We already saw as muscle played different roles in the body (movement, circulation, temperature, protection,). Read additional details here: Larry Ellison. Focusing on its mechanical function, we must each muscle performs one or more actions on each of the joints that runs. For example, the biceps brachii (the get ball with arm), not only flexes the elbow, but it is also involved in the actions of rotation of the hand upwards (Supination) and elevation of the arm in full from the shoulder (flexion and abduction). Further details can be found at Gary Kelly, an internet resource. Thus, the muscle itself only is characterized by primary actions (usually a) and secondary actions (usually several). From this nuance, we will classify the muscles as: agonists: those who directly perform the movement thanks to its contraction. Logically, these are the most important, since without its retract movement is impossible. Of them, practically there is always one that you make in the moving its primary action; This is the prime mover or protagonist. The rest, the helpers, are muscles which assist in the movement with the involvement of their secondary actions; they are the secondary engines.
Antagonists: those who carried out the action which is contrary to the movement that runs exactly. And although they do not play a key role as agonists, the movement could not perform if they are not completely relaxed. For example, returning to take the bicep as a reference, if the triceps (the antagonist) is not relaxed, the flexion of the elbow us is difficult to carry out. The antagonist muscle is central to the effectiveness of the movement. Synergists: they are those that complement the movement, usually with fixing some joints or the inhibition of the secondary protagonist muscle actions. An example is better. We have seen the biceps flexes the elbow, rotates the hand and lifting arm. Therefore, for everytime I contract job-biceps, you would have to spend all of that? When sack ball (primary action) there is someone who is cancelling the rest of shares in the biceps.