The effective mechanical advantage of a.L. 129-1a for knee extension.

The functional significance of shape differences between modern human and australopithecine distal femora remains unclear. Here, we examine the morphological component of the effective mechanical advantage (EMA) of the quadriceps muscle group in a sample of hominins that includes the fossil A.L. 129-1a (Australopithecus afarensis) and modern humans. Quadriceps muscle moment arms were calculated from three-dimensional computer models of specimens through a range of knee flexion. All hominins were compared using the same limb positions to allow us to examine, in isolation, the morphological component of the lengths of the pertinent moment arms. After taking into account the differences in bicondylar angle, the morphological component of the EMA was calculated as the ratio of the quadriceps muscle and ground reaction force moment arms. Our analyses reveal that A.L. 129-1a would have possessed a morphological component of the quadriceps muscle EMA expected for a hominin of its body mass.