The triceps brachii is a large muscle located on the posterior side of the upper arm. It is shaped somewhat like a horseshoe and plays a significant role in nearly all movements of the arm, including pushing, throwing, and extending the elbow.
Anatomy of the Three Heads
The name “triceps brachii” translates to “three-headed muscle of the arm,” indicating its composition of three distinct sections: the Long Head, the Lateral Head, and the Medial Head. These heads are differentiated by their points of origin on the bones of the shoulder and arm. The Long Head is unique because its origin is on the infraglenoid tubercle of the scapula.
Because of its origin on the scapula, the Long Head crosses both the shoulder joint and the elbow joint, making it a bi-articular muscle. In contrast, the Lateral Head and Medial Head both originate directly on the humerus, making them uni-articular. The Lateral Head arises from the posterior surface of the humerus, situated superior and lateral to the radial groove.
The Medial Head originates from the posterior humerus as well, but it is located inferior to the radial groove. This head is considered the deepest of the three, partially covered by the Long and Lateral heads. Although they have separate origins, all three heads eventually merge to form a single, thick tendon that crosses the elbow joint.
Primary Function and Attachments
The primary mechanical action of the entire triceps brachii muscle is the extension of the forearm at the elbow joint, which means straightening the arm. All three heads contribute to this action, working as the main antagonist to the biceps brachii muscle. The common tendon formed by the three heads inserts onto the olecranon process of the ulna.
The Long Head, due to its attachment to the scapula, possesses a secondary function at the shoulder joint. It assists in the extension and adduction of the arm. This head also plays a role in stabilizing the shoulder joint, helping to keep the head of the humerus securely in the glenoid cavity, especially when the arm is held close to the body.
How Head Position Influences Activation
The anatomical differences, particularly the Long Head’s crossing of the shoulder joint, mean that arm position significantly influences which head is preferentially activated. Movements that place the arm in an overhead position, such as overhead triceps extensions, stretch the Long Head across the shoulder joint. This stretch puts the Long Head in a mechanically advantageous position, often leading to greater force generation during the elbow extension.
When the arm is in a neutral position, such as during triceps pushdowns or close-grip bench presses, the Long Head is less stretched. In these exercises, the Lateral and Medial Heads bear a greater proportion of the load. The Medial Head is active in lower-force movements and provides precise control, while the Lateral Head is recruited for movements requiring a higher intensity of force.
Performing an elbow extension with the shoulder elevated to 90 degrees or more shifts the muscular emphasis. This position tends to place a higher demand on the Medial Head compared to the Long Head. Trainers utilize these postural variations to target the specific heads for comprehensive development.