[Posture Control Associated with Quadrupedal and Bipedal Gait in Japanese Monkeys (Macaca fuscata)]

How the CNS deals with instability of upright posture is the core in the control of bipedal gait. In this review, we summarize our recent findings comparing kinematics and EMG activity during quadrupedal and bipedal gait in Japanese macaques. Trunk/hindlimb muscles showed step cycle-modulated activity, which was more active in bipedal than in quadrupedal gait. For bipedal gait, enhanced activity during longer double support phase was predominantly observed in distal hindlimb muscles. Alternate burst activity in bilateral back muscles cyclically brought back the tilted trunk. In monkeys' quadrupedal gait, hindlimbs formed functional pairs with contralateral forelimbs, unlike in non-primate quadrupeds. These diagonal pairs acted differently on movements of the center of mass (COM). For bipedal gait, the hindlimbs solely carried the COM. Our results suggest that, compared to non-primate quadrupeds, hindlimbs in macaques contribute more critically to weight support and balance control even for quadrupedal gait. Additionally, for more unstable bipedal gait, the monkeys' CNS reinforces such hindlimb roles and actively controls the trunk posture in maintaining dynamic balance, in a manner similar to humans. Studies on Japanese macaques will further our understanding of the neural basis for the control of gait in mammals by bridging non-primate quadrupeds and humans.