When held in the anatomical position, the femur of an ape will stand almost vertical within a horizontal plane. In humans, the femur will form a bicondylar angle (i.e., the angle at which the femur lies to the midline of the body), and brings the knees closer together (valgus knee). Because of this angle, the feet fall directly below the center of gravity, balancing the body over the stance leg during the stride cycle, thereby stabilizing bipedal locomotion. Lucy’s femur exhibits a bicondylar angle.

The size of the femoral head is related to body mass. The large femoral head of modern humans reflects their larger body size. Comparing Lucy with modern humans, Lucy has a smaller femoral head and a relatively longer femoral neck. Reduction of the femoral neck length increased the amount of stress the femur was able to support, and may have been a specialization due to a change in bipedal posture or an adaptation for the larger body size of later humans.