PROJECT SUMMARY/ABSTRACT More than a quarter of a million people in the US tear their anterior cruciate ligaments (ACL) each year, and most subsequently undergo surgical reconstruction (ACLR). Despite extensive rehabilitation, individuals persistently walk with asymmetrical knee joint mechanics after ACLR. Even years after ACL reconstruction, long after they have returned to sporting activities, individuals walk with subtle gait asymmetries that are not clinically detectable or perceived by the individual. These small asymmetries, nevertheless, directly contribute to the development of early-onset, post-traumatic knee joint osteoarthritis. Post-traumatic knee joint osteoarthritis develops 5-15 years after ACL injury and carries devastating implications for the individual. Early osteoarthritis is directly associated with decreased sports and recreational participation, pain, and lower self- reported quality of life. As the peak age of ACL rupture and reconstruction is 16 years old in girls and 17 years old in boys, most are on track to develop knee osteoarthritis in young adulthood. The objectives of this fellowship are to apply principles of motor learning to directly target the asymmetric knee joint mechanics in individuals after ACLR. The potential to change walking mechanics short-term using a split-belt (one belt under each foot moving at different speeds) treadmill has been demonstrated in neurologic and uninjured populations. This fellowship will use a split-belt treadmill adaptation paradigm to understand how individuals after ACL reconstruction adapt knee joint mechanics. Further, this work will assess the carry-over of a bout of split-belt treadmill training on over-ground walking symmetry by quantifying changes in knee joint kinetics and kinematics. Specifically, the proposal will determine the extent to which individuals after ACLR can adapt (Aim 1) retain (Aim 2), and overground transfer (Aim 3) improved knee joint mechanics compared to uninjured individuals. This work will provide valuable insight into motor learning in individuals after ACLR, which is necessary to correct detrimental walking mechanics. The long-term goal of this fellowship applicant is to develop targeted interventions to optimize gait and ultimately reduce the rate of posttraumatic knee osteoarthritis after ACLR. Findings from the proposed work will aid in the design of rehabilitation interventions for early post-operative ACL rehabilitation. This research has potential to advance post-operative physical therapy interventions and affect the development of early-onset knee osteoarthritis, potentially improving activity levels and quality of life after ACLR. Further, this proposal will facilitate the applicant’s predoctoral training, laying the foundation for a career as an independent clinician-scientist.