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

## TL;DR

Japanese macaques can walk stably on narrow poles without major changes, adjusting forces to stay balanced, showing how surface shape affects primate movement.

## Contribution

This study reveals how Japanese macaques adjust gait mechanics on different pole widths, highlighting substrate geometry's role in primate locomotion.

## Key findings

- Walking speed and limb flexion remained stable across pole diameters, indicating consistent locomotion.
- Macaques reduced mediolateral forces and hindlimb braking on poles compared to flat surfaces.
- Mechanical energy recovery via pendular exchange was similar on poles and flat surfaces.

## Abstract

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.

Summary: Japanese macaques maintain stable gait during quadrupedal locomotion on moderately narrow poles, with reduced mediolateral forces, indicating the role of substrate geometry in primate gait mechanics for arboreal locomotion.