Water polo is a highly dynamic and physically demanding sport that has heretofore not inspired nearly as much research as other overhead sports such as baseball or swimming. As in almost all overhead sports, water polo puts its players at high risks for shoulder injuries since the motions required to perform the sport at an optimum level push the limits of normal shoulder function. Human shoulders are inherently unstable as a tradeoff to their flexibility. Because of this instability, many structures are required to work concurrently to keep the shoulder in place. These structures include muscles, ligaments and cartilages. It is important to recognize that the shoulder is not a singular joint in itself but rather a highly mobile complex that can perform many nuanced motions. Shoulder instability can cause a variety of injuries such as impingement, which occurs often among overhead athletes. This malady refers to a pathologic limitation of normal subacromial space that causes structures to be in abnormal contact with each other, which causes abrasive and pinching forces. These forces can result in a range of shoulder maladies including bursitis, tendinitis, and tearing of muscles and ligaments. Impingement is very apparent in both swimmers and water polo players due to the repetitive stresses present in each sport. In swimmers, impingement has been relatively well researched; the freestyle stroke utilized in water polo, however, is fundamentally different in that water polo players keep their heads and chests erect out of the water with a stroke known as head-up freestyle. Hitherto unknown are the biomechanics of this stroke and the extent to which impingement in water polo players is caused and/or worsened by this form of freestyle. The proposed research is a two-part study to investigate the biomechanics of water polo head-up freestyle as a first step to understanding its role in impingement in water polo players. First, experienced water polo players with healthy shoulders approved by the IRB committee of Scripps College will be recruited in this study, and their head-up freestyle kinematic sequences and muscle activities will be recorded by synchronized high-speed cinematography and fine-wire electromyography. These data will be compared to those of head-down freestyle in the literature to understand the biomechanical differences between head-up in and head-down. Part II of the study will recruit experienced water polo players with existing shoulder impingement in addition to those with healthy shoulders in Part I, and their head-up freestyle will be recorded in the same manner. These data will be compared to those in Part I to understand how the head-up stroke differs between impinged and healthy shoulders. This research will provide a basis for the understanding of impingement in head-up freestyle to ultimately increase the safety of the players. Further studies should be conducted to compare the patterns of biomechanical differences between healthy and impinged shoulders in head-up vs. head-down, analyze impingement in each phase of the head-up swim stroke, and investigate how the rate of impingement is altered by fatigue and poor technique.