PROJECT S U MMARY Knee injuries, e.g., meniscus tears, increase the risk of developing post-traumatic osteoarthritis (PTOA) by at least 10 fold. PTOA leads to disability with a substantial healthcare cost of $3 billion per year in the US. Although rehabilitation is required after injury, current protocols only address the return to pre-injury activities. Critical knowledge gaps exist regarding how to optimize early rehabilitation efforts to preserve long-term knee cartilage health. Using a well-established rat PTOA model, we propose to characterize responses of knee cartilage to the manipulation of three elements commonly implemented during post-injury rehabilitation: 1) durations of initial non-weight bearing, 2) modifications of physical activity level, and 3) deviations in lower-limb joint kinematics. The use of a pre-clinical rat model provides the experimental control over disease timeline to facilitate a mechanistic study to understand PTOA. Our overall hypothesis is that post-injury interventions that emphasize maintaining pre-injury knee loading profile (e.g. minimizing weight-bearing restriction, regaining physical activity level, or reducing deviations in joint kinematics) will delay PTOA. We will systematically test this hypothesis using innovative experimental approaches and technologies to address the knowledge gaps in the PTOA rehabilitation literature as described in the following Aims. (Aim 1) Non-weight bearing durations in rats after the medial meniscal transection (MMT) surgery performed on the left hind limb will be varied using hind-limb suspension. The comparisons of cartilage health among varying exposures of suspension determines the causal effect of initial non-weight bearing on PTOA progression. The use of hind- limb suspension instead of limb immobilization reduces harmful consequences of joint contracture and is more in line with current practice of prescribing initial non-weight bearing after knee injuries. (Aim 2) Post-injury physical activity levels will be modified by varying the amount of daily treadmill running. Rats’ pre-injury activity levels will first be established to allow standardized quantification of post-injury activity modification (% of pre-MMT). The causal effect of post-injury activity level on PTOA progression can then be determined. (Aim 3) 3D hind-limb joint kinematics will be assessed using biplanar X-ray motion analysis at multiple time points before and after MMT to determine whether early post-injury joint kinematic deviations are predictive of the ultimate severity of PTOA. Using X-ray videos to directly quantify 3D bone movements can overcome the significant errors associated with the use of skin markers. For all three Aims, we will incorporate our contrast enhanced microCT analysis with conventional histopathology to provide complementary evaluations of the 3D microstructure/compositions of knee cartilage. Findings from this innovative work will provide insight into the potential causal e...