural Synergies
Postural strategies can be defined by their functional
goals and described based either on body kinematics
(relationship of body segmental motion) or body
kinetics (relationship of body segmental forces). Two
main types of postural movement strategies can be
used to return the human body to equilibrium when
perturbed while standing: strategies that return the
center of mass back over the base of foot support and
strategies that change the base of support under the
falling center of mass by stepping or reaching. The
fixed-support strategies, that return the body center of
mass over the base of foot support, form a continuum
from the ankle strategy to the hip strategy. The ankle
strategy, in which the body moves as a flexible inverted
pendulum, is appropriate for small amounts of sway
when standing on a firm surface [9]. The hip strategy,
in which the body exerts torque at the hips to quickly
move the body center of mass, is used when standing on
surfaces not allowing adequate ankle torque or when the
body center of mass must be moved more quickly such
as for a faster, larger disturbance [9]. When subjects
suddenly change from standing on a wide to a narrow
surface, or vice versa, there is a gradual adaptation from
an ankle to a hip strategy and vice versa with repeated
perturbations. This gradual change in postural strategies
suggest that they not only depend upon sensory
feedback and current sensory conditions but also upon
prior conditions based on central set.
Change-in-support strategies of stepping and/or
reaching to recover equilibrium in response to perturba-
tions are also common, especially during gait and when
it is not important to keep the feet in place [10].
However, even when subjects step in response to an
external perturbation, they first attempt to return the
body center of mass to the initial position by exerting
angle torque. If a railing or other stable surface is
available, subjects forced to extend their base of support
by external displacement will also use a reach-to-
grasp strategy [11]. Reaching reactions are initiated
even faster than stepping reactions. Change-in-support
Postural Control 3213
P
strategies are often used even under conditions in which
it is biomechanically possible for subjects to return
to equilibrium using a fixed-support strategy. Figure 1
illustrates fixed-support and change-in-support strategies
to correct forward and lateral postural displacements.
Postural synergies are groups of muscles activated
together by the nervous system to maintain equilibrium
[4]. By eliminating the need to control each mus-
cle independently, postural synergies are thought to
simplify the neural control task of selecting and coor-
dinating multiple muscles across the body. Postural
synergies define the muscle activation patterns that are
used by the nervous system to implement various
postural strategies. For example, Fig. 2 shows several
muscles activated in the ankle, hip and mixed ankle-hip
postural muscle synergies in response to forward sway
perturbations.
Anticipatory Postural Adjustments
Voluntary movements are accompanied by anticipatory
postural adjustments that act to counter, in a predictive
manner, postural destabilization associated with a forth-
coming movement [12]. Anticipatory postural adjust-
ments are activated as feedforward postural control,
Postural Control. Figure 1 Shows examples of feet-in-place and stepping strategies to correct forward and lateral
postural displacements. In response to small CoM displacements, humans use a strategy that maintains upright trunk
orientation. In response to more forceful displacements, humans add rapid trunk and hip movements to move the
CoM over the base of foot support. Stepping and reaching strategies can also be used to recover equilibrium by
moving the base of support under the falling CoM. Lateral stepping includes both a cross-over strategy, as shown, and
a step by the loaded leg to widen the stance width.
3214 Postural Control
prior to any sensory feedback indicating postural
instability. For example, prior to taking a step, antici-
patory postural adjustments move the body forward and
onto the stance leg prior to lifting the stepping leg. In
addition, when a standing subject rapidly moves their
arms, leg and trunk muscles are activated more than 50 ms
in advance of the prime mover arm muscles [13].
Anticipatory postural adjustments are specific to the
biomechanical requirements of each specific movement
and adapt when the biomechanical requirements change.
For example, anticipatory postural adjustments in the legs
associated with arm movements are reduced or disappear
when subjects are supported at the trunk and no longer
need the anticipatory postural muscle activity in the legs
for stability [5]. These studies suggest that there is a
preselection of an anticipatory postural muscle synergy
associated with every voluntary movement requiring
postural stability. This pre-selection or preparation of the
sensorimotor nervous system in advance of movement
has been called central set [14].
During locomotion, both anticipatory postural adjust-
ments, via feedforward control, and automatic postural
responses, via feedback control, contribute to postural
stability. Unperturbed walking or running in healthy
individuals consists of placing the feet under a falling
center of body mass so the nervous system must anti-
cipate where the feet need to be to maintain equilibrium
during walking [15]. During bipedal locomotion, the
trunk segment and thus, the body center of mass,
is inherently unstable in the lateral direction and thus
requires frequent corrections of lateral trunk orientation
and/or lateral foot placement. When an individual slips or
trips or makes voluntary movements while walking or
running, the same automatic postural strategies observed
during stance (See Postural Strategies) are added to
the locomotor pattern [16]. Somatosensory feedback
is also used to modify joint stiffness and quick respon-
ses to accommodate unanticipated changes in surface
configuration.
Sensory Integration
Sensory information from the somatosensory, visual
and vestibular systems must be integrated in order
to interpret complex sensory environments because
Postural Control. Figure 2 Plots the change in ankle and hip angles using the ankle and hip strategies and the
continuum of mixed ankle-hip strategies used to return the body to upright stance equilibrium after a forward sway
external perturbation.
