http://www.dreamstime.com/stock-images-running-rimagefree1912884-resi2565486
Here are two methods for stretching the lower part of the hamstrings (the big muscle group in the back of your thigh). Stretching of the hamstrings, quadriceps, and calf/achilles is a vital part of most pre and post workout regimens. The young man on the left is using improper technique, whereas the young lady on the right using stress-free perfect technique. Watch the video below to see if you can pick up on his problem.
Also read one of my original posts on hamstring stretches for more elaboration of the point. When you stretch your hamstrings or hammies, do not try to touch your toes or you may be just stressing out your back.
http://www.drblakeshealingsole.com/2010/05/hamstring-stretch-various-positions-can.html