Attend any baseball game at any level and you’ll find that a good chunk of athletes will be taking their practice cuts in the on-deck circle with a heavy bat; usually this involves swinging a “normal” bat with a weighted donut or sleeve around the barrel or an implement such as a metal rod or sand-filled tube designed to mimic the ergonomics of a typical baseball bat.
It is an action that is ubiquitous in the game and, on the surface, it makes logical sense. Swinging a heavy bat primes the muscles and makes a lighter bat feel, well, lighter in the hand, allowing the athlete to generate a higher swing velocity; this is often referred to as “the overload approach.” It’s been well established that swing velocity correlates strongly with exit velocity, meaning the quicker one swings, the faster the ball leaves the bat. The faster the ball leaves the bat, the farther it flies; it’s simple physics.
However, upon delving into the research, it appears as though warming up with a heavy bat does not actually produce this result.
A term that popped up regularly was “kinesthetic illusion” - sometimes referred to as “kinesthetic aftereffect”. This term describes the athlete’s perception that they are swinging faster after warming up with a heavy bat, when in reality, they are not. Five studies published in 2009 or later (linked below) involving a total of 88 participants who participated in high-school or college baseball found that warming up with a heavy bat did not increase swing velocity in a statistically meaningful way when compared to warming up with a standard bat - no matter the modality used to make the bat “heavy.” (The standard bat was often 33-in., 30-oz. - though it wasn’t until the later studies that this became standardized.)
All five studies also assessed whether or not warming up with a “light” bat impacted the resulting swing velocity with a standard bat. Four studies found that warming up with a light bat did not impact resulting swing velocity, though one study found that it improved swing velocity by (this same study also found that warming up with a heavy bat slowed resulting swing velocity). The overall range of light to heavy bats across all five studies was 6.9-96-oz.
An additional study, looking at 30 collegiate baseball players, found that utilizing weighted batting gloves did not alter swing mechanics compared to swinging a standardized control bat; however, swinging a bat with a weight applied to the barrel did impact swing mechanics.
So not only does warming up with a donut or weighted sleeve not impact - or possibly have a negative impact on - resulting swing velocity, but it may also change the mechanics of the athlete’s swing compared to that of a standard bat. I think that Charles Williams and his research team summed it up best when they concluded: “If presented with the current options, athletes should choose the warm up implement with which they are most comfortable using before an at-bat situation.” After all, kinesthetic illusion - a variant of the placebo effect - can be a powerful tool.
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