hmmm... my experiences with back pain and rebounding happened long before i ever fasted (i got rid of the trampoline about a year ago)... i didn't weigh too much more than i do now, and though i wasn't as healthy back then as i am now, my posture was still pretty good.
now you got me wondering if i was storing toxins in my back and THAT'S what was causing the pain? it would be interesting to see what would happen if i gave it a try now, or better yet, after we finish our next fast, mouseclick :)