This article is from the Stretching FAQ, by Brad Appleton Brad_Appleton@ivhs.mot.com with numerous contributions by others.
When muscles cause a limb to move through the joint's range of motion, they
usually act in the following cooperating groups:
"agonists"
These muscles cause the movement to occur. They create the normal range
of movement in a joint by contracting. Agonists are also referred to
as "prime movers" since they are the muscles that are primarily
responsible for generating the movement.
"antagonists"
These muscles act in opposition to the movement generated by the
agonists and are responsible for returning a limb to its initial
position.
"synergists"
These muscles perform, or assist in performing, the same set of joint
motion as the agonists. Synergists are sometimes referred to as
"neutralizers" because they help cancel out, or neutralize, extra
motion from the agonists to make sure that the force generated works
within the desired plane of motion.
"fixators"
These muscles provide the necessary support to assist in holding the
rest of the body in place while the movement occurs. Fixators are also
sometimes called "stabilizers".
As an example, when you flex your knee, your hamstring contracts, and, to
some extent, so does your gastrocnemius (calf) and lower buttocks.
Meanwhile, your quadriceps are inhibited (relaxed and lengthened somewhat)
so as not to resist the flexion (See "1.6.4 - Reciprocal Inhibition"). In
this example, the hamstring serves as the agonist, or prime mover; the
quadricep serves as the antagonist; and the calf and lower buttocks serve
as the synergists. Agonists and antagonists are usually located on
opposite sides of the affected joint (like your hamstrings and quadriceps,
or your triceps and biceps), while synergists are usually located on the
same side of the joint near the agonists. Larger muscles often call upon
their smaller neighbors to function as synergists.
The following is a list of commonly used agonist/antagonist muscle pairs:
* pectorals/latissimus dorsi (pecs and lats)
* anterior deltoids/posterior deltoids (front and back shoulder)
* trapezius/deltoids (traps and delts)
* abdominals/spinal erectors (abs and lower-back)
* left and right external obliques (sides)
* quadriceps/hamstrings (quads and hams)
* shins/calves
* biceps/triceps
* forearm flexors/extensors
 
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