Anatomical factors restricting hip extension range of motion in Japanese macaques and spider monkeys

Japanese macaques (Macaca fuscata) are capable of bipedal locomotion but exhibit fundamental differences from human walking, including a flexed limb posture and a single-peaked ground reaction force profile. One key factor underlying these differences is their limited hip extension, likely constrained by muscle architecture. This study examines the anatomical restrictions on hip extension in Japanese macaques and spider monkeys (Ateles spp.), which achieve greater hip extension during bipedal walking. We measured passive joint moments before and after the sequential dissection of hip flexor muscles in cadaveric specimens using a custom-built measurement device to quantify passive hip joint resistance across different muscle groups. Our results reveal species-specific differences in hip mobility: Japanese macaques exhibit greater passive resistance to hip extension, primarily due to the adductor muscle group and joint capsule, whereas spider monkeys show lower resistance, with the iliopsoas playing a more substantial role in restricting hip extension. The findings emphasize the role of passive elastic muscle constraints in shaping primate bipedalism and offer comparative insights into the evolution of human locomotion.