As shown in the diagram, the human hamstring occupies the posterior of the body of the femur.
The three muscles of the posterior thigh (semitendinosus, semimembranosus, biceps femoris) flex (bend) the knee, while three of the four extend (straighten) the hip. The short head of the biceps femoris, with its divergent origin and innervation, is not involved in hip extension, and thus is sometimes excluded from the 'hamstring' characterization.
|semitendinosus||ischial tuberosity||medial surface of tibia||tibial|
|semimembranosus||ischial tuberosity||medial tibial condyle||tibial|
|biceps femoris - long head||ischial tuberosity||lateral side of the head of the fibula||tibial|
|biceps femoris - short head||linea aspera near the head of the femur||lateral side of the head of the fibula (common tendon with the long head)||common fibular|
A portion of the adductor magnus is sometimes considered a part of the hamstrings.
Semitendinosus and semimembranosus extend the hip when the trunk is fixed or extend the trunk when the hip 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.