Project Summary Rotator cuff injuries affect 4.5 million Americans and the increase in prevalence of rotator cuff injuries correlates with increasing age. Around 250,000 rotator cuff repairs are performed each year, equating to over three billion dollars in economic cost. The rotator cuff is critically important in activities of daily living, and therefore damage to the rotator cuff can limit independence and quality of life. Rotator cuff tears are commonly treated with surgical reattachment of the tendon; however, surgery is not always advisable due to the high probability of retear. High repair tension during reattachment surgery is not only technically challenging during repair but also is often followed by post-surgical retear, limited functional capacity, and/or shoulder instability due to altered joint contact forces. Thus, a method to predict repair tension could play a key role in surgical selection. Repair tension is intraoperatively measured; therefore, it cannot inform the decision to operate. Surgeons currently must select patients for surgical repair based on other MR imaging parameters associated with poor outcomes and more severe injury. Following injury of the rotator cuff, the muscle-tendon unit undergoes physiological changes including retraction, muscle atrophy, and increased intramuscular fat and fibrous tissue. However, it is unclear whether there is a direct relationship between repair tension and the three main clinical parameters typically used for determining surgical candidacy (intramuscular fat, muscle atrophy, and retraction). Given the financial costs associated with surgical intervention and the physical burden experienced by patients, an improved understanding of the factors that influence rotator cuff injury and repair tension are needed to provide a higher level of care for surgery patients, thus positively impacting the quality of life for millions of Americans. Our overall objective is to understand the mechanical underpinnings of rotator cuff repair to identify relationships between physiological changes and post-surgical outcomes. This research will provide crucial information needed to inform likelihood of surgical success and better predict surgical outcomes, thereby guiding development of a standard of practice for rotator cuff repair surgery. The proposed research combines an experimental and computational framework to (1) determine the relationship between rotator cuff injury-induced physiological changes and repair tension, and (2) determine how physiological changes due to rotator cuff injury and elevated repair tension influences post-surgical outcomes.