Popliteus tendinitis is an overuse condition, where the attachment of the popliteus tendon to the lateral femoral condyle (bone on the outer side of the knee) becomes painful.
Looking at the knee from the side, the circle shows the site of pain from popliteus tendinitis.
Acute calcific tendinitis of the popliteus tendon--an unusual site and clinical syndrome.. Tibrewal SB. Ann R Coll Surg Engl. 2002 Sep; 84(5): 338–341.