Muscles are typically grouped so that the action of one muscle or group of muscles is opposed by that of another muscle or group of muscle. For example, the biceps brachii flexes the elbow and the triceps brachii extends the elbow.
A muscle that accomplishes a certain movement, such as flexion, is called the agonist. A muscle acting in opposition to an agonist is called an antagonist. The biceps brachii is the agonist in elbow flexion, whereas the triceps brachii is the antagonist, which extends the elbow.