Peroneal Tendonitis is not that common in runners, but it probably is the most common cause of pain on the outside of the ankle area in runners.
This usually starts of as just an ache either just above or below that lateral malleolus (ankle bone) where the tendon runs past. It will get worse with activity and is helped with rest. The most common cause of the problem is simple overuse – too much load is applied to the peroneal tendons when they have not been given enough time to adapt or get used to the loads. It is also more likely to occur in those with ankle that roll outwards easily (lower supination resistance) as this makes the peroneal muscles work harder resulting in the inflammation of the tendon that is peroneal tendonitis.
The simplest solution to peroneal tendonitis is simply reducing load and putting a lateral wedge in the shoe to make the foot roll inwards so that the peroneal muscles do not have to work so hard. Loading programs and strengthening of the peroneal muscles are also helpful as that can help the tendon adapt to the loads of higher volumes of running.