Your ankle is built like a carpenter's mortise and tenon joint
Woodworkers join timber by slotting a tongue into a snug rectangular socket. The human ankle uses the same trick, and it is the only joint in the body to do so. That design explains both why the ankle is so steady when the foot points up and why it is the most frequently injured major joint.
Three bones meet here: the tibia and fibula of the lower leg and the talus of the foot. The end of the tibia forms a roof, sometimes called the plafond, French for ceiling, and bony knobs on either side, the medial and lateral malleoli, hang down like walls. Together they make a rectangular socket, the mortise, into which the talus fits. The tibia carries far more of the body's weight across this hinge than the slimmer fibula does.
Strictly, the ankle region holds three joints. The main hinge lets the foot tilt up, called dorsiflexion, or point down, plantarflexion. Beneath it, the subtalar joint does so much of the work of angling the foot that some anatomists call it the lower ankle joint. With the foot pointed, the joint also allows a little sliding and rotation. Normal gaps between the talus and the malleoli are tiny, around 1.70 and 2.13 mm.
Stability depends on position. With the foot flexed upward, the bones lock tightly; with it pointed down, ligaments must do more of the holding. That is why the classic sprain happens when a pointed foot rolls under the leg, tearing the anterior talofibular ligament on the outside, which is weaker than the broad deltoid ligament on the inner side. Bands called retinacula strap the tendons down so they do not lift away from the bend, an effect known as bowstringing.
The ankle also feeds balance. Sensors in the surrounding muscles report their length to the brain, and a 2011 brain-scanning study linked ankle proprioception with balance. Aristotle and Leonardo da Vinci both pondered its role in walking. The word probably comes from roots meaning bent, related to Latin angulus. Some researchers suggest humans gave up nimble toes in exchange for finer control of the ankle.
Source: Ankle
