A lot of muscles are involved in an action. The principle muscle involved is called the primer mover or agonist.
For example an action like uplifting a cup. The primar mover is actually the biceps branchii. However, these muscles are assisted by brachialis which are known as synergist.<u> A synergist behaves like a fixator which stabilizes the bone and thus, assist the action of uplifting the cup.</u>
