Final answer:
The three-headed muscle on the posterior arm is the triceps brachii, which is responsible for forearm extension. It opposes the biceps brachii, which flexes the forearm.
Step-by-step explanation:
The three-headed muscle on the posterior arm is known as the triceps brachii. This muscle is responsible for extending the forearm. It attaches to the infraglenoid tubercle of the scapula and the posterior shaft of the humerus, running down to the ulna. The triceps brachii works in opposition to muscles such as the biceps brachii, brachialis, and brachioradialis, which flex the forearm. The anconeus is a small muscle on the lateral posterior elbow that also helps to extend the forearm, similar to the triceps brachii. Unlike triceps brachii, the biceps brachii is a two-headed muscle that plays a crucial role in flexing the forearm and supinating it.