No butterfingers in baseball: Understanding slip between fingertips and the ball

Before June 3, 2021, baseballs spun more. That’s the day when Major League Baseball (MLB) said it would begin cleaning up a sticky situation: unapproved substances, like pine resin, smeared on baseballs to help pitchers keep a precise grip.

Immediately following the notice, MLB pitchers’ average revolutions per minute dropped until it leveled out at about 86, or 4%, fewer revolutions per minute. Such a reduction appeared to translate to decreased pitching performance, with more batters hitting the ball, but why?

That’s what a team of researchers based in Japan set out to understand.

Using high-speed cameras, they captured six experienced pitchers throwing fastballs at about 130 kilometers per hour and analyzed how different baseball treatments impacted the finger-ball slip distance. This is the distance the fingers slip on the four-seam baseball—the type used in MLB games—as the ball is wound and released.

The research is published in the journal Scientific Reports.

The researchers found that the stickier the surface of the ball, the less the fingers slipped, and reduced slip distance resulted in faster pitches with more revolutions per minute and more directional control.

“This is the first study to focus on the slip between the fingertips and the ball in baseball pitching—an aspect previously discussed only subjectively—and to quantify the slip distance while clarifying its variations under different friction conditions,” said Takeshi Yamaguchi, professor in Tohoku University’s Graduate School of Engineering, who led the research.

“The findings of this research are expected to enhance our understanding of the ball release mechanism under varying friction conditions, contributing to improved pitching performance, injury prevention for pitchers and the development of better equipment.”

When pitching, the ball accelerates forward, leaving the thumb and rolling over the fingertips before it releases from the hand entirely. “It’s the time from when the ball leaves the thumb to leaving the fingertips that is of interest in terms of slipping,” Yamaguchi said.

The researchers had six pitchers test baseballs with one of four treatments applied to their palms and fingertips—no additional coating; water coating; rosin powder coating; and pine resin coating—to see if the time and distance of slipping changed.

“We developed a method to estimate the slip distance between the fingertip and the ball during the release process using high-speed camera analysis, revealing—for the first time—the difference in slip distance under various friction conditions,” Yamaguchi said.

For the slipperiest condition of a water coating, the ball slipped through the whole release process, with a slip distance of 22 millimeters. When coated with rosin powder or pine resin, the slip distance was reduced by more than half to approximately 8 millimeters on average.

The researchers also noticed that when pitching the water-treated balls, the velocity of the pitches dropped significantly compared to the other conditions, especially the rosin-applied condition.

“This unexpected result is thought to be due to the pitcher’s perception of fingertip slippage and subsequent adjustments in their pitching action,” Yamaguchi said.

“In the future, we plan to investigate the changes in pitching movement resulting from different conditions through analysis of whole-body movements and muscle activity. Additionally, we aim to identify pitching techniques that maintain performance with slippery balls while reducing the risk of injury.”