Abstract An estimated 400,000 anterior cruciate ligament (ACL) injuries occur yearly in the United States. Most ACL injuries are non-contact, occurring during rapid deceleration movements such as jumping and cutting. Importantly, the risk of early-onset osteoarthritis (OA) persists regardless of whether ACL reconstruction is performed. Therefore, identifying risk factors for non-contact ACL injury is of great clinical importance. The function of the ACL is to resist tibiofemoral motion during the intense forces associated with dynamic activity. Therefore, measuring the dynamic elongation patterns of the ACL functional bundles will provide imaging biomarkers of elevated tension within the ACL. As the ACL fails under tension, factors associated with elevated ACL bundle elongations reflect increased vulnerability to injury. Non-contact ACL injuries are 2-6x more likely in females as compared to males. Patients with prior ACL injury are up to 15x more likely to sustain injury to their uninjured contralateral knee than uninjured controls. Finally, while controversial, some studies indicate that fatigue increases injury risk. However, there remains limited in vivo data to describe ACL bundle elongations during relevant dynamic activities (such as jumping and cutting) in these groups. Because different studies suggest that different motion patterns are associated with non-contact ACL injury, the kinematic risk factors for this injury remain unclear. Additionally, studies suggest that a number of characteristics of femoral, tibial, and ACL morphology are related to injury risk. However, it remains unclear how these kinematic and morphologic factors interact to influence ACL tension during relevant dynamic activities. Thus, quantifying the influence of these factors on in vivo ACL bundle elongations in these high risk groups addresses a critical barrier to our understanding of what elevates ACL injury risk. Thus, we propose to utilize a novel in vivo multimodal imaging methodology developed by our lab to assess the impact of 6DoF kinematics and characteristics of knee morphology on in vivo ACL bundle elongations (defined as change in length normalized to an unloaded reference length). These relationships will be assessed during jumping and cutting activities in three potentially high risk populations. Specifically, we will investigate these relationships Aim 1.) in both sexes, Aim 2.) in the uninjured knees of patients with prior ACL injury and control knees, and Aim 3.) pre- and post-fatigue. Relevant biological variables including sex, age, BMI, race, isokinetic muscle strength, sport participation, and activity level, will also be considered as covariates that may impact ACL bundle elongations. Furthermore, interactions between covariates will be explored to assess differential effects of one covariate across levels of another. Thus, the proposed studies will have high impact because they will 1.) address a fundamental gap in knowledge regarding th...