Semimembranosus

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Description

Semimembranosis is one of a group of muscles called the Hamstrings. It is located on the posteromedial side of the thigh deep to Semitendinosus. Its origin is the ischial tuberosity on the inferior pelvis and the insertion is the medial tibial condyle. It's primary action is knee flexion, hip extension and knee internal rotation. In the lower part of the thigh, semitendinosus and semimembranosus together form the upper medial boundary of the popliteal fossa.

Origin

A strong membranous tendon attaches to the upper lateral facet on the rough part of the ischial tuberosity.

Insertion

An aponeurotic tendon arises at the distal end of the semimembranosus where it narrows and attaches to a horizontal groove on the posteromedial surface of the medial tibial condyle. The tendon then passes upwards and laterally to form the oblique popliteal ligament.

A bursae separate the muscle from the medial head of the tibia and the medial head of the Gastrocnemius

Nerve

Tibial division of the Sciatic Nerve (root value L5, S1 and 2).

Nerve supply for the skin covering the muscle is L2.

Artery

Branches from the internal iliac, popliteal, and profunda femoris arteries.

Function

• Hip extension
  • Agonists: Gluteus Maximus, semitendinosus, Biceps Femoris (long head), and Adductor Magnus (posterior part)
  • Antagonists: Psoas Major and Iliacus

• Knee flexion
  • Agonists: biceps femoris (long head), biceps femoris (short head), and semitendinosus
  • Antagonists: Vastus Lateralis, Vastus Medialis, Vastus Intermedius, and Rectus Femoris

Gracilis, Sartorius, Popliteus, gastrocnemius, and plantaris assist with flexion of the knee.

• Knee internal rotation of the knee when it is flexed
  • Agonists: popliteus and semitendinosus
  • Antagonists: biceps femoris (long head) and biceps femoris (short head)

• Sartorius and Gracilis assist with internal rotation of the knee when the knee is flexed.

Clinical relevance

Hamstring Syndrome

The 'Hamstring Syndrome' is often confused with the piriformis syndrome. This typically affects athletes who present with pain near the ischial tuberosity. This is generally thought to be an insertional tendopathy at the ischium but may also involve some compression on the sciatic nerve. The pain in hamstring syndrome typically radiates down the posterior thigh or popliteal region and is exacerbated by any activity that stretches the hamstrings. This is often seen in sprinters or hurdlers.

On examination there is exquisite tenderness over the ischial tuberosity and percussion in that region may reproduce the sciatic distribution of pain. Most cases can be managed with rest, anti-inflammatory agents and steroid injections, but occasionally exploration of the sciatic nerve will release of various bands is necessary. The alternative is release or slide of the tendon attachment of the hamstring tendons at the ischial tuberosity.

Baker's cyst

The bursae that separates the muscle from the medial head of the tibia and the medial head of the Gastrocnemius may at times become enlarged with distended fluid. This swelling is termed 'Baker's cyst' (described by Morrant Baker in the 19th century as a cystic mass in the popliteal fossae of children).

Assessment

Palpation

It lies deep to semitendinosus and is difficult to palpate, but can be felt easier when the knee is flexed.