The group of muscles at the back of the thigh is commonly known as the hamstrings. In standard anatomical descriptions, the group includes the biceps femoris, semitendinosus, and semimembranosus.
The hamstrings cross both the hip and knee joints. They help extend the hip, as when rising from a chair or driving the leg backward, and flex the knee. They also contribute to rotation of the lower leg, depending on which muscle is acting and the position of the knee.
The word “hamstring” has an older historical origin. “Ham” was an Old English word for the back of the knee, while “string” referred to the cord-like tendons that can be felt in that region. The name therefore describes their location and appearance rather than a single muscle.
A common mix-up is calling the gluteal muscles or quadriceps hamstrings. The glutes are at the buttocks, the quadriceps are at the front of the thigh, and the hamstrings occupy the back. The phrase “pulled hamstring” usually refers to a strain in one or more of these muscles or their tendons.