What is the definition of a primary mover?

A primary mover, also known as an agonist, is a muscle or group of muscles that is primarily responsible for initiating and executing a specific movement in the body. During physical activity, the primary mover contracts to generate the force needed for that movement, while other muscles may assist or stabilize the action. For example, in a bicep curl, the biceps brachii is the primary mover that flexes the elbow.