We already saw as the muscle performed different functions in the organism (movement, circulation, temperature, protection, ). Centering us in its mechanical function, we emphasize that each muscle conducts one or more battle on each one of the joints that it crosses. For example, the biceps brachial (the one of to remove to ball with the arm), not only flexiona the elbow, but also participates in the actions of turn of the hand upwards (supinacin) and of elevation of the arm completely from the shoulder (flexion and abduction). Therefore, the muscle by itself is characterized to carry out primary actions (generally one) and secondary actions (normally several). From this shade, we will classify muscles like: Agonists: those that directly realise the movement thanks to their contraction. Logically, these are most important, since without its retraction the movement is impossible. Of them, always there will practically be one realises in movement its primary action; this one is the primary motor or protagonist. The rest, the assistants, will be muscles that collaborate in the movement with the implication of their secondary actions; they are the secondary motors.
Antagonists: those that exactly conduct the opposite battle to the movement that is executed. And although they do not play a fundamental role like the agonists, the movement could not be realised if these are not completely relaxed. For example, returning to take the biceps like reference, if trceps (its antagonist) is not relaxed, the flexion of the elbow it will be difficult to us to carry out. The antagonistic muscle is fundamental in the effectiveness of the movement. Sinergistas: they are those that complement the movement, generally with the fixation of some joints or inhibition of the secondary actions of the muscle protagonist. With an example it is understood better. We have seen that the biceps flexiona the elbow, turns the hand and raises the arm.