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PTP 512Neuroscience in Physical Therapy
Postural Control
Reading Assignment
Shumway-Cook: pp. 161-162, 164-193
Min H. Huang, PT, PhD, NCS
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Objectives
Define postural control and distinguish betweenpostural orientation and stability
Describe the concepts of dynamic stability limits
Describe postural control processes, including
the contribution of the motor action componentand the role of sensory functions
Compare and contrast feedback vs. feedforwardpostural control
Discuss the attentional demands of posturalcontrol and its impact on stability during multi-tasks
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INTRODUCTION
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Postural Control Defined
Postural Control
Controlling body
position in space for
StabilityOrientation
Postural Orientation
Ability to maintain an
appropriate relationshipbetween body segments
and between the body
and the environment
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Base of Support (BOS)
BOS is the area of the body in contact with the
support surface
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Gait: Chasing COG
Walking is a state of constant falling
During gait, the COG
falls anterior to the
BOS and the personmust step forward to
re-establish the COG
within the BOS to
avoid falling
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Center of Pressure (COP)
COP is the centerof thedistribution of total forcesapplied to the support surface
COP represents the averagelocation of these forces butNOTthe forces! It is a point ona 2-D plane!
CNS activates muscles tochange the location of COP,which in turns shifts the
location of COG Biodex Balance System
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Systems for Postural Control
Horak et al., 2009
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Postural Stability = Balance
Balance is the ability to keepthe vertical projection of the
center of mass (COM), within
the limits of base of supportCOM is a point in 3-
dimensional space, usually
around L2 in standing
COG is the vertical projection
of the COM on a 2-dimensional
plane, usually the ground
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Horak et al., 1989
McCollum & Leen, 1989
Stability Limits
Stability limits refer to theboundarieswithin which the
body can maintain stability
without changing thebase ofsupport.
Previous concepts of stability
limits only consider the area of
the feet utilized to maintain
balance rather static concept
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Current Concepts of
Dynamic Stability Limits
Stability limits result from the interactionbetween the velocity and position of COM.
Stability limits are the boundaries of the
combined COM velocity and positionpossible withoutthe needs to change thebase of support
Other factors, such as muscle strength, range ofmotion, fears of falls, perceived stability, andvarious aspects of the environment (e.g. lighting,icy vs. dry) also affect the stability limits.
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COM displacement-velocity trajectory. Subjectsstood on a platform that moved unexpectedly.
Left: Stepping response. COP velocity exceed thevelocity threshold.
Right: Non-stepping response. COM did not crossthe stability boundary.
Triangle symbol indicates the initial quiet standingposition.
Pai 2000
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Current Concepts of
Dynamic Stability Limits: Try this.
Lean forward as far as possible and then
1. Throw your arms up as fastas possible
2. Throw your arms up as slowlyas possible
Which condition do you feel more stable?
Stand in your neural upright position
1. Lean backward as fastand as faras possible
2. Lean backward as slowlyand asfar aspossible
Which condition do you feel more stable? Which
condition are you able to lean further backward?
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Modes of Postural Control:
Feedback vs. Feedforward Processes
Slip on the ice or trip over your catvs.
Any voluntary movements
Kandel, 1991
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Mode of Postural Control
Feedback control (Compensatory or reactivepostural responses)
Sensory feedback from unexpectedexternal perturbations triggerspostural
responses
Feedforward control (Anticipatory posturalcontrol)
Postural responses are made prior tovoluntary movement that is potentiallydestabilizing in order to maintain stabilityduring the movement
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ACTION SYSTEMS INPOSTURAL CONTROL
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Quite Stance Postural Control
Body alignment: idealalignment requires the leastamount of energy
Postural Tone: activity in
antigravity postural musclesincreases to counteract thegravity
Samemuscle synergies usedduring perturbed stance alsoplay a role in maintainingquiet stance.
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Postural Control during Perturbed
Stance
Earlier studies of
postural control used a
platform that moves in
the anteroposteriordirection. Subjects
were asked to keep
their feet in place.
These studies
found..
Moving Platform Studies
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Movement Strategies to Recover
Anteroposterior Stability
Ankle Hip Stepping
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Ankle Strategy
Distal to proximalmuscle activation
pattern
Body sways at
ankles with hips and
knees in relatively
extended positions
Utilized in responseto small
perturbations on
firm surface
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Stepping or Reaching
Strategy
When subjects were not asked tokeep feet in place, they more
frequently step or reach, instead of
utilizing ankle or hip strategies to
restore balance (McIlroy & Maki, 1993)
Stepping or reaching are natural
responses, not the last resort to
restore balance!
Older adults more frequently step
than young adults (Mille, 2003)Horak, 2009
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Movement Strategies to Recover
Multidirectional Stability
There is a continuum response patterns that
control stability in the 360-degree of possible
perturbation directions
Shumway-Cook, 2007
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Movement Strategies to Recover
Multidirectional Stability
Complex postural response patterns in 360-degree cannot be explained by simple ankleor hip strategies.
Current concepts: Synergies are flexible. each muscle
belongs to more than one synergy
Within each synergy, each muscle has aunique or fixed weighting factor thatrepresents the level of activation of thatmuscle within the synergy
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Each synergy activates a specific set of muscles in a
fixed amount. Different combinations of synergies
are activated based on continuous sensory
feedback to adjust postural stability.
Ting, 2005
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Clinical Implications of Movement
Strategies for Postural Control
Both quiet stance and recovery of stability in
response to perturbations use common
postural synergies.
