Despite previous research and proposed intervention strategies, upper extremity injuries and surgeries in youth overhead athletes continues to rise. A lack of knowledge about factors such as age, sport specialization, and overuse pathomechanics are likely reasons for the continued high injury rate. Incomplete information about underlying tissue characteristics and insufficient knowledge transfer from the laboratory to field setting, limits the ability of sports medicine professionals to fully understand these injuries. The purpose of this study was to investigate how upper extremity tissue characteristics and injury may differ among those who participate in separate sports, specialize earlier in their sport, and display various biomechanical patterns. Musculoskeletal ultrasound allowed measurement of tissue characteristics, and three-dimensional motion capture techniques examined the relationship between biomechanical variables and injury history. Biomechanical angles from high speed, commercially available video cameras were compared with laboratory data to determine whether clinically applicable tools can identify pitchers with injury history or higher upper extremity joint loads. Differing from non-overhead athlete controls, the results of this study indicated that humeral retrotorsion and range of motion are similar among athletes of various ages. Bilateral soft tissue differences are only present in collegiate athletes. Sport and degree of specialization did not xi ii significantly impact tissue characteristics. These findings suggest that overhead sport participation produces bony adaptations participation before skeletal maturity, whereas the development of bilateral soft tissue differences occurs later. Three-dimensional biomechanical analysis accurately differentiated pitchers with and without injury history. Pitchers with a previous injury presented with limited shoulder abduction at the point of maximal external rotation. While some two-dimensional techniques were valid compared to three-dimensional analysis, they did not accurately identify previous injury. Separating groups based on normative and non-normative pitching mechanics did not differentiate between pitchers who experienced more upper extremity joint loading. Based on these findings, clinicians may want to identify pitchers who drop their arms at maximum external rotation. They should use caution when examining other biomechanics, as variability in pitching mechanics often supersedes injury group identification. Clinicians should also promote age specific intervention programs which specifically address underlying anatomical differences associated with each population.