Inter-pitcher and Intra-pitcher Comparisons on the Relationship between Trunk Kinematics and Ball Speed in Collegiate Baseball Pitchers

Introduction The three-dimensional trunk movements are considered to play important roles in producing higher ball speed in baseball pitchers. However, most of the previous studies have not reported the details on how the variations in trunk kinematics within an individual pitcher associate with ball speed. Methods The purpose of this study was to examine the inter- and intra-pitcher relationship between ball speed and the three-dimensional kinematics of the trunk. Sixteen male collegiate baseball pitchers (age; 20.0 ± 1.1 years, height; 1.77 ± 0.06 m, body mass; 78.0 ± 8.0 kg) participated in this observational study. Kinematic data of their 10 maximum effort pitches were obtained for the correlation analyses with ball speeds. The inter-pitcher analysis included the mean values of each kinematic variable from 10 pitches per pitcher, while intra-pitcher analysis included all 10 pitches within each pitcher. Results The inter-pitcher analysis showed that there was a positive correlation between peak trunk throwing-arm side twist angle and ball speed ( ρ = 0.64, p < 0.01, 90% confidence interval [0.24, 0.86]). This relationship was also observed for the intra-pitcher analysis in three pitchers, indicating that the greater trunk throwing-arm side twist may be an important factor for both the group of pitchers and within an individual pitcher to throw with higher ball speed. However, each intra-pitcher showed different patterns of correlational relationship between trunk kinematics and ball speed. Conclusion Inter- and intra-pitcher relationships between ball speed and the three-dimensional trunk kinematics were different, suggesting that the individuality should be considered along with the general tendency when the baseball pitchers modify their three-dimensional trunk mechanics.