Effects of physical workload on postural sway during load holding task

Postural steadiness (sway) is used to evaluate the dynamics of the postural control system associated with maintenance of balance during standing. Recently, the chaotic behavior of many biomechanical systems has been examined using Lyapunov exponents, fractal dimension or attractor plot (reconstructed phase portrait). The purpose of this study was to investigate the effects of physical workload on postural stability during load holding task by measuring sway of center of pressure (COP). The experimental variables were weight of load (10 kg, 15 kg, and 20 kg) and height of holding a load (50 cm, 70 cm, and 90 cm). Ten male subjects participated in a repeated measures experiment in which 9 lifts were performed. The time of holding a load was for 10 sec. The sampling interval was 1 msec. The basic measures of postural stability (mean distance of the COP, mean velocity of the COP, and sway area) and the chaotic measures (fractal dimension and first Lyapunov exponent) of the COP were calculated. When the holding height increased, the mean velocity of the COP tended to increase but the first Lyapunov exponent of the COP decreased. On the other hand, the fractal dimension of the COP decreased with the increase in load weight. From the above results, it was clarified that physical workloads (load weight and holding height) affect the chaotic measures of the COP. It was implied that the physical workload can be measured by the chaotic features of the COP.