What is the difference between a prime mover and a synergist?

A prime mover, also known as an agonist, is the muscle primarily responsible for initiating and carrying out a specific movement, such as the biceps brachii during a bicep curl. In contrast, a synergist is a muscle that assists the prime mover by stabilizing the joint or helping with the movement, like the brachialis during the same curl. While the prime mover is the main actor in a movement, synergists support and enhance the action for smoother and more effective performance.