The Semimembranosus is aptly named for its long flat tendon from the lower portion of the ischial tuberosity, and its middle muscular portion. It becomes entirely tendinous behind the medial condyle of the femur and inserts into the back of the medial tibial condyle.

The semimembranosus reinforces the posterior aspect of the knee joint, largely forming the popliteal cysts which are associated with the medial head of the gastrocnemius (and may fill the popliteal space).