Answer
Muscles pull , but cannot push. When a muscle contracts it pulls its insertion is moved towards its origin. But muscles usually act in retion hips with other muscles seldom if ever in isolation. Thus for different actions roles may change from prime mover, to antagonist to synergist to fixator --or a combination of these.
Prime Mover (PM)
The prime mover ( or agonist) in a muscular action is the muscle that has the main responsibility for the movement.
Example: The pectoralis major is the prime mover of arm (shoulder) flexion
Antagonist: tHe antagonist is the muscle that opposes the action of the prime mover or agonist
Example: The triceps brachii is the antagonist of elbow flexion
prime movers and antagonists are usually located on opposite sides of a joint on which they act
Example : The brachialis ( PM of elbow flexion) is situated n the opposite side of the elbow joint to
the triceps brachii.
The latissimus dorsi, PM for arm extension, is opposed by pectoralis major , antagonist for
arm extension.
Work Step by Step
Synergists:
A synergist is a muscle that assists the action of the PM . It may do so by adding a little extra force or by reducing undesirable or unnecessary movements that my be associated with the contraction of the PM.
Example. The brachioradialis and the biceps femoris are synergists in elbow flexion. They assist the brachialis which is the PM.
Fixators:. Fixators are synergists that act to stabilize a muscle origin ( usually on a bone) so that the prime mover is provided a stable base for movement of the insertion.
Example; In elbow flexion, the rhomboideus muscle fixes the scapula in a stable position
