Instantaneous postural stability characterization using time-frequency analysis

Postural stability assessment is critical to a more accurate understanding of sway and balance control. The center of pressure (COP) metric has been s hown to be a suitable output measure for time and frequency analysis. Howev er, the center of pressure is a non-stationary signal. Standard time and fr equency analysis methods may not be adequate for monitoring the dynamic cha nges in the center of pressure signal. In this study a time-frequency metho d, based on data-adaptive evolutionary spectral estimation, is applied to m onitor the dynamic changes of the center of pressure in a non-stationary en vironment. Metrics including the instantaneous mean frequency (IMF), instan taneous spectral bandwidth (ISB), and instantaneous average power (TAP) are analyzed to characterize the center of pressure signal in both the anterio r-posterior (AP) and the medial-lateral (ML) planes. Within the confines of this study, the IMF was found to be inversely proportional to TAP. The inv erse proportionality factors were calculated in both eyes-open and eyes-clo sed trials during upright quiet standing. These findings suggest that the t ime-frequency analysis provides instantaneous metrics which describe the am plitude changes and frequency shift of the center of pressure under a varie ty of environmental conditions, thus providing a more reliable quantificati on of postural control. (C) 1999 Published by Elsevier Science B.V. All rig hts reserved.