The Human Ankle

The ankle is often classified as a mortise joint and is a very special joint due to its position in the body. The ankle hinge performs forwards and backwards movement during gait and confers stability so that we can balance during motion and on rough surfaces. The lower leg is connected to the foot by the ankle and this permits the body weight to be transmitted across the joint to the propulsion unit which is the foot. Most of the upper part of the joint consists of the tibia with a small contribution made by the fibula on the lateral side.

The upper ankle mortise closely encloses the talus or ankle bone which itself balances on top of a series of tarsal and mid foot bones of the foot arch. The talar dome on the upper surface makes the joint with the under surface of the tibia but there are two other joints which the talus participates in. The talo-navicular joint is to the front and through which weight is transferred forwards. The complex talo-calcaneal joint is below the talus and takes body weight straight down.

Weight passing vertically down through the tibia is transmitted through the talus in an anterior direction through the navicular and towards the metatarsal bones and backwards to the calcaneum. The metatarsals are slender bones lying almost parallel to each other as they fan out towards the toes, conveying important degrees of stability and mobility to the foot. The foot has arches and in the front of the foot the weight is primarily taken on the metatarsal heads of the first and fifth rays. If the arch collapses the other metatarsal heads of the second to the fourth may also weight bear.

The ankle upward and downward movements are known as dorsiflexion (up) and plantarflexion (down) and the inwards and outwards movements of the foot do not occur at the ankle. The inwards movement is known as inversion, the outwards as eversion, and both of these movements occur at the talo-navicular, forefoot and talo-calcaneal joints. Together these complex joints allow the body weight to be held stable over the feet as the body moves and to allow the feet to cope with irregular surfaces. The foots design allows it to satisfy these competing demands.

The human foot is very well designed to manage the severe demands which are put upon it. The vertical forces which are developed in weight bearing are very significant and the foot has to cope with these and transmit them onwards. The central arch of the tarsal bones takes a good degree of weight, with the interconnecting ligaments between the joints absorbing much of the forces developed inside the foot. The foot muscles are also important in maintaining the structural integrity of the foot under the pressure of body weight and the momentum of large movements.

The tibialis anterior muscle is the prominent long muscle on the outside of the shin and you can see the tendon as it rolls down towards the ankle and crosses over to the inside to some degree. The tendon attaches to three bones which are placed at the top of the arch of the foot, so when the muscle contracts it lifts and supports the arch to some degree. The tibialis posterior muscle comes from the rear calf and round the inner side of the ankle bone to end up inserting close to the tibialis anterior tendon. It pulls the bone to the rear and accentuates the arch again.

Working together, the two above muscles pull the arch upwards from the top and clamp it from the side, making it stable and preventing the arch from being flattened by the weight of the body. This keeps the foot dynamic and allows the spring required in walking and running. Running down the outside of the foot and underneath the sole towards the big toe is the tendon of the peroneus longus muscle. This contributes to the sling effect where the muscles can contract from every direction to hold the foot stable against the destabilising forces of weight bearing combined with active movement.