The popliteus muscle is a small but important muscle at the back of the knee.

The tendon of the popliteus passes into the joint cavity through the capsule at the back of the lateral meniscus, on its way to the side of the femur. The tendon can be seen from inside the knee during an arthroscopy.
Function of the popliteus muscle
The popliteus muscle enables the leg to 'unlock' from the stable standing position of rest by laterally rotating the femur on the tibia.
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CLINICAL PUBLICATIONS
Shape and innervation of popliteus muscle. Hwang K, Lee KM, Han SH and Kim SG. Anat Cell Biol. 2010 Jun; 43(2): 165–168.
Popliteus impingement after TKA may occur with well-sized prostheses. Bonnin MP, de Kok A, Verstraete M, Van Hoof T, Van der Straten C, Saffarini M and JVictor J. Knee Surg Sports Traumatol Arthrosc. 2017; 25(6): 1720–1730.