Scapular dyskinesis type is associated with glenohumeral joint and scapular kinematic alteration during pitching motion in baseball players.

Introduction: Scapular dyskinesis (SD) is associated with an increased risk of throwing-related shoulder injury onset, resulting in abnormalities in glenohumeral joint (GH) and scapular motions during pitching. The effects of SD on shoulder motion during pitching remain unknown. This study aimed to investigate kinematic alterations in GH and scapular motions during pitching in baseball players with type I SD.

Methods: Sixty-seven university and independent-league baseball players with and without SD were included. Pitching motion was measured using an optical three-dimensional motion capture system, and a SD test was conducted to evaluate SD. SD was classified into types I-IV. The inter-rater reliability of SD assessment was calculated using kappa coefficients. Three-dimensional GH and scapular kinematics during pitching motion were analyzed.

Results: The percentage of agreement representing the inter-rater reliability of SD assessment was 77.6% (52/67; kappa coefficient: 0.72). Overall, 24 and 27 participants were categorized into abnormal (type I SD) and normal group (type IV SD), respectively, with normal scapular motion; one individual with type III SD was excluded. The abnormal group exhibited a significantly increased GH external rotation angle (9°) and decreased scapular posterior tilt angle (6°) during the maximum external rotation period compared with the normal group.

Conclusions: Baseball players in the abnormal group showed increased GH motion and decreased scapular motion during pitching. The SD test for the evaluation of type I SD can help predict excessive GH external rotation and decreased scapular posterior tilt during pitching.