Anatomy and Location
The semimembranosus muscle is located in the posterior thigh. It lies deep and medial to the semitendinosus muscle (m. semitendinosus), while its lateral border is adjacent to the biceps femoris (m. biceps femoris). In a cross-section of the upper third of the thigh, the muscle is situated within the posterior fascial compartment.
The muscle gets its name from its broad, flat tendon of origin, which arises from the ischial tuberosity (tuber ischiadicum).
In its distal portion (near the knee), the tendon splits into three distinct bands, forming a complex insertion:
- Direct (anterior) band: Attaches to the posterior-medial surface of the medial tibial condyle.
- Expansion to the fascia: Blends into the fascia covering the medial head of the gastrocnemius muscle.
- Reflected (inferior) band: Courses toward the knee joint capsule, where it continues as the oblique popliteal ligament (lig. popliteum obliquum).
The semimembranosus bursa (bursa m. semimembranosi) is located near the divergence of these three bands, reducing friction against neighboring structures.
Functions
The semimembranosus muscle performs the following actions:
- At the hip joint: Extends the thigh.
- At the knee joint: Flexes the leg.
- When the knee is flexed: Medially rotates the leg (assists in internal rotation).
Blood Supply and Innervation
- Innervation: Tibial nerve (n. tibialis), spinal segments L4–S1.
- Blood Supply: Medial femoral circumflex artery (a. circumflexa femoris medialis), perforating arteries (aa. perforantes), and popliteal artery (a. poplitea).
Topography and Relations
The semimembranosus muscle serves as an important anatomical landmark in the posterior lower extremity:
- Popliteal fossa (fossa poplitea): The tendon of the semimembranosus, together with the semitendinosus tendon, forms the superomedial border of this diamond-shaped space.
- Sciatic nerve (n. ischiadicus): Below the upper half of the thigh, the nerve runs in the fascial plane between the biceps femoris and the semimembranosus.
- Genicular vessels: The superior medial genicular artery (a. superior medialis genus) runs deep to the tendons of the semimembranosus and adductor magnus, wrapping around the medial femoral condyle.
- In the lower-medial thigh near the popliteal fossa, the distal portion of the adductor magnus (m. adductor magnus) lies deep to the semimembranosus and semitendinosus muscles.
Clinical Significance
- Hamstring strain: Common sports injury occurring during sprinting; can present with tenderness near the ischial tuberosity or posterior to the medial joint line.
- Strength testing: Assessed via resisted hip extension while prone, and knee flexion from 30° to 90° with a focus on medial rotation.
- Sciatic nerve pathology: Lesions result in paralysis of the posterior thigh muscles. Needle electromyography (EMG) of the semimembranosus helps detect denervation-reinnervation changes (such as positive sharp waves and fibrillation potentials) 3–4 weeks post-injury.
- Deep space infection: Abscesses or phlegmon can track along fascial planes in the region of the sciatic nerve and hamstring origins.