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Youth baseball frequently results in repetitive strain injuries. Quantitative ultrasound allows real-time imaging with the ability to identify acute markers of tendon change. The study objective was to determine acute quantitative ultrasound changes in the long head of the biceps and infraspinatus tendons of the throwing and nonthrowing shoulders during a pitching performance. We hypothesized the tendons of the pitching arm would exhibit an increased width and decreased echogenicity after pitching and that tendons of the nonpitching arm would not demonstrate such changes.Fifty youth baseball players, ages 9–14 yr, engaged in a simulated pitching performance that consisted of 50 pitches. Subjects underwent serial quantitative ultrasound imaging of the infraspinatus and the long head of the biceps before pitching and after 25 and 50 pitches were thrown.Testing of the change in tendon width revealed the infraspinatus (0.21 mm) and long head of the biceps tendons (0.18 mm) in the throwing shoulder had statistically significant increases (P = 0.03) in tendon width as an acute response to throwing 50 pitches, without such changes in the nonthrowing shoulder (P > 0.05). No tendon width change was found at 25 pitches in either arm or tendon (P > 0.05). No associated changes in echogenicity were found at any time point (P > 0.05).The results of this study suggest that pitching acutely increases tendon width in two biomechanically important tendons of the shoulder as early as the 50 pitch mark. This change could be a normal physiological response or a potential warning sign of future pathology and requires further study.