Testing the limits: the impact of extreme functional hip width on the muscular demands of walking.

The obstetrical dilemma (OD) posits that hominin pelvic morphology reflects a trade-off between obstetric demands and efficient bipedal locomotion - wider pelves facilitate childbirth but increase mechanical demands on stabilizing muscles, potentially raising the energetic cost of walking. Several studies challenge this view, however, demonstrating no strong associations between pelvic width and increased energy expenditure, suggesting that musculoskeletal compensation minimizes locomotor costs via muscle coordination and posture dynamics. Because assessment of evolutionary constraints requires examination of forms outside observed species variation, we used musculoskeletal simulations to evaluate how extreme deviations in functional hip width affect gait parameters. Motion capture and ground reaction data from ten adults (45 trials) drove subject-specific musculoskeletal models where functional hip widths were altered by ±20% and ±40%. Simulations used identical gait kinematics to isolate the effect of pelvic breadth. Wider pelves increased hip abductor force and metabolic power during mid-stance, while narrower pelves showed inverse trends. Compensatory changes in antagonistic muscle groups (e.g. adductors and flexors) limited net effects. Across all pelvic conditions, the dominant contributors to stride and muscle metabolic cost remained consistent, and total muscle metabolic power stayed within observed inter-individual variation. These results support hypotheses suggesting human musculoskeletal systems can adapt to extreme shifts in pelvic width without substantial muscle metabolic penalties. Contrary to core assumptions of the OD, pelvic breadth alone does not constrain walking effectiveness, suggesting that pelvic evolution was shaped primarily by reproductive, developmental, and structural pressures than by locomotor energetics alone.