Association Between Trunk Rotation, Ball Velocity, and Elbow Torque in College Pitchers

Cohen AD, Garibay EJ, Solomito MJ. The association among trunk rotation, ball velocity, and the elbow varus moment in collegiate-level baseball pitchers. Am J Sports Med. 2019;47(12):2816-2820. doi:10.1177/0363546519867934

Background

The trunk plays a key role in the kinetic chain during pitching.  As a result, it is receiving increasing attention for its contributions to the rise in pitching-related elbow injuries.  During the pitching motion, the trunk functions to transfer energy from the lower extremities to the throwing arm and helps develop ball velocity.  Previous research shows sagittal and coronal plane trunk positioning influences ball velocity and elbow varus moment during pitching.

The timing and magnitude of both pelvic and trunk rotation have also been implicated as contributors to upper extremity pitching injuries.  Pelvic and trunk rotation at the maximum external rotation of the shoulder and pelvic rotation at ball release has been shown to be associated with increased ball velocities.  This leads to speculation that trunk rotation may also be associated with elbow stress during the pitching motion.

Purpose

The purpose of this study was to describe the trunk range of motion in the transverse plane throughout the pitching cycle and to determine the associations between trunk rotation, ball velocity, and the elbow varus moment.

Study Population

  • NCAA Division I and III college pitchers (n=99)
  • All pitchers were free of injury for at least 6 months

Research Methods

Using motion capture analysis software and retroreflective markers a biomechanical evaluation of the pitching motion was performed.  Research participants pitched multiple pitch types in random order.  Only fastball pitches were used for data analysis.  The pitching motion was divided by 4 events: lead foot contact (FC) (0% pitch cycle), maximum external rotation of the glenohumeral joint (MER), ball release (BR), and maximum internal rotation of the glenohumeral joint (MIR) (100% pitch cycle).

The specific variables of interest for this study were trunk rotation at FC, MER, and BR; total range of trunk rotation; and maximum rotational velocity of the trunk.  The outcomes of interest were ball velocity and elbow varus moment. The associations between the variables of interest and the outcomes of interest were determined.  To determine the association, a random intercept mixed-effects regression model was used.

Key Results

  • At FC, trunk rotation was -83° (externally rotated away from home plate).
  • At MER, trunk rotation was 12° past square to home plate (internally rotated).
  • At BR, trunk rotation was 23° past square to home plate (internally rotated).
  • At MIR, trunk rotation was 29° past square to home plate (internally rotated).
  • The average maximum rotational velocity of the trunk was 979 deg/s and occurred about 2/3 of the way between FC and MER.
  • At BR, trunk rotation angle was associated with elbow varus moment (P = .019) and ball velocity (P = .016).
  • The maximum trunk rotational velocity was positively associated with ball velocity (P < 0.001) and elbow varus moment (P < 001).

Study Limitations

  • No cause-and-effective relationship can be established based on study design.
  • Extrapolating these results to youth or professional-level pitchers may not be appropriate.

Practical Implications

Based on these results, for every 10° increase above the average trunk rotation angle at BR, ball velocity increases by 0.6 m/s and the internal elbow varus moment increases by 2.5 N•m . This suggests greater internal trunk rotation at ball release is associated with a 1.9% increase in ball velocity along with a 3.2% increase in elbow varus moment.  These findings are consistent with previous research showing ball velocity as possibly the most important contributor to elbow stress during baseball pitching.

Faster trunk rotational velocities also contributes to faster ball velocities.  This may also contribute to increased elbow stress.  For every 100 deg/s increase in rotational velocity between FC and MER, a 0.70 m/s increase in ball velocity can be expected. This would also correspond with a 2.9 N•m increase in elbow varus moment.  The authors theorize that greater trunk rotational velocity requires improved core function to maintain upright trunk positioning during FC to BR.  Further research is needed to confirm this claim.

Even though no cause-and-effect relationship can be established from this study, the findings do support the importance of trunk function for performance enhancement and reducing injury risk.  Rehabilitation and performance enhancement programs should include trunk strengthening and motor control exercises.  Strengthening and motor control exercises should be prescribed to complement traditional rotational power exercises such as medicine ball throws.  Training only trunk power and velocity may predispose the pitcher to injury.

 

 

 

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