Sway ratio - a new measure for quantifying postural stability.

In the search of a reliable postural stability index, two sway time series: the center-of-mass (COM) and the center-of-foot pressure (COP) were recorded simultaneously in elderly subjects standing quiet with eyes open and with eyes closed. From a battery of commonly use sway measures, only the anteroposterior COM and the COP path lengths proved their high sensitivity and discriminative power to the imposed vision conditions. Based upon these indices, a new measure - sway ratio (SR) - was computed, as the COP-to-COM path length ratio. The measure can easily distinguish vision vs. no vision in the elderly. The SR can be successfully accessed base upon the COP signal only. In contrast to traditional sway indices, the SR as a relative measure is insensitive to the length of sampled record and to the signal sampling frequency. Its magnitude can be interpreted as an average amount of balance controlling motor activity that coincides with a unit COM displacement. The SR is recommended as a reliable measure that allows for assessment of postural stability.