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.

site of popliteus tendinitis
Looking at the knee from the side, the circle shows the site of pain from popliteus tendinitis.
Popliteus tendonitis

Pain in the back of the knee

An overview with illustrations.