The triceps, or triceps brachii is a large muscle in the back of the upper arm and is the largest arm muscle. It has three sections to it, long head, medial head and lateral head.
Essentially an extensor muscle of the elbow joint, it also works to balance out the actions of the biceps and brachialis muscles. It stabilizes the elbow joint when the forearm and hand are used for finer movements such as writing.
The long head of the triceps is the power portion of the muscle and can deliver high-intensity force. The long head originates from the scapula, so the triceps muscle acts on and helps to stabilize shoulder motion at the top of the humerus.
The triceps is often ignored with arm movement, such as lifting. This leaves this type of activity the responsibility of the biceps muscle. Weakness in the triceps muscles is contributory to many shoulder injuries.