The most common swimmer\'s injury -- \"swimmer\'s shoulder\" -- can affect as many as 70 percent of competitive swimmers, a U.S. sports physician says. Dr. Scott A. Rodeo, co-chief of the Sports Medicine and Shoulder Service at Hospital for Special Surgery and chairman of the USA Swimming Sports Medicine Committee, says the shoulder is an inherently unstable joint. \"Shoulder stability is controlled by a synchronous pattern of muscle firing,\" Rodeo says in a statement. \"Changes in the way the muscles work due to overload or fatigue can alter shoulder mechanics and cause problems.\" Rodeo says the main causes of shoulder pain in swimmers include: -- Muscle fatigue from overdoing it. -- Degenerative changes in the rotator cuff tendon, or tendonosis. -- Impingement of the rotator cuff during the swimming stroke. The rotator cuff is a group of four tendons that hook up to muscles that stabilize the shoulder joint. -- Shoulder laxity, various muscles and ligaments play a role in shoulder stability. Looseness in the shoulder may lead to injury. \"Most often, shoulder pain is caused by an overuse injury,\" Rodeo says. \"If you think about a competitive swimmer\'s number of stroke revolutions per day, per week, per month, per year, it\'s phenomenal. We\'re talking about one-half million stroke revolutions per year.\"