Which muscle acts primarily as an extensor of the elbow?

The triceps muscle is primarily responsible for extending the elbow joint. This muscle is located at the back of the upper arm and is made up of three heads, which work together to facilitate the action of straightening the arm. When the triceps contracts, it pulls on the olecranon process of the ulna, leading to extension of the forearm relative to the upper arm. This action is crucial for various movements such as pushing, throwing, and performing exercises that involve pressing or extending the arms.

The other muscles mentioned have different primary functions. The pronator teres, for instance, mainly assists with the pronation of the forearm, which involves turning the palm downwards. The biceps brachii primarily serves as a flexor of the elbow, allowing the arm to bend. The brachialis, while it does support elbow flexion, does not play a significant role in extension. Understanding the specific roles of these muscles highlights why the triceps is the correct answer for the primary extensor of the elbow.