Semimembranosus – Origin, Nerve Blood Supply, Function

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 medically related to its tendon and the fibers are oriented anteromedially. 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
• A third party that becomes the oblique popliteal ligament.

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

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.

The semimembranosus is innervated by the tibial part of the sciatic nerve. The sciatic nerve consists of the anterior divisions of ventral nerve roots from L4 through S3. These nerve roots are part of the larger nerve network–the sacral plexus.^[rx] The tibial part of the sciatic nerve is also responsible for the innervation of semitendinosus and the long head of biceps femoris.

Blood supply

• Inferior gluteal artery
• Perforating arteries
• Popliteal artery

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.

Functions

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

• It also helps to medially rotate the knee – the tibia medially rotates on the femur when the knee is flexed. It medially rotates the femur when the hip is extended. The muscle can also aid in counteracting the forward bending at the hip joint.^[rx]
• 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.
• The 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.

Movement

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.

Functional movements

• Stand from sitting
• Walking upstairs
• Standing jump forwards
• Standing jump upwards