Antagonist

An antagonist is a muscle that performs the opposite action of an agonist.^{1International Sports Sciences Association, Corrective Exercise, 1st ed., p. 36.} The triceps extends the elbow and the biceps brachii flexes it, so each is the antagonist to the other.^{1International Sports Sciences Association, Corrective Exercise, 1st ed., p. 36.} The quadriceps act as antagonists during knee flexion.^{2International Sports Sciences Association, Strength and Conditioning, p. 43.} The IPTA textbook notes that an antagonist can sometimes resist the agonist’s action.^{3International Personal Trainer Academy, Certified Personal Trainer Textbook, p. 96.}

An antagonist can also slow or stop movement near the end of its range of motion, helping stabilize the joint and protect ligaments and cartilage from potentially destructive forces.^{4Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 36.} In throwing, the triceps extends the elbow to accelerate the ball, then the biceps brakes that extension.^{4Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 36.}

In practice

• Identify the role by the movement: a muscle that opposes elbow extension acts as the agonist in elbow flexion.^{2International Sports Sciences Association, Strength and Conditioning, p. 43.}
• When the prime mover contracts, the opposing muscle relaxes, a process called reciprocal innervation; ISSA also describes this inhibition as Sherrington’s law.^{5International Sports Sciences Association, Bodybuilding: The Complete Guide to Unlocking Muscle Hypertrophy, 1st ed., p. 15.},^{6International Sports Sciences Association, Fast Track Study Guide: Applied Science, p. 29.} Without reciprocal innervation, ISSA says muscle actions would be jerky and weak.^{5International Sports Sciences Association, Bodybuilding: The Complete Guide to Unlocking Muscle Hypertrophy, 1st ed., p. 15.}