Semitendinosus, as its name suggests, has a long tendon.
Semitendinosus arises from the supero-medial part of the ischial tuberosity of the hip bone, in common with the tendon of origin of the long head of the biceps femoris muscle. From this origin, the semitendinosus muscle runs obliquely, infero-medially behind semimembranosus. Approximately halfway down the thigh, the semitendinosus muscle gives rise to a strong, rounded tendon.
In the lower part of the thigh, semitendinosus and semimembranosus together form the upper medial boundary of the popliteal fossa. Distally, semitendinosus is attached to the upper part of the medial surface of the tibial shaft postero-inferior to the insertion of sartorius and gracilis.
Pes anserinus is a commonly used expression (though not one that is officially recognized in anatomical nomenclature) for the partly joined flattened tendons of the insertion of sartorius, gracilis and semitendinosus. These tendons are inserted onto the medial surface of the upper part of the tibia. The fancied resemblance of this arrangement to a goose's foot is the reason for the name! Within this arrangement, the tendons of gracilis and semitendinosus are relatively thick, strong and cord-like, whereas the sartorius tendon is almost fascial in its appearance.
Semitendinosus is innervated by the tibial component of the sciatic nerve, derived from L5, S1 and 2.
As it is a hamstring muscle, its action is to assist in flexion of the knee and extension of the hip joint. It also medially rotates the hip joint when the hip is extended and medially rotates the lower leg when the knee is semi-flexed.