Anatomy and Topography
The muscle is situated on the posterior aspect of the thigh, occupying a medial position within the hamstring group.
- Topography: Superficially, the muscle is covered by the gluteus maximus (m. gluteus maximus). Laterally, it borders the biceps femoris (m. biceps femoris). Anteriorly and somewhat deeper lies the broader semimembranosus muscle (m. semimembranosus).
- Origin: It originates from the ischial tuberosity (tuber ischiadicum) via a common tendon shared with the long head of the biceps femoris.
- Course: The muscle belly descends and transitions into a long tendon in the middle third of the thigh, spanning the entire lower third. The tendon passes posterior to the medial femoral condyle.
- Insertion: It inserts onto the proximal tibia at the medial surface of the tibial tuberosity (tuberositas tibiae). At this site, the semitendinosus tendon, along with the tendons of the sartorius (m. sartorius) and gracilis (m. gracilis) muscles, forms the pes anserinus. Some superficial fibers of the tendon blend into the fascia of the leg (fascia cruris).
Functions
As a biarticular muscle spanning two joints, the semitendinosus produces 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.
- Additional function: When the lower limb is fixed, it assists in extending the trunk.
Topography and Neighboring Structures
The semitendinosus plays a key role in forming the surface contours and boundaries of the popliteal region:
- Popliteal fossa (fossa poplitea): The tendon of the m. semitendinosus, together with the semimembranosus tendon, forms the superomedial boundary of the popliteal fossa. In comparison, the superolateral boundary is formed by the tendon of the biceps femoris. The neurovascular bundle (popliteal artery and vein, tibial nerve) passes between these tendons.
- Cross-section of the thigh: In the upper third of the thigh, the muscle is visualized within the posterior osteofascial compartment, which also transmits the sciatic nerve (n. ischiadicus).
Blood Supply and Innervation
- Blood supply: Provided by the perforating branches (aa. perforantes) of the profunda femoris artery (a. profunda femoris), which pierce the aponeurosis of the adductor magnus to reach the posterior thigh muscles.
- Innervation: Supplied by the tibial division of the sciatic nerve (n. tibialis) (L4–S1).
Clinical Significance
- Injuries: The muscle is prone to strains and tears (hamstring strain) during sprinting. Injury presents as pain localized near the ischial tuberosity and posterior to the medial femoral condyle.
- Strength testing: Muscle function is evaluated by resisting hip extension with the patient prone, as well as knee flexion at 30–90° while assessing medial rotation strength.
- Neurology: Sciatic nerve mononeuropathy leads to paresis or paralysis of the posterior thigh muscles, including the semitendinosus. To diagnose denervation-reinnervation changes (positive sharp waves, fibrillation potentials), needle electromyography (EMG) is typically performed 3–4 weeks after symptom onset.