Background and objective: This study aimed to assess the relationship between range of motion (ROM) and isometric strength of the shoulder joint, adjusted for humeral head retroversion angle (HHRA), in professional baseball pitchers.

Material and Methods: A total of 18 pitchers from a professional baseball team were included in this study. The isometric strength of internal rotation (IR) and external rotation (ER) were measured using an isokinetic device at 85{}^{\circ } and 30{}^{\circ } ER, and at 25{}^{\circ } IR. A linear regression analysis was then performed.

Results: The HHRA of the dominant arm was approximately 7{}^{\circ } greater than that of the non-dominant arm (P 0.001). As the IR ROM increased by 1{}^{\circ }, the IR isometric strength at 25{}^{\circ } IR was significantly increased by 0.448% body weight (P 0.05). However, as the ROM of IR increased, the IR isometric strength at 85{}^{\circ } and 30{}^{\circ } ER was not significant (P > 0.05), and as the ROM of ER increased, the IR and ER isometric strength were not significant (P > 0.05).

Conclusions: Thus, the increase of IR ROM in professional baseball pitchers was associated with an increase in isometric strength at 25{}^{\circ } IR, after adjustments were made for HHRA.