I had a problem with tendinitis a few years ago in my left hand. I have always been a fanatical guitar player. Sometimes playing 3-4 hours a day. And that started when I was in grade school. I found that I wasn't stretching enough before playing and that in itself was what was causing the problem.
Ironically, for years now I have watched guys walk into the gym and just start lifting without even a few shorts minutes of stretching. If you think about it, you can probably count on one hand the times that you've seen someone walk into a gym and stretch before a workout. That said, many of the problems when it comes to tendinitis can be traced back to improper or an almost complete lack of stretching before a workout. JMHO..