TIME-FREQUENCY ANALYSIS OF POSTURAL SWAY

Postural sway during quiet stance has been used to characterize the po stural control system. Most studies have used center of pressure (COP) measurements and have assumed stationarity, however, recent research has indicated that COP is not stationary. The purpose of this study is to introduce and demonstrate a nonstationary spectral estimation tech nique to examine the time-varying nature of postural sway. Data from t wo experiments were used to verify the usefulness of the spectral esti mator for the analysis of COP. The first data set contains COP recorde d from normal subjects swaying about their ankles in response to a met ronome as it was gradually changed from 2 to 1 Hz. The time-frequency distribution reveals time-varying spectral changes corresponding to fr equency changes made by the subjects. The second set consists of COP f rom normal subjects and vestibularly impaired patients standing quietl y on a force plate with eyes closed for 100 s. The time-frequency dist ributions for the COP were estimated for both sets of data. The COP's appear to be nonstationary with the energies at a given frequency modu lating through time.