actually you are tearing/causeing trama to the muscle fibers every time you move, whether its your fingers or doig a 600lbs squat. I'm not an expert on this by no means just what I've learn through 5 years of exercise science classes. SO I'd imagine their is either less trama being caused because the muscle is use to the motion/stress, the body is more efficient in the repairs, or both. There are probably other factors as well.