The shoulder is the most mobile joint in the body, being able to move in all directions. This mobility also means the structures that make up the joint are more vulnerable to injury, including the rotator cuff muscles.
Tears of the Rotator Cuff
The rotator cuff is composed of four muscles: supraspinatus, infraspinatus, teres minor and subscapularis. These muscles serve to stabilize the shoulder during movement, especially rotational and overhead movement. Both sudden force and repetitive use can lead to tears or fraying of the tendons of these rotator cuff muscles. Quite often the injury doesn't completely tear the tendon or muscle. This is called a partial rotator cuff tear. According to Wheeless' Textbook of Orthopaedics, "by definition, partial tears involve 50% or more of the tendon."
Causes
Tendons, especially in younger people, are actually quite tough, so it takes a lot of force to tear them. Because of this, injuries to the rotator cuff due to sudden forceful movements, such as when pitching a ball, paddling a raft in white water or falling on an outstretched arm, will frequently lead to only a partial tear of a tendon.
The rotator cuff tendons are also subject to constant wear as they move back and forth over the bony structures that make up the shoulder blade. This might also lead to some fraying and tearing of the deep muscle and tendon fibers, frequently at the point where they attach to the top of the humerus, or upper arm bone.
Because of the anatomy of the shoulder, it also isn't unusual for the rotator cuff muscles, and especially the supraspinatus, to become caught---or impinged, in medical terms---by bony spurs under the acromion process, a part of the shoulder blade. This impingement leads to swelling and inflammation, which further damages the impinged muscle, leading to fraying or small tears in the muscle.
Symptoms
Rotator cuff tears are quite common. Many older people have at least one partial or complete rotator cuff tear, usually on their dominant arm. Many people who have a partial rotator cuff tear have no or few symptoms, but others may notice that their shoulder is stiff and may begin to experience weakness in the affected arm. This is because the muscle can't work at full force when it is partially torn. As the weakness increases, shoulder movement may become more painful. Another sign of a partial tear of the rotator cuff is crepitus---a crackling sound---in the joint as the arm is partly raised and then turned away from the body.
