Which muscle is primarily responsible for elbow flexion?

The biceps brachii is the primary muscle responsible for elbow flexion due to its anatomical structure and leverage. It is a two-headed muscle located on the upper arm and spans the shoulder and elbow joints, allowing it to aid both in flexing the elbow and in supinating the forearm. During activities such as lifting or pulling, the biceps is activated to contract and flex the elbow joint, pulling the forearm closer to the upper arm.

In contrast, other muscles listed do not play a primary role in elbow flexion. The deltoid primarily functions to lift the arm and stabilize the shoulder joint. The triceps brachii is responsible for elbow extension, opposing the action of the biceps. The extensor carpi ulnaris serves a different function by aiding in wrist extension and does not contribute significantly to flexing the elbow. Hence, the biceps is the muscle primarily engaged in this action.