Answer
In muscle physiology a prime mover is the muscle that produces most of the force during a particular joint action like flexing the elbow. In this case the prime mover is the brachialis.
A synergist is a muscle that aids the prime mover (PM). The action of the synergist may not be identical or redundant to that of the PM. A synergist may stabilize a joint and restrict undesirable movements. It may also modify the direction, of a movement so that the result of the action of the PM is more specific , coordinated, and controlled.
Work Step by Step
Antagonist. An antagonist is a muscle that opposes the PM:
It may relax to give the PM complete control
It may limit the speed or range of the movement effected by the PM in order to prevent joint injury;
prevent excessive movement ( by PM), or to prevent inappropriates action.
Antagonistic pair. An antagonistic pair is two muscles that act on opposite sides of a joint.
They function so that we can do opposing actions such as pull and push with the same limb:--a single
muscle cannot both push and pull:
In flexion at the elbow joint, the brachialis is the prime mover; the triceps brachii is the antagonist
In extension at the elbow joint, the triceps brachii is the PM and the brachialis is the antagonist
