Dynamic pressure patterns in the hands of olive baboons (Papio anubis) during terrestrial locomotion: implications for cercopithecoid primate hand morphology.

Habitually terrestrial monkeys adopt digitigrade hand postures at slow speeds to increase effective forelimb length and reduce distal limb joint moments. As these primates move faster, however, their hands transition to a more palmigrade posture, which is likely associated with the inability of wrist and hand joints to resist higher ground reaction forces (GRF) associated with faster speeds. Transitioning to a palmigrade posture may serve to distribute GRFs over a larger surface area (i.e., increased palmar contact), ultimately reducing stresses in fragile hand bones. To test this hypothesis, dynamic palmar pressure data were collected on two adult baboons (Papio anubis) walking, running, and galloping across a runway integrated with a dynamic pressure mat (20 steps each; speed range: 0.46-4.0 m/s). Peak GRF, contact area, peak pressure, and pressure-time integral were quantified in two regions of the hand: fingers and palms (including metacarpal heads). At slower speeds with lower GRFs, the baboons use digitigrade postures resulting in small palmar contact area (largely across the metacarpal heads). At faster speeds with higher GRFs, they used less digitigrade hand postures resulting in increased palmar contact area. Finger contact area did not change across speeds. Despite higher GRFs at faster speeds, metacarpal pressure was moderated across speeds due to increased palmar contact area as animals transitioned from digitigrady to palmigrady. In contrast, the pressure in the fingers increased with faster speeds. Results indicate that the transition from digitigrady to palmigrady distributes increased forces over a larger palmar surface area. Such dynamic changes in palmar pressure likely moderate strain in the gracile bones of the hand, a structure that is integral not only for locomotion, but also feeding and social behaviors in primates.