EVALUATION OF NONLINEAR DYNAMICS IN POSTURAL STEADINESS TIME-SERIES

Fractal and correlation dimensions have been computed for time series obtained from tests of balance (postural steadiness). Although these m easures appear to be reliable and differentiate subject groups, it has become clear that random (noise) time series may have finite dimensio ns and appear to demonstrate dynamics characteristic of nonlinear syst ems. Consequently, it is necessary to apply a test to distinguish a ti me series with putative nonlinear dynamics from random noise. A simple predictor was utilized to compare center of pressure (COP) time serie s with surrogate data constructed to have similar time and frequency d omain characteristics. It was found that the original time series was more predictable than the surrogate data, suggesting that the COP data is derived from a nonlinear system.