Answer
The prime mover (muscle) is the muscle that is mainly responsible for a specific movement--applies the greatest force. For example, the movement of flexion of the elbow joint is accomplished with the cooperation of several muscles ( flexors) --biceps brachii, barachioradialis and brachialis. However, the prime mover for this action is the brachialis.
An antagonist( muscle) opposes or reverses a particular action or movement . For example, the antagonist of the elbow flexors is the triceps brachii( and the aconeus) which function to extend the elbow joint.
Work Step by Step
Synergists help prime movers (agonists), in some way, to accomplish their action,
a. By adding extra force to the movement of the prime mover
b. By reducing unnecessary or undesirable movement.
The biceps brachii and the brachoradialis are synergists of the brachialis (PM) in elbow flexion.
Fixators. The function of fixators is to to stabilize a bone or the origin of a muscle so that the prime mover
has a firmer base on which to act. Thus in upper limb abduction, the fixators of the scapula are the serratus anterior, the middle and lower trapezius and the rhomboids.
