RE: knuckle cracking and arthritis
Fair enough question.
From my un-scientific point of view, I think cracking your knuckles help/instigates arthritis. I cracked them all the time in my youth, and clearly have arthritis now, in my fingers.
They hurt when cold, and move Oh-so-slow the rest of the time. Perhaps being a semi-pro waterskier helped aggrivate my current condition, makes sense to me.