Agonist

An agonist, or prime mover, is the muscle most directly responsible for a movement.^{1Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 36.} The biceps brachii is the agonist for elbow flexion, and the hamstring complex is the agonist for knee flexion.^{2International Sports Sciences Association, Strength and Conditioning, p. 43.} At the ankle, the tibialis anterior produces dorsiflexion.^{3International Sports Sciences Association, Corrective Exercise, 1st ed., p. 36.}

Most movements involve several muscles, and a muscle’s role changes with the movement.^{2International Sports Sciences Association, Strength and Conditioning, p. 43.} The IPTA describes the antagonist as producing the opposite movement and synergists as helping the agonist.^{4International Personal Trainer Academy, Certified Personal Trainer Textbook, p. 96.}

In practice

In a biceps curl, the biceps is the agonist and the triceps the antagonist; in elbow extension, they swap roles.^{5International Sports Sciences Association, Bodybuilding: The Complete Guide to Unlocking Muscle Hypertrophy, 1st ed., p. 34.} ISSA describes the opposing muscle relaxing as the agonist contracts, using the triceps as prime mover and the biceps relaxing at bench press lockout.^{6International Sports Sciences Association, Bodybuilding: The Complete Guide to Unlocking Muscle Hypertrophy, 1st ed., p. 15.}

Two-joint muscles can have different roles at each joint: most act as prime movers at the distal joint and assistant movers at the proximal joint.^{5International Sports Sciences Association, Bodybuilding: The Complete Guide to Unlocking Muscle Hypertrophy, 1st ed., p. 34.} In hip extension at a deadlift lockout, ISSA names the glutes as prime movers and the hamstrings as assistant movers.^{5International Sports Sciences Association, Bodybuilding: The Complete Guide to Unlocking Muscle Hypertrophy, 1st ed., p. 34.} Even a dumbbell curl involves muscles at the wrist, shoulder and spine alongside the agonist.^{2International Sports Sciences Association, Strength and Conditioning, p. 43.}