Antagonistic muscle action:
Muscles never work alone. In order for a coordinated movement to be produced, the muscles must work as a group of a team, with several muscles working at one time.
The muscle that produces the desired joint movement is called the agonist or prime mover. For example: flexion of the arm at the elbow, the muscle responsible for flexion (bending of the arm) is the biceps brachii.
In order for the bicep muscles to shorten when contracting, the triceps muscle must lengthen. The triceps in this instance is known as the antagonist, since its action is opposite to that of the agonist.