Identifying risk factors to prevent anterior cruciate ligament (ACL) injuries remains a major objective of musculoskeletal research. Athletic participation in the United States among children and adolescents has increased over the last three decades, resulting in a greater prevalence of ACL injuries. The frequency of ACL injuries among individuals under 15 years of age has risen 924% since 1994 despite the incorporation of injury prevention programs in youth sports. Further, 70% of ACL injuries occur while performing non-contact maneuvers (e.g. single foot landings, pivots). Yet, it remains unclear whether these injuries result from a single overload event or a fatigue failure wherein a period of intervention is available. Injuries are likely multifactorial and influenced by an individual?s natural anatomical variation. Fatigue failure of the ACL via repetitive loading, combined with the natural variation in anatomical structure between individuals, has largely been overlooked as an explanatory mechanism for unanticipated ACL failures. We showed previously that ACL cross-sectional area (CSA) is a significant predictor of peak relative strain. However, injury risk between individuals is not easily delineated because of the large degree of variation in this trait. We hypothesize that the natural variation in ACL CSA is accompanied by coordinated changes in tissue-level mechanical properties and/or other gross anatomical features that together allow the ?ACL-complex? (i.e. the ligament and its entheses) to be mechanically functional under routine loading conditions. We further hypothesize that this coordination, although effective in establishing the strength of the ACL-complex, may result in changes in tissue-level mechanical properties that deleteriously affect the fatigue resistance of the ACL-complex for certain individuals. We will test our hypotheses using a mouse model that mirrors the natural variation in ACL-complex traits among humans. At the completion of this work we will have an established mouse model for generating new hypotheses, along with a better understanding of how the ACL-complex develops postnatally and functionally adapts to mechanical loads. These outcomes are expected to guide interventions aimed at maximizing ACL strength while minimizing ACL rupture among children and adolescents, and to have a broad positive impact in other anatomically unique locations where ligamentous and tendinous injuries routinely occur (e.g. rotator cuff, elbow, spine) and are likely influenced by a similar complex adaptive system wherein multiple traits are coordinated to establish organ-level strength and stiffness.