Glenohumeral external rotation weakness partially accounts for increased humeral rotation torque in youth baseball pitchers

Objectives: To examine differences in shoulder internal rotation (IR) torque among youth pitchers of above and below average relative glenohumeral (GH) rotation strength levels. It was hypothesized that differences in IR torque during the pitching motion would could be explained by differences in relative IR and external rotation (ER) strength.

Design: Descriptive laboratory study.

Methods: Isometric GH rotation strength and upper extremity pitching mechanics were assessed in 78 male youth baseball pitchers (12.7±2.0yrs; 1.63±14.0m; 56.9±12.4kg). Shoulder IR torque during the pitch was examined at maximum humeral external rotation (MER) and throughout the arm acceleration phase (ACC). Multivariate analysis of covariance (MANCOVA) was used to examine the differences in pitching IR torque between GH strength groups.

Results: A significant main effect of ER strength on the dependent variables was present after controlling for fastball velocity (λ=0.855, F_2,72=6.13, p=0.003, η_p²=0.145). Follow up univariate tests indicated significant differences in IR torque between strength groups at MER (F_1,73=12.36, p<0.001, η_p²=0.145) and during ACC (F_1,73=6.65, p= 0.012, η_p²=0.083). Participants who displayed ER strength at or below the group mean experienced greater IR torque at MER and greater average IR torque during ACC than participants who displayed ER strength above the group mean.

Conclusions: Weakness of the GH ER musculature partially accounts for increased shoulder IR torque during pitching.