Semimembranosus muscle

Origin and insertion

Semimembranosus is a relatively large muscle that originates from a small facet on the rough superolateral surface of the ischial tuberosity. The tendon of semimembranosus appears at the level of the mid-thigh and continues caudally toward the point of insertion at the medial condyle of the tibia.

Structure

There are interesting structural details of semimembranosus that helps in identifying the muscle grossly. The muscle begins as a flat and membranous structure that develops a fleshy belly about midway down the thigh. The fleshy component is medially related to its tendon and the fibers are oriented inferomedially. The tendon actually trifurcates distally to give:

• the main part that inserts on the medial tibial condyle,
• a second part that fuses with the popliteal fascia
• and a third part that becomes the oblique popliteal ligament.

Another important feature of semimembranosus is that its lateral border forms the superomedial wall of the popliteal fossa. 

Relations 

The semimembranosus muscle has numerous adjacent muscular and neurovascular structures along its course. Semimembranosus is deep to semitendinosus, superficial to adductor magnus and medial to biceps femoris along its entirety. The proximal part of the muscle is covered by gluteus maximus and medial to adductor minimus. Distally, semimembranosus crosses over and becomes medially related to the medial head of gastrocnemius before inserting on the medial tibial condyle. The distal portion of semimembranosus is also medial to the adductor canal, which accommodates the vessels of the lower limb.

There is a U-shaped bursa that encompasses the semimembranosus tendon. It separates the tendon from the medial tibial plateau, medial head of gastrocnemius, semitendinosus and the medial cruciate ligament. 

The largest nerve in the human body – the sciatic nerve – travels lateral to semimembranosus until it reaches the apex of the popliteal fossa. At this point, the popliteal artery and vein are lateral to and partially covered by semimembranosus.

Don't forget to quiz yourself on the semimembranosus and other muscles of the thigh to consolidate your knowledge! 

Innervation

Semimembranosus is innervated by the L5, S1 and S2 nerve roots. These fibers access the muscle through the tibial division of the sciatic nerve.

Blood supply

The femoral and popliteal arteries give off deep perforating branches that supply the semimembranosus muscle. Occasionally, the inferior gluteal artery supplies the proximal part of the muscle. 

Do you feel overwhelmed by the anatomy of so many muscles? Kenhub can help you! Check out the muscle anatomy reference charts to ease your learning and improve your reviewing efficiency!

Functions

Semimembranosus extends across both the hip and knee joints and is consequently responsible for multiple movements about the joints. However, semimembranosus works in conjunction with the other hamstring muscles to carry out its function. When the feet are firmly planted on the ground, semimembranosus causes extension at the hip, which pulls the upper torso to go into an erect position. Semimembranosus (along with semitendinosus) can also cause internal rotation of the thigh when the hip is fully extended. When the legs are suspended off the ground, it causes flexion of the knee and internal rotation of the leg on the thigh.

Semimembranosus and the other posterior thigh muscles are inactive whenever an individual is standing symmetrically. However, once the individual tilts too far forward, semimembranosus is activated and counteracts the forward movement; thus stabilizing the hip.

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