Gait transition mechanism from quadrupedal to bipedal locomotion in the Japanese macaque based on inverted pendulum

The ability of non-human primates to transition from quadrupedal to bipedal locomotion offers critical insights into both the evolution of human bipedalism and the principles of complex motor control. While quadrupedal and bipedal gaits in non-human primates have been studied, the dynamic mechanisms underlying the transition between these gaits remain poorly understood. Japanese macaques trained to walk bipedally have been reported to utilize inverted pendulum dynamics to achieve efficient bipedal locomotion. Given the intrinsic instability of inverted pendulum systems, which can induce large changes in movement with minimal control input, we hypothesized that this mechanism also contributes to the gait transition. To test this, we developed a neuromusculoskeletal model of the Japanese macaque that integrates a detailed musculoskeletal structure with a physiologically inspired motor control system. Through forward dynamics simulations, we generated a variety of movement patterns by systematically parameterizing motor commands, including failed transitions that are difficult to capture experimentally. We then applied dynamical systems analysis using on an inverted pendulum model to examine the underlying principles of the transition process. Our results demonstrate that successful gait transitions depend on generating an inverted pendulum motion through appropriate control of the forward step length of one hindlimb. These findings provide mechanistic insights into how Japanese macaques coordinate their complex musculoskeletal systems to perform skilled, full-body movements in the gait transition, offering a deeper understanding of both advanced motor control and the evolution of human bipedalism.