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Semimembranosus muscle

The semimembranosus is one of the three hamstring muscles of the posterior thigh. It lies deep to the semitendinosus and medial to the biceps femoris. Characterized by its broad, membranous proximal tendon, the muscle descends along the posterior thigh and inserts on the tibia. It plays a major role in hip extension, knee flexion, and rotation of the leg. Clinically, it is significant in sports injuries, hamstring strains, and as a landmark in imaging and surgery.

Synonyms

  • Medial hamstring muscle

  • Semimembranosus of posterior thigh

Origin and Insertion

  • Origin:

    • Superolateral impression of the ischial tuberosity (common hamstring origin)

  • Course:

    • Descends obliquely down the medial posterior thigh, deep to semitendinosus

    • Forms a long flattened tendon near the knee

  • Insertion:

    • Posteromedial surface of the medial tibial condyle

    • Expansions contribute to:

      • Oblique popliteal ligament of the knee

      • Fascia over popliteus muscle

Nerve Supply

  • Tibial division of the sciatic nerve (L5–S2)

Arterial Supply

  • Perforating branches of the profunda femoris artery

  • Inferior gluteal artery

  • Popliteal artery (distal contribution)

Venous Drainage

  • Follows arterial supply into profunda femoris vein → femoral vein

  • Popliteal vein (distally)

Function

  • Extension of the thigh at the hip joint

  • Flexion of the leg at the knee joint

  • Medial rotation of the leg when the knee is flexed

  • Stabilization of hip and knee during dynamic activities

Clinical Significance

  • Commonly involved in hamstring strains and tendinopathies

  • Avulsion injuries may occur at the ischial tuberosity in athletes

  • Distal tendon expansions may be confused with pathology on imaging

  • Landmark in knee imaging and surgeries, especially around the posteromedial corner

MRI Appearance

T1-weighted images:

  • Muscle shows low-to-intermediate signal intensity

  • Surrounding fat appears bright

  • Tendinous origin and insertion: dark linear signal

T2-weighted images:

  • Normal muscle: low-to-intermediate signal

  • Edema, strain, or tear: bright hyperintense areas within muscle belly or tendon

STIR (Short Tau Inversion Recovery):

  • Normal muscle: Intermediate signal

  • Pathology (edema, strain, inflammation): bright hyperintensity

  • Useful for detecting subtle hamstring injuries

T1 Fat-Sat Post-Contrast:

  • Normal muscle: mild homogeneous enhancement

  • Pathology: heterogeneous or nodular enhancement in myositis, tumor, or abscess

  • Tendinous tears: rim enhancement with non-enhancing central defect

CT Appearance

Non-Contrast CT:

  • Muscle appears as soft tissue density in posterior medial thigh

  • Chronic fatty atrophy: areas of low attenuation

  • Hematoma: hyperdense acutely, becomes hypodense over time

Post-Contrast CT:

  • Normal muscle: mild uniform enhancement

  • Pathology: heterogeneous or irregular enhancement (inflammation, tumor, abscess)

  • Abscess: rim enhancement with central hypodensity

MRI image

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MRI image

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CT image

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Semimembranosus muscle ct coronal image

MRI images

Semimembranosus muscle image

MRI images

Semimembranosus muscle mri image