Coordination of trunk motion during bipedal walking in the frontal plane: A comparison between Homo sapiens, Macaca fuscata, and an exploratory study on a gibbon

Objectives In human walking, the pelvis lists toward the swing side during the support phase while the thorax lists toward the stance side. In contrast, during bipedal walking in chimpanzees, both the pelvis and thorax list toward the stance side during the support phase, making their body mass oscillation larger than that in humans. However, aside from a few reports on chimpanzees and macaques, studies on the relationship between trunk movements and step width during bipedal walking in nonhuman primates are limited. Materials and Methods We collected three‐dimensional trunk kinematic data during bipedal walking in five humans and five Macaca fuscata. While walking, the positions of cluster markers, which defined trunk segments, were recorded by multiple video cameras. Segmental xyz coordinates were digitized, and frontal rotations and translations were calculated using motion analysis software. Results The kinematic approximation of the safety ratio, which is the center of mass motion relative to the step width, was similar in all species. Nevertheless, the relationship between intratrunk movements and step width varied among the species. Discussion Our results support the hypothesis that trunk movement in the frontal plane in humans is unique among anthropoid primates. Because the safety ratio is within a similar range, regardless of habitual or facultative bipeds, this further supports the use of fossil footprints to estimate the center of mass motion.