Originally Posted by: AkiraHmm, this will end up putting a lot of strain on your thumb, which isn't too good.

I'd take a good hard look at your technique, chances are you're moving your hand in ways which cause it stress; economy of motion is paramount. Don't press down on the strings harder than needed, this seems weird at first but it will benefit you no end. Best thing to do is try pressing down on the string very lightly, whilst picking it, this will usually produce a muted note, press down a bit more until you can hear the note, doing this will show you how much pressure to apply.

And of course playing fast for long amounts of time will hurt you, it's not normal for tendons to move that much.

Also, look at the positioning of your arm, try moving it about in different positions to see if some positions are more comfortable than others.

Remember, if it starts to hurt when you play, then stop! It may seem frustrating but it's not worth carrying on and damaging your tendons. Take frequent breaks.

If I may add something... Ok, maybe it will sound stupid, but... You shoul, i dont know how you guys say it? Lower your action? Or something. So that your strings are really close to the fretboard... then you don"t need to apply so much pressure. I know, it is obvious... But just to say it. Maybe it will help you out. Sorry if it sounds childish.