Leg of ape
Quadrupedal animals like apes, have femurs in which the ball joint, the part that joins the pelvis, sits directly over the inside of the knee. The angle subtended by the femur at the knee in quadrupedal walkers is less than that of bipedal walkers.

Leg of Australopithecus afarensis
This diagram shows the femur with the same shape and structure as that of modern humans, but it is a little shorter. It subtends the same angle at the knee as that of a modern human and the inner bump of the knee joint is larger than the outer one. This shows that this hominin was also a bipedal walker.

Leg of modern human
This modern Homo sapiens bone shows the structure of the femur of an upright walker or bipedal animal. The ball joint, the part that joins the pelvis, sits directly over the outside of the knee. The angle subtended by the femur at the knee in bipedal walkers is greater than that of quadrupedal walkers. This results in the inner bump of the knee joint being longer than the outer bump.