The shoulder has the largest range of movement of any joint in the body.  The shoulder blade moves on the chest wall.  The humerus (arm bone) forms a ball and socket joint with the shoulder blade. The ball, or head of the humerus, is quite large and the socket relatively small and shallow.  The head is stabilised in the socket by the rotator cuff.  The rotator cuff is made up of the tendons of four muscles which are attached deep on the shoulder blade.  The muscle that rests on top of the humeral head and shoulder blade is known as the supraspinatus.  It is most at risk of wear against the point of bone on the shoulder blade known as the acromion.  This wear will occur where the muscles of the rotator cuff do not control the head of the humerus in the socket well enough with  use of the arm away from the body.