Week 7.2 Posture & Balance

Define postural control

Control of body position in space for the dual purpose of orientation and stability

Define posture

Ability to maintain an appropriate position of the whole body with respect to the environment and task

Define balance

Ability to control the centre of mass relative to gravity and the base of support

Define centre of mass (COM)

Point where body mass is equally distributed (≈10 cm below navel in standing)

Define base of support (BOS)

Area of the body in contact with the support surface

Define centre of pressure (COP)

Point of application of ground reaction forces

Relationship between COP and COM

COP moves continuously around COM to keep COM within BOS

COP is an indirect measure of what?

Postural sway

Three major components influencing postural control

Task, individual, and environment

How do postural control requirements change?

They vary with task demands and environmental context

Definition of postural stability

Ability to control COM relative to gravity and BOS

Four main challenges to balance

Gravity, muscle forces, limb interactions, external perturbations

Three types of balance tasks

Static (steady-state), anticipatory (proactive), reactive

Static balance definition

Control of COM in predictable and unchanging conditions

Anticipatory balance definition

Activation of postural muscles in advance of voluntary movement

Reactive balance definition

Recovery of stability following unexpected perturbation

Feedforward control definition

Anticipatory postural adjustments based on prior experience

Feedback control definition

Corrective responses based on sensory input after perturbation

Which balance type relies on feedforward control?

Anticipatory balance

Which balance type relies on feedback control?

Reactive balance

Static and dynamic balance relationship

A continuum rather than separate entities

Three primary sensory inputs for postural stability

Somatosensory, vestibular, vision

Approximate contribution of somatosensory input to balance

~70%

Approximate contribution of vestibular input to balance

~20%

Approximate contribution of vision to balance

~10%

Primary role of vision in balance

Orientation relative to surrounding objects

Primary role of somatosensory system in balance

Body position and motion relative to support surface

Primary role of vestibular system in balance

Head position and motion relative to gravity

Which sensory system is relied on most early in learning?

Vision

Which sensory systems are relied on more after skill mastery?

Somatosensory and vestibular

Early post-brain injury balance reliance

Heavy reliance on vision

Structures of the vestibular apparatus

Otolith organs (utricle, saccule) and semicircular canals

Fluid in bony labyrinth

Perilymph

Fluid in membranous labyrinth

Endolymph

Sensory receptors of vestibular system

Hair cells

Function of semicircular canals

Detect angular acceleration of the head

How many semicircular canals per ear?

Three

Names of semicircular canals

Anterior (superior), posterior, horizontal

Orientation of semicircular canals

Perpendicular to one another

Purpose of counterbalanced canal design

Precise detection of head movement by comparing both sides

What happens when head turns to the right?

Right canal depolarizes, left canal hyperpolarizes

What type of movement increases canal firing rate?

Acceleration or deceleration

Function of semicircular canals in vision

Stabilize gaze during head movement

Function of otolith organs

Detect linear acceleration and static head tilt

Two otolith organs

Utricle and saccule

What are otoconia?

Calcium carbonate crystals atop gelatinous layer

Utricle orientation and function

More horizontal; detects horizontal movement

Saccule orientation and function

More vertical; detects vertical movement

Otolith firing pattern

Tonic (sustained) with head tilt

Vestibular information projects first to where?

Vestibular nuclei and cerebellum

Two key outputs of vestibular nuclei

Eye movement control and postural control

Which tract connects vestibular nuclei to eye muscles?

Medial longitudinal fasciculus (MLF)

Cranial nerves involved in VOR

CN III, IV, VI

Define vestibulo-ocular reflex (VOR)

Reflex that keeps eyes on target during head movement

Example of VOR when head turns right

Eyes move left

Vestibulospinal tracts function

Control postural orientation and stability

Two vestibulospinal tracts

Medial and lateral vestibulospinal tracts

Function of medial vestibulospinal tract

Regulates head and neck position (VCR)

Function of lateral vestibulospinal tract

Controls extensor tone for upright posture

Role of cerebellum in vestibular system

Distinguishes active vs passive movement and refines balance

Vestibular cortical projection areas

Parietal cortex and insula

Function of vestibular cortex

Generates head-in-space perception

Physiological vertigo cause

Conflict between sensory systems

Pathological vertigo cause

Imbalance of vestibular signals due to lesion

Most common cause of vertigo

BPPV

Cause of BPPV

Displacement of otoconia into semicircular canal

Most commonly affected canal in BPPV

Posterior canal (~90%)

Purpose of Dix-Hallpike maneuver

Diagnosis of posterior canal BPPV

Purpose of Epley maneuver

Treatment of posterior canal BPPV

Which balance strategy is used for small perturbations?

Ankle strategy

Ankle strategy characteristics

Distal muscles activated first; firm surface; small perturbations

Which balance strategy is used for large or fast perturbations?

Hip strategy

Hip strategy characteristics

Proximal muscles activated first; small BOS or compliant surface

Change-in-support strategy definition

Stepping or grasping to expand BOS

Descending motor system supporting anticipatory balance

Extrapyramidal (reticulospinal) pathways

Descending motor system supporting voluntary movement

Pyramidal (corticospinal) pathways

Origin of reticulospinal tract

Reticular formation

Role of reticulospinal tract in balance

Postural muscle tone and anticipatory support

Examples of abnormal postural control

↑ sway, slowed anticipatory control, poor alignment with gravity

Conditions associated with abnormal balance

Parkinson's disease, stroke, cerebellar disease, aging

Effect of aging on balance

Increased sway and reliance on hip/stepping strategies

Cognitive factors affecting balance

Attention, executive function, dual-tasking

Effect of dual-tasking on balance

Increased reaction time and instability

Key message: sensory systems in balance

Somatosensory, vestibular, and visual systems all contribute

Key message: balance control types

Static, anticipatory, and reactive balance all decline with age and neurological disease