The femur

In quadrupeds, the femur forms a straight line between the hip and the knees, which is an ideal arrangement to bear weight on all four limbs. When walking bipedally, however, this arrangement would mean that with each step, the body’s center of gravity would have to be swung inefficiently in a circle around the supporting leg. 

In bipeds, since the pelvis is so wide, the leg attaches to the pelvis more laterally than in a quadruped. Thus, the femur has to angle inward to distribute weight from the hip to the knees. This angle is known as the valgus angle (or bicondylar angle). This angle brings the knees and lower limbs directly over the center of gravity and increases stability. This angle is achieved in part because the medial part (condyle) of the distal femur is larger than the lateral part.

The tibia

There are differences in the morphology of the tibia that parallel the differences in the femora.  In bipeds, the proximal surface is flat where the tibia articulates with the femur. The medial condyle is larger than the lateral condyle and more oval shaped, similar to what is seen in the femur. In quadrupeds, both condyles of the proximal tibia surface are more equal in size and circular.  

The overall size of the proximal condyles are larger in bipeds than in quadrupeds because of the increased weight being transferred from the femur to the foot. In addition, the tibial shaft of bipeds is more robust and straight, whereas in quadrupeds it is more gracile and curved.

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