yeah, ive had some of those pains before myself. im gonna look for the thread i started a while back when i was freaking out about it and maybe it will help. when at keyfest, a question of mine that JR answered was about these pains some get from playing, and he also said just to relax while your playing. and if you feel your muscles cramp or tighten up, just take a short break and actually shake your hands out a bit. i havent had such pains since before i started doing that, and its been working great for me. some finger/hand strength techniques as well as some attention to posture and position of arms and hands might help a little as well. JR also reccomended if and when you do "table-top" practicing, for finger exercizes, your whole arm should be flat on the table and not at an incline or anything. this seems to work and it might get your arm trained to be level as it's supposed to. i was ignorant to proper techniques before and still am no expert on this stuff, but i think there is a piano teacher or two around here that could help you out with that.
ahh, heres that thread!
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