While the capacity to walk upright is not unique to humans, other primates can only achieve this for short periods and at a great expenditure of energy.The human adaption to bipedalism has also affected the location of the bodys center of gravity, the reorganization of internal organs, and the form and biomechanism of the trunk. In humans, the double S-shaped vertebral column acts as a great shock-absorber which shifts the weight from the trunk over the load-bearing surface of the feet. The human legs are exceptionally long and powerful as a result of their exclusive specialization for support and locomotion—in orangutans the leg length is 111% of the trunk; in chimpanzees 128%, and in humans 171%. Many of the legs muscles are also adapted to bipedalism, most substantially the gluteal muscles, the extensors of the knee joint, and the calf muscles. Skeleton
The major bones of the leg are the femur (thigh bone), tibia (shin bone), and adjacent fibula, and these are all long bones. The patella (kneecap) is the sesamoid bone in front of the knee. Most of the leg skeleton has bony prominences and margins that can be palpated and some serve as anatomical landmarks that define the extent of the leg. These landmarks are the anterior superior iliac spine, the greater trochanter, the superior margin of the medial condyle of tibia, and the medial malleolus. Notable exceptions to palpation are the hip joint, and the neck and body, or shaft of the femur.
The extensor hallucis longus has its origin on the fibula and the interosseus membrane between the two other extensors and is, similarly to the extensor digitorum, is inserted on the last phalanx of big toe ("hallux"). The muscle dorsiflexes the hallux, and acts similar to the tibialis anterior in the weight-bearing leg. Two muscles on the lateral side of the leg form the fibular (peroneal) group. The fibularis (peroneus) longus and fibularis (peroneus) brevis both have their origins on the fibula, and they both pass behind the lateral malleolus where their tendons pass under the fibular retinacula. Under the foot, the fibularis longus stretches from the lateral to the medial side in a groove, thus bracing the transverse arch of the foot. The fibularis brevis is attached on the lateral side to the tuberosity of the fifth metatarsal. Together, these two fibularis muscles form the strongest pronators of the foot. The fibularis muscles are highly variable, and several variants can occasionally be present. Of the posterior muscles three are in the superficial layer. The major plantar flexors, commonly referred to as the triceps surae, are the soleus, which arises on the proximal side of both leg bones, and the gastrocnemius, the two heads of which arises on the distal end of the femur. These muscles unite in a large terminal tendon, the Achilles tendon, which is attached to the posterior tubercle of the calcaneus. The plantaris closely follows the lateral head of the gastrocnemius.