Hamstring

In human anatomy, the hamstring is a group of tendons contracted by four posterior thigh muscles (posterior femoral muscles, semitendinosus, semimembranosus and biceps femoris) that make up the borders of the space behind the knee, or their corresponding tendons.

In quadrupeds, it is the single large tendon found behind the knee or comparable area.

Etymology
The word ham originally referred to the fat and muscle behind the knee. String refers to tendons, and thus, the hamstrings are the string-like tendons felt on either side of the back of the knee.

The three muscles of the posterior thigh (semitendinosus, semimembranosus, biceps femoris long & short head) flex (bend) the knee, while all but the short head of biceps femoris extend (straighten) the hip. The three 'true' hamstrings cross both the hip and the knee joint and are therefore involved in knee flexion and hip extension. The short head of the biceps femoris crosses only one joint (knee) and is therefore not involved in hip extension. With its divergent origin and innervation it is sometimes excluded from the 'hamstring' characterization.

A portion of the adductor magnus is sometimes considered a part of the hamstrings.

Functions
The hamstrings cross and act upon two joints - the hip and the knee.

Semitendinosus and semimembranosus extend the hip when the trunk is fixed; they also flex the knee and medially (inwardly) rotate the lower leg when the knee is bent.

The long head of the biceps femoris extends the hip as when beginning to walk; both short and long heads flex the knee and laterally (outwardly) rotates the lower leg when the knee is bent.

The hamstrings play a crucial role in many daily activities, such as, walking, running, jumping, and controlling some movement in the trunk. In walking, they are most important as an antagonist to the quadriceps in the deceleration of knee extension.

Imaging
Imaging the hamstring muscles is usually performed with an ultrasound and/or MRI. The biceps femoris is most commonly injured, followed by semitendinosus. Semimembranosus injury is rare. Imaging is useful in differentiating the grade of strain, especially if the muscle is completely torn. In this setting, the level and degree of retraction can be determined, serving as a useful roadmap prior to any surgery. Those with a hamstring strain of greater than 60mm in length have a greater risk of recurrence.

Use in surgery
The distal semitendinosus tendon is one of the tendons that can be used in the surgical procedure ACL reconstruction. In this procedure, a piece of it is used to replace the anterior cruciate ligament (ACL). The ACL is one of the four major ligaments in the knee.

Training
In strength training, squats, knee curls and the Romanian deadlift are combined with other lower body exercises to develop the hamstrings.

Hamstring extension is key to determining flexibility, assessed by performing a sit and reach test.