WUNDERLICH, R.E.*; SINOPOLI, M.C.; KILKENNY, P.; James Madison University: Plantar pressure distribution during bipedalism in Propithecus verreauxi

Propithecus verreauxi, an indrid primate specialized for vertical clinging and leaping (VCL), exclusively uses bipedalism during terrestrial locomotion. They use a unique galloping bipedal gait with lead and trail limbs switching every 5-7 steps. Whereas the human foot, specialized for leverage with its elongated, rigid midtarsal region, longitudinal arch, and short abducted hallux, is oftentimes considered the hallmark of bipedalism, indrids have been described as possessing feet designed for grasping rather than leverage and specialized for a I-II grasp (Gebo, 1988). We collected plantar pressure distribution and force plate data on five Propithecus verreauxi moving bipedally in a semi-natural setting at the Duke University Primate Center. We hypothesized that lead limb switch reduces load on the trail limb, and that they load their first and second digits preferentially. During bipedal galloping, Propithecus touches down laterally on the trail foot and pushes off medially. The lead limb touches down medially on the hallux and toes off laterally. Higher lateral than medial forefoot load indicates higher axial (fourth) digit than second digit load, leading us to suggest they either do not use a I-II grasp at all or they change their foot loading pattern when on a terrestrial substrate. There is no significant difference between peak vertical force on trail versuslead limbs, however, the lead limb exhibits similar propulsive and braking forces whereas the trail limb exhibits very low propulsive relative to braking force. The use of lead limb switch, therefore, cannot be attributed to reduction of load on the trail limb but may be attributed to muscle recovery and/or different plantar load patterns.