EXPRIMENTAL ANALYSIS OF POSTURAL MECHANISM USING A NEW MOVABLE PLATFORM

Abstract

A new system has been developed which can estimate postural mechanism using a movable platform. An outstanding characteristic is that the center of pressure (COP) for each leg can be measured. Based on experimental measurement of COP in the static and dynamic conditions of the platform, the maximum error was set within the allowable regions. The main theme of the present research is to develop parameters to assess how to control postural stability under a preprogrammed response pattern, under the command of the central nervous system. By measuring the angular displacements mainly in the lower extremities (by PSD cameras), several experiments estimated postural response for (1) forward/backward movement and (2) lateral inclination stimulation. Subjects are 25 males (ranging from 24 to 72 years old). As a first stage, we conducted a power spectrum estimation (using an autoregressive model of the time seris analysis) of data on COP and angular displacements. As a pre-programmed response pattern in the foreward/backward movement, its response patterns were complex, but the ankle and hip strategy were found at a specified stage of the intensity of stimulation for each subject. The older group made use of the hip strategy to hold a postural stability under comparatively smaller stimulation than the younger group. In the case of hip strategy, the spectral curve often shows a peak around 1 Hz. In the lateral inclination stimulation, we found that the hip was one of the main parts involved in controlling a stable and smooth response. In a stronger stimulation, a spectral curve often shows a remarkable second peak from 2.0 to 2.5 Hz, besides a 1 Hz-peak similar to the forward/ backward movement. This suggests a more complex response pattern of the hip and trunk. Generally a vectral presentation of COP was useful to express the accuracy of the response pattern, giving the latency and the difference between two legs. Frequency analysis was also useful to represent some parameters of postural responses by a movable platform, and could be used to present a limited stable response under a specified frequency stimulation.