Semimembranosus muscle

Semimembranosus muscle
Muscles of the gluteal and posterior femoral regions. Semimembranosus labeled at bottom left.
Semimembranosus labeled at bottom right.
Latin	musculus semimembranosus
Gray's	subject #128 479
Origin	Ischial tuberosity
Insertion	Medial surface of tibia
Artery	profunda femoris, gluteal artery
Nerve	sciatic (tibial, L5, S1, S2)
Actions	Hip extension, Knee flexion
Antagonist	Quadriceps muscle

The semimembranosus is a muscle in the back of the thigh. It is the most medial of the three hamstring muscles.

Structure

The semimembranosus, so called from its membranous tendon of origin, is situated at the back and medial side of the thigh.

Its origin is the ischial tuberosity and it inserts on the medial condyle and nearby margin of tibia; intercondylar line and lateral condyle of femur; and the ligament of popliteal region. It arises by a thick tendon from the upper and outer impression on the tuberosity of the ischium, above and medial to the biceps femoris and semitendinosus.

The tendon of origin expands into an aponeurosis, which covers the upper part of the anterior surface of the muscle; from this aponeurosis muscular fibers arise, and converge to another aponeurosis which covers the lower part of the posterior surface of the muscle and contracts into the tendon of insertion.

It is inserted mainly into the horizontal groove on the posterior medial aspect of the medial condyle of the tibia.

The tendon of insertion gives off certain fibrous expansions: one, of considerable size, passes upward and laterally to be inserted into the posterior lateral condyle of the femur, forming part of the oblique popliteal ligament of the knee-joint; a second is continued downward to the fascia which covers the Popliteus muscle; while a few fibers join the tibial collateral ligament of the joint and the fascia of the leg.

The muscle overlaps the upper part of the popliteal vessels.

Innervation

The semimembranosus is innervated by the tibial nerve, a branch of the sciatic nerve. The tibial nerve consists of the anterior divisions of ventral nerve roots from L4 through S3. These nerve roots are part of a larger nerve network called the sacral plexus.^[1] The tibial nerve is also responsible for innervating the skin of the posterior leg as well as plantar skin.

Actions

The semimembranosus helps to extend (straighten) the hip joint and flex (bend) the knee joint.

It also helps medially rotate the knee: the tibia medially rotates on the femur when the knee is flexed. Medially rotates the femur when the hip is extended. Can aid in counteracting the forward bending at the hip joint.^[1]

Variations

It may be reduced or absent, or double, arising mainly from the sacrotuberous ligament and giving a slip to the femur or adductor magnus.

Additional images

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  Right hip bone. External surface.

• 

  Bones of the right leg. Posterior surface.

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  The popliteal, posterior tibial, and peroneal arteries.

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  Back of left lower extremity.

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  Semimembranosus muscle

See also

• Semitendinosus
• Biceps femoris

References

1. ^ ^{a b} Saladin, Kenneth S. Anatomy & Physiology: the unity of form and function. 5th ed. New York: McGraw-Hill, 2010. Print.

External links

• LUC semm
• SUNY Labs 14:st-0408
• SUNY Figs 14:01-07 - "Muscles (hamstrings) of the posterior compartment of the thigh."
• SUNY Figs 14:02-06 - "Muscles that form the superficial boundaries of the popliteal fossa."
• Anatomy at Dartmouth knee/surface/surface4
• PTCentral

This article was originally based on an entry from a public domain edition of Gray's Anatomy. As such, some of the information contained within it may be outdated.