The trapezius is a large, triangular muscle located in the upper back and neck. It extends from the base of the skull down to the middle of the back and out to the shoulders. This muscle plays a crucial role in moving and stabilizing the shoulder blades, allowing for various arm movements. The trapezius is divided into three parts: the upper, middle, and lower fibers. Each section has specific functions, such as elevating the shoulders, retracting the shoulder blades, and assisting in the rotation of the neck. Strengthening the trapezius can improve posture and reduce neck and shoulder pain.