Triceps brachii muscle

In humans, the triceps brachii muscles are the two muscles on the back of the upper part of the arm. They are used to straighten the arm from a bent position. Its name is based on the three bundles of muscle fibers that make up the muscle. The various bundles are attached to either the radial nerve or the axillary nerve. 
One common test is to hit the triceps to observe the reflex reaction which indicates whether the nerves into the arm are functioning properly.
Triceps brachii muscle at Wikipedia
Category:Anatomy