Location of Injury: Shoulder
Likely cause: Repetitive overhead activities, like serving tennis balls or swimming the crawl, can inflame or even cause mini tears in the tendons of the rotator cuff, the part of the shoulder that connects the upper-arm bone with the shoulder blade and allows circular motion. Not warming up may be a factor, too.
What to avoid: Overhead motions that irritate already painful muscles. Also avoid exercises that strain the shoulders, such as military presses, water-skiing, sit-ups with the hands behind the head, and yoga positions like downward dog and plow.
What you can do: Run, use an elliptical machine, bike (but stick to recumbent or stationary bicycles), or swim the breaststroke.
What would make it better: Shoulder-stabilizing exercises―such as internal and external rotation moves using stretchy bands (see familydoctor.org)―strengthen the muscles that keep the ball of the arm bone in the shoulder socket.