The triceps brachii is a large muscle that covers the entire back side of the upper arm. It is divided into three sections, known as the lateral (outer) head, medial (middle) head, and long (inner) head.
The long head of the triceps originates on the scapula just below the shoulder joint.
All three heads come together into a common tendon which inserts on the olecranon process of the ulna.
Elbow joint extension.