Note: the semitendinosus and semimembranosus, the continuation into the sacrotuberous ligament, and the innervation via the tibial branch of the sciatic nerve.

Semitendinosus:

Origin:    Tuberosity of ischium by tendon common with with the long head of the biceps femoris

Insertion:   Proximal part of the medial surface of the tibia   The tendon of the semitendinosus contributes the distal fibers of the pes anserinu,biceps femoris) lay in the posterior compartment of the thigh.

The semitendinosus can be palpated with relative ease by locating the space between the two large bands that comprise the hamstring tendons just superior to the posterior knee.

Nerve:   Tibial nerve; a branch of the sciatic nerve via the sacral plexus, originating from nerve roots L5 – S2 and sometimes L4 (3).

Action:

Hip: Extension and medial rotator

Knee: Flexion and tibial internal rotation.

 

Semimembranosus

 

Origin: Tuberosity of ischium, proximal and lateral to the common tendon of the biceps femoris and semitendinosus .

Insertion: Posteromedial aspect of the medial condyle of the tibia, with expansions into the posterior oblique ligament, poplitial ligament, popliteus fascia, posterior joint capsule.

The semimembranosus is also superficial muscle of the posterior thigh, bordered by the long head the semitendinosus laterally and adductors medially. All of hamstring muscles lies in posterior compartment of the thigh.

The semitendinosus can be palpated with relative ease by locating the space between the two large bands that comprise the hamstring tendons just superior to the posterior knee.

Nerve: Tibial nerve; a branch of the sciatic nerve via the sacral plexus, originating from nerve roots L5 – S1 and sometimes L4 and/or S2 .

Action:

Hip: Extension and medial rotation

Knee: Flexion and tibial internal rotation

 

Note: the insertion of the semimembranosus tendon into the complex posterior oblique ligament, the popliteal ligament, and investment of these structures into the distal MCL.

 

Integrated Function:

Stabilization:

The semimembranosus helps in hip and knee stabilization, specifically the posterior medial compartment where it reinforces the capsule and increases tension on the posterior oblique ligament, popliteal ligaments, popliteal fascia and posterior capsule.

The semitendinosus stabilizes the medial knee, hip and may stabilize the sacroiliac joint via the sacrotuberous ligament and common tendon with the biceps femoris.

Eccentrically Decelerates:

The semitendinosus and semimembranosus eccentrically decelerate hip flexion, external rotation and abduction, as well as, knee extension and tibial external rotation.

Synergists:

The semimembranosus and semitendinosus are synergists of hip extension along with the biceps femoris, adductor magnus, and the prime mover of hip extension, the gluteus maximus.

Both the semimembranosus and semitendinosus help in knee internal rotation, along with the gracilis, popliteus, and medialgastrocnemius.

The semitendinosus help the biceps femoris in sacral extension.

 

 

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