Shape analysis of postural sway area

Abstract

Measurement of the human body centre of pressure (COP) movement (postural sway) with a force platform is a standard procedure for the assessment of postural stability. Recently we proposed a method where the outline of the sway region is expressed in terms of Fourier coefficients determined by asymmetric fitting and minimal outline bending. The index of sudden excursions (ISE) is defined as the ratio of this sway area to the one obtained by the standard principal component analysis. The meaning and the discriminative power of this index is still an open question, which is addressed in present work. Stabilometric data were simulated by considering movements of COP in a potential that consisted of a flat central ellipsoidal region and increased quadratically outside it. Using random walk procedure, combined with the Metropolis algorithm, positions of COP were simulated. The parameter called temperature was defined so that it was related to the probability of COP movements against the potential. A large number of data were calculated for a set of different values of the temperature and the ISE values were determined. For the series of 3000 points the average values of ISE were found at first to increase with temperature from 1.3 to 1.85 as the temperature increased from 0 to 0.195 and then started to decrease towards 1.2 with temperature. It was shown that ISE is sensitive to the actual shape of the measured postural sway area. It increases if there is a small number of large excursions of the COP outside the central region, which are mostly missed by the conventional analysis.