The broad objective of this study is to examine the effects of moderate and more intense aerobic exercise as an intervention on cognitive performance, physical functioning and health-related quality of life in patients with chronic (more than 12 months post-injury) traumatic brain injury (TBI). Importantly, structural and biological brain changes will be measured to examine whether functional outcomes are related to exercise-induced adaptations. It is hypothesized that in the chronic phase of persons with TBI, there will be improved: 1) cognitive function, 2) physical fitness and fatigue severity, 3) motor performance and balance, and 4) mood and depressive symptoms, in those that performed the exercise intervention compared to a control group. It is also hypothesized that these functional improvements will be related to exercise intensity, improved cortical connectivity, dopamine transmission gene scores, and blood biomarkers related to neuroand angio-genesis.