BACKGROUND: The incidence of shoulder and elbow injuries in adolescent baseball players is rapidly increasing. One leading theory about this increase is that breaking pitches (such as the curveball) place increased moments on the dominant arm and thereby increase the risk of injury. HYPOTHESIS: There is no difference in the moments at the shoulder and elbow between fastball and curveball pitches in adolescent baseball pitchers. STUDY DESIGN: Controlled laboratory study. METHODS: Thirty-three adolescent baseball pitchers with a minimum of 2 years of pitching experience underwent 3-dimensional motion analysis using reflective markers aligned to bony landmarks. After a warm-up, pitchers threw either a fastball or curveball, randomly assigned, from a portable pitching mound until 3 appropriate trials were collected for each pitch technique. Kinematic and kinetic data for the upper extremities, lower extremities, thorax, and pelvis were collected and computed for both pitch types. Statistical analysis included both the paired sample t test and mixed model regression. RESULTS: There were lower moments on the shoulder and elbow when throwing a curveball versus when throwing a fastball. As expected, speed for the 2 pitches differed: fastball, 65.8 +/- 4.8 mph; and curveball, 57.7 +/- 6.2 mph (P < .001). Maximal gle-nohumeral internal rotation moment for the fastball was significantly higher than for the curveball (59.8 +/- 16.5 N.m vs 53.9 +/- 15.5 N.m; P < .0001). Similarly, the maximum varus elbow moment for the fastball was significantly higher than for the curveball (59.6 +/- 16.3 N.m vs 54.1 +/- 16.1 N.m; P < .001). The wrist flexor moment was greater in the fastball, 8.3 +/- 3.6 N.m, than in the curveball, 7.8 +/- 3.6 N.m (P < .001), but the wrist ulnar moment was greater in the curveball, 4.9 +/- 2.0 N.m, than in the fastball, 3.2 +/- 1.5 N.m (P < .001). Relatively minor motion differences were noted at the shoulder and elbow throughout the pitching motion, while significant differences were seen in forearm and wrist motion. The forearm remained more supinated at each point in the pitching cycle for the curveball but had less overall range of motion (62 degrees +/- 20 degrees ) than with the fastball (69 degrees +/- 17 degrees ) (P < .001), and the difference in the forearm pronation and supination moment between the pitches was not significant (P = .104 for pronation and P = .447 for supination). The wrist remained in greater extension during the fastball from foot contact through ball release but did not have significantly different total sagittal range of motion (53 degrees +/- 11 degrees ) when compared with the curveball (54 degrees +/- 15 degrees ) (P = .91). CONCLUSION: In general, the moments on the shoulder and elbow were less when throwing a curveball than when throwing a fastball. In each comparison, the fastball demonstrated higher moments for each individual pitcher for both joints. CLINICAL RELEVANCE: The findings based on the kinematic and kinetic data in this study suggest that the rising incidence of shoulder and elbow injuries in pitchers may not be caused by the curveball mechanics. Further evaluation of adolescent and adult baseball pitchers is warranted to help determine and subsequently reduce the risk of injury.