Postural Control 3215
P
sensory information from a single sensory channel can
be ambiguous and misleading. Postural control depends
on the central neural interpretation of convergent
sensory information from somatosensory, vestibular,
visual systems. Thus, the nervous system controls
posture via estimates of position and motion of the body
and the environment by combining sensory inputs
from several modalities. In addition, kinematic and
kinetic body information must be integrated for
control of posture. Sensory systems that signal kinemat-
ic position and motion of the body provide negative
feedback control to minimize postural motion whereas
sensory systems that signal kinetic force input provide
positive feedback control to maximize joint torque
when tilting [17]. Interpretation of sensory information
by integrating sensory information across modalities is
also thought to involve internal models of the bodys
sensory and motor dynamics, also called the body
schema, as well as internal models of the environment.
These internal models are based on expected sensory
inputs from prior experience and provide the basis for
central set. Errors between expected and actual sensory
information is thought to be the basis for disorientation,
dizziness, and motion sickness in both pathology and
challenging environments.
Somatosensory inputs for posture include pressure
information from skin in contact with surfaces, limb
segment orientation from muscle proprioceptors and
joint receptors, as well as muscle length, velocity and
force information. Somatosensory inputs from many
different types of peripheral sensory receptors converge
onto neurons in the spinal cord to encode intersegmen-
tal and limb orientation in space [18]. Somatosensory
inputs are important for triggering the earliest automatic
postural responses in response to external perturbations.
Thus, people with neuropathies that slow conduction of
somatosensory inputs such as from diabetes or multiple
sclerosis have longer than normal latencies of automatic
postural responses. Somatosensory inputs are also
important for providing information about the direction
of perturbation and about the texture and stability of the
support surface so that appropriate postural strategies
can be selected. Somatosensory inputs can provide
confusing, ambiguous information about body center of
mass motion because they cannot distinguish between
body motion over a stable surface and surface motion
under a stable body, such as when standing on a moving
boat or pier.
Vestibular inputs for posture are important for
orientation of the trunk and head to gravity, especially
when the surface is unstable. The vestibular system
consists of two types of structures located in the inner
ear, the labyrinths that encode head rotational
acceleration and the otoliths that encode head linear
acceleration, including gravity. The labyrinth consists
of three, fluid-filled, semicircular canals that each sense
a different direction of head rotation via motion of hair
cells imbedded in the cristae, the sensory tissue.
Within the otoliths, the utricle senses horizontal linear
acceleration such as during walking and the saccule
senses vertical acceleration such as during falling.
Vestibulospinal inputs are particularly important for
controlling orientation of the head and trunk in space
but are not necessary to trigger automatic postural
responses to external perturbations [19]. Vestibular
inputs can provide confusing, ambiguous information
about body center of mass motion because they cannot
distinguish, on their own, between head motion over
a stable body and head motion accompanying body
center of mass motion. Vestibular information is thought
to help the somatosensory system distinguish a stable
from an unstable surface and then become increasingly
important for controlling postural orientation the more
unstable is the surface (see sensory re-weighting,
below). Thus, patients who have lost all vestibular
function can still stand and walk and show normal
latencies of automatic postural responses to a slip or
trip although they will orient to moving surfaces and
become unstable when vision is not available [20].
Vestibular inputs must be interpreted via somatosen-
sory inputs for the nervous system to control posture.
For example, galvanic vestibular stimulation from
direct current behind the ears can activate or inhibit
the vestibular nerve and result in vestibulospinal
responses. Vestibulospinal responses consist of medium
latency activation of a group of muscles that tilt the
body toward the side of the inhibited vestibular nerve
when standing. The direction of body tilt depends on the
direction the head is facing with respect to the base of
foot support [21]. The muscles activated depend on
which muscles can exert forces against the surface such
that leg muscles are activated in free stance but arm
muscles are activated with holding onto a stable surface
[22]. Vestibular control of head orientation in space also
depends on the close interaction between the vestibular
and somatosensory systems via the vestibulocollic
and cervico-collic reflexes.
Visual information provides knowledge of body
sway and orientation in the environment and provides
advanced information about potentially destabilizing
situations. Vision can provide information about the
direction and speed of body sway.For example, forward
body sway is signaled by the visual system as backward
visual flow across the peripheral retina and looming
across the central retina. Visual information also allows
perception and body orientation with respect to the
vertical and horizontal visual environment (see percep-
tion of visual vertical, below). Thus, standing subjects
exposed to slowly moving visual surrounds will sway
with reference to the visual motion, even when unaware
of it. Visual inputs can provide confusing, ambiguous
information about body center of mass motion because
3216 Postural Control
they, alone, cannot distinguish between body motion
with respect to a stable visual surround and visual
surround motion with respect to a stable body. For
example, when stationary subjects view large moving
scenes, especially in their peripheral vision, they often
momentarily perceive self-motion in the opposite
direction. When actually moving the body through
space, vision also provides advanced, or feedforward,
information to position body parts to avoid obstacles,
navigate complex terrain, and plan motor strategies. For
example, subjects tend to view obstacles in order to plan
foot placement and clearance about 3 steps before they
reach the obstacles [23].
