No Effect of Scapular Position on 3-Dimensional Scapular Motion in the Throwing Shoulder of Healthy Professional Pitchers

Context: Differences in 3-dimensional (3D) scapular motion have been reported between healthy baseball position players and healthy nonoverhead athletic controls, as well as players diagnosed with shoulder impingement syndrome. These alterations are theorized to be the result of adaptations due to the demands of repetitive throwing. However, comparisons between the throwing and nonthrowing shoulders are commonly used to infer normal motion. Objective: The purpose of this study was to compare 3D scapular kinematics between the throwing and nonthrowing shoulders in asymptomatic professional male baseball pitchers. Design: Cross-sectional study. Setting: Laboratory. Participants: 45 asymptomatic professional baseball pitchers participating without restrictions during preseason training. Interventions: An electromagnetic tracking system was used to assess 3D scapular orientation at rest and during weighted (2.3-kg) shoulder flexion across discrete humeral-flexion angles (rest, 30 degrees, 60 degrees, 90 degrees, 120 degrees, and maximum). Main Outcome Measure: 3D scapular upward/downward rotation (UR/DR), anteroposterior (AP) tilt, and internal/external rotation (IR/ER). Separate mixed-model ANOVAs (Side x Angle) for each scapular motion were used to compare the throwing and the nonthrowing shoulder across all angles. Results: There were significant side-to-side differences with scapular UR/DR (P < .001), AP tilt (P < .001), and IR/ER (P < .001). The throwing scapula displayed greater mean UR (increase = 3.6 degrees, SE = 0.50) and anterior/posterior tilt (increase = 2.1 degrees, SE = 0.60) and less mean IR (decrease = 2.1 degrees, SE = 0.66) than the nonthrowing shoulder averaged across all arm angles. Conclusions: In asymptomatic professional pitchers, the throwing shoulder's scapular position differs across all arm angles from that of the nonthrowing shoulder, but the motion does not differ. Scapular asymmetry that is consistent throughout arm elevation may be indicative not of pathology but, potentially, of a normal adaptation of the pitching shoulder.