Spinal cord injuries (SCI) are among the most debilitating conditions an individual can sustain with the estimates of SCI incidence in the United States at 12,000 new cases per year. The loss of innervation to the tissues muscle below the level of the lesion results in reduced physical activity which leads to an array of secondary complications including muscle atrophy, cardiovascular and metabolic disease, obesity and vascular dysfunction. This further leads to exercise intolerance, reduced quality of life and depression. Although current rehabilitative programs focus on improving muscle strength in this population, the efficacy of these programs is challenged by the injury related motor impairment, which limits the exercise intensity and subsequent positive muscular adaptations. Therefore, development of an exercise program that promotes maximal muscular adaptations to light intensity exercise could greatly improve the efficacy of rehabilitation in the SCI population and help restore functional capacity and quality of life for these individuals. Blood flow restriction (BFR) exercise has shown tremendous promise for improving muscle size and strength in a variety of healthy and clinical populations, however the benefits of BFR exercise for those with SCI has not been established. Thus, the purpose of this Merit proposal is to conduct a comprehensive study that explores the benefits and risks of BFR exercise in the incomplete SCI population. In general individuals with chronic incomplete SCI will be recruited to partake in two 8-week training periods (20 sessions) that involve traditional knee extension/flexion exercise or knee extension/flexion exercise with blood flow restriction. There will be a series of measurements before and after the 8-week intervention to look at changes in muscle and vascular function. Specific Aim 1 will determine how the 8-weeks of BFR exercise influenced muscle strength (Biodex isokinetic dynamometer), muscle cross sectional area and volume (CTscan) and fatigue resistance. Specific Aim 2 will determine how this novel 8-week training intervention impacts peripheral vascular function. Specifically, changes in nitric oxide mediated endothelial function will be determined through tests of flow mediated dilation, changes in endothelial function of the microvascular network will be determined through assessments of reactive hyperemia and changes in arterial stiffness will be determined through measurements of pulse wave velocity. Specific Aim 3 will focus on the safety of BFR exercise for the SCI population. Those with SCI are at greater risk for thrombosis and DVT compared to able bodied individuals. Although unlikely, the introduction of temporary blood stasis during BFR exercise might augment this risk. Thus, the third aim of this study will be to determine changes in innate immune activation and thrombosis risk. Specifically, blood will be collected at multiple timepoints throughout the training intervention and analyzed for hypoxia-inducible factor 1-alpha, neutrophil extra cellular traps (which act as prothrombotic scaffolds), neutrophil-platelet aggregates and inflammatory cytokines. Ultimately, if the improvements in muscle and vascular function following BFR resistance exercise is greater than the traditional resistance exercise often performed in rehabilitation settings, without increasing risk for DVT, it should be incorporated into the long-term rehabilitation programs for Veterans with SCI.