The ability to orient the body with respect to gravity,
the support surface, visual surround and internal
references and to automatically alter how the body is
oriented in space, depending on the context and task
requirements is an important attribute of postural control.
For example, a subject may automatically orient their
body perpendicular to the support surface unless the
support surface becomes unstable, when they will orient
themselves to gravity or to their visual surround.
Sensory re-weighting is an important mechanism
for changing the relative contributions made by
different sensory systems for postural control. Figure 3
shows a model of sensory integration for postural
control in which somatosensory, vestibular and visual
inputs can change weighting depending on changes in
the environment. Using this model, studies have shown
that in a well-lit environment with a firm base of
support, healthy subjects refy on somatosensory 70%,
vision 10% and vestibular 20% [20]. However, when
healthy subjects stand on an unstable surface, they
increase sensory weighting to vestibular and vision as
they decrease dependence on surface somatosensory
inputs for postural orientation [24]. Ability to reweight
sensory information depending on the sensory context
is important for maintaining stability when moving
from one sensory context to another, such as from a
moving boat to firm ground. Individuals with loss of
somatosensory, vestibular or visual input from patholo-
gy are limited in their ability to reweight postural
sensory dependence and thus, are at risk of falls in
particular sensory contexts. In addition, some central
nervous system disorders may impair the ability to
quickly reweight sensory dependence, even when the
peripheral sensory systems are intact. Subjects can use
sensory substitution to replace one sensory modality
for another to help control posture. For example, light
touch on a cane can be used to substitute haptic sensory
cues for missing vestibular or somatosensory inputs
dues to pathology and thereby reduce postural sway in
stance [25,26]. Biofeedback systems that provide
visual, auditory or somatosensory inputs to the nervous
system correlated with body sway have also been
shown to provide effective sensory substitution to
improve postural stability in patients with loss of
sensory information.
Healthy individuals also have a conscious perception
of vertical spatial orientation. Perception of verticali-
ty, or upright, may have multiple neural representations
[27]. In fact, perception of visual vertical, or ability to
align a line in the dark with gravity, is independent of
perception of postural (or proprioceptive) vertical, or
ability to align the body in space without vision [28].
For example, the internal representation of visual, but
not postural, vertical is tilted in subjects with unilateral
vestibular loss, whereas the internal representation of
Postural Control. Figure 3
Postural Control 3217
P
postural, but not visual vertical is tilted in some subjects
with stroke. A tilted or inaccurate internal representa-
tion of vertical will result in automatic postural
alignment that is not aligned with gravity.
Neuroanatomy of Posture Control and Clinical
Implications
Control of posture is distributed in the nervous system
and the musculoskeletal system such that pathology
almost anywhere in the nervous system or musculoskel-
etal system can impair postural equilibrium and/or
postural orientation. The spinal cord is sufficient for
maintaining antigravity support and locomotor patterns
but not for maintaining balance [29]. Sensory pathways
in the spinal cord carry somatosensory information about
limb orientation as well as motor pathways such as
the medially located vestibulospinal and reticulospinal
pathways for activating postural muscle synergies. In
the brainstem, the vestibular nuclei are important for
integrating sensory information across modalities for
postural orientation and the reticular formation is likely
involved in organizing postural synergies. The important
role of the cerebellum in posture can be seen by the
severe problems with postural stability and postural
orientation in patients with damage to the cerebellum.
Damage to the spinocerebellum, specifically, impairs
postural stability by causing larger than normal automatic
and anticipatory postural adjustments and by impairing
the ability to optimize postural strategies based on prior
experience [30]. In contrast, damage to the vestibulocer-
ebellum results in difficulty using vestibular or visual
information to orient the body with reference to gravity or
visual references. The basal ganglias importance to
postural control can be seen by the frequent falls in
patients with pathology involving the basal ganglia, such
as Parkinsons disease. The basal ganglia is important for
quickly changing postural strategies when conditions
change, for regulating posturalmuscle tone, forgenerating
forceful anticipatory and reactive postural responses and
for perception of postural orientation [31]. The cerebral
cortex is involved in postural control in as many complex
ways as voluntary movement [32]. The cortex is involved
in changing postural responses with alterations in cognitive
state, initial sensory-motor conditions, prior experience,
and prior warning of a perturbation, all representing
changes in central set. In addition, the supplementary
motorcortex is involved in generatinganticipatorypostural
adjustments and the primary motor cortex participates in
longer latency postural responsestoperturbations.Parietal
and temporal association cortical areas are involved
in perception of spatial orientation and in formulating
the internal models of the body and the environment
so important to postural control. Thus, damage to almost
any part of the cortex from a cerebral vascular accident
can impact postural stability or orientation.
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Postural Equilibrium
Definition
A state in which the body is either at rest, moving at
constant velocity or executing a repeatable (periodic)
pattern of motion. A stable system is one that returns to
a state of equilibrium after it has been perturbed.
