The trapezius muscle is a large, flat muscle that extends across the upper back and neck. It is shaped somewhat like a trapezoid, which is how it gets its name. The muscle has three distinct regions: the upper (or superior) fibers, middle fibers, and lower (or inferior) fibers, each of which has specific functions.
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Upper Fibers: These fibers originate from the base of the skull and the cervical vertebrae and insert into the lateral third of the clavicle. They primarily function to elevate the scapula (shoulder blade), such as when shrugging the shoulders.
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Middle Fibers: These fibers run horizontally and originate from the thoracic vertebrae. They insert into the acromion and spine of the scapula. The middle fibers are mainly responsible for retracting the scapula, pulling the shoulder blades toward the spine.
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Lower Fibers: These fibers originate from the lower thoracic vertebrae and insert into the scapular spine. They assist in depressing the scapula and help in upward rotation of the shoulder blade.
Overall, the trapezius plays a critical role in stabilizing and moving the shoulder girdle and is essential for many upper body movements, including lifting, reaching, and rotating the arms. It is innervated by the accessory nerve (cranial nerve XI) and receives blood supply from various sources, including the transverse cervical artery.
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