Gait compliance alters ground reaction forces in human walking: implications for the evolution of bipedalism

Despite decades of inquiry, the evolution of bipedalism remains a mystery. Some have argued that a compliant walking gait, with deep hip and knee flexion to moderate ground reaction forces, was used by early human ancestors, marking our relatively stiff modern gait as a recently acquired feature of our genus. Building on previous compliant walking studies, we test the hypothesis that vertical ground reaction forces are attenuated in compliant walking through increases in contact time. Twenty-four adults walked on an instrumented runway using a normal and a compliant gait at a selfselected pace. Vertical, mediolateral and fore-aft ground reaction forces were assessed using both standard discrete and novel continuous methods. We report mixed evidence for the effect of contact time on peak vertical force in the first third of stance during compliant walking. Our data show greater vertical forces at midstance and reduced vertical forces in the last third of stance during compliant walking. Vertical impulse did not differ between gaits. Compliant walking minimized medial and fore-directed forces and increased lateral and aft-directed forces compared with stiff walking. We identified robust increases in lateral and aft impulses. In addition to discrete analysis of force trace peaks, we employed continuous waveform analysis of force traces that confirmed and further illuminated these patterns. Our data clearly demonstrate that compliant walking has lower vertical forces in late stance, with lower medial and fore forces and higher lateral and aft forces across the gait cycle. These results point toward key changes in leg and foot mechanics and advance our understanding of advantages and challenges associated with the evolution of bipedalism.