Ankle

What it is

The talocrural joint: a hinge formed by the lower ends of the tibia and fibula gripping the upper surface of the talus between them, like a spanner on a nut. The two leg bones form a socket — the mortise — and the talus is the tenon.

The talus is a remarkable bone. It has no muscles attaching to it at all; it is moved entirely by the bones and ligaments around it, and much of its surface is articular cartilage.

The joint is held by strong ligaments on both sides. The outer ligaments are the ones injured in the common ankle sprain; the inner deltoid ligament is much stronger and less often torn.

The talus is wider at the front than the back. That single fact explains the joint's behaviour: dorsiflexed, the wide part is wedged into the mortise and the joint is tight and stable; plantarflexed, the narrow part sits in it and the joint has more play. Ankles are most vulnerable when pointed down, which is exactly the position of a foot landing badly.

What it does

Dorsiflexion and plantarflexion — lifting the foot up and pointing it down. That is all this joint does. Turning the sole in and out happens below it, at the subtalar joint.

What loads it

Everything on your feet, and it takes several times body weight in running and landing.

What is commonly got wrong about it

That inversion sprains happen at the ankle joint. The rolling motion is largely subtalar; what tears are the outer ankle ligaments as the joint is forced beyond its range in plantarflexion.

That limited dorsiflexion is always tight calves. It restricts squat depth for many people, and the restriction can be muscular, capsular, or bony — the front of the tibia meeting the neck of the talus. Only the first responds well to stretching, which is why some people gain range quickly and others never do.

That ankle sprains are trivial. They are the most common musculoskeletal injury and a substantial proportion of people go on to have ongoing symptoms or repeat sprains. That is a clinical matter and not something this site addresses.