This is why I swim; it gets all the right muscles worked in just the right way, from the gut to fingertip. Water resistance is perfect for the pianist, IMO.
I think the primary muscles to strengthen are the lats, traps, abs and deltoids. The muscles of the arm are secondary; the tiny muscles of the hand must be exercised very gently, if at all; that's why I like to use chunks of memory foam in any "grasping" activities; it's extremely gentle yet effective. Heavy exercising of the hand is BAD, even injurious, IMO.
We must consider the piano and all it's idiosyncrasies, before engaging in any 'muscle building'.
I think Peter (pepasch) is right. The real strength comes from the larger muscles of the arm (and I would argue, the shoulders and the trunk are where any 'power' should be generated.)
The fingers are merely there to carry and coordinate the power from the larger muscles. It's all in the physics of it...too long to explain here.
(Not there's anything wrong with strong hands!)
Greetings from Pete
BTW, I didn't do any special hand exercising for this one; I did swim for about an hour a day for three months: http://server3.pianosociety.com/protect ... ournet.mp3