Semimembranosus muscle

Location and anatomy

The semimembranosus muscle is located on the posterior aspect of the thigh. It lies medially and somewhat deeper than the semitendinosus muscle (m. semitendinosus), and its lateral margin borders the biceps femoris muscle (m. biceps femoris). On a cross-section of the upper third of the thigh, the muscle is visualized within the posterior osteofascial compartment.

The structure got its name due to the wide, flat tendon by which it originates from the ischial tuberosity (tuber ischiadicum).

In its distal part (closer to the knee), the muscle tendon splits into three bundles, forming a complex attachment:

• Inner bundle — attaches to the posteromedial surface of the medial condyle of the tibia.
• Middle bundle — blends into the fascia covering the medial head of the gastrocnemius muscle.
• Outer bundle — extends toward the capsule of the knee joint, where it continues as the oblique popliteal ligament (lig. popliteum obliquum).

At the site where these three bundles diverge lies the semimembranosus bursa (bursa m. semimembranosi).

What is its function?

The semimembranosus muscle performs the following actions:

• At the hip joint — extends the thigh.
• At the knee joint — flexes the leg.
• When the knee joint is flexed — rotates the leg medially.

Blood supply and innervation

• Innervation: tibial nerve (n. tibialis), L4–S1 segments.
• Blood supply: medial circumflex femoral artery (a. circumflexa femoris medialis), perforating arteries (aa. perforantes), and popliteal artery (a. poplitea).

Topography and adjacent structures

The semimembranosus muscle serves as a topographic landmark on the posterior aspect of the lower limb:

• Popliteal fossa (fossa poplitea): the tendon of the semimembranosus muscle, together with the tendon of the semitendinosus muscle, forms the upper-medial boundary of this diamond-shaped space.
• Sciatic nerve (n. ischiadicus): below the upper half of the thigh, this nerve runs in the space between the biceps femoris and semimembranosus muscles.
• Knee vessels: the superior medial genicular artery (a. superior medialis genus) runs beneath the tendons of the semimembranosus and adductor magnus muscles, curving around the medial femoral condyle.
• In the lower medial region of the thigh, at the border with the popliteal fossa, the distal part of the adductor magnus muscle (m. adductor magnus) is located beneath the semimembranosus and semitendinosus muscles.

Clinical significance

• Sports injuries: hamstring strain occurs during sprint running; tenderness may occur along the ischial tuberosity and posterior to the medial condyle.
• Strength tests: to evaluate muscle function, prone resisted hip extension tests are performed, as well as knee flexion at a 30–90° angle with assessment of medial rotation.
• Sciatic neuropathy: nerve damage leads to paresis or plegia of the posterior thigh muscles. To detect denervation-reinnervation changes, including positive sharp waves and fibrillation potentials, needle electromyography (EMG) of the semimembranosus muscle is performed 3–4 weeks after symptom onset.
• Purulent processes: a phlegmon may form in the area of the sciatic nerve.

Class and exam questions

What else to study on this topic

• Semitendinosus muscle (m. semitendinosus)
• Biceps femoris muscle (m. biceps femoris)
• Popliteal fossa (fossa poplitea)
• Sciatic nerve (n. ischiadicus)

Other topics in the "Anatomical Structures" section

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