Kinematic and kinetic adjustments of Japanese macaques during quadrupedal walking on horizontal poles

Primates employ a diagonal-sequence gait, exhibit compliant forelimbs and place greater weight on the hindlimbs, features regarded as adaptations to arboreal locomotion. Although substrate diameter is an ecologically critical factor influencing gait mechanics, its effects remain underexplored in larger catarrhine monkeys. This exploratory study examined how horizontal pole diameter affects the kinematics and kinetics of quadrupedal walking in Japanese macaques (Macaca fuscata). Four trained macaques walked on metal poles of three diameters (48.6, 76.3 and 114.3 mm) mounted on force plates while ground reaction forces, joint kinematics, center of mass (COM) dynamics and mechanical energy exchange were analyzed. Contrary to predictions, walking speed, stance duration, mediolateral COM fluctuations and limb flexion did not differ significantly between narrow and wide poles, indicating stable locomotion across tested diameters. The only detectable differences were a modest lowering of COM and a modest decrease of stride length on narrower poles, which was probably of negligible functional significance. In contrast, comparisons with flat-surface walking revealed substrate-specific adjustments: macaques minimized mediolateral forces, showed reduced hindlimb braking impulses and lifted the hindlimb earlier on poles. Despite these differences, mechanical energy recovery via pendular exchange remained similar (35-50%) between pole and flat-surface walking. These results demonstrate that Japanese macaques maintain stable quadrupedal locomotion on moderately narrow poles without major postural changes, while modulating mediolateral force production to reduce destabilizing moments in the frontal plane. Substrate geometry therefore plays a key role in shaping primate gait mechanics and offers insights into the evolutionary adaptations underlying arboreal locomotion.