Final answer:
The triceps brachii, anconeus, and brachioradialis are the muscles attached to the posterior humerus shaft. They contribute to the extension of the forearm and elbow.
Step-by-step explanation:
The muscles attached to the posterior humerus shaft are the triceps brachii, anconeus, and the brachioradialis. These muscles play a role in extending the forearm and elbow.