Macaques trained to perform bipedally use grounded running, skipping and aerial running, but avoid walking. The preference for grounded running across a wide range of speeds is substantially different from the locomotion habits observed in humans, which may be the result of differences in leg compliance. In the present study, based on kinematic and dynamic observations of three individuals crossing an experimental track, we investigated global leg properties such as leg stiffness and viscous damping during grounded and aerial running. We found that, in macaques, similar to human and bird bipedal locomotion, the vector of the ground reaction force is directed from the center of pressure (COP) to a virtual pivot point above the center of mass (COM). The visco-elastic leg properties differ for the virtual leg (COM-COP) and the effective leg (hip-COP) because of the position of the anatomical hip with respect to the COM. The effective leg shows damping in the axial direction and positive work in the tangential component. Damping does not prevent the exploration of oscillatory modes. Grounded running is preferred to walking because of leg compliance. The transition from grounded to aerial running is not accompanied by a discontinuous change. With respect to dynamic properties, macaques seem to be well placed between bipedal specialists (humans and birds). We speculate that the losses induced in the effective leg by hip placement and slightly pronograde posture may not pay off by facilitating stabilization, making bipedal locomotion expensive and insecure for macaques.
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