Semitendinosus muscle

Origin and insertion

Semitendinosus muscle attaches between the ischial tuberosity of pelvis and proximal end of tibia. This muscle is specific by having almost half of its mass (semi-) made of a rounded tendon (-tendinosus). 

Semitendinosus shares the origin with the long head of biceps femoris. Both muscles originate with a common tendon from an posteromedial impression on the superior part of ischial tuberosity. 

The muscle fibers then descend in the posterior thigh and around the middle of the thigh, they give way to a cord-like tendon. The semitendinosus tendon takes the inferomedial course, passing posteriorly to the medial condyle of femur and laterally to the medial condyle of tibia. As it passes the medial condyle, it partially blends with the tendons of the gracilis and sartorius muscles and forms a common insertion tendon called the pes anserinus. Pes anserinus inserts to the medial surface of proximal end of tibia.

Relations

Together with semimembranosus, semitendinosus comprises the superomedial border of popliteal fossa. Its tendon can be palpated when the knee is flexed against resistance (for example while squatting), as a most lateral and posterior tendon of the superomedial border.

The ischial attachment of the semitendinosus lies deep to gluteus maximus muscle. Being in the same plane and located medial to biceps femoris, the muscle belly and its tendon course posteriorly to adductor magnus and semimembranosus in the thigh.

Tibial attachment has a couple of interesting relations; 

• Within pes anserinus, semitendinosus inserts along a vertical line posteriorly to sartorius and posteroinferiorly to gracilis. 
• All three muscles in pes anserinus are separated from each other with a bursa. Moreover, pes anserinus as a whole is separated from the medial collateral ligament of the knee by the anserine bursa.

In order to remember the muscles that comprise the pes anserinus you can use the following mnemonic;

Say Grace Sarah

• Sartorius muscle
• Gracilis muscle
• Semitendinosus muscle

Innervation

Semitendinosus is innervated by the tibial division of sciatic nerve (L5-S2).

Blood supply

This muscle is supplied by branches from three large arterial sources;

• Femoral artery - via deep femoral artery and its first perforating branch, and medial femoral circumflex artery
• Internal iliac artery - through inferior gluteal artery 
• Popliteal artery - via inferior medial geniculate artery.

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).

The function of the muscle depends on the position of the body parts it moves and which one of its attachments is fixed (i.e. origin or insertion).

There are two functions of semitendinosus related to the hip joint when its tibial attachment is fixed;

• When the body’s in the anatomical position, it internally rotates the thigh 
• If the trunk is flexed anteriorly, this muscle extends the thigh

In case ischial attachment is fixed, semitendinosus has two functions on the knee joint;

• When the lower limb is in the anatomical position, it flexes the leg 
• If the knee is semiflexed, semitendinosus internally rotates the leg acting together with all the hamstring muscles.

Besides stabilizing the pelvis together with all the short hip muscles that attach between the pelvis and proximal femur, semitendinosus also acts to stabilize the knee. In fact, due to the location of their insertion points, all the hamstrings act as the accessory medial stabilizers of the knee, complementing the function of the medial collateral ligament of knee.