First-order statistics of human stabilogram Bronislaw Grzegorzewski a, * , Andrzej Kowalczyk b a Department of Biophysics, The Ludwik Rydygier Medical University in Bydgoszcz, ul. Jagiello nska 13, 85-067 Bydgoszcz, Poland b Institute of Physics, The Nicholas Copernicus University, ul. Grudzia z dzka 5, 87-100 Toru n, Poland Accepted 24 July 2001 Abstract We investigated the statistical properties of the centre of pressure (COP) vector during quiet standing, as measured on a force platform. To derive a statistical model for the data, the COP vector, locally averaged COP, and deviations from the average COP were analysed. Whereas indefinite multimodal distributions for the components of the COP were observed, the density functions of deviation from the averaged COP had reproducible properties. The anterior– posterior and mediolateral components of the deviations had in common that they were correlated variables with a non-circular statistic. The main axes of the equiprobability density curves were rotated with respect to the natural anterior–posterior and mediolateral directions. By means of more detailed statistical analysis of the data, two distinct populations of subjects were identified. In some cases we have found Gaussian distributions of the components of deviation from the average COP. In most cases the statistics was non-Gaussian. The average contrast, a new dimensionless characteristic quantity, independent of platform calibration, was introduced as a useful indicator of non-Gaussian effects in the postural control sys- tem. Ó 2001 Elsevier Science B.V. All rights reserved. PsycINFO classification: 2330; 2340 Keywords: Upright posture; Stabilogram; Statistical analysis Human Movement Science 20 (2001) 853–866 www.elsevier.com/locate/humov * Corresponding author. Tel.: +48-52-3270-429; fax: +48-52-3226-229. E-mail address: grzego@aci.amb.bydgoszcz.pl (B. Grzegorzewski). 0167-9457/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII:S0167-9457(01)00054-9