Anatomy, Back, Trapezius

The trapezius muscle is a large superficial back muscle that resembles a trapezoid. It extends from the external protuberance of the occipital bone to the lower thoracic vertebrae and laterally to the spine of the scapula. The trapezius has upper, middle, and lower groups of fibers. The ventral rami of C3 and C4 innervate the sensory functions of the trapezius. The spinal accessory nerve (cranial nerve XI) innervates the motor function of the trapezius. The function of the trapezius is to stabilize and move the scapula. The upper fibers can elevate and upwardly rotate the scapula and extend the neck. The middle fibers adduct (medially retract) the scapula. The lower fibers depress and aid the upper fibers in upwardly rotating the scapula. These motions allow for the scapula to rotate against the levator scapulae and the rhomboid muscles. This rotation, in conjunction with the deltoid muscle, is essential for throwing objects.