Originally Posted by Virtua Tennis
Are you using more chest muscles to serve or are you using more shoulder to serve. If you want to protect your shoulder you need to learn to use more chest muscles like Sampras or Roddick who lean back and use more chest and less like Sharapova who drops the shoulder down.
Try watching this http://www.youtube.com/watch?v=GgdXawklcZk
Respectfully, your comment about using more chest muscles is not what serving, or the film you posted is about. It's about tilting or if you will positioning the chest in a way to keep the relation of the arm movement to the shoulder the least impinged.
It's actually the muscles in the back that often prevent shoulder injury because with more of the back involved, the strain is taken off the rotator cuff. Without the back, and of course your core and legs, you're arming the ball and that's too much to ask of many player's shoulders.