This article is from the Stretching FAQ, by Brad Appleton Brad_Appleton@ivhs.mot.com with numerous contributions by others.
This stretch is sometimes called the "runner's start" because the position
you are in resembles that of a sprinter at the starting block. It mainly
stretches the psoas muscle located just above the top of the thigh.
Crouch down on the floor with both hands and knees on the ground. Put one
leg forward with your foot on the floor so that your front leg is bent at
the knee at about a 90 degree angle. Now extend your rear leg in back of
you so that it is almost completely straight (with just an ever so slight
bend) and so that the weight of your rear leg is on the ball of your rear
foot with the foot in a forced arch position. Now we are in the position to
stretch (notice that your rear leg should be in pretty much the same
position that it would assume if you were performing a front split).
Keeping your back straight and in line with your rear thigh, exhale and
slowly try to bring your chest down to the floor (you shouldn't need to
bend much further than the line your front knee is on). You should feel
the stretch primarily in the upper thigh of your rear leg but you should
also feel some stretch in your front hamstring as well. Hold this position
for at least 15 seconds. If you wish to also stretch your rear quadricep
from this position, you can shift your weight back so that your rear leg
makes a right angle with your knee pointing toward the floor (but don't let
it touch the floor). Now, without bending your rear leg any further, try to
force your rear knee straight down to the floor.
Now repeat the same stretch(es) with your other leg in front.
For an isometric stretch, you can do this same stretch in front of a wall
and instead of putting your hands on the floor, put them in front of you
against the wall and then push against the wall with the ball of your foot
(without decreasing the "stretch" in your psoas).
 
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