The large muscle spanning the upper back and neck, forming a diamond shape across both shoulders, is the trapezius.
The trapezius is a broad, paired surface muscle that extends from the base of the skull and neck down to the lower thoracic spine. It spreads outward toward the clavicle, acromion and spine of each scapula, giving the two sides together their characteristic trapezoid or diamond-like outline. Its name reflects that geometric shape.
The muscle is divided functionally into upper, middle and lower portions. The upper fibers help elevate the shoulder and support the arm; the middle fibers pull the shoulder blades toward the spine; and the lower fibers help depress and rotate the scapula. Working together, these actions stabilize the shoulder and help raise the arm above the head.
The trapezius is often confused with the deltoid because both influence shoulder movement, but they occupy different regions. The deltoid caps the shoulder joint, while the trapezius covers much of the upper back and neck. The muscle receives motor innervation primarily from the accessory nerve, also called cranial nerve XI.