We got some custom wood bats made up by the Dream Bat Co for the overload (32-32, 33-33, 34-34) and purchased some cheap old slowpitch softball bats from a local retailer for the underload. There was a study done in NSCA Journal that recommended staying within 13% of the game bat's weight so we stay as close to that as possible. (We also have our kids swing wood all the time, so the weight bump isn't too dramatic on the overload.)
The protocol we used was as follows:
10 overload swings
10 underload swings
10 game weight swings
* keep things moving, but make sure there are appropriate rest periods - working in groups of 2 or 3 worked best
The bat speed gains are good. We didn't do a control group or anything so I can't report exact scientific results. It certainly won't replace good mechanics and a well designed and implemented strength program. I actually like the over/under better for the timing aspect. Switching between the bats forces the kids to be more aware of the barrel. I like mixing in a little league bat too to keep 'em honest.
