Just as a post script to what was said above, getting finger strength from alternate means for the sake of thinking it will improve your playing is not, I think, gonna net the result you want. Yes, your fingers will get stronger but part of finger strength is not how 'muscular' they are. You won't be lifting weight with them...but that you're trying to build muscle memory over actual strength. Sure, stronger fingers are useful in a way but it's dexterity and fluidity of motion that will win the day. You want the right muscles to be improved and I think that is best served by playing. A stronger finger won't make you more fluid. That your fingers have a comfort on the fretboard and the strength build from those motions will be most useful for you.