Postural Strategies
Postural Instability
Definition
Impairment of balance when standing, walking, or
turning.
Postural Muscle Tone
VICTOR S. GURFINKEL
Neurological Sciences Institute, Oregon Health &
Science University, Portland, OR, USA
Definition
Postural tone is the steady contraction of muscles that
are necessary to hold different parts of the skeleton in
proper relation to the various and constantly changing
attitudes and postures of the body.
Description of the Theory
Decerebrate Posture
Because postural muscle tone is completely suppressed
by narcosis, postural tone has mainly been studied using
an experimental model called the decerebrate animal
[1]. In the decerebration of mammals, the cerebral
cortex and thalamus are surgically inactivated by inter-
collicular cross-section of the brain stem under general
anesthesia. Once the effects of the anesthesia have
dissipated, the condition known as decerebrate rigidity
can be seen. This condition is characterized by a strong
extended neck, trunk, tail and limbs, which resist
attempts to flex them. In decerebrate rigidity, there is
no sensation of pain. Because of this rigidity, when the
decerebrate cat is placed on its four limbs, tension in
the limb muscles is enough to maintain its body posture.
This muscle tone has been named postural tone[1].
In decerebrate animals, the neuronal structures of the
brain stem and spinal cord are in an active condition.
Therefore, this model is useful for studying many
questions of neurophysiology. For example, on a
background of high muscle tone, it is possible to
study not only influences of excitation, leading
to the enhancement of muscle tone, but also to
study inhibition, which results in the suppression of
muscle tone.
In the past, many researchers have been devoted to
studying the nature of decerebrate rigidity. It was
found that deafferentation (i.e., sectioning appropriate
dorsal roots) abolishes decerebrate rigidity of limb
muscles [1]. In addition, it was found that the tonus of
the extensor muscles is autogenous, in that each muscle
is dependent on afferent nerve fibers from the muscle
itself (myotatic componentin decerebrate rigidity).
These findings were reproduced many times [2]. This
showed that the origin of decerebrate rigidity cannot
be completely explained by the myotatic component.
The actual situation is more complex. In studies of
decerebrate cats, it was shown that in addition to pro-
prioception, there are other sources of postural tonic
activity that are connected to the position of the head
Postural Muscle Tone 3219
P
... Maintaining a posture requires controlling the center of mass (COM) through active postural muscle activation and integration of sensory feedback while maintaining a fixed base of support (BOS). 8 Unlike static posture, transitional movements involve voluntary movement of COM and changes in BOS through incorporation of preplanning and anticipatory control based on central set 8 and sensory feedback for corrections. 8,9 In contrast to static or transitional movements, RPRs involve involuntary, externally driven motion of the COM or BOS that is corrected using rapid, time-constrained, automatic postural responses 7,8 to quickly stop the initial falling motion by moving the COM or changing the BOS 10,11 and stabilizing the body through subsequent postural adjustments. ...
... Maintaining a posture requires controlling the center of mass (COM) through active postural muscle activation and integration of sensory feedback while maintaining a fixed base of support (BOS). 8 Unlike static posture, transitional movements involve voluntary movement of COM and changes in BOS through incorporation of preplanning and anticipatory control based on central set 8 and sensory feedback for corrections. 8,9 In contrast to static or transitional movements, RPRs involve involuntary, externally driven motion of the COM or BOS that is corrected using rapid, time-constrained, automatic postural responses 7,8 to quickly stop the initial falling motion by moving the COM or changing the BOS 10,11 and stabilizing the body through subsequent postural adjustments. ...
... 8 Unlike static posture, transitional movements involve voluntary movement of COM and changes in BOS through incorporation of preplanning and anticipatory control based on central set 8 and sensory feedback for corrections. 8,9 In contrast to static or transitional movements, RPRs involve involuntary, externally driven motion of the COM or BOS that is corrected using rapid, time-constrained, automatic postural responses 7,8 to quickly stop the initial falling motion by moving the COM or changing the BOS 10,11 and stabilizing the body through subsequent postural adjustments. 8 As RPRs occur faster than voluntary movement (70-100 milliseconds vs 180-250 milliseconds), 8,12 there is little time for preplanning or feedback-driven control. ...
Article
Objective: Balance testing after concussion or mild traumatic brain injury (mTBI) can be useful in determining acute and chronic neuromuscular deficits that are unapparent from symptom scores or cognitive testing alone. Current assessments of balance do not comprehensively evaluate all 3 classes of balance: maintaining a posture; voluntary movement; and reactive postural response. Despite the utility of reactive postural responses in predicting fall risk in other balance-impaired populations, the effect of mTBI on reactive postural responses remains unclear. This review sought to (1) examine the extent and range of available research on reactive postural responses in people post-mTBI and (2) determine whether reactive postural responses (balance recovery) are affected by mTBI. Design: Scoping review. Methods: Studies were identified using MEDLINE, EMBASE, CINAHL, Cochrane Library, Dissertations and Theses Global, PsycINFO, SportDiscus, and Web of Science. Inclusion criteria were injury classified as mTBI with no confounding central or peripheral nervous system dysfunction beyond those stemming from the mTBI, quantitative measure of reactive postural response, and a discrete, externally driven perturbation was used to test reactive postural response. Results: A total of 4747 publications were identified, and a total of 3 studies (5 publications) were included in the review. Conclusion: The limited number of studies available on this topic highlights the lack of investigation on reactive postural responses after mTBI. This review provides a new direction for balance assessments after mTBI and recommends incorporating all 3 classes of postural control in future research.
