You are certainly right that sore shoulders are common, especially as a person ages. About half of all middle-aged tennis players suffer from shoulder pain, according to a 2012 study in The British Journal of Sports Medicine, and youngsters aren’t immune either. The same study reported that about a quarter of competitive tennis players under 20 hurt their shoulders every year.
Many of these injuries involve the rotator cuff, the group of muscles and tendons at the back of the shoulder that stabilize the joint. Studies show that forces equivalent to at least 120 percent of a person’s body weight slam through the rotator cuff during a typical tennis serve or baseball pitch. To withstand that pounding, the rotator cuff needs to be strong.