A versatile gluteus medius drove a seamless transition to bipedalism

The functional role of the gluteus medius differs between extant humans and other primates. In nonhuman primates, the muscle rotates the pelvis in the horizontal plane and assists forelimb reach. By contrast, in humans, it pulls the pelvis upward in the frontal plane and contributes to mediolateral stability during bipedal gait. In the earliest evolutionary stage of bipedalism, however, a functional flexibility would have been required to cope with both bipedalism and locomotion using all four limbs. This study conducted a moment arm analysis based on a model-kinematics matching approach in Japanese macaques (Macaca fuscata), which lack any morphological adaptations for bipedalism. We hypothesized that it was postural change itself that triggered the functional transition of the gluteus medius. The primary action of the gluteus medius was found to shift from medial rotation to abduction with a change from quadrupedal to bipedal walking. This is likely to have been caused by a change in the geometric relationship between the muscular force vector and femoral orientation. This indicates that the gluteus medius has the faculties to change its action regardless of iliac morphology while also suggesting that the behavioral changes preceded morphological adaptations in the evolutionary process of the hip abductor apparatus. In the earliest evolutionary stage, a versatile gluteus medius likely facilitated mediolaterally stable bipedal gaits while ensuring the kinematic demands of quadrupedal locomotion, which led to a seamless locomotor transition to bipedalism.