This article is from the Stretching FAQ, by Brad Appleton Brad_Appleton@ivhs.mot.com with numerous contributions by others.
This mainly stretches your groin and upper inner-thigh area, but also makes
some demands on your lower back. It is often called the "butterfly stretch"
or "frog stretch" because of the shape that your legs make when you perform
it.
Sit down with your back straight up (don't slouch, you may want to put your
back against a wall) and bend your legs, putting the soles of your feet
together. Try to get your heels as close to your groin as is *comfortably*
possible. Now that you are in the proper position, you are ready to
stretch. For the passive stretch, push your knees to the floor as far as
you can (you may use your hands to assist but do *not* resist with the
knees) and then hold them there. Once you have attained this position, keep
your knees where they are, and then exhale as you bend over, trying to get
your chest as close to the floor as possible. Hold this stretch for about
20 seconds.
The isometric stretch is almost identical to the passive stretch, but
before you bend over, place your hands on your ankles and your elbows in
the crooks of your knees. As you bend over, use your elbows to "force" your
knees closer to the floor while at the same time pushing "up" (away from
the floor) with your thighs to resist against your arms.
 
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