The only way I could see this working at all would be with the mouse wheel, and moving it from 1% - 100% (as you previously said) but even then, with reaction time during MP it would be too hard to judge every single joint's power.
Also, every player wanting to use this feature would need the mouse wheel. I cant see how else you would do it, maybe a code like /jointpower rightpec 50 or something, but then that would only work in spars and SP.
Also, there have been many posts like this, where people would suggest different mechanics ideas like joint angles or something, and well, all of them were shot down.
I mean, I see where you're coming from, but I think it changes the game too much, its too hard to judge the "power" of a joint from 1-100, and dev's wont implement it.
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edit, maybe if you were to go in increments of 25 would work.
ex. moving the mouse wheel one click would be 25%, another 50%
This would also make the people who use the mouse wheel as their joint mover slightly angry ;)