The trapezius muscle (or simply trapezius, Latin: musculus trapezius) is a significant muscle of the back lying in the most superficial layer. It extends from the occipital bone down to the lower thoracic vertebrae and laterally to the scapula. The trapezius moves and stabilizes the scapula. This shape of this muscle resembles a trapezium or diamond-shaped quadrilateral. The trapezius has three functional parts:
- upper (descending) part - supports the upper arm;
- middle (transverse) part - retracts the scapula;
- lower (ascending) part - medially rotates and depresses the scapula.
The trapezius originates from the spinous processes of the seventh cervical vertebrae and the following twelve thoracic vertebrae (C7 - T12), the nuchal ligament, also from the occipital protuberance, and the superior nuchal line - landmarks on the occipital bone.
The trapezius inserts onto the lateral third of the clavicle, the acromion process, and the spine of the scapula.
Upon activation, the trapezius rotates, raises, lowers, and adducts (retracts) the scapula. It also aids in the rotation of the head. When the spinal origins are stable, it moves the scapula, but when the scapula is fixed, it provides movements in the spine. The trapezius also is important in stabilizing the scapula and supporting the weight of the arm.
Several nerves participate in the innervation of the trapezius. The accessory nerve (CN XI) provides motor innervation to the trapezius muscle. Motor innervation, together with proprioception, comes from the third and fourth cervical spinal nerves (C3 - C4) that arise from cervical plexus.