Popliteus is a small but important muscle at the back of the knee on the outside. The tendon can become inflamed causing popliteus tendinopathy. This happens as an overuse injury in sports which involve a lot of knee stability such as cross country running.
Pain is felt at the back of the knee on the outside. It usually starts gradually and may only be painful after exercise initially. It may feel tight behind the knee. As the condition progresses the pain may become more intrusive, interfering with participation, or occurring at rest.
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 ultrasound scan or MRI scan can be useful to exclude other problems. Treatment options may include very specific physiotherapy, or injections. Once you have recovered, good physio should be able to prevent this from coming back.