Joint classification puts the ankle joint into the category of a mortise joint and it is specialised for its function. A front and rear movement action is performed by the ankle hinge for walking and it stabilises the lower leg on rough ground and when moving. The ankle forms the connection between the lower leg and the foot to facilitation the transfer of the weight of the body to the mobile and stable foot. The upper joint structures are mostly made up of the tibial surface with the fibula making up a small part of the lateral side of the joint.
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.
As the weight comes down through the tibia it is distributed forwards through the navicular on to the metatarsals in the forefoot, down and backwards to the calcaneal joint and the heel. The metatarsals spread out in the front of the foot, lying in parallel and with mobile joints between them, allowing a great degree of mobility in the foot to cope with unstable surfaces. With the foot arches the weight is borne on the heads of the first and fifth metatarsals primarily, although if the foot develops problems the remaining metatarsals can also bear weight and become problematic.
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 foot is amazingly engineered to cope with the strong requirements which it is asked to. When bearing weight the direct downward forces are of a very high level and these are routed through the foot and transmitted further. The tarsal bones make up the foot arch and the forces are taken by their arched structure and by the ligaments which connect the individual foot joints. The muscles of the foot also have a strong role in managing to keep the foot structurally stable against the weight of the body and the forces generated by large body movements.
On the outside of the shin lies the prominence of the tibialis anterior muscle, with its tendon obvious and prominent as it runs across the front of the ankle towards the inside to insert into the foot. Three bones which make up the top of the arch of the foot are the insertion points for the tibialis anterior tendon and it helps lift the arch as the muscle contracts. From the posterior calf the tibialis posterior muscle's tendon runs around the inner ankle bone and finally inserts near the tendon of the tibialis anterior, drawing the bone posteriorly and increasing the arch.
The arch is pulled up to some degree by these two muscles working in concert to pull it up and stabilise it from the side against the weight of the body. The spring of the foot, vital in running and walking, is maintained by this. Another important muscle is the peroneus longus which runs down the leg and its tendon runs under the outside of the foot to insert over towards the first toe. This arrangement stabilises the foot from any direction as there are muscles which pull from each direction to maintain the foot posture against the forces generated by movement and body mass.