Above graphic courtesy of Iemily.com
The biceps femoris, semi-tendinosus and the semi-membranosus collectively make up the hamstring muscle group, which is one of the largest, and certainly the longest, in the human body. These muscles are in action every time you kick your butt in a treble-hop-back or a back click, or lift your leg into a front kick. The hamstrings are a very tricky muscle; they are the easiest muscles to strengthen and train. However, they are also one of the easiest to injure and take a long time to heal. Click here for some very good info on the biomechanics of the hamstrings.
Some basic hamstring stretches
Sit on the floor with the leg you will be stretching in front of you. Bend the other leg so the foot touches the knee of the stretching leg. Now bend forward slowly and try to touch your toes. Focus on bending from the waist. You do not have to touch your forehead to your knee (like the girl in the picture below) if you can't. If you flex your foot, you will also incorporate the calves into the stretch.
Source of above graphic unknown
Stand with your feet shoulder-width apart. Slowly bend down and try to touch the floor if you can. Don't bend your knees, but don't lock them either. Keep your back straight and focus on bending from the waist. This stretch is very effective because it also targets the calves and the back.
Above graphic courtesy of spineuniverse.com
Lie down on your back. Keeping the stretching leg straight, bring it in towards you as far as you can. The stretch will be much easier if you point the foot. If you flex it you will also stretch the calves.
Above graphic courtesy of www.spine.org
