What is the primary action of the triceps brachii?

The primary action of the triceps brachii muscle is elbow extension. This muscle is located at the back of the upper arm and is composed of three heads: the long head, the lateral head, and the medial head. When the triceps brachii contracts, it pulls on the olecranon process of the ulna, straightening the elbow joint, which results in the extension of the forearm. This movement is essential for various activities, such as pushing, throwing, and performing any movement that requires the arm to be straightened.

In contrast, the other options relate to different movements or muscles. Elbow flexion, for example, is primarily performed by the biceps brachii and brachialis muscles. Shoulder abduction involves raising the arm sideways away from the body, which is primarily facilitated by the deltoid and supraspinatus muscles. Shoulder flexion entails raising the arm in front of the body, primarily activated by the anterior deltoid and pectoralis major. Therefore, the triceps brachii's role in elbow extension is distinct and crucial for proper arm function.