Final answer:
Muscles for extending the elbow attach to the humerus, with the triceps brachii muscle playing a key role by attaching to the ulna and extending the forearm at the elbow joint.
Step-by-step explanation:
Muscles attach to the rough surface of the humerus allowing for the extension of the elbow joint. Specifically, the distal end of the humerus articulates with the forearm bones to form the elbow joint. Here, the capitulum of the humerus articulates with the head of the radius, as part of the elbow joint. Extension of the elbow is primarily performed by the triceps brachii muscle, which attaches to the olecranon process of the ulna and is responsible for extending the forearm at the elbow.