The hamstrings are a group of muscles located at the back of the thigh. The names of these muscles include Semitendiosus, Semimembranosus and Bicep Femoris.
These muscles start at the pelvis and run down to the top of the back of the shin bone. They are important for good posture because they can effect the tilt of the pelvis, as well as the position the hip and the knee.
To correctly stretch the hamstring requires proper positioning of the pelvis as well the knee.
This video shows a few great techniques for stretching the hamstrings muscles. Look for the neutral position of the back while performing these stretches correctly. The video also includes a common mistake people make when stretching the hamstrings.
A hamstring stretch can also put a stretch the sciatic nerve, which starts in the lower back and runs down the back of the leg. Stop if stretching the hamstring causes pain.
For best results hold the stretches for at least 30 seconds, Repeat several times each leg.