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.

Figure 1

Spatiotemporal parameters. Stride frequency, duty factor (d) and the term 1 + (d/(1 − d))² from Eq. (11) are plotted for chimpanzees (red circles; data from Pontzer et al.) and humans (blue squares; data from Pavei et al.). Species-specific linear and polynomial regression equations are shown, together with their coefficient of determination (R²).

Figure 2

Mechanical work, metabolic cost, and efficiency. Internal kinetic mechanical work (W_INT,k), total mechanical work (W_TOT), metabolic cost, and locomotor efficiency are plotted as a function of speed. Data from Pavei et al. for humans. Error bars: standard deviation. Solid lines: regression lines for chimpanzees (red) and humans (blue). Shaded area in panel (d): maximum efficiency range for isolated muscles contracting concentrically.

Figure 3

Mechanical work and efficiency assuming a flexed hindlimb. In addition to the data presented in Fig. 2, this plot shows how assuming a flexed lower limb for chimpanzees impacts modeled W_INT,k, W_TOT, and efficiency. In the flexed limb model, a mean knee flexion angle of 125° (with 180° representing knee full extension) and a mean angle of the foot relative to the vertical of 80° was considered (see Supplementary Material S1). Error bars: standard deviation.