Function. Attaching between the hip and leg, the semitendinosus produces movements on both hip and knee joints producing thigh extension, internal rotation, pelvis stabilization (hip joint), leg flexion and internal rotation (knee joint).Click to see full answer. Keeping this in consideration, what is the action of the Semimembranosus?It’s primary action is knee flexion, hip extension and knee internal rotation. In the lower part of the thigh, semitendinosus and semimembranosus together form the upper medial boundary of the popliteal fossa.Likewise, what is the origin insertion and action of the Semitendinosus? The semitendinosus muscle begins at the inner surface of the base of the pelvis (known as the tuberosity of the ischium) and the sacrotuberous ligament. It inserts at the medial tibial condyle. One may also ask, what is the function of the Semitendinosus? The semitendinosus muscle lies between the other two. These three muscles work collectively to flex the knee and extend the hip. The muscle also helps to medially rotate the tibia on the femur when the knee is flexed and medially rotate the femur when the hip is extended.What is the action of the biceps femoris?The functions of the biceps femoris are flexion and external rotation of the knee. The fibers of the long head of the biceps femoris also aid in extension of the thigh.