The hamstrings are mainly comprised of an inner and outer section. The inner section includes the semimembranosus (lower aspect) and the semitendinosus (upper aspect). The outer section has two heads; the biceps femoris: long head (upper aspect) and the biceps femoris: short head (lower aspect).
Hamstring injuries are among the most common in the leg region. It is highly advised to properly stretch before performing any hamstring exercises involving these muscles.