Tarsus (skeleton)

Seven bones sit in your foot.

They are near your ankle. One big bone makes your heel. This bone holds your weight.

Other bones form the middle of your foot. These bones make arches. The arches act like a spring. They soak up the shock when you move. These bones connect to your toes. They help you walk and run. Your foot is a very busy place!

The tarsus is a group of seven bones in each foot.

These bones sit between your leg and your toes. They make up two main parts. The first part is the hindfoot. It has the talus and the calcaneus. The calcaneus is your heel bone. It is the largest bone in the tarsus. This bone helps you carry your weight.

The second part is the midfoot. It has five bones. These are the cuboid, the navicular, and three cuneiform bones. These bones form arches in your foot. The arches act like a shock absorber. This helps soak up the impact when you move.

Your foot bones also help you move. The joints between these bones allow for many motions. Some joints make the foot flexible. Other joints make the foot rigid. This happens when the bones lock together. This helps you walk or run. In many animals, these bones look different. For example, birds have bones that fuse together. This creates a new leg part.

The tarsus is a group of seven bones in each foot.

These bones sit between your leg and your toes. They are located between the tibia and fibula and the metatarsus. The tarsus is divided into two main parts. The hindfoot contains the talus and the calcaneus. The calcaneus is the largest bone in the tarsus. It is the weight-bearing bone in your heel.

How these bones work is quite amazing. The talus bone connects to the tibia and fibula. This connection forms the ankle joint, also called the talocrural joint. Below the talus is the subtalar joint. This joint connects the talus to the calcaneus. The midfoot has five irregular bones. These are the cuboid, the navicular, and three cuneiform bones. Together, they form arches that act as a shock absorber.

These bones move in very specific ways. The subtalar joint allows for inversion and eversion. Inversion is when the foot turns inward. Eversion is when the foot turns outward. The subtalar joint acts like a screw or a spiral. This helps change the rotation of your leg into foot movement. The transverse tarsal joint also helps. This joint includes the talonavicular and calcaneocuboid joints. They work with the subtalar joint to change foot stiffness.

Scientists have studied how these bones change over time. In very old animals like Trematops, the tarsus had three rows of bones. There were three bones at the top and four in the middle. There were also five bones at the bottom. Most animals today have fewer bones because they fused together. In reptiles and mammals, there are usually only two top bones. The talus and the calcaneus are the main ones in humans.

Different animals use their feet in different ways. In mammals, the calcaneus forms a heel. This is a place for the Achilles tendon to attach. Birds have a very different foot structure. Their tarsus has mostly disappeared. The top bones fused with the tibia. The bottom bones fused with the metatarsals. This creates one single bone called the tarsometatarsus. This makes the bird's leg have a third segment.

The tarsus is a complex cluster of seven articulating bones located in each human foot. These bones sit between the lower leg and the metatarsus, which leads to the toes. The tarsus is essential for supporting body weight and allowing for sophisticated movement. It is organized into two distinct regions: the hindfoot and the midfoot. The hindfoot consists of the talus and the calcaneus. The midfoot is made of five irregular bones: the cuboid, the navicular, and three cuneiform bones. These midfoot bones form the arches of the foot to act as a shock absorber.

Mechanically, the tarsus functions through several interconnected joints. The talus, or ankle bone, connects superiorly to the tibia and fibula. This connection creates the ankle joint, also known as the talocrural joint. Below the talus lies the subtalar joint, which connects it to the calcaneus, or heel bone. The calcaneus is the largest bone in the tarsus and serves as the primary weight-bearing bone in the heel. The midfoot bones are connected to the hindfoot and forefoot by muscles and the plantar fascia. This network allows the foot to transition between being flexible and being rigid.

The subtalar joint is responsible for complex motions in three different planes. It produces two specific movements called inversion and eversion. Inversion is the inward turning of the foot, while eversion is the outward turning. The axis of rotation for this joint is directed 42 degrees upward from the horizontal plane. It is also oriented 16 degrees medially from the midline of the foot. Interestingly, the subtalar facets form a shape like an Archimedean spiral or a screw. During inversion, the calcaneus rotates clockwise and moves forward along this screw-like axis.

Average subtalar motion involves 20 to 30 degrees of inversion and 5 to 10 degrees of eversion. During a normal walking cycle, the heel strikes the ground in slight inversion. This is quickly followed by eversion of 10 to 15 degrees. Another important structure is the transverse tarsal joint, also called Chopart's joint. This joint is formed by the talonavicular and calcaneocuboid joints. It features two axes of motion to help control foot stability. The talonavicular joint allows for 7 degrees of flexion-extension and 17 degrees of pronation-supination. The calcaneocuboid joint allows for 2 degrees of flexion-extension and 7 degrees of pronation-supination.

The interaction between these joints determines the stiffness of the foot. When the subtalar joint is in eversion, the two joints of the transverse tarsal joint become parallel. This state allows for more movement within the joint. However, when the subtalar joint is in inversion, the axes of the transverse joint become convergent. This convergence locks the joints and makes the midfoot rigid. This ability to switch between a flexible and a rigid state is vital for efficient movement.

Evolutionary history shows that the tarsus has changed significantly over time. Primitive tetrapods, such as Trematops, possessed a tarsus with three distinct rows of bones. The first row had three proximal tarsals named the tibiale, intermedium, and fibulare. The second row contained four centralia bones. The third row consisted of five distal tarsals. In most modern tetrapods, this pattern has been modified through the loss and fusion of bones. In reptiles and mammals, there are typically only two proximal tarsals. These are the calcaneus and the talus.

Different animal groups show unique adaptations of these bones. In mammals, the talus forms a hinge joint with the tibia. The calcaneus has also evolved to form a heel for the Achilles tendon to attach. These specific features are not found in reptiles, which have a simpler bone structure. Birds show an even more extreme change in their tarsus. In birds, the proximal tarsals have fused with the tibia. The distal bones have fused with the metatarsals to create a single tarsometatarsus bone. This evolution effectively gives the bird leg a third segment.