(5) RELATIONSHIPS BETWEEN KINEMATICS OF THE PELVIS AND TORSO AND EXIT VELOCITY IN BASEBALL HITTERS

Purpose: The purpose of this study was to assess the relationships between baseball hitting kinematics and exit velocity (EV) of the hit, as well as determine if any hitting kinematics contribute to predictions of EV.

Methods: All data were collected and published freely online by The Open Biomechanics Project performed by Driveline Baseball. All batters performed four to nine swings using a K-motion hitting vest and multiple high-speed cameras coupled with 55 markers to record hitting kinematics. The swing with the highest EV was used for this analysis, providing a sample size of n=97. The following metrics were used for the present study: EV (mph), bat speed (mph) at contact in the X-axis, sweet-spot velocity (mph) at contact in the X-axis, attack angle (°) at contact in the X-axis, pelvis angle (°) at the first movement in the X, Y, and Z-axes, pelvis angle (°) at the foot plant in the X, Y, and Z-axes, pelvis angle (°) at the heel strike in the X, Y, and Z -axes, torso angle (°) at the first movement of the X, Y, and Z-axes, torso angle (°) at the foot plant in the X, Y, and Z-axes, and torso angle (°) at the heel strike in the X, Y, and Z-axes. Pelvis and torso angles were measured using a K-Motion Vest. Pearson product-moment correlation coefficients examined the relationships between EV and all kinematic measures. Stepwise regression analyses examined which kinematic measures contributed to predictions of EV.

Results: EV exhibited a significant moderate positive correlation with bat speed (r = 0.695, p < 0.001), a significant high positive correlation with sweet spot velocity (r = 0.853, p < 0.001), and a significant negligible negative correlation with Y-axis pelvis angle at first movement (r = -0.248, p = 0.014). No other significant relationships existed between EV and all other kinematic measures (|r| ≤ 0.190, p ≥ 0.062). The results of the stepwise regression demonstrated that sweet spot velocity at contact, bat speed at contact, and Z-axis pelvis angle at heel strike contributed to predictions of exit velocity (r² = 0.760, p < 0.001).

Conclusions: The primary results indicate that faster bat speed at contact, faster sweet-spot velocity, and lower pelvis angles in the Y-axis at first movement were significantly related to EV. Also, sweet-spot velocity at contact, bat speed at contact, and Z-axis pelvis angle at heel strike together account for 76% of the variability in EV. Although hitting kinematics are pertinent to improving hitting performance, the present correlations suggest that pelvis and torso angles during different phases of the hit may have little overall influence on EV, a commonly assessed marker of hitting performance and power. The general lack of significant relationships between kinematic metrics and predicted EV might suggest that measures such as strength, power, and potentially kinetic measurements during the swing may be worth examining to better understand overall hitting performance as assessed by EV. PRACTICAL APPLICATIONS: The present study demonstrates that faster swings, as assessed by bat speed and sweet-spot velocity, may have a greater impact on EV than kinematics about the pelvis and torso. Therefore, if a higher EV is the goal for hitting, then strength and conditioning professionals should emphasize improvements in power and velocity, which should then yield improvements in EV. While kinematics are important, other hitting performance factors as assessed by EV should be considered.

Acknowledgements: None