Your upper-arm muscles may be divided into those of the pectoral girdle, or shoulder, the arm, the forearm and the hand. The triceps muscle is found in the arm and, while it is the major muscle involved in forearm extension, it is not involved in the shrugging of the shoulder, making shoulder shrugs an ineffective triceps workout.
Shoulder Shrugs
When you shrug your shoulders, you are elevating your scapulae, or shoulder blades. This movement is acheived by the action of two main muscles: the trapezius and the levator scapulae. The trapezius originates at the skull and cervical spine and inserts at the scapula and clavicle. The levator scapulae originates at the top of the cervical spine and inserts along the medial side of the shoulder blade. When these muscles contract, the shoulder blade is raised, as with shrugging the shoulders.
The Triceps Brachii
The triceps brachii muscle is a large muscle found on the posterior side of the arm. The muscle is named for its three-headed origin: at the scapula and two places on the humerus. The triceps inserts at the elbow. Thus, it contracts, extending the forearm. The triceps is the prime mover of forearm extension.
Working the Triceps Muscle
Since the triceps muscle is the main extensor of the forearm, any exercise designed for the triceps should resist that extension. Examples include pushups, the bench press and the military press. Triceps exercises should always involve forearm extension, or straightening the elbow, not forearm flexion, or bending the elbow. Exercises that involve elbow flexion, like curls, utilize the biceps brachii muscle.
Antagonistic Pairs
The triceps brachii and biceps brachii are an antagonistic pair of muscles. While the biceps is responsible for forearm flexion, the triceps is responsible for forearm extension. It is important to work these muscles equally, so that one muscle does not become much stronger than the other. When antagonistic pairs of muscles become unbalanced, mobility is lost and the joint becomes more susceptible to injury.
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