The semimembranosus is a muscle in the back of the thigh. It is the most medial of the three hamstring muscles.The semimembranosus helps to extend (straighten) the hip joint and flex (bend) the knee joint. It also helps medially rotate the knee: the tibia medially rotates on the femur when the knee is flexed. Medially rotates the femur when the hip is extended. The muscle can also aid in counteracting the forward bending at the hip joint.