Agonists
Agonists are muscles that act as prime movers. The agonists are not only responsible for accelerating the concentric movement; they also decelerate the eccentric portion of the movement and perform isometric contraction as well.
Antagonists
Antagonists are muscles that act in direct opposition to prime movers. To contract any muscle, the opposing muscle must release most of its tension through an action known as reciprocal inhibition.
Synergists
Synergists are muscles that assist prime movers during movement patterns. The degree of assistance they offer depends on the amount of load, direction of resistance, the overall capabilities of the primary movers.
Stabilizers
Stabilizers are muscles that stabilize or support a body segment while the prime movers and synergists perform the movement. Muscles designed for stabilization are usually classified as “tonic muscles”. These tonic muscles are primarily constructed for endurance and have low force output. Refer to Volume 3 – Chapter 1: Flexibility for more details on tonic and phasic muscle relationships.
Neutralizers
Neutralizers are muscles that counteract the unwanted action of other muscles. Without proper functioning of neutralizers, joint mechanics are altered and faulty motor patterns develop, which can cause compensation and an increased incidence of injury or joint degeneration.