Triceps medial head
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 medial head of the triceps originates on the lower portion of the back of the humerus over a wide space extending nearly two-thirds of the length of the bone.
All three heads come together into a common tendon which inserts on the olecranon process of the ulna.
Elbow joint extension.