Abstract Persistent quadriceps muscle dysfunction following anterior cruciate ligament (ACL) injury and reconstruction (ACLR) is a critical clinical concern impacting the success of postoperative rehabilitation and prognosis of long- term joint health. Our novel working hypothesis is that worse quadriceps muscle quality, caused by a progressive increase in non-contractile tissue within the muscle, post-ACL injury and ACLR is a critical and understudied mechanism underlying poor clinical outcomes. There is a significant scientific gap in our understanding of the mechanistic pathway linking changes in quadriceps muscle quality to poor clinical outcomes. Our preliminary work using ultrasonography to assess changes in muscle quality supports our working hypothesis, in that patients with an ACL injury demonstrate muscle tissue image signal changes consistent with increased non- contractile tissue makeup in the first 3 months post-ACLR. Furthermore, ultrasonographic muscle tissue signal changes are associated with lesser knee extensor strength and worse physical performance (i.e., hop testing). Unfortunately, rigorous studies to test our working hypothesis have not been conducted to comprehensively evaluate the effect of ACL injury on muscle quality and the impact of poorer muscle quality on clinical outcomes. Recent magnetic resonance imaging (MRI) innovations now allow for high-precision and rapid evaluation of muscle quality throughout the entire muscle volume and in individual quadricep heads. We can be opportunistic and leverage an ongoing rehabilitation-based parent clinical trial already collecting critical clinical outcomes of knee extensor strength, gait biomechanics, physical performance and patient-reported outcomes (PRO), and tibiofemoral cartilage composition at preoperative, and 1-, 6-, and 12-month post-ACLR timepoints. We are proposing to test our working hypothesis with an ancillary R21 by conducting the following specific aims: 1) determine longitudinal changes in quadriceps muscle quality following ACL injury and ACLR preoperatively and at 1-, 6- and 12-months post-ACLR; and 2) determine the associations between changes in muscle quality and a) knee extensor strength; b) gait biomechanics; c) PRO and physical performance; and d) tibiofemoral cartilage composition. Our study is innovative as: 1) we are testing a novel hypothesis that quadriceps muscle quality changes early post-ACL injury and that it is a critical mechanism associated with poor clinical outcomes; 2) we are utilizing a novel MRI technique capable of collecting highly-sensitive measures of muscle quality in individual heads of the quadriceps throughout the entire muscle volume; and 3) associations between muscle quality and a comprehensive host of clinical outcomes (i.e. knee extensor strength, gait biomechanics, physical performance and PRO, and tibiofemoral cartilage composition) can be evaluated as part of the ancillary R21. The proposed R21 is significant as identif...