Transcript Document

PTP 512
Neuroscience in Physical Therapy
Postural Control
Reading Assignment
Shumway-Cook: pp. 161-162, 164-193
Min H. Huang, PT, PhD, NCS
Objectives
• Define postural control and distinguish between
postural orientation and stability
• Describe the concepts of dynamic stability limits
• Describe postural control processes, including
the contribution of the motor action component
and the role of sensory functions
• Compare and contrast feedback vs. feedforward
postural control
• Discuss the attentional demands of postural
control and its impact on stability during multitasks
INTRODUCTION
Postural Control Defined
• Postural Control
– Controlling body
position in space for
 Stability
 Orientation
• Postural Orientation
– Ability to maintain an
appropriate relationship
between body segments
and between the body
and the environment
Base of Support (BOS)
• BOS is the area of the body in contact with the
support surface
Gait: Chasing COG
“Walking is a state of constant falling”
• During gait, the COG
falls anterior to the
BOS and the person
must step forward to
re-establish the COG
within the BOS to
avoid falling
Center of Pressure (COP)
• COP is the center of the
distribution of total forces
applied to the support surface
• COP represents the average
“location” of these forces but
NOT the forces! It is a point on
a 2-D plane!
• CNS activates muscles to
change the location of COP,
which in turns shifts the
location of COG
Biodex Balance System
Systems for Postural Control
Horak et al., 2009
Postural Stability = Balance
• Balance is the ability to keep
the vertical projection of the
center of mass (COM), within
the limits of base of support
 COM is a point in 3dimensional space, usually
around L2 in standing
 COG is the vertical projection
of the COM on a 2-dimensional
plane, usually the ground
Stability Limits
• Stability limits refer to the
boundaries within which the
body can maintain stability
without changing the base of
support.
• Previous concepts of stability
limits only consider the area of
the feet utilized to maintain
balance rather static concept
Horak et al., 1989
McCollum & Leen, 1989
Current Concepts of
Dynamic Stability Limits
• Stability limits result from the interaction
between the velocity and position of COM.
• Stability limits are the boundaries of the
combined COM velocity and position
possible without the needs to change the
base of support
• Other factors, such as muscle strength, range of
motion, fears of falls, perceived stability, and
various aspects of the environment (e.g. lighting,
icy vs. dry) also affect the stability limits.
COM displacement-velocity trajectory. Subjects
stood on a platform that moved unexpectedly.
Left: Stepping response. COP velocity exceed the
velocity threshold.
Right: Non-stepping response. COM did not cross
the stability boundary.
Triangle symbol indicates the initial quiet standing
position.
Pai 2000
Current Concepts of
Dynamic Stability Limits: Try this…….
• Lean forward as far as possible and then
1. Throw your arms up as fast as possible
2. Throw your arms up as slowly as possible
Which condition do you feel more stable?
• Stand in your neural upright position
1. Lean backward as fast and as far as possible
2. Lean backward as slowly and as far as
possible
Which condition do you feel more stable? Which
condition are you able to lean further backward?
Modes of Postural Control:
Feedback vs. Feedforward Processes
Kandel, 1991
Slip on the ice or trip over your cat
vs.
Any voluntary movements
Mode of Postural Control
• Feedback control (Compensatory or reactive
postural responses)
– Sensory feedback from unexpected
external perturbations triggers postural
responses
• Feedforward control (Anticipatory postural
control)
– Postural responses are made prior to
voluntary movement that is potentially
destabilizing in order to maintain stability
during the movement
ACTION SYSTEMS IN
POSTURAL CONTROL
Quite Stance Postural Control
• Body alignment: ideal
alignment requires the least
amount of energy
• Postural Tone: activity in
antigravity postural muscles
increases to counteract the
gravity
• Same muscle synergies used
during perturbed stance also
play a role in maintaining
quiet stance.
Postural Control during Perturbed
Stance
• Earlier studies of
postural control used a
platform that moves in
the anteroposterior
direction. Subjects
were asked to keep
their feet in place.
• These studies
found………..
Moving Platform Studies
Movement Strategies to Recover
Anteroposterior Stability
Ankle
Hip
Stepping
Ankle Strategy
• Distal to proximal
muscle activation
pattern
• Body sways at
ankles with hips and
knees in relatively
extended positions
• Utilized in response
to small
perturbations on
firm surface
Hip Strategy
• Proximal muscles
activated first
• Produces large,
rapid motion at hip
joints
• Utilized when
standing on narrow
BOS (e.g. beam), on
soft surface, or
during larger, faster
perturbations
Stepping or Reaching
Strategy
• When subjects were not asked to
keep 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
Movement Strategies to Recover
Mediolateral Stability
• Controlled primarily
Head movement
through lateral hip and
trunk movement
Hip movement
• Proximal to distal
(ABD, ADD)
muscle activation
pattern
Ankle movement
• Loading and unloading
Head motion occurs
of the two legs
in the opposite
controlled by hip
direction of hip
abductors & adductors
and ankle
movement
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
Movement Strategies to Recover
Multidirectional Stability
• Complex postural response patterns in 360degree cannot be explained by simple ankle
or hip strategies.
