On the quantitative characterization of human body sway in experiments with long-term performance

Two different conditions of standing were examined in young male subjects with an intact vestibular system by means of stabilography. Low-frequency sways were eliminated by means of a digital high-pass filter. The parameters of the histograms of amplitudes, the autocorrelation functions, and the power-spectral density were estimated. The quantitative parameters of the stabilograms are compared for the two standing positions, both for the frontal and sagittal planes. Quotients of estimates of variance components calculated on the basis of analysis of variance are stated as measures of reliability for the parameters of histograms of amplitudes and power-spectral density in defined frequency bands. The reliability of the standard deviations of amplitudes was much better than the reliability of skewness and excess. The reliability of spectral density differed for frequency ranges investigated. Generally, the quotients of estimates of variance components were higher for the spectral density in the frequency bands of sagittal stabilograms than in those of frontal ones. Performance significantly affected several parameters of histograms and the distributions of power-spectral density. Standard deviation of amplitudes and power-spectral density proved to be suitable for the quantitative characterization of stabilograms.