Located on the posterior or back of the upper arm, the triceps brachii extends the forearm at the elbow and the humerus at the shoulder and is the prime mover for arm extension. The forearm can flex and extend at the elbow because of the upper-arm muscles.
The three heads of the triceps brachii originate at the scapula and humerus. Before they insert at the olecranon of the ulna, these three heads merge. The opposite of extension, flexion, occurs through action of three muscles: the brachialis, biceps brachii, and brachioradialis. These flexors are all on the anterior or front of the upper arm and attach between the humerus and scapula down the forearm to the radius and ulna. The biceps brachii also supinates or twists the forearm.