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 lateral head of the triceps originates on the back of the humerus from the middle of the shaft to almost the very top.
All three heads come together into a common tendon which inserts on the olecranon process of the ulna.
Elbow joint extension.