How to Get Slower and Screw Up Motor Patterns
Dumbbell shadow boxing is one of the stupidest exercises on the planet. It literally does nothing good. I really think it's only done by people who want to look tough in the gym or athletes that just don't know any better, because there's no logical reason to do it.
First, you're doing a horizontal movement (punching forward) against a vertical resistance – the dumbbell only provides vertical resistance because of gravity. So the resistance doesn't even overload the action you want to strengthen.
Second, it just teaches you to be slow. If a movement is overloaded and speed decreases by 10% or more, it will have a negative impact on the performance of the unloaded movement. I'm talking about explosive movements here. So, punching with dumbbells makes you a slower puncher.
I'm not saying that slower speed exercises to increase strength aren't beneficial. I'm saying that doing the SAME movement as the explosive exercise, but slower, will mess up your motor pattern and make you slower.
Weights That Make Athletes Slower
A friend of mine was a high level sprinter. He hired this guy who fancied himself a track & field guru (although he never trained any elite sprinter). He had my friend do most of his sprinting with weighted insoles in his shoes. Not heavy either, just a few ounces. In a few months his 60 meter time slowed by 3/10ths of a second! And when filmed we could see a severe disruption of his running mechanics.
Gaining strength with basic barbell lifts can improve your power and speed potential. But overloading the same movement you're trying to make faster will actually make it slower.
When I was getting my Masters degree I did an experiment. I had people do a vertical jump, then do a set of 5 vertical jumps with a weight vest that was 20% of their bodyweight, then test their vertical again. (There was enough rest to avoid the fatigue effect.)
I asked them, "Did you jump higher before or after the set with the vest?" They all said they jumped higher after. Well, they didn't. They all jumped lower after the set with the vest. They THOUGHT they jumped higher because of the difference in perceived speed. They felt so much faster when compared to the vest set that they thought they got faster. Again, they did not.
Baseball Players Too
The same thing happens when baseball players use weighted bats as warm-ups.
Coach Nick Tumminello notes that studies found the heavy bat not only alters the batter's perceptions of bat heaviness and swing speed, but also slows down his actual swing speed for up to five swings after using the weighted bat!
Yes, some professional boxers still shadow box with dumbbells, but that just shows you what amazing athletes they are. They can do dumb things and still perform at the elite level. You can't. Be smarter than the guy who gets punched in the head a lot.