This study will compare the effects of a 3-month, individually prescribed progressive exercise training program on: 1) chronic low back pain (cLBP), depression and PTSD symptoms, and 2) neurobiological and related neuropsychological mechanisms by which our exercise-training paradigm may foster exercise maintenance. More specifically, the investigators hypothesize relationships between exercise-training associated augmentation of neuropeptide Y (NPY) system function and improved capacities for reward and self-regulation-neuropsychological capacities posited to underlie intrinsic motivation and self-efficacy, which in turn have been shown to predict exercise maintenance. This study will focus on Veterans with cLBP/PTSD. The study design includes a baseline, acute, cardiopulmonary exercise assessment (CPX) that will inform the exercise prescription for the 12-week "progressive exercise" training program, comprised of three 30-45 minute in clinic exercise sessions per week (walking or running--depending on the ability/capacity of the participant). All exercise sessions will be supervised by an exercise physiologist in the Clinical Studies Unit (CSU) at VA Boston Healthcare System. Intermittent telephone calls by the researchers will provide additional motivational support and problem solving. Implementation of the prescribed exercise regimen will also be supported by the use of heart rate and actigraph monitors programmed for the participant to achieve their prescribed heart rate range (HRR). Also, a "midpoint" and "endpoint" CPX assessment will track changes in NPY system function and delineate their impact on pain, depression and PTSD symptoms, as well as the factors proposed to foster exercise maintenance. All three CPX tests will be performed in accordance with guidelines published by the American College of Cardiology. Among Veterans with cLBP/PTSD, the investigators hypothesize that the capacity to release NPY in response to vigorous exercise (i.e., acute CPX testing) will be associated with improvements in pain, depression and PTSD symptoms, as well as the putative factors that predict exercise maintenance. Data from this R21 will be used to demonstrate feasibility and inform the further development of individually prescribed, motivationally based exercise regimens that could be used as adjuncts to cognitive and other therapeutic PTSD, depression or chronic pain interventions to reduce cLBP, depression and PTSD, as well as the negative consequences of these disorders over the long-term.

\*Note: mandatory covid precautions due to the pandemic led to an initial suspension of all study activities in 2020 after just one of the consented enrolled participants was randomized into the 'progressive exercise' study arm. That participant completed only half of the 12 week intervention. In addition, this participant had one adverse event after a stress test which was determined to be unrelated to the study intervention. Despite intentions to resume the research, a decision was made to terminate the study in early 2022 given the recruitment and intervention challenges with the pandemic surge and the end of funding. No data wee collected related to any of the study outcome measures so no outcome measures are reported.