Hamstring stretch in doorway

Lie on your back in a doorway, with one leg through the open door.
Slide your leg up the wall to straighten your knee. You should feel a gentle stretch down the back of your leg. Hold the stretch for at least 15 to 30 seconds.
Do not arch your back.
Do not bend either knee.
Keep one heel touching the floor and the other heel touching the wall. Do not point your toes.
Repeat with your other leg.
Do 2 to 4 times for each leg.
If you do not have a place to do this exercise in a doorway, there is another way to do it:

Lie on your back and bend the knee of the leg you want to stretch.
Loop a towel under the ball and toes of that foot, and hold the ends of the towel in your hands.
Straighten your knee and slowly pull back on the towel. You should feel a gentle stretch down the back of your leg. Hold the stretch for at least 15 to 30 seconds.
Repeat with your other leg.
Do 2 to 4 times for each leg.