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 or not the kinematics and kinetics of modern humans are appropriate proxies for the patterns in extinct hominins. In this work we treat modern human kinematics and kinetics as a null hypothesis by applying them to musculoskeletal models with modern human-like and australopithecine-like hips and examine the resulting joint reaction and muscle forces. 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 and hip joint reactions that are similar in the modern human- and australopithecine-like forms.

Using previously developed musculoskeletal models that approximate gluteus maximus with twelve muscle elements, gluteus medius with twelve, and gluteus minimus with three, we calculated joint reaction and muscle forces using inverse dynamics analyses and a muscle redundancy algorithm. We used data from 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. Hip joint reaction forces are similarly heterogeneous: the australopithecine-like hip exhibits lower forces in the direction of travel but higher in the vertical. 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.