Because of the shoulder's anatomy, it is easily subject to injury in people of any age, especially if they are engaged in activities involving overhead movement. However, aging can increase the chance of injury. Studies report that deterioration of the rotator cuff occurs in a large percentage of individuals, and the percentage goes up with each successive decade of life. In this excerpt from a longer article from a recent Johns Hopkins Arthritis Bulletin, Drs. Steve A. Petersen and Edward G. McFarland discuss rotator cuff injury.

Q. What happens when the rotator cuff is injured?

A. Crucial to the shoulder's many movements -- and yet often overlooked -- are the four small, powerful muscles that, with their tendons, make up the rotator cuff. These muscles -- supraspinatus, infraspinatus, teres minor, and subscapularis -- are located on or near the shoulder blade and tightly attach the upper arm bone to the shoulder socket, providing rotation possibilities, stability, and the ability to raise the arm.

Because the muscles are located well inside the shoulder, most people are completely unaware of their function -- until something goes wrong. Unfortunately, when the arm is repeatedly raised above the head -- the common repetitive motions involved in many activities, such as pruning hedges, swimming, tennis, and golf -- the rotator cuff tendons slowly start to fray and wear out.

It is not known exactly why this occurs but one common theory is that the tendons rub against the bony underside of the acromion part of the shoulder blade, causing them to swell and even tear slightly because of the reduced space in the joint. The result can be inflammation, pain, and eventual weakness, especially if the tendon tears.

I always tell my patients that one of the sad things about the rotator cuff is that as you "mature," your rotator cuff also matures. When that happens, it often becomes irritated and eventually tears. By age 50, about 50% of the population is walking around with rotator cuff tearing of some degree. Think of cutting through a rope with a knife and seeing strands of rope hanging down. That's what a partial tear of the rotator cuff looks like: The tendons are frayed and are just hanging there. With a full-thickness tear of the rotator cuff tendon, the tendon is cut through and is no longer attached to the bone.

As you age, you progress from rotator cuff irritation to partial-tear to full-thickness tears. Strangely enough, by age 80 upwards of 40% of the population has full-thickness tears of the rotator cuff and doesn't even know it.