Anatomy of the Foot and Ankle
The ankle joint is formed by the leg bones (tibia and fibula) and the talus. This joint allows the foot to move up down. The sideways (inversion/eversion) movement of the heel is mainly the joint under the talus (sub-talar joint). Normally the three bones of the ankle joint are covered by smooth articular cartilage which allows the bones to glide over each other and the joint to move up down freely.
The Achilles tendon is the thick cord felt behind the ankle which attaches the powerful calf muscles to the back of the heel. It is the biggest and strongest tendon in the body and made of fibres called collagen. The Achilles takes all the force during walking and running as the calf muscles contract to pull the foot down. Therefore, if the tendon is injured it is difficult to walk normally.
The two oval-shaped bones under the big toe joint are called the medial and lateral Sesamoids. They measure approximately 1x 1.5 cm and lie within the two tendons that move the big toe (flexor hallucis brevis FHB). They work in the same way as the knee cap acts to straighten the knee. Therefore, the surface of the bones should be very smooth (covered with articular cartilage) to allow the bones to glide smoothly over the head of the first metatarsal bones (see diagram).
The peroneal muscles (peroneus brevis and peroneus longus) lie in the lateral (outside) part of the lower leg. They form tendons which lie within a sheath which restrains them as they pass behind the outer bone of the ankle joint (fibula) before attaching to bones in the foot. The peroneal tendons are responsible for pulling the foot downwards and outwards and contribute to stabilising the ankle joint.
Source - foot and ankle injuries