The work to swing limbs in humans versus chimpanzees and its relation to the metabolic cost of walking

Abstract

Compared to their closest ape relatives, humans walk bipedally with lower metabolic cost (C) and less mechanical work to move their body center of mass (external mechanical work, W_EXT). However, differences in W_EXT are not large enough to explain the observed lower C: humans may also do less work to move limbs relative to their body center of mass (internal kinetic mechanical work, W_INT,k). From published data, we estimated differences in W_INT,k, total mechanical work (W_TOT), and efficiency between humans and chimpanzees walking bipedally. Estimated W_INT,k is ~ 60% lower in humans due to changes in limb mass distribution, lower stride frequency and duty factor. When summing W_INT,k to W_EXT, between-species differences in efficiency are smaller than those in C; variations in W_TOT correlate with between-species, but not within-species, differences in C. These results partially support the hypothesis that the low cost of human walking is due to the concerted low W_INT,k and W_EXT.