Muscle Action Roles (Agonist, Antagonist, Synergist)

All muscles function by developing or releasing tension, but the same muscle may vary in its specific role depending on the action required. There are five basic roles (Thompson, 1989):

1. Agonists / primary movers — most directly responsible for executing the desired movement.
2. Secondary movers — assist the primary muscle group.
3. Antagonist muscles — on the opposite side of the joint; responsible for the opposite joint action.
4. Guiding muscles / neutralizers / synergists — on either side of the muscle most involved; help eliminate extraneous movements.
5. Fixators / stabilizing muscles — fixate or hold a joint or body part so other parts can move.

A basic governing principle is reciprocal innervation — a hard-wired neural circuit that automatically relaxes the antagonist as the agonist contracts. For example, during elbow flexion the biceps brachii, brachialis, and brachioradialis contract while the triceps brachii is reciprocally innervated to relax; during elbow extension the reverse occurs.

• 5 muscle roles: agonist (primary mover), secondary mover, antagonist, synergist/neutralizer/guiding, fixator/stabilizer.
• Reciprocal innervation: antagonist relaxes automatically as the agonist contracts.
• Example: elbow flexion — biceps/brachialis/brachioradialis contract, triceps relaxes.

• biceps-brachii / triceps — antagonist pair example.
• muscle-balance — training agonist and antagonist.
• push-pull — system exploiting agonist/antagonist pairing.