|Derivation||Akin to trapezoid, a four-sided shape with two parallel sides.|
|Origin||From a short horizontal line on the base of the skull to all of the vertebrae down to the 10th thoracic.|
|Insertion||The spine of the scapula, the medial side of the acromion process of the scapula, and the lateral third of the clavicle.|
|Action||The upper third raises the scapula; with the scapula fixed, it draws the head upright, backward, toward the shoulder, and rotates the face to the opposite side. The middle third draws the scapula towards the spine; the lower third draws it downward. Both sides together extend the head.|
Trapezius extends over the upper back, nape, and shoulders. Its form varies in thickness and shape as it moves from the back of the body to the front.
From the back, the two sides of trapezius cover the upper back in the form of a diamond. It is much thicker toward the center than at the upper and lower ends, that are strongly influenced by underlying forms.
The key to understanding the shape of trapezius is locating the spine of the scapula, where it attaches along the upper border. When the arms are down, the lower border forms an obtuse angle with the spine of the scapula. When the arm is raised, which is accomplished by lifting the scapula and rolling its lower angle laterally, this angle flattens, almost forming a straight line from the tenth cervical vertebra to the acromion process of the scapula.
The form of trapezius is influenced by supraspinatus (
soo-pruh-spine-ATE-us), which lies over the spine of the scapula and helps to raise the arm. It is also influenced by the rhomboids, which contributes further to the bulk of the middle third of trapezius and are apparent especially towards their insertion on the lower angle of the scapula.