Our feet are incredible pieces of bio-engineering, allowing us to stand upright, walk, run, and carry loads. This article shows how the component parts of our feet work together with split-second timing over a wide variety of ground surfaces and gradients.

Up to three times body weight is placed through each foot during running. The foot has therefore evolved uniquely to allow humans to walk upright and to perform as a specialised structure for stability and shock absorption.

There are many structures in the foot that assist in this, some of them highly specialised, and all have to function with split-second timing to enable us to walk and run. Although the way this occurs varies between individuals, the basic pattern is the same for everyone.

Bones and Joints
 

The foot consists of 26 bones and in many ways resembles the hand. It has adapted uniquely, allowing humans to be the only animal that can effectively walk upright. The foot is divided into three main sections, the forefoot, mid-foot and rear-foot.

The rear-foot contacts the ground and absorbs impact by transforming it into leg rotation. The mid-foot comprises a unique set of bones structured in an arch to provide support. The forefoot allows us to push off the ground later in the step.

Rear-foot

The main joint in the rear-foot is the sub-talar joint, which is found just below the ankle joint and allows the heel to turn in and out in relationship to the leg and ground. Although they have their own distinct functions, the sub-talar and ankle joints work together as a unit to convert movement from leg to foot and vice versa. Efficient movement is required in these joints to allow us to pivot over a stable foot during walking and running.
The two main joints in the mid-foot are the talo-navicular joint and calcaneo-cuboid joints. These joints work together to allow the arch to lower and rise in response to sub-talar joint motion. If you stand and physically lower your arch you will notice that your heel turns outwards whilst your knee turns inwards.

This motion occurs at the sub-talar joint but requires motion in the talo-navicular and calcaneo-cuboid joints to allow it to occur. If the mid-foot were rigid, the motion would not occur and the foot would be unstable on uneven ground.

Forefoot

The main joints in the forefoot are the metatarso-phalangeal joints, which form the ball of the foot. They are formed by the long bones (metatarsals) of the toes. When walking and running, these joints bend when the heel lifts from the ground. If they are stiff due to injury or disease (e.g. arthritis) then movement is limited and we have to compensate the way in which we walk and run.

Tendons and Muscles

Tendons are the tissues that connect muscle to bone. Because muscles arise on one bone and attach to another, they must cross a joint. Therefore when a muscle contracts the force is transmitted through the tendons and pulls on the joint causing it to move. Muscles cannot push, so a joint is returned to its original position by a second force, either gravity or the pulling action of another muscle (called an antagonist).

Muscles can contract to move a joint in two ways:
1 Concentrically, which happens when muscle length shortens, bringing the two ends of a muscle closer together. If you place a weight in your hand and bend your elbow to bring your hand towards your shoulder so that the biceps muscle becomes prominent, this is concentric contraction as the muscle fibres are contracting and shortening.

2 By eccentric contraction. This occurs if a muscle is trying to control movement, as the muscle's fibres are contracting but lengthening. If you hold the same weight in your hand with the biceps contracted and then lower your hand slowly, this motion is being controlled by eccentric biceps contraction. Muscles function far more efficiently eccentrically than concentrically and, therefore, during walking and running, it is this type of function that is occurring most of the time, aiding stability and function.

Foot muscles and tendons

While there are many small muscles and tendons on the sole of the foot, the main muscles that control foot and ankle movement are located in the lower leg. Each muscle and tendon has a specific function and causes a specific movement. At a fairly basic level, the four main tendons around the ankle are the tibialis posterior, tibialis anterior, peroneus longus and Achilles tendon.  The tibialis posterior muscle and tendon run just behind the inside ankle bone (medial malleolus). During walking and running this motion helps to slow down subtalar joint motion (pronation).

The tibialis anterior muscle and tendon run down the front of the ankle and insert into the base of the big toe and first cuneiform. They help to slow down lowering of the forefoot during walking and running. 

The peroneus longus muscle and tendon run behind the outside ankle bone, beneath the cuboid bone and insert underneath the base of the big toe and cuneiform bones. It helps to stabilise the ankle joint and big toe against the ground. 

The calf muscle and Achilles tendon help to lift the heel from the ground towards the end of each step. The calf muscle consists of three muscles - the gastrocnemius, popliteus and soleus. The main muscle, gastrocnemius, attaches just above the knee and can therefore affect both knee and ankle function.

Ligaments and Fascia  
Ligaments join bones with bones. They are extremely tough structures, found around all joints that help control the stability of the joint and guide the direction of movement.

Given the numbers of bones in the foot there are a huge number of ligaments that help to maintain stability. The two main ligaments around the ankle joint are called the anterior talo-fibula and calcaneo-fibula ligaments. These are well-known as they are the ones commonly injured with an ankle strain.

The foot has a unique structure called the plantar fascia. It's rather like a ligament and runs from the bottom of the heel to the ball of the foot. When the heel lifts from the ground, causing the ball of the foot metatarso-phalangeal joints to bend, this places tension in the fascia and helps to stabilise the unique arch structure of the foot. It is not surprising, given the forces that are placed on the foot, that this structure can often become inflamed and painful. The condition is called plantar fasciitis.

Basic Function  

The basic pattern is the same for everyone. Obviously this set of movements occurs in a specific sequence with split second timing. Abnormal function can disrupt the whole mechanism and increase the likelihood of injury in the foot, ankle, knee, leg, hip and back.

Normally the outside of the back of the heel is the first point of contact. At this point, the heel would be turned to face slightly inwards towards the other leg. As the foot hits the ground and lowers, the heel will roll down due to motion at the joint below the ankle, the sub-talar joint.

This motion is called pronation and is slowed by the tendon towards the back of the ankle, the tibialis posterior tendon. A few degrees of pronation are normal as this helps to absorb shock and also causes the rest of the foot to become mobile and therefore able to adapt to varying surfaces and angles. As the inside forefoot lands, the tendon towards the front of the ankle, the tibialis anterior, controls the motion. The body passes over the foot and the heel begins to rise, then turn inwards again, with a motion called supination. This causes the foot to become rigid so that it is a good lever for pushing off.

This motion is assisted by contraction of the peroneus longus muscle, as it stabilises the big toe against the ground, and tensions the plantar fascia, increasing stability. The calf muscle contracts rapidly, helping lift the heel and push the lower leg into the swing phase of the step.

It is important to appreciate and look after your feet, particularly as they are so important for enjoying activity.