Antagonist muscles work against agonist muscles to provide a pulling and pushing motion to areas of the body where muscles are attached to bones that form a joint. Examples of antagonistic muscles include biceps and triceps. In order to conduct a back and forth movement, the biceps act as the agonists to lift the forearm while the triceps relax as the antagonists to support the biceps' contractions, or vice versa.
The BBC explains that muscles are only capable of conducting pulling motions. Without the assistance of antagonist muscles, joints would not be capable of moving in their sockets. According to the Houston Chronicle, agonist muscles are also referred to as prime movers. They initiate a bone's movement in the joint. Muscles do not strictly function as either the agonist or the antagonist, though.
In the example of the relationship between the biceps and the triceps, the roles are reversed during an exercise such as a tricep pressdowns. In this case, the triceps contract as the prime movers or agonists, and the biceps provide lengthening support to the triceps as the antagonists. Usually, the motion of the antagonist muscles is passive.
However, there are certain occasions when the antagonist muscles also contract in support of the agonist muscles. This occurs more frequently when heavy weights are used. For example, when doing bicep curls with heavy weights, the triceps also contract as the forearms are lowered to slowly guide the weights back to their starting position.