Granted I pratice martial arts, and some of it comes with the territory.  But like, no one else seems to get hurt that much.  I try to take precautions to not get hurt, like cross training, weight lifting, and warming up right, but I still get hurt.  

Any advice?

Your biomechanics might be wrong too, like how you do things.

If you're in school you should have an athletic trainer, or some kind of health professional that can check you out. It could be so many things. Like for your back/knees it could be your shoes, you might need orthotics or something.

Or it could be one big problem, that is causing most of the others. Who knows.

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"A wise girl kisses but doesn't love,
listens but doesn't believe,
and leaves before she is left"
-Marilyn Monroe

I will see about the trainer at my school.

I have really good flexibility, as I can do the splits all three ways almost.  like a few inches off.

the masters at my school said they got hurt many times, and many of the same injuries, so maybe its just unavoidable?