The primary muscles of the shoulder can be divided into two categories: scapulothoracic and scapulohumeral. The scapulothoracic muscles are the muscles that connect the shoulder blades to the thoracic spine (mid-back) and ribs and are mainly responsible for keeping the shoulder blades stable against the torso. The scapulothoracic muscles include the trapezius, rhomboids, and serratus anterior. The trapezius and rhomboids work together to retract or squeeze the shoulder blades together, preventing the shoulders from rounding forward and shrugging up toward the ears, while the serratus anterior keeps the shoulder blades flat against the rib cage. The scapulohumeral muscles are the muscles that connect the shoulder blades to the upper arms; essentially, these are the muscles around the shoulder joint. The primary scapulohumeral muscles include the four muscles that make up the rotator cuff (supraspinatus, infraspinatus, teres minor, subscapularis) and deltoid. The rotator cuff functions to provide stability and maintain alignment of the shoulder joint when you move your arms. The deltoid is the large shoulder muscle at the top of the arms that is seen in the mirror and it functions to move the arm to the front of the body, back of the body, and away from the side of the body.
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