The shoulder joint itself is surrounded by muscle, and in fact, most of the structural support is provided by these muscles. The rotator cuff is comprised of four muscles that run from the shoulder blade to the top of the arm bone, which act to support and move the shoulder joint. A rotator cuff tear is when the tendons which connect these muscles to the bone become irritated and painful. This typically occurs from two mechanisms, either a traumatic event (lifting something too heavy, falling) or an overuse injury resulting in repeated microtrauma. This can occur from months to years of use over time.