Ground-reaction-force profiles of bipedal walking in bipedally trained Japanese monkeys

Ground-reaction-force (GRF) profiles of bipedal locomotion in bipedally trained Japanese macaques (performing monkeys) were analyzed in order to clarify the dynamic characteristics of their locomotion. Five trained and two ordinary monkeys participated in the experiment. They walked on a wooden walkway at a self-selected speed, and three components of the GRF vector were measured using a force platform. Our measurements reveal that trained monkeys exhibited vertical-GRF profiles that were single-peaked, similar to those of ordinary monkeys; they did not generate the double-peaked force curve that is seen in humans, despite their extensive training. However, in the trained monkeys, the peak appeared relatively earlier in the stance phase, and overall shape was more triangular than that of the more parabolic profile generated by ordinary monkeys. Comparisons of vertical fluctuation of the center of body mass calculated from the measured profiles suggest that this was larger in the trained monkeys, indicating that storage and release of potential energy actually took place in their bipedal walking. This energetic advantage seems limited, however, because efficient exchange of potential and kinetic energy during walking were not completely out of phase as in human walking. We suggest that anatomically restricted range of hip-joint motion impedes the inherently quadrupedal monkeys from generating humanlike bipedal locomotion, and that morphological rearrangement of the hip joint was an essential precondition for protohominids to acquire humanlike bipedalism.