Postural sway and perception of the upright stance stability borders.

Decline in the perception of the borders of postural stability due to increase in sway was evaluated in young and elderly subjects. Ranges of lateral and anteroposterior postural sway were measured in eleven young and eleven elderly subjects during maximum voluntary excursions of center of gravity while leaning forward, backward, left, and right. In both age groups, displacement of the center of gravity out of the reference position resulted in increases in the range of sway in the plane corresponding to the direction of lean. Young subjects who further displaced their center of gravity within the base of support also exhibited significantly elevated anteroposterior sway range while leaning forward and backward, both in eyes-closed and in eyes-open experimental conditions. The elderly subjects, however, showed greater mediolateral oscillation of center of gravity while leaning forward with their eyes open. No significant intergroup differences in the anteroposterior sway range during leans in the mediolateral plane were found. However, a greater mediolateral component of sway range at lateral borders of stability was observed in the young adults. Analysis of signal-to-noise ratios indicated a greater decline in stability control in the elderly, due to impairment of perception of postural stability borders.