Which muscle primarily contributes to flexion of the elbow joint?

The biceps brachii is the primary muscle responsible for flexion of the elbow joint. This muscle is located in the upper arm and consists of two heads: the long head and the short head. When the biceps contracts, it pulls the forearm towards the shoulder, effectively bending the elbow. This action is crucial for various activities such as lifting and pulling.

The biceps also plays a role in supination of the forearm, but its main function regarding elbow flexion makes it the standout choice for this question. The other muscles listed have different primary functions; for example, the triceps brachii is primarily involved in extension of the elbow, while the pronator teres and supinator have roles in forearm pronation and supination, respectively, rather than elbow flexion. Understanding the specific functions of these muscles is key to recognizing the biceps brachii's prominence in elbow flexion.