Identification of time-varying effects of sensory perturbations for postural stability analysis

Center of pressure (COP) measurements are often used to identify balance problems. One method commonly used to process COP signals is time-frequency analysis, with higher frequencies of postural sway interpreted as tightly controlled posture. However, standard Fourier transform based methods are poorly suited to COP signals due to their inherent non-stationarity. A new method of mean frequency estimation is proposed for COP signals based on the Fourier-Bessel (FB) expansion that is suitable for use in non-stationary signals. In addition, FB expansion provides better frequency resolution and low frequency detectability. Seventeen subjects were tested under eyes open and eyes closed conditions, with different vibration frequencies applied for the eyes closed condition in order to further perturb sensory information. Mean frequency as calculated by FB expansion was able to distinguish between eyes open and eyes closed conditions (p < 0.05), but no effect of vibration could be detected.