The hamstring muscles are found at the back of the thigh. They are three muscles which act on both the hip and knee joints.
The hamstring group of muscles are made up of:
All three hamstring muscles act to bend the knee and to extend the hip (move the leg backwards).
They all originate at the Ischial tuberosity of the pelvis. This is the base of the pelvis, sometimes known as the sitting bone.
Semimembranosis and Semitendinosus both insert into the inside back of the knee joint, to the upper Tibia.
Biceps Femoris is a little different. This muscle has two heads (which is what biceps means). One originates with the other hamstrings at the Ischial tuberosity and the other from the mid part of the posterior Femur (thigh bone). It then insets at the outer back of the knee joint, to the Tibia and the head of the Femur.
All three muscles are innervated by the Sciatic nerve, which is why sciatic pain is often referred into the hamstrings. This kind of pain is usually stemming from either the lower back or the buttocks.
Common hamstring injuries include hamstring strains which are a tear to one or more of these three muscles.