Identify a primary mover in the biceps curl exercise.

The biceps brachii is the primary mover in the biceps curl exercise due to its primary function in flexing the elbow joint. During the biceps curl, as you lift the weight by bending your arm at the elbow, the biceps brachii contracts to create this movement. This muscle is located at the front of the upper arm and is specifically engaged during any action that involves elbow flexion.

While the other muscles mentioned play roles in upper arm movements, they do not serve as the primary mover in a biceps curl. For instance, the triceps brachii is involved in extending the elbow and is engaged when lowering the weight, but it does not facilitate the curling action. The deltoid primarily assists with shoulder movements rather than flexing the elbow, and the brachialis, although it aids in elbow flexion, does not have the same prominent role as the biceps brachii, which is larger and more visibly active during this specific exercise. Thus, the biceps brachii stands out as the key muscle responsible for the primary action performed in a biceps curl.