Synergistic muscles are muscles that have a similar and mutually helpful function or action. Mosby's medical dictionary defines synergistic muscles as groups of muscles that contract together to accomplish the same body movement. Synergistic muscles act as movable joints; they are sometimes referred to as neutralizers because they help to cancel out the motion of agonist muscles.
Wikipedia explains that agonist muscles often occur in pairs, such as the biceps and triceps. One member of the pair contracts while the other relaxes. Reverse motions require antagonistic pairs located on the opposite side of a joint or bone. Not all muscles are part of an antagonistic pair, such as the deltoids.
Synergistic muscles are required for many types of movements, Wikipedia notes. Muscles are only able to contract to 40 percent of their fully stretched length. This means that muscles that cover a wide number of joints may be unable to stretch far enough to move all of the joints. When this occurs, synergistic muscles located in that region of the body help the primary muscle to complete the movement. Synergistic muscles that serve this function are known as helping synergists. True synergistic muscles only neutralize unwanted joint action.