Comparative analysis of primate hind limb muscle moment arms using subject-specific 3D musculoskeletal models

Variation in primate hind limb anatomy underpins differences in locomotor repertoire, substrate use, and phylogenetic history. Muscle moment arms determine the leverage with which muscles generate joint torques, yet comprehensive comparative data across primates remain limited. Here, we present subject-specific 3D musculoskeletal models of the hind limb across a broad sample of primates, including hominoids and a cercopithecoid. We quantified and compared leverage patterns across hip, knee, ankle, and metatarsophalangeal joints. Our results show that proximal muscles exhibit larger posture-dependent variation than distal muscles, supporting greater versatility in joint excursions among arboreal taxa. Orangutans, gibbons, and siamangs display large and variable moment arms in hip flexors, abductors, and rotators, consistent with multi-planar control during suspensory and climbing behaviours. In contrast, humans exhibit peak leverage at mid-range sagittal-plane postures, reflecting optimisation for upright bipedal stance and propulsion. Macaques show intermediate leverage patterns across the hind limb. Across joints, closely related taxa retain broadly similar muscle moment arm profiles. The dataset establishes a robust baseline for comparing locomotor function as inferred from muscle moment arms between different species with variable morphology and highlights the functional differences shaping primate hind limb diversity.