Trapezius gets its name from its trapezium-like shape; the corners being the neck, the two shoulders, and the thoracic vertebra, T12.
A person can feel trapezius on themselves by holding a weight in front the them in one hand, and with the other, touching between the shoulder and the neck. They should feel a sheet of muscle become active.
Trapezius actually arises, down the midline, from the external occipital protruberance, the nuchal ligament, the medial part of the superior nuchal line, and the spinous processes of the vertebrae C7-T12.
It inserts, at the shoulders, into the lateral third of the clavicle, the acromion process, and into the spine of the scapula.
Its muscle fibres at the neck, run downward and laterally towards the arm. The fibres from the vertebrae run upward, also towards the shoulder.
Because the fibres run in different directions, it has a variety of actions. It elevates, retracts, adducts and rotates the scapula. The superior fibres elevate the scapula, the middle fibres retract it, and the inferior fibres depress it. When the superior and inferior fibres act together they superiorly rotate the scapula.
Trapezius's major nerve supply is the XIth cranial nerve. Cervical nerves C3 and C4 receive information about pain in this muscle.