The biceps muscle in the leg has a tendon behind and to the outside of the knee. This can become inflamed as part of an over use injury in sports that involve rapid acceleration and deceleration. Microscopic tears accumulate faster than they can be repaired causing inflammation.
You will feel pain in the tendons at the back of the knee on the outside. Initially the pain is only felt following exercise, but as the condition progresses it may become more intrusive and interfere with sports and day to day activities. Certain biomechanical problems and tightness of the muscle make this condition more likely.
If you think you have this condition it is well worth getting it checked out by Mr Willis-Owen. In the mean time you should avoid any aggravating activities which may prolong the problem. It generally won't get better on its own. The first thing to do is to reduce the problem activities. Anti-inflammatories and ice are helpful after exercise.
Treatment involves first confirming the diagnosis, then establishing the cause. An MRI scan can be useful to exclude other problems. Treatment options may include very specific physiotherapy, shockwave treatment, or injections. Once you have recovered, good physio should be able to prevent this from coming back.