The triceps brachii is the large, three-headed muscle situated on the back of the upper arm, occupying the entire posterior compartment. This muscle is the primary engine for straightening the elbow, a movement fundamental to nearly all pushing and reaching activities. Its functions extend beyond simple arm extension to include roles in shoulder movement and joint stability. The triceps is largely antagonistic to the biceps brachii on the front of the arm; when one contracts to bend the arm, the other relaxes to allow the movement.
Anatomical Structure of the Triceps
The triceps brachii is defined by its three distinct points of origin, or “heads”: the long head, the lateral head, and the medial head. These three muscle segments share a common point of insertion, forming a thick, single tendon near the elbow. The long head is unique because it originates from the infraglenoid tubercle of the scapula, meaning it crosses both the shoulder and elbow joints.
In contrast, the lateral head and the medial head both originate directly from the humerus. The lateral head arises from the posterior surface of the humerus, superior to a nerve groove, while the medial head originates lower down on the humerus, inferior to that same groove. All three heads converge distally to insert onto the olecranon process, the prominent bony projection at the top of the ulna. The nervous supply for all three heads is provided by the radial nerve, which carries signals from spinal segments C6 through C8.
The Primary Function of Elbow Extension
The most recognized and powerful action of the triceps brachii is the extension of the elbow joint. This action directly opposes the forearm flexion performed by the biceps brachii and brachialis muscles. All three heads of the triceps contribute to this extension, with their combined force pulling the ulna back from the flexed position.
The medial head is considered the workhorse for low-resistance movements, remaining active throughout the entire range of elbow extension. The lateral and long heads are recruited more strongly when the arm is straightening against significant resistance or when high force is required. When pushing an object away from the body, the triceps performs a concentric contraction, shortening to generate the necessary force. In contrast, when slowly lowering a heavy object, the triceps performs an eccentric contraction, lengthening under tension to control the descent.
Secondary Actions and Joint Stabilization
Beyond its primary role in straightening the arm, the triceps brachii performs secondary actions due to the unique attachment of its long head. Because the long head originates on the scapula, it is the only part of the muscle that acts on the shoulder joint. This allows the long head to assist with shoulder extension, such as pulling the arm backward toward the torso.
The long head also contributes to adduction, the movement of pulling the arm back toward the midline of the body. The long head plays a significant role in stabilizing the shoulder joint, especially when the arm is raised overhead, helping to keep the head of the humerus securely in the socket. At the elbow, the triceps, particularly the medial head, assists in joint stabilization. This stabilization is essential for fine motor control, allowing the forearm to be held steady during tasks that require precision, like handwriting or typing.
Role in Everyday Movement
The powerful extension capability of the triceps translates directly into many fundamental movements. Any action that involves pushing something away from the body relies heavily on the triceps. Pushing open a heavy door or pressing a shopping cart forward are common examples of triceps engagement.
The ability to move the arm against gravity is powered by this muscle group, such as when reaching up to place an item on a high shelf. When rising from a seated position, the triceps engage to push off from the armrests of a chair, lifting the body’s weight. Activities requiring throwing, like tossing a ball, utilize the triceps as they provide the final accelerating push to extend the arm.