Heel-strike mechanics reveal evolutionary trade-offs in hominin bipedalism.

The human walking step is initiated by a distinctive heel-strike. Our closest living relatives, the African apes, are among the few other animals that are also thought to heel-strike. Qualitative similarities between human and African ape heel-strikes have been invoked in theories about the origins of hominin bipedalism, but some quantitative data suggest possible interspecies differences in foot-strike mechanics that hint at more complex evolutionary scenarios. However, these data are currently too sparse to fully characterize these differences or understand their functional consequences for hominin evolution. To address these gaps, we collected detailed three-dimensional marker-based kinematic and ground reaction force data in humans and in chimpanzees walking bipedally and quadrupedally. We found that chimpanzees used 2.4 to 8.6 times greater ranges of foot-strike angles than humans and often did not heel-strike. Statistical models revealed a possible explanation for this finding, showing that in both chimpanzees and humans, heel-striking is associated with high impact peak forces and loading rates. We also found that humans expend 26 to 41% more metabolic energy when they contact the ground with the distal foot before the heel, indicating an important adaptive trade-off: Heel-striking lowers the energetic cost of bipedal walking but increases potentially damaging impact loading rates. These results suggest that early hominins with primitive lower limb anatomy faced a choice between high impacts and relatively uneconomical walking, either of which likely constrained their terrestrial mobility. The later evolution of larger heels and lower limb joints enabled safe, economical heel-striking and greater daily ranging.