Chris Novellino1, Anthony Fava1, Adam Nebel1, Christopher O'Grady2, Jack Iandoli3, Hillary Plummer4, Thomas van Hogerwou1, Gretchen Oliver, FACSM1. 1Auburn University, Auburn, AL. 2Florida Bone and Joint Specialists, Gulf Breeze, FL. 3OhioHealth Department of Orthopedic Surgery, Columbus, OH. 4San Diego Padres, San Diego, CA.

BACKGROUND: Forces on the shoulder after ball release (BR) in baseball pitching are postulated to increase risk of injury to the rotator cuff due to translation of the humeral head. PURPOSE: To examine the relationship of peak shoulder forces on infraspinatus muscle activity during the deceleration phase of the pitch. METHODS: Thirty-three youth baseball pitchers [11.6±1.5y, 156.8±13.1cm, 49.4±12.0kg] active on a team roster and free from injury for the past six months participated. Electromyographic (EMG) data of the infraspinatus were collected and processed using a root-mean-square calculation with a period of 100ms and a sampling interval of 1/1500 seconds. Two, five second maximal voluntary isometric contractions (MVIC) of the infraspinatus were performed. Afterwards, pitchers threw five maximal effort four-seam fastballs to a catcher. Kinematic data were collected using an electromagnetic tracking system. The mean of the two MVIC peaks was used to normalize EMG data. Mean shoulder peak forces (distraction, anterior, and superior) as a percentage of body weight (BW) were analyzed during the deceleration phase (BR to maximum shoulder internal rotation). Linear regression was performed to predict infraspinatus activity from each kinetic variable at the shoulder. RESULTS: Shoulder superior force was -51.8±22.9%BW, distraction was 76.2±22.0%BW, and anterior was -51.4± 20.1%BW, while infraspinatus activity was 32.7±15.1%MVIC. There was a moderate correlation between superior force and infraspinatus activity (r=-.31 p=.08). Weak correlations were found between distraction force (r=.18 p=.32), and anterior force (r=.17 p=.35) on infraspinatus EMG activity. Superior force did not predict infraspinatus activity (F(1, 31)=3.23, p=.08, R2=.09, Radj=.07). Neither distraction force (F(1, 31)=1.02 p=.32) nor anterior force (F=.88 p=.35) predicted infraspinatus activity. CONCLUSIONS: Increased superior force was correlated with higher maximum infraspinatus EMG activity, yet the individual forces did not predict maximal muscle activation. The infraspinatus undergoes eccentric loading to decrease the rate of glenohumeral internal rotation and may play a minor role in mitigating superior translation of the humeral head. Examining shoulder forces and muscle activity in additional muscles can help explain mechanisms of injury.

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