Location and anatomy
The muscle is located on the posterior surface of the thigh, occupying a medial position within the posterior muscle group.
- Topography: superiorly, the muscle is covered by the gluteus maximus (m. gluteus maximus). Laterally, it borders the biceps femoris (m. biceps femoris). Anteriorly and slightly deeper lies the broader semimembranosus muscle (m. semimembranosus).
- Origin: originates from the ischial tuberosity (tuber ischiadicum) via a common tendon with the long head of the biceps femoris.
- Course of fibers: the muscle belly descends and, in the middle third of the thigh, transitions into a long tendon that occupies the entire lower third of the muscle. The tendon passes posterior to the medial femoral condyle.
- Insertion: inserts into the tibial tuberosity (tuberositas tibiae). At this site, the tendon of the semitendinosus muscle, together with the tendons of the sartorius (m. sartorius) and gracilis (m. gracilis) muscles, forms a structure known as the pes anserinus (pes anserinus). Some of the superficial tendon fibers blend into the crural fascia.
Functions
The semitendinosus muscle crosses two joints and performs the following movements:
- At the hip joint: extends the thigh.
- At the knee joint: flexes the leg. When the knee is flexed, it performs medial rotation (internal rotation) of the leg.
- Additionally: when the lower extremity is fixed, the muscle participates in trunk extension.
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Topography and adjacent structures
The semitendinosus muscle plays an important role in forming the contours and boundaries of the popliteal region:
- Popliteal fossa (fossa poplitea): the tendon of m. semitendinosus along with the tendon of the semimembranosus muscle forms the superomedial border of the popliteal fossa. For comparison, the superolateral border is formed by the tendon of the biceps femoris muscle. Passing between these tendons is the neurovascular bundle (popliteal artery and vein, tibial nerve).
- Transverse section of the thigh: in the upper third of the thigh, the muscle is visualized in the posterior osteofascial compartment. The sciatic nerve (n. ischiadicus) runs in this same compartment.
Blood supply and innervation
- Blood supply: provided by perforating branches (aa. perforantes). These are branches of the deep femoral artery (a. profunda femoris) that pierce the tendons of the adductor muscles to reach the posterior compartment of the thigh.
- Innervation: supplied by the tibial nerve (n. tibialis) (L4–S1).
Clinical significance
- Injuries: the muscle is frequently subject to strains and tears (hamstring strain) during sprinting. Injury manifests as pain along the ischial tuberosity and posterior to the medial condyle.
- Strength tests: muscle function is assessed by extending the thigh against resistance with the patient in a prone position, as well as by flexing the knee to 30–90° while evaluating the strength of medial rotation.
- Neurology: mononeuropathy of the sciatic nerve leads to paresis or plegia of the posterior thigh muscles, including the semitendinosus. To diagnose denervation-reinnervation changes (positive sharp waves, fibrillation potentials) 3–4 weeks after symptom onset, needle electromyography (EMG) is performed.
Class and exam questions
Which muscles form the pes anserinus?
The superficial pes anserinus is formed by the tendons of three muscles: semitendinosus, sartorius, and gracilis. They attach together to the tibial tuberosity.
How does the semitendinosus muscle participate in forming the popliteal fossa?
Its tendon, together with the tendon of the semimembranosus muscle, forms the supero-medial boundary of the diamond-shaped popliteal fossa.
What symptoms occur when the nerve innervating this muscle is damaged?
Mononeuropathy of the sciatic nerve leads to paresis or plegia of the posterior thigh muscles, including the semitendinosus. To diagnose denervation-reinnervation changes 3–4 weeks after symptom onset, needle EMG is performed.
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What else to study on this topic
- Semimembranosus muscle (m. semimembranosus)
- Biceps femoris muscle (m. biceps femoris)
- Popliteal fossa (fossa poplitea)
- Sciatic nerve (n. ischiadicus)
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