Muscles in the Posterior Compartment of the Thigh

The muscles in the posterior compartment of the thigh are collectively known as the hamstrings. They consist of the biceps femoris, semitendinosus and semimembranosus – as a group they act to extend at the hip, and flex at the knee. They are innervated by the sciatic nerve.

The hamstrings form prominent tendons medially and laterally at the back of the knee. This explains the phrase ‘hamstringing the enemy’ – whereby these tendons are cut.

In this article, we shall look at the attachments, actions and clinical relevance of the muscles in the posterior thigh.


Muscles in the Posterior Compartment

Biceps Femoris

Fig 1.0 - Posterior view of the hamstrings.

Fig 1.0 – Posterior view of the hamstrings.

Like the biceps brachii in the arm, the biceps femoris muscle has two heads – a long head and a short head.

It is the most lateral of the muscles in the posterior thigh – the common tendon of the two heads can be felt laterally at the posterior knee.

  • Attachments: The long head originates from the ischial tuberosity of the pelvis. The short head originates from the linea aspera on posterior surface of the femur. Together, the heads form a tendon, which inserts into the head of the fibula.
  • Actions: Main action is flexion at the knee. It also extends the leg at the hip, and laterally rotates at the hip and knee.
  • Innervation: Sciatic Nerve.

Semitendinosus

The semitendinosus is a largely tendinous muscle. It lies medially to the biceps femoris, and covers the majority of the semimembranosus.

  • Attachments: It originates from the ischial tuberosity of the pelvis, and attaches to the medial surface of the tibia.
  • Actions: Flexion of the leg at the knee joint. Extension of thigh at the hip. Medially rotates the thigh at the hip joint and the leg at the knee joint.
  • Innervation: Sciatic nerve.

Semimembranosus

The semimembranosus muscle is flattened and broad. It is located underneath the the semitendinosus.

  • Attachments: It originates from the ischial tuberosity, but does so more superiorly than the semitendinosus and biceps femoris. It attaches to the medial tibial condyle.
  • ActionsFlexion of the leg at the knee joint. Extension of thigh at the hip. Medially rotates the thigh at the hip joint and the leg at the knee joint.
  • Innervation: Sciatic nerve.
Fig 1.1 - Cross section of the thigh, showing the position of the major muscle groups.

Fig 1.1 – Cross section of the thigh, showing the position of the major muscle groups.

Clinical Relevance: Damage to the Hamstrings

Muscle Strain

A hamstring strain refers to excessive stretch or tearing of the muscle fibres. They are often seen athletes involved in running or kicking sports.  Damage to the muscle fibres is likely to rupture the surrounding blood vessels – producing a haematoma (a collection of blood). The haematoma is contained by the overlying fascia lata.

Treatment of any muscle strain should utilise the RICE protocol – rest, ice, compression and elevation.

Avulsion Fracture of the Ischial Tuberosity

An avulsion fracture occurs when a fragment of bone breaks away from the main body of bone.

In an avulsion fracture of the ischial tuberosity, the hamstring tendons ‘tear off’ a piece of the ischial tuberosity. Such an injury usually occurs in sports that require rapid contraction and relaxation of the muscles – such as sprinting, football and hurdling.

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