Hip stabilization in an australopithecine-like hip: the influence of shape on muscle activation

Hip stabilization through muscular activation of the gluteals is a key feature of hominin walking, but the role of pelvic shape on muscular activation remains uncertain. Coupled with this is the uncertainty regarding whether the kinematics and kinetics of modern humans are appropriate in extinct hominins. We apply modern human kinematics and kinetics to musculoskeletal models with modern human-like and australopithecine-like hips. We test the prediction that the hip functional complex that includes biacetabular breadth, femoral neck length, and iliac blade flare, produces hip abductor muscle activations that are similar in the modern human- and australopithecine-like forms. Using previously developed musculoskeletal models, we calculated muscle forces using inverse dynamics analyses and a muscle redundancy algorithm for ten individuals who walked at their normal velocity. We found that the shape of the australopithecine-like pelvis produces absolutely higher muscle activations in gluteus medius and gluteus minimus, but lower muscle activations across a long period of stance in gluteus maximus compared to the modern human-like pelvis when kinematics and size are held constant. These results suggest that, while the australopithecine-like pelvis is compatible with human walking patterns, influences on pelvic shape other than accommodating muscle and joint reaction forces during walking are present.