... Una de las áreas dedicadas a la búsqueda de estos patrones es la biomecánica. Sus esfuerzos se enfocan fundamentalmente en el establecimiento de patrones de movimiento característicos de personas sanas y en la comparación de estas con las variaciones generadas por trastornos patológicos que comprometen el sistema osteo-muscular [1,2,3] . En ese sentido, es posible evaluar de forma más objetiva el estado y evolución de las patologías que afectan el sistema neuro-motor, siendo la alteración del equilibrio uno de los signos más comunes en la actualidad [4] , evaluado tradicionalmente mediante un estudio de estabilometría [5,6,7] . ...
... La estabilometría es el estudio del equilibrio que permite analizar el control postural y su relación con la estabilidad en una posición bípeda [8] . Este control involucra la integración de la información sensorial de la periferia corporal (sistemas visual, vestibular y propioceptivo) [9,10] , en particular de los mecano-receptores de las plantas de los pies [11,12] y los receptores que informan sobre la posición corporal y orientación [3] . ...
Article
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RESUMEN La inestabilidad de la postura y alteración del balance corporal son signos comúnmente observados en sujetos con afecciones que comprometen el sistema músculo-esquelético, visual, vestibular y propioceptivo, siendo la estabi-lometría la principal herramienta de valoración clínica. Sin embargo, no se ha podido establecer un gold standard o patrones cuantitativos que permitan clasificar sujetos sanos y patológicos basados en el grado de alteración del equilibrio. Objetivo: El objetivo principal de este estudio fue realizar un análisis estabilométrico preliminar con 38 sujetos sanos (19 mujeres y 19 hombres), con el fin de identificar un patrón de desplazamiento característico de la postura. Metodología: Este estudio se basó en la aplicación del test de Romberg. Se realizaron tres mediciones en cada paciente con ojos abiertos (OA) y ojos cerrados (OC), ubicados sobre una plataforma de fuerza en posición vertical. Resultado: se identificó un patrón característico de desplazamiento en los ejes medio-lateral X y ante-ro-posterior Y, así como una estabilidad notable sobre el eje vertical Z, alrededor del centro de presión. Limitaciones: Sin embargo, debido al tamaño de la muestra, no se encontraron resultados concluyentes sobre las diferencias de índice de masa corporal, sexo o edad en el grupo de estudio. Originalidad: A pesar de ello, se encontraron paráme-tros prometedores para la evaluación de la estabilometría en personas jóvenes sanas, fortalenciendo con ellos las herramientas objetivas de valoración clínica. Conclusión: Esta investigación permitió identificar características de movimiento comunes para pacientes normales, que pueden ser consideradas como un patrón objetivo para el seguimiento y evaluación de tratamientos en pacientes con trastornos del equilibrio de origen patológico.
... The active alignment of the trunk and head concerning gravity, support surfaces, the visual surround, and internal references is referred to as postural orientation. Also, postural stability refers to the coordination of movement strategies used to stabilize the center of body mass during both self-initiated and externally induced disturbances of stability (6,7). Copyright According to Dewar et al., research should focus on establishing links between postural control impairments, treatment options, and outcome measures (4). ...
Article
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Background: This study aimed to assess the test-retest intra-rater reliability and convergent validity of digital photography (DP) in detecting the postural orientation of children with cerebral palsy (CP). Methods: The study recruited children with various types of CP with the Gross Motor Function Classification System level I or II and spasticity < 2 on the Ashworth Scale, without any visual or cognitive impairments. Children who had undergone any surgical intervention or received a botulinum toxin injection within the previous six months were excluded. A digital camera was fixed at 1.5 meters from the participants at the height of 90 cm. Non-reflective markers were attached to eight anatomical landmarks to localize the upper and lower center of mass on both sides. The same examiner took three digital photos to detect intra-rater reliability using the intraclass correlation coefficient (ICC). Pearson's correlation and linear regression analysis were used to assess the convergent validity of the DP method compared with the Pediatric Balance Scale (PBS) scores. Results: Thirty children (7.44 ± 2.38 years) were assessed to test the reliability of DP, and 55 others (8.06 ± 2.19 years) participated in the convergent validity study. Intra-rater reliability was found to be perfect (ICC > 0.995) and there was a strong significant negative correlation between DP measures and PBS scores (Pearson's correlation > 0.75) with high adjusted R2 (R2 > 0.567), indicating goodness of fit between the measures. Conclusions: Digital photography (DP) is a reliable and valid method for assessing postural orientation in children with various types of CP.
