In order to better understand the background behind the origin of human bipedality, the functional significance of the human upright posture in terms of stability should be more investigated. Therefore, in this study, we examined postural stability of human upright posture against external perturbations and compared it with that of ape-like bent-hip, bent-knee posture. Five male participants were ask to stand on a stabilometer and forward and backward horizontal surface perturbations were applied. Fluctuation of the center of pressure, electromyographic signals, and body sway were monitored.
The results show the human upright posture, despite of its relatively higher location of the center of body mass (COM), is not significantly less stable against the external perturbations than the ape-like flexed posture. Moreover, it is suggested that postural adjustments against external perturbations become more difficult in the flexed posture, since muscles have to be more activated not only to cope with the body sway, but also to sustain the flexed posture against the gravity. Therefore, the upright posture seems to be more advantageous than the mechanically more stable flexed posture as a whole. Due to this inherent advantage, it is deduced that the human ancestors could probably transfer to the upright posture in a relatively early stage, once bipedality becomes necessary.