Identify the primary extensor of the elbow joint.

The primary extensor of the elbow joint is the triceps brachii. This muscle is located on the posterior aspect of the upper arm and is responsible for extending the forearm at the elbow joint. The triceps brachii consists of three heads: the long head, lateral head, and medial head, all of which converge to form a single tendon that attaches to the olecranon process of the ulna. When the muscle contracts, it pulls on this tendon, leading to extension of the forearm.

In contrast, the biceps brachii primarily functions as a flexor of the elbow, aiding in lifting and turning the forearm, while the brachialis also plays a role in flexion. The deltoid muscle is involved in shoulder movements and does not assist in elbow extension. Therefore, the role of the triceps brachii in extending the elbow highlights its importance as the primary extensor in this joint action.