Pitching-related upper extremity injuries affect a large number of high school baseball pitchers. Therefore, it is important to develop an intervention strategy to prevent these injuries. One of the suggested risk factors for the pitching-related upper extremity injuries is an improper technique that results in an added stress on the shoulder and elbow joints. Therefore, the purpose of the study was to examine the effects of selected pitching technique parameters on joint loading, performance, and overall quality of pitching technique, by focusing on the observable technical errors of the trunk. The specific technical errors examined were: 1) open shoulder, 2) backward lean at stride foot contact, 3) lateral lean at stride foot contact, 4) lateral lean at maximal shoulder external rotation (LLMER), and 5) inadequate forward trunk tilt at ball release. The pitching biomechanics of 73 high school baseball pitchers were captured using a motion capture system, two high speed cameras, and a radar gun. The presence of each error was determined by the raters who reviewed the pitching trial videos. The joint loading, performance, and quality of pitching technique were compared between the pitchers who did and did not demonstrate each error. We observed that the pitchers with LLMER demonstrated a higher ball speed but also experienced an increased joint loading at the shoulder and elbow joints. This suggests that LLMER is a strategy that pitchers take in order to achieve a higher ball speed at an expense of increased joint loading. Additionally, there was a trend that an inadequate (<25°) and excessive (>45°) forward trunk tilt (FTT) angle at the ball release may influence the joint loading that are linked to injuries. These technical errors should be avoided considering the negative consequences of injuries. Since these technical parameters can be observed using video cameras, screening of pitching technique can be used to identify pitchers with these technical errors. Possible strategies to modify LLMER and FTT angle at ball release were also identified. Using these strategies, it may be possible to correct the technical errors and thereby prevent the pitching-related upper extremity injuries.