Tune into tonight's baseball All-Star game and you'll see a familiar ritual: Batters standing in the on-deck circle will swing a weighted bat (or even a heavy, pipelike club) while they wait to hit. The exercise is intended to improve players' bat speed, with the idea being that the regular bat feels lighter after taking cuts with the heavier one. But a new study suggests batters who add ounces to their practice swings may be making an error.

Practicing with a heavier bat significantly slows down the velocity of the bat head—depriving the batter of slugging power, exercise researchers at California State University, Fullerton, say. Swinging light or normal weight lumber just before stepping up to the plate helps players become accustomed to swinging fast, repetition that is key to athletic training, the researchers say.

For the study the researchers recruited 19 recreation league baseball players (all men) and had them take five practice swings with bats of different weights: a light bat, weighing just 9.6 ounces; a standard-weight bat (31.5 ounces); and a heavy warm-up bat (55.2 ounces). After resting for 30 seconds, players then took five "real" swings with the standard-weight bat. The researchers recorded bat speeds using a computer that calculated the time it took the bat head to pass between two sensors spaced 43.2 centimeters apart, the length of home plate.

During warm ups, players swung fastest with the light and standard bats, averaging 101.4 and 82.1 kilometers per hour, respectively. Both were far quicker than hacks with the heavy bat, which averaged just under 67.6 kilometers per hour. More important, players practicing with the light or standard bats were able to maintain higher velocities on their real swings—averaging 83.7 and 80.5 kilometers per hour, respectively. The heavy bat practice group averaged 77.2 kilometers per hour with their real swings—not as quick as the other two groups.

Steven Zinder, a researcher who helped conduct the study, said the findings do not prove that swinging faster makes a hitter better, although from a biomechanical perspective that makes sense. "If you want to swing faster, you need to practice by swinging faster," says Zinder, now an assistant professor of sports medicine in the exercise and sport science department at the University of North Carolina at Chapel Hill. "Training to swing fast made you swing the normal bat faster. With the heavy bat, you're training yourself to swing more slowly."

Although no follow-up studies are planned, Zinder says it would be intriguing to convince a team to swing light bats in the on-deck circle and see if it improved their batting averages and home run production. The findings appear online in the Journal of Strength and Conditioning Research.