• Current concepts:
– Synergies are flexible. each muscle
belongs to more than one synergy
– Within each synergy, each muscle has a
unique or fixed weighting factor that
represents the level of activation of that
muscle within the synergy
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
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 not limit training to the activation of a
specific synergy, e.g. ankle or hip strategy
Adapting Strategies
• 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
ANTICIPATORY POSTURAL
CONTROL
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. 2
and why?
Anticipatory Postural Adjustments
(APAs)
In A, Gastroc was
activated prior to
biceps.
In B, subject was
support at the
shoulders so the
arm movement did
not disturb posture.
Thus, APAs were
not needed.
Cordo & Nashner, 1982
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 also
utilized in APAs.
• In Cordo and Nashner’s study (1982), A & B had
different “Central Set”, which refers to the state
or readiness of the nervous system that is
determined by the context of a task
Clinical Implications
• APAs increase with ↑ movement magnitudes
and speed. APAs more frequently present
with faster movements and heavier loads
• Practice can affect the timing of APAs, e.g.
dancers activate APAs much earlier in a leglifting task than untrained individuals
• APAs are reduced when a support is
given Your patients will never improve
balance if they practice balance tasks while
holding on to // bars!
Neural Systems Controlling
Postural Orientation and Stability
Spinal Cord
Brainstem
• Spinal cats can
• Regulation of
activate extensor
postural tone
muscles to support
• Integration of
body but their
sensory information
postural stability is
• Contribute to
poor
anticipatory
• Postural stability is
postural control for
NOT organized at the voluntary
spinal cord level
movements
Neural Systems Controlling Postural
Orientation and Stability
• Basal ganglia-cortical loop controls postural
set, i.e. the ability to modify the postural
muscle activation patterns to changes in the
task or environmental conditions
Patient with
Parkinson’s
Disease
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 perturbations
of the support surface (Horak and Diener, 1994).
PERCEPTUAL SYSTEMS IN
POSTURAL CONTROL
Sensory Contributions to Balance
• CNS processes information from sensory
receptors throughout the body to determine
the body’s 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”
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 on
its accuracy as a reference for body motion.
 Try this. Stand on one leg with eyes open vs.
closed. Which sense(s) may be “weighted”
when you close your eyes?
Visual Contributions
Moving Room
Experiment
• ↑ sway in young
children and old
adults with room
oscillation (may be
due to reduced
somatosensation)
• Vision may not be
reliable
– Self motion vs.
object motion?
Lee and Aronson, 1974
Somatosensory Contributions
• Somatosensory contributions typically
dominate postural control in response to
transient or fast surface perturbations (in
this type of situation, visual and vestibular
inputs do not help)
• Lightly touching a stable surface reduces
sway significantly. The somatosensory
inputs from the touch, rather than the
contact force through touching a surface
(Jeka, 1994; Lackner, 1999)
Vestibular Contributions
• Typically vestibular system contributes less
than somatosensory system
• For example, CNS cannot tell whether it is
just head bending forward or the whole body
is leaning forward
• Vestibular system provides a frame of
reference relative to the gravity
Testing Adaptation of Postural to
Changing Sensory Conditions:
Sensory Organization Test (SOT)
SOT Normal Results
• Adults and children over age 7 easily
maintain balance on all conditions
• Least sway on conditions 1, 2, & 3 where the
support service is providing accurate sensory
information
• Greatest sway on conditions 5 & 6 because
only one set of sensory inputs (vestibular) are
accurate and available
 Visual cues are more important when the
balance task becomes more challenging
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 individuals
increase postural sway with ↑ difficulty of
secondary cognitive tasks
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 will
affect balance is still unclear
• Executive function may be the most
important cognitive function required to
maintain normal balance under dual-task
paradigms
Testing Executive Function during
Walking: Walking Trail Making Test (WTMT)
Wright, 2011
Balance-impaired older adults with depression
required ↑time to step accurately under
cognitively challenging conditions that require
executive function.
MDD: Major
depressive
disorder
*
ND: Nondepressed
Wright, 2011