This article is from the Stretching FAQ, by Brad Appleton Brad_Appleton@ivhs.mot.com with numerous contributions by others.
When an agonist contracts, in order to cause the desired motion, it usually
forces the antagonists to relax (See "1.4 - Cooperating Muscle Groups").
This phenomenon is called "reciprocal inhibition" because the antagonists
are inhibited from contracting. This is sometimes called "reciprocal
innervation" but that term is really a misnomer since it is the agonists
which inhibit (relax) the antagonists. The antagonists do *not* actually
innervate (cause the contraction of) the agonists.
Such inhibition of the antagonistic muscles is not necessarily required.
In fact, co-contraction can occur. When you perform a sit-up, one would
normally assume that the stomach muscles inhibit the contraction of the
muscles in the lumbar, or lower, region of the back. In this particular
instance however, the back muscles (spinal erectors) also contract. This is
one reason why sit-ups are good for strengthening the back as well as the
When stretching, it is easier to stretch a muscle that is relaxed than to
stretch a muscle that is contracting. By taking advantage of the
situations when reciprocal inhibition *does* occur, you can get a more
effective stretch by inducing the antagonists to relax during the stretch
due to the contraction of the agonists. You also want to relax any muscles
used as synergists by the muscle you are trying to stretch. For example,
when you stretch your calf, you want to contract the shin muscles (the
antagonists of the calf) by flexing your foot. However, the hamstrings use
the calf as a synergist so you want to also relax the hamstrings by
contracting the quadricep (i.e., keeping your leg straight).