PTP 512Neuroscience 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 multi-tasks

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 3-dimensional space, usually around L2 in standing

COG is the vertical projection of the COM on a 2-dimensional plane, usually the ground

Horak et al., 1989 McCollum & Leen, 1989

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

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

Slip on the ice or trip over your cat vs.

Any voluntary movements

Kandel, 1991

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 through lateral hip and trunk movement

• Proximal to distal muscle activation pattern

• Loading and unloading of the two legs controlled by hip abductors & adductors

Head motion occurs in the opposite direction of hip

and ankle movement

Head movement

Hip movement (ABD, ADD)

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 360-degree 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

• 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

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 leg-lifting 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 • Spinal cats can

activate extensor muscles to support body but their postural stability is poor

• Postural stability is NOT organized at the spinal cord level

Brainstem• Regulation of

postural tone • Integration of

sensory information • Contribute to

anticipatory postural control for voluntary movements

• 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

Neural Systems Controlling Postural Orientation and Stability

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

Lee and Aronson, 1974

• ↑ 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?

• 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)

Somatosensory 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

Vestibular Contributions

Testing Adaptation of Postural to Changing Sensory Conditions:

Sensory Organization Test (SOT)

• 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

SOT Normal Results

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 (W-TMT)

Wright, 2011

MDD: Major depressive disorderND: Non-depressed

Balance-impaired older adults with depression required ↑time to step accurately under cognitively challenging conditions that require executive function.

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Wright, 2011