In runners if there is pain on the outside of the ankle joint and there is no history of trauma, then the most likely problem is what is known as peroneal tendonitis. The peroneal muscles are on the outside of the leg and there tendons go around the outside of the ankle joint to then go to the outside and the bottom of the foot. The primary function of these muscles are to stabilize and support the foot.
If a tendonitis develops in the tendons of these peroneal muscles in runners, then the cause is most likely overuse. That is doing too much too soon and the tendons are not given a chance to adapt to the increasing loads that are placed on them from that too rapid increase in the distances and speeds being run. It is important that after hard longer runs that the body is given adequate rest before the next load is applied. if a load is applied too soon before the tissues have had time to recover, then there is an increased risk for an injury.
The pain of peroneal tendonitis usually just starts of as a minor ache, either just above or below the ankle bone on the outside of the ankle. Initially there is no swelling, but that will tend to develop later as the pain increases if the problem is not addressed.
To deal with peroneal tendonitis, the runners needs to cut back the running to tolerable levels to allow the tendons to heal. Podiatrists often use a lateral wedge to treat peroneal tendonitis in the short term as this decreases the activity of the peroneal muscles, so there is less load on the tendons. This goes under the heel in the running shoes. After the pain in the tendons starts to settle down, then a gradual and slow increase is needed in the distances run to allow the tendon to adapt to those loads are needed. A strengthening program is also very helpful.