A5 Balance Problems

Topic

Postural control maintains the body's balance within a range of normal stability parameters. When postural control deteriorates relative to these normal parameters, balance issues arise that manifest in three distinct ways.

The first manifestation of this deterioration in postural control is increased oscillation of the center of pressure: as postural control deviates from normal stability parameters, the point where the body exerts force against the ground—the center of pressure—sways more than expected while the person attempts to remain standing.

The second manifestation is a reduction in the limit of stability margin: as postural control degrades, the body is left with an increasingly narrow margin between its current position and the limit beyond which it can no longer regain balance without taking a step or bracing against something.

The third manifestation is a slowing of corrective responses to perturbations: when postural control deteriorates, the body takes longer to generate the response that corrects a perturbation—such as a stumble, a push, or uneven ground—before that perturbation exceeds the limit of stability and causes a fall.

The slowing of corrective responses depends, in part, on motor control: when motor control degrades, the signals correcting posture in response to a perturbation take longer to generate and reach the muscles, further delaying the corrective response and bringing the body closer to the limit of stability.

In addition to motor control, compensatory kinematics play a role in the limit of stability margin: when the body fails to adjust the position of the arms, trunk, or legs in time—within the scope of compensatory kinematics—to offset a shift in the center of pressure, the limit of stability margin shrinks further, because these compensatory adjustments are what normally expand that margin. These three manifestations—increased oscillation of the center of pressure, a reduced limit-of-stability margin, and slowed corrective responses—collectively represent the functional expression of balance problems; the more they combine, the greater the risk that a perturbation will overwhelm the postural control system's ability to maintain body balance, resulting in a fall.

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