Which term best describes a muscle that promotes a certain movement?

The term that best describes a muscle that promotes a certain movement is "agonist." Agonist muscles are those that contract to produce a specific action or movement, functioning as the primary movers in a given activity. When you perform an action such as lifting your arm, the agonist muscle is the one responsible for the lifting motion, effectively creating movement at a joint.

In contrast, other types of muscles serve different roles. Antagonist muscles oppose the action of the agonist; for example, while the bicep acts as the agonist in flexing the elbow, the tricep serves as the antagonist. Stabilizers are muscles that help maintain a position or stabilize joints during the movement but do not create the movement itself. The term "flexor" refers specifically to muscles that decrease the angle between two body parts, like bending an arm, but does not encompass all movements that an agonist might perform. Therefore, the most accurate term for a muscle that drives movement is the agonist.