In humans, the calcaneus is the largest of the tarsal bones and the largest bone of the foot. It is placed at the lower and back part of the foot, serving to transmit the weight of the body to the ground, and forming a strong lever for the muscles of the calf. It is irregularly cuboidal in form, having its long axis directed forward and lateralward. The tarsal bones of the ankle are arranged in proximal and distal groups somewhat like the carpal bones of the wrist. Because of the load-bearing role of the ankle, however, their shapes and arrangement are conspicuously different from those of the carpal bones, and they are thoroughly integrated into the structure of the foot. In the calcaneus, several important structures can be distinguished: The posterior half of the bone is the tuber calcanei. On its lower edge on either side are its lateral and medial processes (serving as the origins of the abductor hallucis and abductor digit minimi). The calcaneal (Achilles) tendon is inserted into a roughened area on its superior side, the cuboid bone articulates with its anterior side, and on its superior side are three articular surfaces for the articulation with the talus bone. Between these superior articulations and the equivalents on the talus is the tarsal sinus (a canal occupied by the interosseous talocalcaneal ligament). On the medial side of the bone, below the middle talar facet, is the sustentaculum tali (which serves for the attachment of several other ligaments). On the lateral side is commonly a tubercle called the peroneal trochlea, under which is a groove for the tendon of the peroneus longus. The talus, calcaneus, and navicular are considered the proximal row of tarsal bones. In the calcaneus, an ossification center is developed during the 4-7th week of fetal development. With normal axial alignment in the hindfoot, the axes of the tibia and calcaneus lie on a vertical line (pes rectus). If the calcaneal axis is turned medially the foot is in an everted position (pes valgus), and if it is turned laterally the foot is in an inverted position (pes varus). Muscle attachments The calcaneus serves as the insertion point for three muscles: the gastrocnemius, soleus, and plantaris. These muscles have a variety of functions to include plantar flexion of the foot, flexion of the knee, aids in walking, running, jumping, and steadies leg on ankle during standing. All of these muscles are innervated by the tibial nerve and compromise the posterior (flexor) compartment of the leg.