I'm sorry, but I can just visualize some unintended consequences for that guitar waving around while you work on your explosive acceleration. One slip...
. If I wanted to do those exercises, I would use a hammer. Dirt cheap, available in a wide variety of weights, and dual purpose.
Consider devising some streching exercises targeting the same muscles. The weight work tears down the muscle a little, and it will tend to shorten and tighten when it heals.
But really, aren't you strong enough now? I could be wrong, and it's possible you'll prove me wrong, but I would expect better results from devoting my time to speed and dexterity proficiency exercises on a consistent daily basis.