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Movement of the arm requires joint mobility and muscular stability. Moving the arm requires appropriate muscle facilitation and mobility in the shoulder joint, the scapulo-thoracic joint (which is not a "true" joint, more of a physiological joint), the acromioclavicular joint (AC) and the sternoclavicular joint (this is the only joint connecting the arm to the axial skeleton).
Take a look at the pics on the left. There are a lot of muscles that can affect the movement of the arm, the scapula, the shoulder, and the clavicle. So... basically what I am pointing out here is that optimal muscle function of the shoulder girdle is key to a healthy pain free shoulder joint, neck and arm.
Muscle, bone and connective tissue adapt to how we use them. And typically during a day our arms are positioned out in front of the body: computing, driving, pushing a stroller/walker/cart. This positioning, along with all of our other daily habits of grasping, clenching and tensing can lead to shortened muscles, winged shoulder blades and internal rotation of the shoulder joint which can lead to impingement and eventual rotator cuff tendonitis/tear.