Agonist: Any muscle that is the primary mover in a joint motion. Also called Prime Mover.
Antagonist: Any muscle that acts against an agonist. Your muscles are paired around your joints in a push-pull relationship, so that if an agonist moves your body one direction, its antagonist moves you the opposite direction. When an agonist relaxes to allow the antagonist on the other side of the joint to contract and vice versa, it is called Reciprocal Inhibition. However, when one of your agonist muscles is too tight, like your chest muscles, for example, then its functional antagonist, in this case your upper back muscles, are inhibited — neither will function optimally. This is called Altered Reciprocal Inhibition.
Synergist: Any muscle that assists the prime mover in a joint motion.
During a movement there is an action phase and a recovery phase. During the action phase your prime movers concentrically contract and shorten, while your antagonists eccentrically contract and lengthen. That’s the power of the movement. During the recovery phase your prime movers eccentrically contract and lengthen as they control your descent and your antagonists concentrically contract. During both phases, your syngerist muscles are actively assisting the prime movers.
A good way to imagine this is to think of your muscles as your gas and brake pedals. When it’s time to go, you step on the gas and power through your movement. But when it’s time to slow down, you have to gradually decrease your speed by stepping on the brake pedal in a controlled fashion. Both are very crucial to making the movement successful and injury-free.
Agonist: Pectoralis Major (pecs)
Antagonists: Trapezius – middle fibers (traps), Posterior Deltoids (post delts), and Rhomboids
Synergists: Triceps and Anterior Deltoids (ant delts)
Action Phase: Pushing up off of the floor
Recovery Phase: Controlled lowering back down to the floor
Suggested Reading: The Importance of Eccentric Muscle Movement (<– coming soon!)