I am no expert on this but do feel able to refute the suggestion that power and strength are what is needed here. I prefer to use the term 'grace' in that you need to get the muscles in your hands, which are already plenty strong enough to do the job, to work together efficiently to allow you to execute your ideas without tensing up and gripping too hard.
One of the issues I have identified is the [i]psychological[/i] impact of playing on your hands. I can play for hours at home on my own without any difficulty but the minute I get to play with a band at higher volumes (without any reall attempt to soundchecks etc), I tense up and play harder, thereby wearing myself out quicker. This is not going to be improved by powerballs and weights but by learning to get the best out of my bass set-up, pick-up configuaration and amp/cab set-up, to learn to control my unconscious impulse to dig in in an effort to compete with the other musicians and to consider other issues such as speaker placement etc.
Personally, I feel that the strength that you need to climb rocks is not the same as the muscular dezxterity and finese you need to play our chosen instrument. Good warm ups and a sensible approach to the differing demands of practice, rehearsal and live work work for me.
I also advocate for proper rest, fluids, a working knowledge of Alexander techniques and an ergonomic keyboard (to allow you to rest your wrists)!!