An analysis of directional changes in the center of pressure trajectory during stance

This study proposes a new approach of posturography analysis, which enables the evaluation of directional changes in the center of pressure trajectory. The concept is similar to that of so-called "stabilogram diffusion analysis". Instead of calculating the distance traveled by the center of pressure in a certain time interval, this new method calculates the cosine of the instantaneous velocity vectors of the center of pressure, which corresponds to the amount of change in the sway direction (cosine=1 corresponds to the identical direction; cosine=-1 corresponds to the opposite direction). This method was applied to the analysis of the experimental data in which postural sway was collected under four conditions: open eyes, closed eyes, and two auditory biofeedback conditions. In the biofeedback conditions, auditory signal was given to the subjects when the center of pressure swayed out of a certain area. As results, the differences in the postural reaction under these conditions were clearly shown using the new method. The results indicated that the subjects reacted to the auditory signals by swaying in the opposite direction with biofeedback. It was also found that the eyes open condition exhibited a more random-like profile. As this method analyzes the directional change in the postural sway, this method can be utilized cooperatively together with such a method as stabilogram diffusion analysis, which analyzes the magnitude of sway.