Peroneal tendonitis is an uncommon problem with the tendons on the lateral side of the ankle joint. The condition in most cases occurs in runners where the strains on these structures are therefore higher. There are two peroneal muscles on the outside of the leg whose tendons go across the outside of the ankle joint with one tendon attaching on the lateral side of the foot at the base of the fifth metatarsal. The other tendon goes underneath the foot to attach to an spot near the middle of the arch of the foot. The muscles have many different functions, but a principal one is to prevent the rearfoot rolling outwards and ending up with a sprained ankle. Because they work hard during that action, the load on the tendons might be too much for the tendon to take and they end up having a peroneal tendinopathy.
Typically the condition commences with pain either over or just beneath the lateral ankle bone without or with some puffiness. In some the swelling develops later. With continued activity the symptoms becomes more constant and gradually worse. A typical feature in those with peroneal tendinopathy is a decreased supination resistance. Because of this it's easy for the rearfoot to supinate or roll laterally. This could cause the peroneal tendons to be really active, so if you then combine it with higher level of sporting activity, then the tendon is at higher risk for an overuse injury.
The treating of Peroneal Tendonitis almost always begins with reducing the strain by decreasing activity levels and also the use of shoe wedging or foot orthoses to pronate or tip the feet inwards so the muscle doesn't have to function as hard. Ice and anti-inflammatory drugs could also help decrease the pain and swelling. Over the medium to longer term increasing load by the way of exercise needs to be put on the tendon in order that it can get accustomed to the stresses placed on it. In some situations, surgery is often needed.