The trapezius muscle is a large, triangular, paired muscle located on the posterior aspect of the neck and thorax. It is one of the broadest and most superficial muscles of the upper back and trunk. The trapezius muscle has three parts: the upper, middle, and lower fibers. The trapezius muscle is mainly postural, allowing and supporting the spinal column to remain erect when the person is standing. It is also used for active movements such as side bending, rotation of the head, elevating and depressing the shoulders, and internally rotating the arm. The trapezius muscle is innervated by the spinal accessory nerve (cranial nerve XI) and the ventral rami of C3 and C4. Trapezius muscle strain is a common injury that happens when you stretch the muscle too far.