The semimembranosus and semitendinosus muscles lie side by side on the inside back of the thigh. They are much thinner than the biceps femoris and have a unique relationship. The semimembranosus has a long upper tendon and a short lower tendon, and the semitendinosus has a short upper tendon and a long lower tendon. Because of this, the muscle masses of each of these muscles are in line with one another and form a cylindrical mass. Therefore, when well developed they appear as one long muscle
The semimembranosus attaches to the tuberosity of the ischium at the upper end.
The semimembranosus attaches to the posterior medial condyle of the tibia at the lower end.
Knee joint flexion. The semimembranosus also extends the thigh and assists in hip joint medial rotation.