I think this idea works well with swinging heavier baseball bats, but it doesn't translate as well to guitar, for one reason.

You need a lot of fine-motor control and nuance to play guitar. You're not just looking for power and strength. If you just build muscle and make your hands "strong" you're getting away from the guitar strings being a delicate instrument.

As a parallel example, one of the things that makes a drummer like Vinnie Colaiuta play with such finesse is that he is playing a wide-range of quiet and loud notes simultaneously. Listen to all of his ghost notes and accents. He wouldn't learn that from swinging a sledge-hammer at his snare drum over and over.

My point is, it's not all about being able to press the strings harder. Your fingers are all naturally strong enough to push a guitar string down. What they might not be ready for is pushing a string down at the required angle, or holding one weird chord position for a long time. Think of how many variations there are. It's not just one single muscle motion like doing pushups. Playing guitar is subtle twisting and turning from multiple angles and varying degrees of pressure on a grid of strings. Each note on each string from each position has its own combination of strength and dexterity. You can't play the low E string with the same feel that you apply to the high E. And on the higher frets, the angles are all different. The fret distances all change sizes.

Very few instruments have all of the notes crammed so close together like the guitar does. From the 12th to 14th fret, you've got 18 notes in a 2" x 1.5" area. That's crazy. To be able to equally play all of those (especially in weird combinations in jazz chords) takes a lot of specialized practice.

A lot of it has to do with training your mind / nerves to be able to plant your fingers in any position instantly. The raw muscle strength comes in handy, too, but I think the best way to develop all of this combined nuance is to keep playing your guitar in as many unusual ways as you can, play all over the neck with different tempos and levels of dynamics, get your mind and hands conditioned, and don't hurt yourself.

If these putties and balls help, then that's great. But none of these will truly condition your hands for playing guitar like playing guitar will.

-Carl.