PROJECT SUMMARY About 250,000 anterior cruciate ligament (ACL) injuries occur annually in the United States and are primarily treated with a surgical reconstruction (ACLR). However, for at least 1 in 3 young adults with an ACLR, the injury and surgery are pivotal life events that lead to chronic pain, diminished long-term quality of life, and an increased risk for osteoarthritis (OA). The 1st year post-ACLR is critical to a patient’s long-term outcomes as there is evidence that levels of pain and quality of life experienced at 1 year post-ACLR will remain unchanged for up to 10 years. Additionally, at 1 year post-ACLR, ~33% of young adults present with knee structural pathologies. Unfortunately, there are no evidence-based strategies to identify people during the 1st year post-ACLR who are at risk for chronic symptoms or structural pathology. Hence, we cannot identify susceptible populations and lack critical insights into which modifiable risk factors could decrease their risk and prevent lifelong disability. Potentially modifiable risk factors that are common throughout the 1st year post-ACLR are alterations in walking biomechanics and insufficient levels of physical activity. Since poor symptoms and pre-radiographic structural pathology are risk factors for the development of knee OA, understanding modifiable risk factors that relate to poor symptomatic and structural outcomes during the 1st year post-ACLR is needed for future studies to identify therapies that prevent OA. The objective of this study is to determine how longitudinal changes in PA and walking biomechanics assessed at 3, 6, 9, and 12 months post-ACLR relate to poor symptomatic or structural outcomes during the 1st year post-ACLR. This will be the first study that applies the following outcomes to young adults at multiple visits during the 1st year post-ACLR: 1) classification criteria for early OA symptoms, 2) clinically accessible whole-knee ultrasound scoring system to detect multiple structural pathologies. PA will be assessed with research-grade accelerometers to quantify steps per day and weekly minutes of moderate to vigorous PA during a 7-day period following each study visit. Walking biomechanics will be assessed in a motion capture laboratory to quantify vertical ground reaction force and internal knee adduction moment. My central hypothesis is that participants with a moderate change in PA and walking biomechanics post-ACLR will be less likely to have poor symptomatic and structural outcomes during the 1st year when compared to people with a rapid increase or no change in PA and walking biomechanics. The expected outcome of this work is to identify: 1) at- risk patients post-ACLR who are the ideal participants for clinical trials aimed at preventing poor symptomatic and structural outcomes, and 2) when PA or walking biomechanics need to be targeted during the 1st year post- ACLR. This proposal will also provide the PI with the training and mentoring to develop a novel ...