Synergistic muscles are groups of muscles that work together to cause the same movement. Muscles that oppose each other are called antagonistic muscles. Antagonist muscles must oppose the action of an agonist muscle so that movement can occur. For example, when the biceps brachii on the front of the upper arm contracts and shortens (agonist), the triceps brachii must relax and lengthen (antagonist) so that the arm can flex.