Abstract
Background:
Hip-shoulder separation (HSS) is a biomechanical factor linked with subsequent rotational velocity of the trunk and improved ball velocity in baseball pitchers. One contributing factor to HSS may be the external oblique musculature, which generates torque that facilitates contralateral trunk rotation.
Hypothesis:
External oblique muscle activity will influence trunk rotation contributing to HSS, which may be useful for training and lend to a component for athlete injury-risk profiling.
Methods:
Thirty-two youth pitchers (10-15y) each threw five fastballs from regulation distance for their age. Surface electromyographic (EMG) data (1500Hz) of the glove-side external oblique (EOglove) and full-body kinematics (150Hz) were collected with an electromagnetic tracking system. EMG signals were band-pass filtered (80-250Hz) and analyzed using a root-mean square calculation with a .01s sampling period. Kinematic data were filtered with a zero-lag 4th-order Butterworth filter (13.4Hz). Mean EMG and kinematic values were extracted during the preparation phase and at max HSS (negative vector value). EMG data were normalized to the maximum voluntary contraction expressed as a percentage. The preparatory phase began when the pelvis or trunk first exceeded 10% of its peak rotation velocity toward the target and ended at max HSS. After removing outliers (±2sd), data for 31 pitchers remained, from which means were aggregated for each participant. A path analysis was then performed to better understand the relationship between EOglove muscle activity, trunk rotation velocity, and max HSS angle.
Results:
The path model (Figure 1) was a good fit for the data (χ2(1) = .154, p = .694), and included one non-significant (EOglove -> trunk rotation velocity; p = .282) and one significant path (trunk rotation velocity -> HSS; p = .007).
Conclusion:
Findings suggest that greater EOglove muscle activity had minimal effect on decreasing trunk rotation velocity toward the target (standardized β = ‒.19). Accordingly, factors such as abdominal muscular strength or other inter-limb coordination patterns may be more influential. In contrast, while oblique activity showed weak associations with early trunk rotation velocity, decreased trunk rotation velocity had a strong effect on increasing HSS (standardized β = .44). This indicates that reducing trunk rotation velocity during the preparation phase is beneficial to HSS. Since adolescent-aged pitchers are often less effective at utilizing pelvis-trunk rotational kinematics to attenuate arm kinetics compared to elite pitchers, training interventions focused on controlling early trunk rotation velocity could improve performance related to HSS and help reduce arm kinetics, potentially decreasing injury risk.
