Using the differential method to identify the skeletal muscle mass index cutoff point for professional baseball batters: an observational and pilot study

The purpose of this study was to determine the skeletal muscle mass index (SMMI) cutoff using the differential method based on a second-order polynomial regression model and to validate the reliability of the identified cutoff using bootstrapping method in professional baseball batters. Thirty-four batters from the single team in the Chinese Professional Baseball League (CPBL) were assessed. The TrackMan system was used to analyze each player ‘s maximum exit velocity across the major- and minor- leagues during the 2024 season. Body composition measurements obtained at the beginning of the month in which each player’s highest exit velocity occurred were included in the analysis, with one observation per player. A second-order polynomial regression model was fitted to examine the relationship between body composition and baseball exit velocity. The cutoff point was then determined using the differential method, and its predictive accuracy was evaluated by bootstrapping method. As free fat mass index (FFMI) (R 2 = 0.41, p < 0.001) and SMMI (R 2 = 0.39, p = 0.001) increase, the maximum exit velocity increased simultaneously, exhibiting a relationship described by a polynomial regression curve. The impact of the muscle mass index and maximum exit velocity cutoff point are calculated as FFMI = 23.8 kg/m 2 and SMMI = 13.4 kg/m 2 , respectively. This study demonstrated that the differential method can be used to identify cutoff points for FFMI and SMMI in professional baseball batters. These cutoff values may serve as useful reference points for evaluating body composition in relation to maximum exit velocity.