This article is from the Stretching FAQ, by Brad Appleton Brad_Appleton@ivhs.mot.com with numerous contributions by others.
"Isometric stretching" is a type of static stretching (meaning it does not
use motion) which involves the resistance of muscle groups through
isometric contractions (tensing) of the stretched muscles (See "1.5 - Types
of Muscle Contractions"). The use of isometric stretching is one of the
fastest ways to develop increased static-passive flexibility and is much
more effective than either passive stretching or active stretching alone.
Isometric stretches also help to develop strength in the "tensed" muscles
(which helps to develop static-active flexibility), and seems to decrease
the amount of pain usually associated with stretching.
The most common ways to provide the needed resistance for an isometric
stretch are to apply resistance manually to one's own limbs, to have a
partner apply the resistance, or to use an apparatus such as a wall (or the
floor) to provide resistance.
An example of manual resistance would be holding onto the ball of your foot
to keep it from flexing while you are using the muscles of your calf to try
and straighten your instep so that the toes are pointed.
An example of using a partner to provide resistance would be having a
partner hold your leg up high (and keep it there) while you attempt to
force your leg back down to the ground.
An example of using the wall to provide resistance would be the well known
"push-the-wall" calf-stretch where you are actively attempting to move the
wall (even though you know you can't).
Isometric stretching is *not* recommended for children and adolescents
whose bones are still growing. These people are usually already flexible
enough that the strong stretches produced by the isometric contraction have
a much higher risk of damaging tendons and connective tissue. Kurz
strongly recommends preceding any isometric stretch of a muscle with
dynamic strength training for the muscle to be stretched. A full session of
isometric stretching makes a lot of demands on the muscles being stretched
and should not be performed more than once per day for a given group of
muscles (ideally, no more than once every 36 hours).
The proper way to perform an isometric stretch is as follows:
1. Assume the position of a passive stretch for the desired muscle.
2. Next, tense the stretched muscle for 7-15 seconds (resisting against
some force that will not move, like the floor or a partner).
3. Finally, relax the muscle for at least 20 seconds.
Some people seem to recommend holding the isometric contraction for longer
than 15 seconds, but according to `SynerStretch' (the videotape), research
has shown that this is not necessary. So you might as well make your
stretching routine less time consuming.