Nonlinear dynamics of human postural sway during upright stance

We present a procedure for characterizing the postural sway during quiet upright stance using methods of nonlinear dynamics. The displacements of the centre of foot pressure in anterior-posterior and lateral directions were measured using a force platform. We examined the changes of the postural control mechanisms when visual or proprioceptive information are reduced. We applied four analytic procedures to test the nature of posturographic time series: Kolmogorov-Smirnov test for amplitude distribution; shapes of autocorelation functions; structure functions and log-log plots of power spectra. Nongaussian distribution and nonlinear dynamics were established by all tests. The power spectra (PS) exhibited an exponential decreasing shape, which is commonly referred to as "1/f noise". The fractal structure of the process was characterised by fractal dimension (Df) and shape of structure functions. Our results show that the maintenance of postural stability in upright stance and reduced sensory inputs is nonlinear dynamic process with fractal structure.