... The postural equilibrium controls stability during both static (i.e., quiet standing) and dynamic (i.e., walking and reaching) situations. Postural orientation involves positioning body alignment with respect to gravity, the support surface, visual environment, and other sensory reference frames [22]. ...
Chapter
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Proprioception is the sense of position or the motion of the limbs and body in the absence of vision. It is a complex system having both conscious and unconscious components involving peripheral and central pathways. The complexity of sensorimotor systems requires deep knowledge of anatomy and physiology to analyze and localize the symptoms and the signs of the patients. Joint sense and vibration sense examination is an important component of physical examination. This chapter consists anatomy, motor control, postural control related to proprioception with neurologic clinical correlation and also the information about the changes of proprioception after orthopedic surgeries and discuss with the available literature.
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Perception of the spatial vertical is important for maintaining and stabilizing vertical posture during body motion. The velocity storage pathway of vestibulo-ocular reflex (VOR), which integrates vestibular, optokinetic, and proprioception in the vestibular nuclei vestibular-only (VO) neurons, has spatio-temporal properties that are defined by eigenvalues and eigenvectors of its system matrix. The yaw, pitch and roll eigenvectors are normally aligned with the spatial vertical and corresponding head axes. Misalignment of the roll eigenvector with the head axes was hypothesized to be an important contributor to the oscillating vertigo during MdDS. Based on this, a treatment protocol was developed using simultaneous horizontal opto-kinetic stimulation and head roll (OKS-VOR). This protocol was not effective in alleviating the MdDS pulling sensations. A model was developed, which shows how maladaptation of the yaw eigenvector relative to the head yaw, either forward, back, or side down, could be responsible for the pulling sensation that subjects experience. The model predicted the sometimes counter-intuitive OKS directions that would be most effective in re-adapting the yaw eigenvector to alleviate the pulling sensation in MdDS. Model predictions were consistent with the treatment of 50 patients with a gravitational pulling sensation as the dominant feature. Overall, pulling symptoms in 72% of patients were immediately alleviated after the treatment and lasted for 3 years after the treatment in 58% of patients. The treatment also alleviated the pulling sensation in patients where pulling was not the dominant feature. Thus, the OKS method has a long-lasting effect comparable to that of OKS-VOR readaptation. The study elucidates how the spatio-temporal organization of velocity storage stabilizes upright posture and how maladaptation of the yaw eigenvector generates MdDS pulling sensations. Thus, this study introduces a new way to treat gravitational pull which could be used alone or in combination with previously proposed VOR readaptation techniques.
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Proprioception considered as the obtaining of information about one’s own action does not necessarily depend on proprioceptors. At the knee joint, perceptual systems are active sets of organs designed to reach equilibrium through synergies. Many surgical procedures, such as ACL reconstruction in personalized medicine, are often based on native anatomy, which may not accurately reflect the proprioception between native musculoskeletal tissues and biomechanical artifacts. Taking an affordance-based approach to this type of “design” brings valuable new insights to bear in advancing the area of “evidence-based medicine (EBM).” EBM has become incorporated into many health care disciplines, including occupational therapy, physiotherapy, nursing, dentistry, and complementary medicine, among many others. The design process can be viewed in terms of action possibilities provided by the (biological) environment. In anterior crucial ligament (ACL) reconstruction, the design goal is to avoid ligament impingement while optimizing the placement of the tibial tunnel. Although in the current rationale for tibial tunnel placement, roof impingement is minimized to avoid a negative affordance, we show that tibial tunnel placement can rather aim to constrain the target bounds with respect to a positive affordance. We describe the steps for identifying the measurable invariants in the knee proprioception system and provide a mathematical framework for the outcome measure within the knee.
Article
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This study explored the fatigue effect on postural control (PC) across menstrual cycle phases (MCPs) in female athletes. Isometric maximal voluntary contraction (IMVC), the center of pressure sway area (CoParea), CoP length in the medio-lateral (CoPLX) and antero-posterior (CoPLY) directions, and Y-balance test (YBT) were assessed before and after a fatiguing exercise during the follicular phase (FP), mid-luteal phase (LP), and premenstrual phase (PMP). Baseline normalized reach distances (NRDs) for the YBT were lower (p = 0.00) in the PMP compared to others MCPs, but the IMVC, CoParea, CoPLX, and CoPLY remained unchanged. After exercise, the IMVC and the NRD decrease was higher at PMP compared to FP (p = 0.00) and LP (p = 0.00). The CoParea, CoPLX, and CoPLY increase was higher in the PMP compared to FP (p = 0.00) and LP (p = 0.00). It was concluded that there is an accentuated PC impairment after exercise observed at PMP.
Article
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Multiple theories regarding motor learning and postural control development aim to explain how the central nervous system (CNS) acquires, adjusts, and learns postural behaviors. However, few theories of postural motor development and learning propose possible neurophysiologic correlates to support their assumptions. Evidence from behavioral and computational models support the cerebellum’s role in supervising motor learning through the production of forward internal models, corrected by sensory prediction errors. Optimal Feedback Control Theory (OFCT) states that the CNS learns new behaviors by minimizing the cost of multi-joint movements that attain a task goal. By synthesizing principles of the OFCT, postural sway characteristics, and cerebellar anatomy and its internal models, we propose an integrated learning model in which cerebellar supervision of postural control is governed by movement cost functions.
