The trapezius is one of the major muscles of the back and is responsible for moving, rotating, and stabilizing the scapula (shoulder blade) and extending the head at the neck. It is a wide, flat, superficial muscle that covers most of the upper back and the posterior of the neck. Like most other muscles, there are two trapezius muscles – a left and a right trapezius – that are symmetrical and meet at the vertebral column.
The trapezius arises from ligaments at its origins along the nuchal crest of the occipital bone and the spinous processes of the cervical and thoracic vertebrae.It extends across the neck and back to insert via tendons on the clavicle, acromion, and spine of the scapula. The name trapezius is given to this muscle due to its roughly trapezoidal shape; the long base of the trapezoid is formed at the origins along the vertebrae and the short base forms at the insertions along the scapula and clavicle.
- The superior fibers cover the posterior and lateral sides of the neck with their tendons connecting to origins along the occipital bone and insertions on the clavicle.
- Just below this region is the narrow band of the middle fibers, which extend from origins along the superior thoracic vertebrae and insert into the acromion process of the scapula.
- Finally, the inferior fibers cover a wide region of the back from their origins along the inferior thoracic vertebrae and insert into the spine of the scapula.
The functions of the trapezius are diverse and are best understood by actions of the individual bands of muscle fibers within the trapezius. The superior fibers typically act upon the scapula by elevating it (as in shrugging) or by bracing the shoulder when a weight is carried. When other muscles hold the scapula in place, the superior fibers of both trapezius muscles can extend the head at the neck by pulling the occipital bone closer to the scapula. The middle fibers work to retract and adduct the scapula by pulling the shoulder blade closer to the spine. The inferior fibers depress the scapula by pulling it closer to the inferior thoracic vertebrae. To rotate the scapula, the inferior and superior fibers work together to push the inferior angle of the scapula laterally and raise the acromion. Finally, the trapezius stabilizes the scapula to prevent extraneous movement by lightly contracting all of its fiber bands simultaneously.