Training in one context, e.g. quite stance,
may transfer to improve stability in the other
context, e.g. recovery of perturbed stance
Do notlimit training to the activation of aspecific synergy, e.g. ankle or hip strategy
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Adaptation is the ability to modify response
according to the task demands
With repeated perturbations, movement
strategies change (within 5-15 trials!)
Several studies in normal adults found
reduced sway with repeated exposure to
platform movements
Adapting Strategies
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ANTICIPATORY POSTURALCONTROL
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Anticipatory Postural Adjustments
(APAs)
Work with a partner. Stand with your arm
outstretched, at about waist height, palm up.
Place a heavy book on your outstretch palm.1. Have your partner remove the book
2. Lift the book using your opposite arm
Are the responses different between 1 vs. 2and why?
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Anticipatory Postural Adjustments
(APAs)
In A, Gastroc was
activated prior to
biceps.
In B, subject wassupport at the
shoulders so the
arm movement did
not disturb posture.
Thus, APAs were
not needed.
Cordo & Nashner, 1982
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Anticipatory Postural Adjustments
(APAs)
Postural muscles are activated prior to the
prime movers that produce movement
Same postural synergies utilized during quiet
stance and postural perturbations are alsoutilized in APAs.
In Cordo and Nashners study (1982), A & B had
different Central Set, which refers to the stateor readiness of the nervous system that is
determined by the context of a task
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Clinical Implications
APAs increase with movement magnitudesand speed. APAs more frequently presentwith faster movements and heavier loads
Practicecan affect the timing of APAs, e.g.
dancers activate APAs much earlier in a leg-lifting task than untrained individuals
APAs are reduced when a support is
given
Your patients will never improvebalance if they practice balance tasks whileholding on to // bars!
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Neural Systems Controlling
Postural Orientation and Stability
Spinal Cord
Spinal cats canactivate extensor
muscles to supportbody but theirpostural stability ispoor
Postural stability isNOT organized at thespinal cord level
Brainstem
Regulation ofpostural tone
Integration ofsensory information
Contribute toanticipatorypostural control forvoluntarymovements
N l S C lli P l
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Basal ganglia-cortical loop controls posturalset, i.e. the ability to modify the postural
muscle activation patternsto changes in the
task or environmental conditions
Neural Systems Controlling Postural
Orientation and Stability
Patient withParkinsonsDisease
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Neural Systems Controlling Postural
Orientation and Stability
Cerebellar-cortical loop controls the
adaptation of postural muscle activation
amplitudes, i.e. scaling, in response to
changes in task and environmental conditions Patients with damage to cerebellum were
unable to modify postural muscle activation
amplitudes even after repetitive perturbationsof the support surface (Horak and Diener, 1994).
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PERCEPTUAL SYSTEMS INPOSTURAL CONTROL
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Sensory Contributions to Balance
CNS processes information from sensory
receptors throughout the body to determine
the bodys position in space
Vision (especially peripheral vision)
Somatosensation (proprioception,
cutaneous, joint receptors)
Vestibular system Each sense provides a different frame of
reference, i.e. map
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Sensory Weighting Hypothesis
Postural control system is able to reweight
sensory inputs in order to optimize stability in
altered sensory environments.
The gain of a sensory input will depend onits accuracy as a reference for body motion.
Try this. Stand on one leg with eyes open vs.
closed. Which sense(s) may be weightedwhen you close your eyes?
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Visual Contributions
Moving Room
Experiment
Lee and Aronson, 1974
sway in youngchildren and old
adults with room
oscillation (may be
due to reduced
somatosensation)
Vision may not be
reliable
Self motion vs.
object motion?
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Somatosensory contributions typicallydominatepostural control in response to
transient or fast surface perturbations (in
this type of situation, visual and vestibularinputs do not help)
Lightly touching a stablesurface reduces
sway significantly. The somatosensory
inputs from the touch, rather than the
contact force through touching a surface(Jeka, 1994; Lackner, 1999)
Somatosensory Contributions
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Typically vestibular system contributes less
than somatosensory system
For example, CNS cannot tell whether it is
just head bending forward or the whole bodyis leaning forward
Vestibular system provides a frame of
reference relative to the gravity
Vestibular Contributions
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Testing Adaptation of Postural toChanging Sensory Conditions:
Sensory Organization Test (SOT)
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Adults and children over age 7 easilymaintain balance on all conditions
Least sway on conditions 1, 2, & 3 where thesupport service is providing accurate sensoryinformation
Greatest sway on conditions 5 & 6 becauseonly one set of sensory inputs (vestibular) are
accurate and availableVisual cues are more important when the
balance task becomes more challenging
SOT Normal Results
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Cognitive Contributions to Balance
Dual-task paradigms
performance in either task because of
limited capacity in information processing
to handle both tasks simultaneously
Different postural and secondary tasks
affect postural control differently
Older or balance-impaired individualsincrease postural sway with difficulty of
secondary cognitive tasks
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Cognitive Contributions to Balance
As the difficulty for maintaining stability
, there is in attention resources required
by the postural control system
What type of secondary cognitive task willaffect balance is still unclear
Executive function may be the most
important cognitive function required tomaintain normal balance under dual-task
paradigms
T ti E ti F ti d i
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Testing Executive Function during
Walking: Walking Trail Making Test (W-
TMT)
Wright, 2011
B l i i d ld d lt ith d i
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MDD: Majordepressivedisorder
ND: Non-depressed
Balance-impaired older adults with depression
required time to step accurately under
cognitively challenging conditions that require
executive function.
*
Wright, 2011