Effect of grip-enhancing agents on sliding friction between a fingertip and a baseball

The friction between a pitcher's fingers and a baseball is crucial for ball control and spin. Major League Baseball (MLB) recently strengthened its ban on unauthorized friction-enhancing agents, except for rosin, significantly impacting pitching performance, with spin rates decreasing by about 4%. While the effect of rosin on friction has been studied previously, the impact of sticky substances remains unclear. This study addresses this gap by investigating how rosin and sticky substances alter the friction coefficient between a fingertip and a baseball, comparing MLB and NPB baseballs, and exploring the effect of mudding and sanding on friction. This research is important for understanding how grip-enhancing agents affect pitching and could inform discussions regarding regulations.

Prior research by Yamaguchi et al. (2020) showed that rosin application to baseball leather increased friction under wet conditions and reduced friction variability among individuals. However, no studies had examined the effects of sticky substances on finger-baseball friction. Anecdotal evidence from MLB pitchers, such as Yu Darvish, suggests that the slipperiness of MLB baseballs contributes to the use of unauthorized sticky substances. Japanese pitchers frequently comment on the greater slipperiness of MLB balls compared to NPB balls, although a quantitative comparison of friction coefficients was lacking before this study.

Nine right-handed adult males participated in sliding friction tests. Friction was measured using a six-axis force sensor between an index fingertip and leather sheets (with and without seams) extracted from MLB and NPB baseballs. Three conditions were tested: no application, rosin powder application, and sticky substance application. The maximum friction coefficient (μmax) was calculated from normal and horizontal forces. Statistical analysis using two-way repeated measures ANOVA and post hoc t-tests was performed to compare mean μmax, within-participant coefficient of variation (CV) of μmax, and between-participant CV of μmax across different conditions. Additionally, tests were conducted to assess the effect of mudding/sanding MLB and NPB balls on friction. Fingertip moisture levels were also measured.

Rosin application to MLB baseballs increased the mean μmax by 27% (with seam) and 23.9% (without seam). Sticky substances further increased μmax by 54.9% (with seam) and 61% (without seam). Rosin significantly reduced within- and between-participant variation in μmax, while sticky substances increased within-participant variation. NPB balls exhibited significantly higher friction coefficients than MLB balls (16.6% higher with seam, 23.1% higher without seam). This difference remained even after mudding/sanding. A strong positive correlation was observed between finger moisture and friction coefficient under no-application conditions. The presence of seams increased friction under no-application and rosin conditions, and also showed a trend toward increasing friction under sticky conditions. Mudding/sanding did not consistently increase friction.

This study provides the first quantitative comparison of friction enhancement effects on MLB baseballs. Sticky substances increase friction, potentially boosting spin rates as described by the equation ω0 = R/I ∫t0t Ft dr, where higher tangential force (Ft) leads to higher spin rate. However, the nonlinear relationship between normal force and friction coefficient with sticky substances may lead to less consistent ball control. Rosin, on the other hand, offers increased friction with consistent and stable friction across pitchers and normal forces, promoting a fairer playing environment. The lower friction coefficient of MLB balls compared to NPB balls suggests that material modifications to MLB balls could be explored to potentially enhance friction, reducing reliance on unauthorized sticky substances. The fact that mudding/sanding did not consistently increase friction points to the complex interplay of factors influencing grip.

This research demonstrates the distinct effects of rosin and sticky substances on finger-baseball friction. Rosin provides a consistent friction increase, beneficial for fair play, while sticky substances provide a larger, but less consistent, friction increase, potentially affecting ball spin rate and control. The significant difference in friction between MLB and NPB balls highlights potential for material modifications to enhance grip in MLB balls. Future research should investigate friction in actual pitching scenarios using multiple fingers and different sliding directions, while also considering various skin conditions and mudding/sanding methods.

The study used flat leather sheets rather than the curved ball surface, potentially underestimating friction. Only the index finger was used, and the sliding direction was limited. The participants lacked calluses, potentially affecting the results. The mudding/sanding methods might not perfectly replicate game conditions. Further, the specific tanning processes and chemicals used in MLB and NPB ball leather were not fully known, which may affect the friction coefficients.