732 Balance

vestibular system

the sensory system located in the inner ear that detects changes in head position and movement contributing to balance, spatial orientation, and coordination

visual system

the sensory system responsible for processing visual information including the eyes, optic nerves, and brain structures involved in vision

postural control

the ability to maintain or regain the body’s center of mass within the base of support during both static and dynamic activities

vestibular apparatus

the set of organs in the inner ear responsible for detecting physical changes in head position and movement including the three semicircular canals, the utricle, and the saccule

semicircular canals

three fluid filled tubes in the inner ear that detects the angular acceleration and deceleration of the head, playing a critical role in balance and coordination

utricle

detects linear acceleration and tilt in the horizontal plane

saccule

detects linear acceleration and tilt in the vertical plane

vestibulo-occular reflex (VOR)

a reflex that allows the eyes to remain focused on an object while the head is in motion, crucial for maintaining visual stability and balance

visual field

the area of the environment that can be perceived by the eyes at any given moment, providing visual cues for maintaining balance and postural stability

proprioception

the sense of body position and movement including information about the position and movement of the limbs and joints

muscle spindles

sensory receptors within muscle fibers that detect changes in muscle length and provide proprioceptive feedback to the CNS

golgi tendon organs

sensory receptors located at the junction of muscles and tendons that detect changes in muscle tension and provide proprioceptive feedback to the CNS

joint receptors

sensory receptors located in the joins that detect changes in joint position and movement, providing information about joint angle and contributing to postural stability

postural sway

the natural, continuous movement of the body’s center of mass while maintaining an upright posture used as an indicator of postural control

spatial orientation

the perception of one’s position in space relative to the environment

feedforward

prepares for movement

feedback

adjusts during or after perturbation

static balance

the ability to maintain balance and postural stability while the body is stationary or not moving

dynamic balance

the ability to maintain balance and postural stability while the body is in motion or changing positions

how much somatosensory input does the somatosensory cortex account for in postural control

70%

how much visual input is there in postural control

20%

how much vestibular input is there in postural control

10%

when the somatosensory cortex is impaired…

the CNS increases reliance on visual and vestibular inputs

when the visual input is unreliable…

the CNS increases reliance on the vestibular and somatosensory input

when the vestibular function is impaired…

patients become visually dependent and demonstrate disproportionate instability in low-light conditions or when vision is occluded

balance and strength

balance requires strength and control in various muscle groups

reaction time and balance

muscle strength plays a significant role in enhancing muscular reaction times and when balance is disturbed, strong muscles contract quickly and forcefully to counteract the disturbance and prevent a fall

joint stability and balance

Muscles work to stabilize joints by maintaining alignment and distributing forces that are applied during movement. If muscles are weak, the joint may become unstable, potentially leading to a loss of balance and increased risk of injury

endurance and balance

stronger muscles have greater endurance which is beneficial for maintaining balance over longer periods, such as during longer walks or standing for extended durations

what is vision responsible for in balance

• perception of environment

• motion perception

• sense of horizon

static visual cues for vision and balance

visual information obtained from stationary objects or features in the environment

static cue- relative size

larger objects appear clsoer

static cue- occlusion

blocked objects appear farther

static cue- perspective

converging lines indicate depth

static cue- shadows

indicate object position and orientation

dynamic visual cues for vision and balance

visual information obtained from moving objects or changes in the viewer’s position

dynamic cue- motion parallax

nearby objects move faster across the visual field than distant objects

dynamic cue- optic flow

the full-field pattern of visual movement indicating the direction and speed of self motion

optic flow + vestibular head motion signals=

perception of self motion during gait

ankle strategy

recruited in response to small, slow perturbations on a firm, full-contact surface; this strategy requires adequate ankle range of motion and intact plantar surface contact

hip strategy

is recruited for larger, faster perturbations or when the support surface is narrow or compliant rendering the ankle strategy ineffective. Rapid hip flexion/extension generates shear forces between the body and surface to reposition center of mass

stepping strategy

is activated when the center of mass is displaced beyond the limits of stability and cannot be recovered by ankle or hip strategies alone. A rapid step re-establishes the base of support- delayed or absent step is a strong clinical indicator of fall risk

postural control sensory processing

detection and interpretation of sensory input from the vestibular, visual, and somatosensory systems which identifies the body’s position, movement, and threats to balance

sensorimotor integration postural control

the brain combines multi-source sensory input into coherent picture of body position and movement; this integrated signal drives motor planning

motor execution postural control

skeletal muscles generate corrective forces to maintain the center of mass within base of support which relies on both feedforward and feedback control

factors affecting balance- age

decline in vestibular, visual, and somatosensory acuity; decreased muscle strength and recovery ability

factors affecting balance- health status

neurological disorders, orthopedic conditions, and vestibular disorders all can impair balance

factors affecting balance- muscle strength & flexibility

weakness in hip abductors, ankle dorsiflexors, and core limits the forces needed for postural correction; reduced ROM impairs adaptive responses

factors affecting age- cognitive function

attention, memory, and executive function support sensory monitoring and motor planning; impairments (TBI, dementia) increase fall risk

medial vestibulospinal tract

projects bilaterally to cervical and upper thoracic motor neurons and coordinates head-neck-trunk alignment during movement

lateral vestibulospinal tract

projects ipsilaterally to the full length of the spinal cord, facilitating extensor tone and inhibiting flexor tone in the ipsilateral limb resisting fall towards the ipsilateral side

vestibular dysfunction →

greater dependence on somatosensory input

compliant or uneven surface →

greater reliance on vestibular input

mechanoreceptors

in the skin detecting pressure, vibration, and skin stretch

plantar foot receptors

estimate the body’s position relative to the support surface and help coordinate postural responses, especially on uneven or compliant surfaces

anticipatory postural adjustments (APAs)

preprogrammed muscle activations that precede voluntary movement by 50-150ms, stabilizing the body against the postural disturbance the movement will create

in what conditions are APAs absent or delayed

in those with Parkinson’s disease, cerebellar ataxia, stroke, and aging

decreased sense of touch

reduced sensitivity to touch can affect an individual’s ability to detect contact with surfaces and pressure changes beneath feet or hands which can affect their ability to adjust their balance in response to changes in the surface they’re walking or standing on

loss of vibratory sensation

vibratory sense provides information about movement and the body’s interaction with the environment- impaired vibratory sense can lead to difficulties in detecting small adjustments needed for balance

vestibular system function

• gaze stability

• postural stability/balance

• orientation in space

vestibulospinal reflex

skeletal muscle extensor activity triggered by ipsilateral vestibular stimulation and the purpose is to stabilize the body (maintain upright head position) coordinating head and neck movements with the body

sensory re-weighting

the CNS actively re-weights sensory inputs based on reliability in the current environment