The trapezius is a large, flat and broad diamond-shaped muscle of the back. It extends from the occipital bone down to the lower thoracic vertebrae and laterally to the scapula. This muscle moves and stabilizes the scapula. The trapezius is composed of three different functional parts:
- Descending (superior) part - fibers of this part descend towards the insertion site; they elevate and rotate the scapula upward;
- Transverse (middle) part - fibers of this portion are directed horizontally and run towards the shoulders; they retract the scapula;
- Ascending (inferior) part - fibers go in the superior direction to reach the insertion site; they medially rotate and depress the scapula.
Origin: descending part - external occipital protuberance, medial third of the superior nuchal line; transverse part - nuchal ligament, spinous processes of all cervical and the first three thoracic vertebrae (C1 - T3); ascending part - spinous processes of all thoracic vertebrae below the third thoracic vertebra (T4 - T12).
Insertion: acromial end of the clavicle, acromion and spine of the scapula. The descending part inserts on the lateral third of the clavicle, the transverse part ends at the acromion and scapular spine, while the ascending part inserts on the spine of the scapula.
Action: overall, it retracts, elevates, depresses and rotates the scapula. The descending part elevates the scapula and draws it superomedially. It also extends and bends the head and neck to the ipsilateral side and provides rotation of the head to the contralateral side. The transverse part retracts the scapula, while the ascending part depresses and medially rotates the scapula.
Innervation: motor innervation - accessory nerve (CN XI); motor and sensory innervation - anterior rami of the third and fourth cervical spinal nerves (C3 - C4) that arise from the cervical plexus.