An individualized linear model approach for estimating scapular kinematics during baseball pitching.
Assessment of scapulothoracic and glenohumeral contributions to shoulder function during baseball pitching are limited by challenges in accurately measuring dynamic scapular orientation. A recently validated individualized linear model approach that estimates scapular orientation based on measurable humerothoracic orientation has yet to be adapted for pitching and may improve upon currently recommended methods such as the acromion marker cluster (AMC). This study evaluates the ability of a pitching-specific individualized linear model to estimate scapular orientation in static positions throughout a throwing motion by comparing against palpation and an AMC. Individualized linear models were created for 14 collegiate pitchers by determining scapulothoracic and humerothoracic orientations at static arm postures throughout their individual dynamic throwing motions. Linear model and AMC estimates were compared against palpation at intermediate test positions within the throwing motion that were excluded from model creation. Linear model estimates were similar to palpation at all test positions and on all scapulothoracic axes while AMC estimates differed on internal/external rotation and anterior/posterior tilt during cocking (p = 0.001, p = 0.018) and follow-through (p = 0.003, p = 0.006). Linear model root mean square error (RMSE) values were smaller than AMC values for all positions/axes. Linear model RMSE values (2.8-6.3°) were within a range of published values previously deemed acceptable, while AMC values (5.1-15.8°) went beyond this range. The linear model approach accurately estimates static scapular orientation throughout a pitching motion and improves upon current methods. Future applications to dynamic pitching may facilitate understanding of how scapulothoracic and glenohumeral joint function relate to injury risks, rehabilitation, and performance.