On the fractal properties of natural human standing

We analyzed the temporal evolution of the displacement of the center of pre ssure (COP) during prolonged unconstrained standing (30 min) in non-impaire d human subjects. The COP represents the collective outcome of the postural control system and the force of gravity and is the main parameter used in studies on postural control. Our analysis showed that the COP displacement during human standing displays fractal properties that were quantified by t he Hurst exponent obtained from the classical rescaled adjusted range analy sis. The average fractal or Hurst exponent (H) was 0.35 +/- 0.06, The prese nce of long-range correlations from a few seconds to several minutes due to the fractal characteristics of the postural control system has several imp ortant implications for the analysis of human balance. (C) 2000 Elsevier Sc ience Ireland Ltd. All rights reserved.