Final answer:
The triceps brachii is the primary muscle used in a tricep extension, functioning as the main extensor of the forearm.
Step-by-step explanation:
The primary muscle used in a tricep extension is the triceps brachii. During a tricep extension, this muscle is responsible for extending the forearm at the elbow. The triceps brachii, along with the anconeus, are the main extensors of the forearm.
Other muscles like the biceps brachii, brachialis, and brachioradialis serve as flexors of the forearm, but they are not involved in the tricep extension movement. When performing tricep extensions, the biceps would relax while the triceps contract to achieve the motion.