What are synergist muscles?
A synergistic muscle is a muscle which works in concert with another muscle to generate movement. They act on movable joints.
Synergistic muscles help neutralise extra motion from the agonists (muscles) to make sure that the force generated works within the desired plane of motion. They stabilise muscle movements and keep them even. By working synergistically muscles also reduce the amount of work they need to do, which can improve endurance.
Sometimes synergistic muscles also form a part of a fixator group and are essential to facilitate the fixation action. It becomes essential to use these fixators to fix certain of the joints so that others can be moved effectively. For example fixation of the wrists during full flexion of the fingers in clenching the fist.