In human anatomy, the hamstring refers to one of the three posterior thigh muscles, or to the tendons that make up the borders of the space behind the knee. In modern anatomical contexts, however, they usually refer to the posterior thigh muscles, or the tendons of the semitendinosus, the semimembranosus and the biceps femoris. In quadrupeds, it refers to the single large tendon found behind the knee or comparable area.
As shown in the diagram, the human hamstring occupies the posterior of the body of the femur.
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.
Another commonly accepted origin is that legs of ham used to be hung by a hook through the space between the thighbone and the tendons behind the knee. Ham/pork used to be more common in England than beef and lamb.
The four muscles of the posterior thigh (semitendinosus, semimembranosus, biceps femoris long & short head) flex (bend) the knee, while three of the four 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.
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.
Straining of the hamstring, also known as a pulled hamstring, is defined as an excessive stretch or tear of muscle fibers and related tissues. Such injuries are very common in several sports and the common phrase “to be hamstrung” arises from this kind of injury and its potential to sideline an otherwise-healthy athlete. Hamstring injuries may result when an individual’s training strengthens the quadriceps to the point where they become out of balance with the hamstrings, so that a powerful contraction from the quadriceps injures the hamstrings.
The distal semitendinosis 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.
In strength training, the hamstrings are trained in isolation with the leg curl exercise, as well as a part of several other lower body exercises.


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