The shoulder is one of the most mobile joints in the body. It flexes, extends, abducts, adducts, and rotates the arm. This amount of movement comes with a price, though: the shoulder is not as stable as the other joints. The muscles surrounding this vital joint accommodate the movement and necessary stability of the shoulder.
The trapezius is one of the largest shoulder muscles. It has three sections. The upper part supports the weight of the arm, the middle retracts the scapula, and the lower part rotates and depresses the scapula. This large muscle originates vertically from the occipital bone at the back of the head through the cervical and thoracic sections of the spine and attaches to the clavicle and scapula.
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