Multidimensional performance trade-offs in bipedal standing of common chimpanzees and implications for human bipedal evolution.

Bipedalism is a hallmark of human evolution, and investigating performance trade-offs in common chimpanzees (Pan troglodytes), our closest living relative, elucidates evolutionary constraints of our own lineage. Through musculoskeletal modeling and robotics-inspired analysis, we respectively simulated thousands of bipedal postures optimized for whole-body stability, head-top flexibility, head-top robustness, and femoral-head vertical support. Optimal joint configurations for each metric revealed insights into chimpanzees' postural strategies during foraging, vigilance, and arboreal navigation. Moderate hip and knee flexion with slight ankle dorsiflexion yielded the most stable postures but compromised head mobility and vertical support. These trade-offs explain chimpanzees' limited bipedal capabilities and short bout durations observed in the wild. Our findings illuminate biomechanical challenges shaping hominin bipedal evolution, provide insight into early hominin postural strategies, and offer implications for developing more efficient humanoids and biomimetic robots.