I disagree that weighted balls are bad for you. As everything - it depends upon how you use them. My son and I use 4oz and 6oz balls. 1oz accounts for a 20% increase in weight - more than enough to help build arm strength but not so much is alters throwing motion and/or damage shoulder. Overloading and underloading programs are widely accepted as proven to help in building strength/speed. We throw 25 with heavy - then 25 with light and then go to normal ball(5oz). We do the same with a bat - he has weights he attaches to bat we use in practice - then we use a wiffle ball bat and hit wiffle balls. Fast twitch muscle fibers need to be worked as much as heavier slower muscles. A good swing is a good swing - obviously - you don't want to swing so much weight that it does not match your normal swing.