A comparative study on the methods for estimating the feedback delay time of spontaneous postural sway control during quite upright standing.

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The present study has confirmed that(1) the center of foot pressure(COP) fluctuations during quiet stance are near the border of fractional Brownian motions and noises(1/f fluctuations) and(2) therefore, detrended fluctuation analysis(DFA) is valid and reliable to analyze the COP position and velocity data. The COP data analyzed with DFA in this study showed that(1) the transition from persistent to anti-persistent correlations(crossover) occurs in the velocity of spontaneous postural sway, not in the position, which indicates that the control of spontaneous postural sway is mainly velocity-based ;(2) the scaling exponents characterizing the correlation structure of COP velocity data provide information that may be useful to understand the functional state of the postural control system during quiet stance ; and(3) laterality or the dominance of one side of the foot often exists in the control of spontaneous postural sway.