Anatomical Adaptations for Bipedalism: Femur

The femur is a critical link between the pelvis, vertebral column, and the feet in a biped because all of the body's mass is transferred to the feet via this bone during standing and movement. The gluteal muscles that link the femur to the pelvis provide the propulsive force for locomotion.

The rounded femoral head articulates with the pelvis at the acetabulum (hip joint). The femoral shaft is generally straight, ending in two bulbous condyles. These are larger and more elliptical in bipeds compared to the relatively smaller and rounder condyles seen in quadrupeds. The distal end of the femur articulates with the tibia (lower leg) and patella (knee cap) at the knee joint.

More force is exerted on the hip joint and the femoral head as the acetabulum moves further away from the body's center of gravity. A femoral head with a larger diameter helps to counteract these forces since the larger size absorbs more of the stress. Thus, a larger femoral head is an indicator of larger body weight. Another adaptation to counteract the increased stress on the hip joint is a longer femoral neck which increases the mechanical advantage of the lesser gluteal muscles by lengthening their lever arm.

Australopithecus has a smaller femoral head and a relatively longer femoral neck compared to later Homo who have a relatively and absolutely larger femoral head.^9,17