Is step width decoupled from pelvic motion in human evolution?

Humans are the only primate that walk bipedally with adducted hips, valgus knees, and swing-side pelvic drop. These characteristic frontal-plane aspects of bipedalism likely play a role in balance and energy minimization during walking. Understanding when and why these aspects of bipedalism evolved also requires an understanding of how each of these features are interrelated during walking. Here we investigated the relationship between step width, hip adduction, and pelvic list during bipedalism by altering step widths and pelvic motions in humans in ways that both mimic chimpanzee gait as well as an exaggerated human gait. Our results show that altering either step width or pelvic list to mimic those of chimpanzees affects hip adduction, but neither of these gait parameters dramatically affects the other in ways that lead to a chimpanzee-like gait. These results suggest that the evolution of valgus knees and narrow steps in humans may be decoupled from the evolution of the human-like pattern of pelvic list. While the origin of narrow steps in hominins may be linked to minimizing energetic cost of locomotion, the origin of the human-like pattern of pelvic list remains unresolved.