Which muscle is categorized as a flexor muscle in the arm?

The biceps brachii is classified as a flexor muscle in the arm because its primary action is to flex the elbow joint and supinate the forearm. When the biceps brachii contracts, it pulls the forearm upwards towards the shoulder, which is the characteristic movement of flexion at the elbow.

In contrast, the triceps is primarily responsible for extension of the elbow, the deltoids are shoulder abductor muscles, and the latissimus dorsi plays a role in extending, adducting, and medially rotating the shoulder. These functions do not contribute to flexion at the elbow, thus reinforcing the role of the biceps brachii as the main flexor muscle in the arm.