Fig. 8

The knee joint and bipedalism. The part of the thigh bone (femur) that contributes to the knee joint is shown in chimpanzees (Pan troglodytes), A. africanus, and humans (Homo sapiens). Note how the shaft of the femur approaches the horizontal plane of the knee joint at a more acute angle in A. africanus and H. sapiens than in P. troglodytes. This is known as having a valgus knee, and it is functionally important because this angulation allows the knee joints to be positioned closer together than the hip joints. This facilitates balance and enhances energetic efficiency during bipedal walking and running. A valgus knee joint is also present in A. afarensis