"The accessory semimembranosus muscle is a rare accessory muscle of the posterior compartment of the thigh. It arises from the distal aspect of the semimembranosus muscle belly and courses through the popliteal fossa between it and the semitendinosus muscle medially and the biceps femoris laterally. It does not have an osseous insertion, instead attaching to the proximal medial head of the gastrocnemius muscle 1."