Between the head and the toes of the feet, joints have many different shapes and sizes. Many have something in common near the joint area, a pouch made to fit each joint filled with a fluid called synovial fluid, which cushions the movement and pressure exerted on the joint, protecting it. These small cushions are called bursae (bursae in Latin).

When a capsule becomes irritated, either by a direct blow or because the nearest joint performs the same movement repeatedly (such as tennis serve), it can cause bursitis. Bursitis can also develop when the body has to change the balance or movement to accommodate the differences, for example, when a person has one leg longer than another.