Biomechanical assessment of quiet standing and changes associated with aging.

The kinematics of standing balance were analyzed in 24 normal subjects, aged 21 to 78 years, to examine differences attributable to age, visual input, and sex. Movements of individual body segments, displacement of the center of gravity (COG), and position of the center of pressure (COP) were measured, and total path length and variability about the mean position were derived from the resulting values. Aging was associated with an increase in variability of the COG, head, and hip, but not in path length. The changes, which may be clinically interpreted as excess postural sway, do not show stability deficits as a consequence of aging. On the contrary, older subjects seem to adopt a postural control strategy that achieves comparable stability during quiet standing. Eye closure increased the anterior-posterior COP path length without corresponding changes in the COG, indicating an increase in small accelerations without associated instability. There was more medial-lateral movement in women than in men. Quantitative electromyographic measures showed that, in general, quiet standing requires very little muscular activity. We conclude that the task of quiet standing produces no evidence of postural instability concurrent with aging. The altered postural control strategy may be less effective when balance is suddenly or severely compromised.