Pain at the back of the thigh, namely in the hamstrings, is widespread. The hamstrings are a set of three muscles (the biceps femoris, semitendinosus, and semimembranosus) that run from the pelvis to the knee. Overuse, muscle strain or tear, improper posture, insufficient warm-up before physical activity, or an abrupt increase or decrease in physical activity are common causes of this pain.