Semitendinosus

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Description

Semitendinosus is one of the three muscles that make up the hamstrings muscle group, and it is located at the posterior and medial aspect of the thigh. The semitendinosus is so named due to it having a long tendon of insertion.

The muscle is fusiform and ends a little below the middle of the thigh in a long round tendon which lies along the medial side of the popliteal fossa; it then curves around the medial condyle of the tibia and passes over the medial collateral ligament of the knee-joint, from which it is separated by a bursa, and is inserted into the upper part of the medial surface of the body of the tibia, nearly as far forward as its anterior crest.

The semitendinosus is more superficial than the semimembranosus (with which it shares very close insertion and attachment points). However, because the semimembranosus is wider and flatter than the semitendinosus, it is still possible to palpate the semimembranosus directly.

The Semitendinosus muscle is highlighted in red

At its insertion it gives off from its lower border a prolongation to the deep fascia of the leg and lies behind the tendon of the sartorius, and below that of the gracilis, to which it is united. These three tendons form the Pes Anserinus, thus named due to the appearance resembling a webbed “goose’s foot”.

Anatomy

Origin

It arises, by a common tendon origin with the long head of the biceps femoris, from from the lower medial facet of the lateral section of the ischial tuberosity. The semitendinosus muscle mainly originates from the medial surface of the tendon of the long head of the biceps femoris, and also originates from the ischial tuberosity with a thin tendon and a muscular part.

Insertion

The semitendinosus tendon inserts at the upper part of the medial surface of the tibia, behind the attachment of sartorius and infero-anterior to the attachment of gracilis.

Nerve Supply

Tibial portion of the sciatic nerve (L5, S1, 2).

Artery

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

Function

Primary actions 1. Extension of the thigh at the hip (Anim) Agonists: gluteus maximus, semimembranosus, biceps femoris (long head), and adductor magnus (posterior part) Antagonists: psoas major and iliacus

2. Flexion of the leg at the knee (Anim) Agonists: biceps femoris (long head), biceps femoris (short head), and semimembranosus Antagonists: vastus lateralis, vastus medialis, vastus intermedius, and rectus femoris

Gracilis, sartorius, popliteus, gastrocnemius, and plantaris assist with flexion of the knee.

3. Internal rotation of the knee when the knee is flexed (Anim, Movie) Agonists: popliteus and semimembranosus Antagonist: biceps femoris (long head) and biceps femoris (short head)

Sartorius and gracilis assist with internal rotation of knee.

Closed kinetic chain action When the distal attachment of a muscle is fixed and unable to move, the muscle will perform in what is known as the closed kinetic chain. In these instances, muscles will cause movement on the bone segment on the opposite side of the joint. Although every muscle in the body is capable of a reverse action, the muscles working in the closed kinetic chain are most often the muscles of the lower extremity that move the joints in flexion or extension.

1. Flexion of the thigh at the knee Agonists: biceps femoris (long head), biceps femoris (short head), and semimembranosus Antagonists: vastus lateralis, vastus medialis, vastus intermedius, and rectus femoris

Gross motor movement

This muscle is involved in many common daily activities as well as in some specific exercises, such as:

• Walking
• Walking upstairs
• Jogging
• Standing jump forwards
• Standing jump upwards

Clinical relevance

A grafted semitendinosus tendon (sometimes combined with gracilis tendon) may be used to replace an unrepairable cruciate or collateral ligament, a service that was previously provided by a graft from the patellar ligament. While either donor site may be used, the hamstring choice provides less post-operative consequences to kneeling.