Abstract
Even when humans stand in a comfortable posture, their center of gravity fluctuates constantly. Many researchers have been studying this phenomenon in various ways since the 19th century. The fluctuation of the center of pressure under the foot has been measured by using a force plate. Many results have already been reported, but most of them discussed static measures, such as mean position and standard deviation, of the fluctuation. Some of them have paid attention to the power spectrum and the correlation function for the measured fluctuation, and analyses with a computer are increasing. For this purpose, data should be collected appropriately. A few researchers, including the authors, have considered that the fluctuation of the center of pressure can be assumed to be a random stochastic process. Therefore, the autoregressive model is applicable to the representation of its dynamics. In this paper we show how to apply the two-dimensional autoregressive model to data measured by two force plates. Moreover, we investigate the adequate sampling rate and the number of data based on a statistical method. The main results are as follows: (1) The sampling rates which were suitable to measure the statistics of the fluctuation were obtained according to the conditions of standing posture. (2) Suitable sampling rates to analyze the power spectrum and correlation function were obtained. (3) A statistical test showed that almost all the measured data were stationary with respect to the standard deviation. (4) The estimated AR models were able to reproduce the characteristics of the measured data qualitatively when we applied the two-dimensional AR model to the measured data, the number of which was 512.
