When the forearm is extended at the elbow joint, the muscle acts as the prime mover.

When the forearm is extended at the elbow joint, the triceps brachii muscle acts as the prime mover.

Explanation:

Triceps brachii -

Triceps brachii is one of the largest muscle which is presents on the back of the upper limbs of many vertebrates.

The origin of the muscle is Infraglenoid tubercle of scapula.

And

The insertion of the muscle is Posterior olecranon process of ulna.

Actions of Triceps Brachii Long Head are extending the forearm at elbow and extends the arm at the shoulder.

Hence ,

When the forearm is extended at the elbow joint, the triceps brachii muscle acts as the prime mover.