Trapezius, the most superficial of the upper back muscles, derives its name from the irregular, four-sided shape that its fibres occupy - a trapezium. A large, flat muscle its origin runs from skull to lower thorax:
- the medial third of the superior nuchal line, medially to
- the external occipital protuberance, then passing inferiorly to
- the ligamentum nuchae, down to the level of C7
- the spinous processes of vertebrae C7 to T12 and the intervening supraspinous ligaments
Between C6 and T3, the origin is aponeurotic and covers a diamond-shaped interval which extends most laterally outwards to the edge of T1. The rest of the origin is muscular.
Trapezius converges on the pectoral girdle and may be considered in terms of the continous insertion of three groups of fibres of different origin:
- upper fibres: pass inferolaterally from the skull and cervical spine to insert into the posterior border of the lateral third of the clavicle
- middle fibres: pass horizontally from the level of C7 to T2 to insert into medial border of the acromion and the superior lip of the crest of the scapular spine; a bursa separates these fibres from the bare area at the base of the scapular spine
- lowest fibres: via a tendon into the tubercle on the medial end of the spine of the scapula
It has important functions in retracting, elevating, rotating and stabilising the scapula. Its nerve supply derives from the spinal accessory nerve and the cervical plexus - see submenu.
Trapezius forms one of the boundaries of the posterior triangle of the neck and the triangle of auscultation.