Semitendinosus Muscle is one of the hamstring muscles located in the posterior-medial thigh. It originates from the inferior-medial impression on the superior part of the ischial tuberosity and shares a tendon with the biceps femoris. The long distal tendon of the semitendinosus starts below the mid-thigh on the posterior of the semimembranosus and turns around the medial condyle of the tibia to insert onto the superomedial aspect of the tibia. The tendons of the semitendinosus, sartorius, and gracilis eventually conjoin to form the pes anserinus. The semitendinosus can also attach to the crural fascia of the leg and is usually the only attachment to this fascia [rx].
The semitendinosus muscle is a member of the posterior component of the thigh which also includes the biceps femoris and the semimembranosus muscles. These three muscles together are collectively referred to as the hamstring muscle complex which serves as hip extensors and knee flexors that are integral to gait and running. Injuries to the hamstring muscle complex are commonly seen in athletes and are usually brought on through explosive or high speed running.
Origin of Semitendinosus Muscle
- Origin – Tuberosity of ischium by a tendon common with the long head of the biceps femoris.
- Semitendinosus muscle attaches between the ischial tuberosity of pelvis and the proximal end of the tibia. This muscle is specific by having almost half of its mass (semi-) made of a rounded tendon (-tendinosis). Semitendinosus shares the origin with the long head of biceps femoris. Both muscles originate with a common tendon from a posteromedial impression on the superior part of the ischial tuberosity.
- Insertion – Proximal part of the medial surface of the tibia and deep crural fascia. The tendon of the semitendinosus contributes to the distal fibers of the pes anserinus tendon – a tendon that is comprised of the distal tendons of the semitendinosus, gracilis, and sartorius.
- The semitendinosus is a superficial muscle of the posterior thigh, bordered by the long head of the biceps femoris laterally, and the semimembranosus medially (along with portions of the adductor Magnus). All of the hamstring muscles (including the biceps femoris) lay in the posterior compartment of the thigh, with the semitendinosus bordered anteromedially by the posteromedial intermuscular septum.
- The semitendinosus can be palpated with relative ease by locating the space between the two large bands that comprise the hamstring tendons just superior to the posterior knee. As you course your fingers up the “valley” created by the division in the hamstrings. You can feel musculotendinous junction invest into the long cylindrical
Nerve Supply
- Semitendinosus is innervated by the tibial division of sciatic nerve (L5-S2).
- A lower motor neuron exits to the sacral plexus exiting through the spinal levels L5-S2. From the sacral plexus, the lower motor neuron travels down the sciatic nerve.[rx]
Blood Supply
This muscle is supplied by branches from three large arterial sources;
- Femoral artery – via the deep femoral artery and its first perforating branch, and medial femoral circumflex artery
- Internal iliac artery – through the inferior gluteal artery
- Popliteal artery – via an inferior medial geniculate artery.
Functions of Semitendinosus Muscle
- 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. It counteracts forward bending at the hips as well.[rx]
- 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).
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.
Extension of the thigh at the hip
- Agonists: gluteus maximus, semimembranosus, biceps femoris (long head), and adductor magnus (posterior part)
- Antagonists: psoas major and iliacus
The semitendinosus is also a weak medial rotator of the hip.
Flexion of the leg at the knee
- Agonists: biceps femoris (long head), biceps femoris (short head), and semimembranosus
- Antagonists: vastus lateralis, vastus medialis, vastus intermedius, and rectus femoris
Gracilis, sartorius, popliteus, gastrocnemius, and plantaris assist with flexion of the knee.
Internal rotation of the knee when the knee is flexed
- Agonists: popliteus and semimembranosus
- Antagonist: biceps femoris (long head) and biceps femoris (short head)
Sartorius and gracilis assist with internal rotation of the knee.
References
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