The trapezius muscle covers most of the upper back and the back of the neck, explains InnerBody. The trapezius is composed of left and right muscle portions, which meet at the vertebral column. Together, the two sections form a triangular shape with points being at the shoulders and thoracic vertebrae.
Functions of the trapezius muscle include moving, rotating and stabilizing the shoulder blades and extending the head and neck. InnerBody explains that the trapezius contains three bands of muscle fibers: superior fibers, middle fibers and inferior fibers. The superior fibers elevate and brace the shoulder blade, while middle fibers pull the shoulder blade closer to the upper thoracic vertebrae of the spine. The inferior fibers pull the shoulder blade closer to the lower thoracic vertebrae of the spinal column.