trapezius muscle

The trapezius muscle is a large, broad superficial muscle of the posterior neck and back. It gains its name from its diamond shape. Along with sternocleidomastoid muscle, it is invested by the superficial layer of the deep cervical fascia, which splits around it.

Summary

Gross anatomy

Origin 

It has many origins due to its large size:

  • medial third of superior nuchal line
  • inion and external occipital protuberance
  • nuchal ligament
  • spinous processes of C7-T12 vertebrae and the corresponding supraspinous ligaments
Insertion
  • clavicle: lateral third
  • scapula: acromion and spine 
Relations

Innervation

Action 

  • all fibers: retract the scapula medially
  • upper fibers only: elevate and rotate scapula up to aid in shoulder abduction
  • middle fibers only: retract scapula
  • lower fibers only: depress and rotate scapula down
  • upper and lower fibers together: rotate spinous process of scapula superiorly
  • with the shoulders fixed:
    • bilateral contraction extends neck
    • unilateral contraction produces ipsilateral flexion