Kevlar messed my shoulder up when I was 15.
I had absolutely no problems with my shoulder and had a huge serve for a 15 yr old.
Within two weeks of Kevlar, my rotator cuff was messed up forever. Now I can still bring the heat, but I have too be super careful off the court, and stick to a proper rehab and maintenance program.
My shoulder will never be the same. Thank you Kevlar (and idiot coach who recommended I use it).