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Background: The incidence of upper extremity injury in baseball pitchers has increased over the past decade. This has resulted in a large body of research mainly focused on the kinematic and kinetic patterns of the elbow and shoulder to elucidate the cause of these injuries, with little attention on studying the associations of forearm rotation and upper arm joint moments. Hypothesis: There will be significant differences in forearm kinematics and kinetics when comparing the fastball pitch with the curveball pitch. There will be a positive association between forearm kinetics, specifically supination and pronation moments, and the elbow varus moment. Study Design: Descriptive laboratory study. Methods: A total of 78 pitchers were recruited for this study, and they underwent a pitching analysis using motion analysis techniques. A random-intercept, mixed-effects regression model was used to determine differences in forearm kinematics and kinetics when pitching a fastball compared with a curveball, as well as to determine if associations existed between forearm supination and pronation moments and the elbow varus moment. Results: There were a number of significant differences in forearm position when pitching a fastball compared with a curveball, with the curveball producing a more supinated forearm position. Significant associations were found between increasing forearm supination moments and an increase in elbow varus moment for both the fastball and the curveball (P = .002 and P < .001, respectively). For every 1-N·m increase in the supination moment, there was a 1-N·m and 1.1-N·m increase in the elbow varus moment for the fastball and curveball, respectively. Conclusion: The results demonstrated that the forearm position was not associated with the elbow varus moment. However, the supination moment was associated with the elbow varus moment. Clinical Relevance: On the basis of these findings, pitching coaches and trainers can understand the implications of proper lower arm pitching mechanics in the later portion of the pitch cycle as a potential risk of injury and, therefore, can develop coaching strategies to reduce incorrect positioning, especially when players are pitching the curveball. [ABSTRACT FROM AUTHOR]