During arm extension, when the arm is being straightened, the triceps muscle is considered the agonist muscle. The agonist muscle is also sometimes called the prime mover, and is the muscle that generates the primary movement by contracting.
Most muscle groupings establish a pair of muscles that work together to move a given joint. For each movement, one muscle is designated as the agonist muscle, while the other is considered the antagonist muscle. The agonist muscle is always the muscle that contracts to create the particular movement, while the antagonist muscle contracts to return the joint to its original resting position. When extending an arm into a straight position, the triceps is considered the agonist muscle and the biceps is the antagonist, since the triceps muscle generates the motion to straighten the arm and the biceps contracts to return the arm to the original position. If the motion in question is an arm flexion, raising the wrist and bending the elbow, then the biceps generates the primary motion and is considered the agonist, while the triceps returns the arm to its original position, making it the antagonist muscle. Agonist muscles are often assisted by synergist muscles that work to reduce excessive force from agonist muscles and create a broader range of contractions for a particular joint.