Tarsus (skeleton)

Lore & Background

In humans, the largest bone in the tarsus is the calcaneus, which is the weight-bearing bone within the heel of the foot. The talus bone, or ankle bone, is connected superiorly to the tibia and fibula to form the ankle joint (talocrural joint) and inferiorly, at the subtalar joint, to the calcaneus. Together, the talus and calcaneus form the hindfoot. The five irregular bones of the midfoot—the cuboid, navicular, and three cuneiform bones—form the arches of the foot, which serve as a shock absorber. The midfoot is connected to the hind- and forefoot by muscles and the plantar fascia. The complex motion of the subtalar joint occurs in three planes and produces subtalar inversion and eversion. Along with the transverse tarsal joint (talonavicular and calcaneocuboid joint), the subtalar joint transforms tibial rotation into forefoot supination and pronation. The axis of rotation in the joint is directed upward 42 degrees from the horizontal plane and 16 degrees medially from the midline of the foot. The subtalar facets form a screw or Archimedean spiral (right-handed in the right foot) about which subtalar motion occurs. Average subtalar motion is 20-30 degrees inversion and 5-10 degrees eversion. In primitive tetrapods, such as Trematops, the tarsus consists of three rows of bones: three proximal tarsals, a second row of four centralia, and a row of five distal tarsals. In reptiles and mammals, there are normally just two proximal tarsals: the calcaneus (equivalent to the amphibian fibulare) and the talus (probably derived from a fusion of multiple bones). In mammals, including humans, the talus forms a hinge joint with the tibia, and the calcaneus is modified to form a heel for the attachment of the Achilles tendon. In birds, the tarsus has disappeared, with proximal tarsals fusing with the tibia and distal bones fusing with the metatarsals to form a single tarsometatarsus bone.

Reader's Guide

The tarsus is significant as the structural foundation of the foot and ankle, enabling weight-bearing, locomotion, and shock absorption. Its seven bones—the talus, calcaneus, cuboid, navicular, and three cuneiforms—form the hindfoot and midfoot, articulating with the lower leg bones above and the metatarsals below. The largest tarsal bone, the calcaneus, bears the body's weight through the heel. The subtalar joint's complex three-plane motion, with an axis oriented 42 degrees upward and 16 degrees medially, allows inversion and eversion essential for adapting to uneven terrain. The transverse tarsal joint (Chopart's joint) has two axes of motion, contributing to foot flexibility or rigidity depending on subtalar position. In comparative anatomy, the tarsus shows evolutionary variation: primitive tetrapods had three rows of bones, while reptiles and mammals typically have two proximal tarsals (calcaneus and talus). Mammals uniquely developed a hinge joint between the talus and tibia and a calcaneal heel for the Achilles tendon. Birds have fused tarsal bones into a tarsometatarsus. These adaptations highlight the tarsus's role in diverse locomotor strategies across vertebrates.

Did You Know?

• The largest bone in the human tarsus is the calcaneus, which is the weight-bearing bone within the heel.
• The subtalar joint's axis of rotation is directed upward 42 degrees from the horizontal plane and 16 degrees medially from the midline of the foot.
• In primitive tetrapods such as Trematops, the tarsus consisted of three rows of bones: proximal tarsals, centralia, and distal tarsals.
• In birds, the tarsus has disappeared, with proximal tarsals fusing with the tibia and distal bones fusing with the metatarsals to form a single tarsometatarsus bone.