Studying the bipedal locomotion of non-human primates is important for clarifying the evolution of habitual bipedalism in the human lineage. However, quantitative descriptions of three-dimensional kinematics of bipedal locomotion in non-human primates are very scarce, due to difficulties associated with measurements. In this study, we performed a kinematic analysis of bipedal locomotion on two highly trained (performing) Japanese macaques walking on a treadmill at different speeds and estimated three-dimensional angular motions of hindlimb and trunk segments, based on a model-based registration method. Our results demonstrated a considerable degree of axial rotation occurring at the trunk and hip joints during bipedal locomotion, suggesting that bipedal locomotion in Japanese macaques is essentially three-dimensional. In addition, ranges of angular motions at the hip and ankle joints were larger and the knee joint was more flexed in the mid-stance phase with increasing speed, indicating that gait kinematics are modulated depending on speed. Furthermore, macaques were confirmed to have actually acquired, at least to some extent, the energy conservation mechanism of walking due to pendular exchange of potential and kinetic energy, but effective utilization of this mutual exchange of energy was found to occur only at comparatively low velocity. Spring-like running mechanics were probably more exploited at higher speed because the duty factor was above 0.5. Fundamental differences in bipedal strategy seem to exist between human and non-human primate bipedal locomotion.
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