Article
"Subjective evaluations of balance performance, like the modified Balance Error Scoring System (mBESS), are highly popular. Alternatively, quantitative measures may offer additional clarity in identifying balance dysfunction. A novel measure to define balance impairments is time to boundary (TTB), which represents the amount of time available to make corrective postural adjustments prior to the centre of pressure (CoP) reaching the edge of the base of support. The purpose of this investigation was to assess TTB and traditional measures of CoP displacement of young adults performing the mBESS on a BTrackS balance plate. Path length and TTB were calculated in anterior-posterior (AP) and medio-lateral (ML) directions, respectively. AP and ML path lengths were largest in Single stance (109.2 & 118.1 cm, respectively) and smallest in Dual stance (27.1 & 36.4 cm, respectively). The average AP and ML TTBs were higher in Dual (10.67 & 7.27 s, respectively) compared to Single (3.54 & 1.20 s, respectively) or Tandem (10.11 & 1.94 s, respectively) stances, and lower in Single stance compared to Tandem. Given the effect sizes for TTB were greater than those of path length in both directions, TTB more adequately differentiates these stance conditions than path length or subjective scores."
Article
This study aims to estimate the control law employed by the CNS (Central Nervous System) to keep a person in balance after a sudden disturbance. For this aim, several experiments were carried out, in which the subjects were perturbed sagittally by using a single-axis tilt-platform and their motions were recorded with appropriate sensors. The analysis of the experimental results leads to the conjecture that the CNS commands the muscular actuators of the human joints according to a PD control law but it updates the control gains and the set points continuously. This conjecture is accompanied with a major assumption that the CNS is able to acquire perfect and instantaneous position and velocity feedback by means of an appropriate fusion of the signals coming from the proprioceptive, somatosensory, vestibular, and visual sensory systems. In order to verify the conjectured control law, an approximate biomechanical model was developed and several simulations were carried out to imitate the experimentally observed motions. The time variations of the set points and the control gains were estimated out of the experimental data. The simulated motions were observed to be considerably close to the experimental motions. Thus, it is concluded that the CNS indeed uses an adaptive PD control law as conjectured here. However, the experiments also indicate that the mentioned adaptation scheme is quite variable even for the same subject tested repeatedly with the same perturbation. In other words, the decisions of the CNS on the variations of the control parameters are hardly predictable.
Article
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Balance and posture of the body is essential to most human locomotion. Because humans are bipeds with about 2/3 of their mass located 2/3 of body height from the ground the control system is critical. In the elderly balance control degenerates. Falls represent a major health problem and the fear of falls is the major deterrent to daily mobility. Many measures have evolved to assess balance, varying from crude balance tasks to sophisticated perturbations. This paper summarizes the balance control task as it relates to standing and walking and details current assessment techniques and equipment. Additional information is provided by the authors to demonstrate from an electromyographical and biomechanical perspective the mechanisms and characteristics of the postural control system in both standing and walking.
Conference Paper
We present the outline of a dual kinetic-kinematic postural control model. It is based on concepts of inter-sensory interaction (sensor fusion) which we consider instrumental for sensorimotor integration. Separation into kinetic and kinematic control signals begins at the level of the sensors (e.g., vestibular system - otoliths: force field meters, canals: head angular speedometers). Sensor fusion mechanisms are used to yield separate internal representations for foot support kinematics, force fields such as gravity, and contact forces such as pull or push having impact on the body. These representations are fed as global set point signals into local proprioceptive control loops of the joints. Fed into an ankle joint proprioceptive loop for body-on-support stabilization, they yield compensation of support tilt, gravity and contact forces, even when these stimuli are combined and, furthermore, voluntary lean is superimposed. Model simulations parallel our experimental findings so far obtained.
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It is generally accepted that human bipedal upright stance is achieved by feedback mechanisms that generate an appropriate corrective torque based on body-sway motion detected primarily by visual, vestibular, and proprioceptive sensory systems. Because orientation information from the various senses is not always available (eyes closed) or accurate (compliant support surface), the postural control system must somehow adjust to maintain stance in a wide variety of environmental conditions. This is the sensorimotor integration problem that we investigated by evoking anterior-posterior (AP) body sway using pseudorandom rotation of the visual surround and/or support surface (amplitudes 0.5-8degrees) in both normal subjects and subjects with severe bilateral vestibular loss (VL). AP rotation of body center-of-mass (COM) was measured in response to six conditions offering different combinations of available sensory information. Stimulus-response data were analyzed using spectral analysis to compute transfer functions and coherence functions over a frequency range from 0.017 to 2.23 Hz. Stimulus-response data were quite linear for any given condition and amplitude. However, overall behavior in normal subjects was nonlinear because gain decreased and phase functions sometimes changed with increasing stimulus amplitude. "Sensory channel reweighting" could account for this nonlinear behavior with subjects showing increasing reliance on vestibular cues as stimulus amplitudes increased. VL subjects could not perform this reweighting, and their stimulus-response behavior remained quite linear. Transfer function curve fits based on a simple feedback control model provided estimates of postural stiffness, damping, and feedback time delay. There were only small changes in these parameters with increasing visual stimulus amplitude. However, stiffness increased as much as 60% with increasing support surface amplitude. To maintain postural stability and avoid resonant behavior, an increase in stiffness should be accompanied by a corresponding increase in damping. Increased damping was achieved primarily by decreasing the apparent time delay of feedback control rather than by changing the damping coefficient (i.e., corrective torque related to body-sway velocity). In normal subjects, stiffness and damping were highly correlated with body mass and moment of inertia, with stiffness always about 1/3 larger than necessary to resist the destabilizing torque due to gravity. The stiffness parameter in some VL subjects was larger compared with normal subjects, suggesting that they may use increased stiffness to help compensate for their loss. Overall results show that the simple act of standing quietly depends on a remarkably complex sensorimotor control system.
