Muscle · Hips

Biceps femoris

Musculus biceps femoris

Biceps femoris is the hamstring on the outer side of the back of the thigh, and the tendon you can grab behind the outside of the knee. It has two heads that lead very different lives: the long head starts on the sit bone and crosses both the hip and the knee, while the short head starts on the thigh bone itself and only crosses the knee. That makes the long head the two-joint muscle — lengthened at both ends at once during the late swing of a sprint, which is why it is the single most commonly torn muscle in running sport. Both heads end together on the head of the fibula, the bony point on the outside of the knee, where the muscle doubles as part of the corner that stops the knee twisting.