Throughout most of my playing career, until about midway through college, I warmed up on deck just like my idols in the Major Leagues, by swinging a weighted bat. This was either with a donut or a Brats bat, or some combination that would help me swing the bat with more velocity when I stepped in the box. This ritual can still be seen on just about every field in America, from little league to the Major Leagues.
The logic behind using a weighted bat on deck is simple. Hitters believe that after using it, their normal bat feels lighter and can be swung faster. After all, creating more bat speed is one of the most important factors in productive hitting. However, there have been numerous studies that show that this ritual may actually hurt bat speed.
In a 2011 article in the Wall Street Journal, Craig Wolff explains how swinging a heavier bat on deck hinders bat speed. He mentions a study done by Coop DeRenne that found swinging a bat 10-13% heavier than your normal bat decreases swing velocity by 3-5 mph. The study also states that the heavier the bat, the slower your bat speed at the plate. In another study, Szymanski et. al found that of all the weighted bat implements, the donut was found to decrease bat speed the most.
Swinging a bat is a violent action that requires explosiveness and a high amount of fast twitch muscle fiber recruitment. The study done by DeRenne states that the reason for the decrease in bat speed after swinging a weighted bat is a higher activation of slow twitch muscle fibers and muscular fatigue. Slow twitch muscles are the muscles that endurance athletes rely on for long distance, slow activities. These are not not what we need to create bat speed.
My general advice to hitters that I work with is to warmup with the bat that you are bringing to the plate. Besides the facts stated above, I believe that one of the most important factors in hitting is timing. When a hitter warms up with an implement that is not the bat they are using in the game, their timing can be throw off. However, many hitters still believe that the perceived feeling of a lighter bat gives them a pyschological advantage. Although they may feel like they have an edge, the reality is that their bat speed will be slower and they will be less productive at the plate.