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Human dynamic behavior in space is very complex in that it involves many physical, perceptual and motor aspects. Sensorimotor physiol-ogists in the past thus dissected it into simple elementary mechanisms. Difficulties arise when we try to derive from these elelnentary mech-. anisms an integrative understanding of the behavior. I hold that the problem can be overcome by applying, in the form of a systems analysis approach, psychophysics as well as measurements of complex motor behavior such as multisensory control of posture. I present here a survey mainly of our own work on this topic, from which I infer that the control of both the perceptual and the motor aspects of spatial behavior take place in one common kinematic reference system. The envisaged reference system represents a rather faithful internal reconstruction of physics, i.e., of the kinematic aspects of mechanics (kinematic meaning movement-related). The system is hierarchically structured, consisting of linked references which are anchored in gravito-inertial space. The perceptual notion of space is derived mainly through vestibular sensors.
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Estimates of the subjective visual and postural vertical were obtained from five patients with acute peripheral vestibular lesions and 20 normal subjects. The visual vertical was assessed by asking the subjects to align a target line to earth vertical by means of remote control. Postural vertical judgments were obtained by exposing them to rotational displacements in the roll plane while sitting on a motor-driven chair and requiring them to align their body to vertical using a joystick control. While the patients showed strong deviations of the visual vertical towards the lesion side, their postural vertical judgments remained veridical. We conclude that the above perceptions are not processed identically and that the participating sensory systems are differently weighted during these tasks.
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Interest in understanding the human vestibulospinal reflex has increased enormously over the past three decades, because this reflex is the primary effector of maintenance of posture and balance. On a posture platform, forces exerted by the triceps surae (TS) and tibialis anterior muscles are measured to calculate center of mass sway. We wished to determine whether the TS response is a direct component of the vestibulospinal reflex. Ten healthy human beings were stimulated with sinusoidal galvanic currents delivered over their mastoid processes. Sway response on a posture platform and TS electromyogram (EMG) were recorded for the following conditions: (1) standing unrestrained; (2) standing completely restrained above the leg; and (3) sitting unrestrained. Results were similar for all subjects. Computer-aided analysis for case 1 reveals that TS EMG and horizontal body sway responses are generated at the same frequency as the stimulating current, with a phase lag of 90 degrees. For case 2, body sway response and any component of the TS EMG over the unstimulated condition were absent in all subjects. For case 3, body sway persisted, but no TS EMG above the unstimulated condition was recorded. As the TS EMG disappears when the standing subject is restrained from swaying or in the unrestrained seated subject, we conclude that the TS EMG response is compensatory to motion of more superior portions of the musculoskeletal system; it is not part of the vestibulospinal reflex.
Article
This study investigated the effect of initial stance configuration on automatic postural responses in humans. Subjects were tested in both bipedal and quadrupedal stance postures. The postural responses to horizontal translations of the supporting surface were measured in terms of the forces at the ground, movement of the body segments, and electromyographic (EMG) activity. Postural responses to the same perturbations changed with initial stance posture; these responses were biomechanically appropriate for restoring centre of mass. A change in stance configuration prior to platform movement led to a change in both the spatial and temporal organization of evoked muscle activation. Specifically, for the same direction of platform movement, during bipedal stance muscles on one side of the lower limb were activated in a distal to proximal sequence; during quadrupedal stance, muscles on the opposite side of the lower limb were activated and in a proximal to distal sequence. The most significant finding was an asymmetry in the use of the upper limbs and the lower limbs during postural corrections in quadrupedal stance. Whereas antagonists of the upper limb were either co-activated or co-inhibited, depending on the direction of translation, lower limb antagonists were reciprocally activated and inhibited. Human subjects in a quadrupedal stance posture used the lower limbs as levers, protracting or retracting the hips in order to propel the trunk back to its original position with respect to the hands and feet. Postural responses of the subjects during quadrupedal stance were remarkably similar to those of cats subjected to similar perturbations of the supporting surface. Furthermore, the same predominance of lower limb correction is characteristic of both species, suggesting that the standing cat is a good model for studying postural control in humans.