UT logo UT logo
eFossils
Lessons > Step by Step: The Evolution of Bipedalism
 

Anatomical Adaptations for Bipedalism: Knee (Distal Femur, Proximal Tibia & Patella)

The bicondylar angle, seen in humans, is characteristic of bipedalism. The angle brings the feet in line with the body's center of gravity, resulting in greater stability.

The bicondylar angle, seen in humans, is characteristic of bipedalism. The angle brings the feet in line with the body's center of gravity, resulting in greater stability.

The femur sits nearly perpendicular to the ground in a quadruped, while the femur sits at an angle in a biped. The A. afarensis Lucy exhibits a distinctly angled femur.

A critical adaptation for efficient bipedal locomotion relates to the need to keep the body's center of gravity balanced over the stance leg during the stride cycle. Birds have solved this issue by having the entire leg (from the hips all the way to the feet) as close as possible to the center line. In humans, the hips are wide apart, but the shaft of the femur is angled so that the knee joint is closer to the midline than the hips. This angle is called the carrying or bicondylar angle, and the knee joint is referred to as a valgus knee. The effect is to bring the knees closer together, placing the feet directly below the center of gravity.

Compared to modern humans, an ape femur is almost vertical within a horizontal plane. In quadrupeds the positioning of the center of gravity during locomotion is less critical since the quadruped is usually supported by two or more legs during the stride cycle rather than just one as with humans. Australopithecines have a human-like bicondylar angle.9,18