Instantaneous spectral characteristics of postural stability using time-frequency analysis

Postural stability assessment is paramount in understanding control of stability. The center of pressure (COP) metric has shown to be a suitable output measure for time and frequency analysis. However, the COP is also a non-stationary signal. Standard time and frequency analysis methods may not be useful for monitoring the dynamic changes in the COP signal. In this study, a time-frequency method, based on data-adaptive evolutionary spectral estimation, has been applied to monitor the dynamic changes in the COP in a non-stationary environment. Metrics including the instantaneous mean frequency (IMF), instantaneous spectral bandwidth (ISB) and instantaneous average power (IAP) are analyzed to characterize the COP signal in both the anterior-posterior and the medial-lateral planes. IMF and ISB were shown to be exponentially proportional to IAP. The decay factors were calculated in both eyes-open and eyes-closed trials during upright quiet standing