In a study of 80 elite swimmers, between the ages of 13 and 25 years, the authors found that 91% of the swimmers reported shoulder pain. A deeper look revealed 84% showed positive shoulder impingement signs, and 70% had MRI evidence of tendinopathy of the supraspinatus (one of the rotator cuff muscles). A number of factors might be related to shoulder pain in swimmers, including overuse, changes in internal and external rotation of the shoulder, and history of previous shoulder injury.
Source: American Journal of Sports Medicine, January 2016