Performance and Injury Correlates of X-Factor Among Baseball Hitters

Abstract

PURPOSE: Abdominal strains related to trunk torsional forces commonly occur during baseball batting. A large degree of trunk rotation, or X factor (XF), is proposed to increase bat speed. However, baseball swing initiation is driven by pelvis rotation, which requires adequate hip joint mobility. This study investigates the correlations between XF, bat speed, passive hip range of motion, and dynamic hip rotation during the baseball swing. METHODS: 22 players (3 professional, 1 amateur, 5 college, 13 high school), mean age 18.09 ± 3.90 years, underwent 3D biomechanical baseball swing analysis. XF (resultant transverse angle of the intersecting rays through both shoulder joints and the pelvis midpoint) and lead and trail hip rotation angles were measured for 1 swing. Passive, prone hip rotation was assessed via goniometer. Spearman rho correlations and one-way ANOVA were performed. RESULTS: Peak XF and XF at the top of the back swing did not correlate with peak bat speed (p = 0.088 and p= 0.832). There was no significant difference in peak XF across level of play [F(2,19)= 2.637, p= 0.098] (Figure 1). Passive internal trail hip rotation differed between college and high school players (p= 0.049) and was inversely correlated with bat speed at ball contact (rs = - 0.526, 0.029). Peak bat speed positively correlated with max trail hip rotation angle during the swing (rs=0.570, p= 0.006). No correlation of peak XF and passive hip rotation measures or hip rotation angles during the swing reached significance set at p < 0.05. CONCLUSIONS: Findings do not support the use of XF as an indicator of bat speed. The variation in XF values may result from coaching differences or joint mobility compensation patterns. Athletes with limited hip rotation may attempt to compensate through generation of a large XF, possibly increasing the risk of back and oblique abdominus injuries. Clinicians caring for baseball players should screen for trail hip joint rotational mobility limitations.Figure 1: X-factor by Competetion Level