Roles of Muscles in Movement (Agonist, Antagonist, Synergist, Fixator)

All muscles function by developing or releasing tension. The same muscle may vary from time to time in its specific role, depending on the type of action desired. There are basic roles a muscle or muscle group may perform, depending on the situation:

1. Agonists / primary movers — the muscle(s) most directly responsible for executing the desired movement.
2. Secondary movers — the muscle(s) that assist the primary group to execute the desired movement.
3. Antagonist muscles — located on the opposite side of the joint or body part from the movers; responsible for the opposite joint action.
4. Guiding muscles / neutralizers / synergists — located 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 that other joints or body parts can move.

A basic governing principle is reciprocal innervation: a hard-wired neural circuit that automatically relaxes the antagonist muscle as the agonist is commanded to contract. For example, when the biceps brachii, brachialis, and brachioradialis are stimulated to contract during elbow flexion, the triceps brachii is reciprocally innervated to relax; in elbow extension, the triceps contracts while the flexors relax.

• Agonist / primary mover: most directly responsible for the movement.
• Secondary mover: assists the primary group.
• Antagonist: opposite side of joint; opposite joint action.
• Synergist / neutralizer / guiding muscle: eliminates extraneous movement.
• Fixator / stabilizer: holds one joint so others can move.
• Reciprocal innervation: agonist contracts while antagonist is signaled to relax.

• three-muscle-types — skeletal muscle as the voluntary type performing these roles.
• muscle-balance — training agonist/antagonist pairs to avoid imbalance.