After pretty much playing all day, my hand is killing me!

its mainly because when im playing my fingers stiffen up, so after a while it starts hurting my hand. I release i need to relax my hand more but soon as im moving the mouse alot my hand tightens and the pain comes back!

dont think its the mouse, because its kinda always been like this (i take brakes inbetween games usually, so helps the problem)

ive always wondered how the pro's play for so long without getting hand cramp or whatever!