Automatic determination of the different control mechanisms in upright position by a wavelet method

A recent model to analyze the center of pressure trajectories is based on fractional Brownian motion. By doing so, one notes that standing still is described by different mechanisms following the frequency. Previous studies exhibit the existence of a control mechanism which stabilizes the upright position at a large enough time scale (from 0.3 s to 1.2 s depending on the method and on the authors) or equivalently at low frequencies. The different mechanisms are separated by a critical time scale or equivalently a critical frequency. This critical frequency is fundamental to understand the control mechanism of upright position: only physiological phenomenon at frequencies larger than this critical frequency could contribute to the task of maintaining equilibrium. A new statistical method is introduced based on recent progress in signal processing: the wavelet analysis. The algorithm is entirely automatic. Seventeen healthy young subjects were studied under quiet-standing conditions, the mean value of critical frequency is 1.8 Hz corresponding to a mean critical time scale 0.68 s. The algorithm is entirely automatic.