In the meantime, you can visit Dr. Mark's website www.pianomap.com, which goes into an extensive discourse on this subject.
Thanks A MILLION for providing this link.
I dont' have the problems discussed in the topis of this thread, but I read it carefully anyway as I DO have severe problems with my hands right now and feared that I would have to rest for several weeks ... bad timing as I soon will have my first piano lesson in many decades, and now when I have found the motivation to practice - and to practice a lot - it is torture not being able to do that.
But I have had big problems with cramps in my hands when playing something fast and difficult. And after that pain that can last for hours and just was getting worse, especially in the joints of my thumb and 4th and 5th finger.
I followed the link and read that playing with "curled fingers" is a very bad thing to do. Well, when I started playing the piano, 36 years ago, I learnt that position on my very first lesson! Which meant that you had to keep your finger nails very short in order not to make "clac" sounds against the keys ... The reason this is bad is, according to Dr. Mark, that you have to make two contradicitve movements at the same time: bending the finger AND lifting it. (Try doing it while holding your hand right out in the air - you can feel that this is not very comfortable.) So he suggested a position with "naturally" curled fingers instead.
I thought

"great, so now I have to unlearn a habit that is 36 years old?" but it turned out to be easier than I feared - after all, this "naturally" curled position is what I use all the time when I type on the computer keyboard, just because nobody ever told me to do something else and it is, yes, the natural position.
So I tried to play the piano the same way and voilá! No cramps! No pain! I feel SO relieved right now.
Yes, you may call me stupid, as I did not figure this out by myself a very long time ago.
Still think I must be careful right now, though. I practice only short sessions at a time ...