Chaotic analysis of body sway

In this study, the chaotic features of postural steadiness were investigated using a reconstructed phase portrait, fractal dimension and first Lyapunov exponent, as well as the basic measures of postural steadiness. The experimental variables were presence of visual information (eyes-open and eyes-closed) and postures (standing straight on both feet: SBF, standing straight on left foot only: SLF, standing straight on right foot only: SRF). The first Lyapunov exponent tended to be larger for SRF and SLF conditions as compared with SBF condition, which showed that the postural control system under SLF and SRF conditions was more chaotic. As for SLF and SRF conditions, the first Lyapunov exponent tended to be smaller for the eyes-closed condition than for the eyes-open condition. On the other hand, the first Lyapunov exponent for the SBF condition was smaller irrespective of the presence of visual information. The fractal dimensions of SLF and SRF conditions were less than 2.0 for both the eyes-open and eyes-closed conditions. Moreover, the fractal dimension of the eyes-closed condition tended to be lower than that of the eyes-open condition. We can conclude that the postural control system under SLF and SRF conditions behaves more chaotically than the SBF condition, and leads to the instability of COP