Sustainability of Pulmonary Rehab Gains

People with moderate to severe COPD trap air in their lungs. This lessens their ability to exercise. The investigators developed a computerized breathing retraining program to help patients empty their lungs during exercise. The investigators believe that learning to control breathing will help patients be more active. The purpose of this study is to test the effects of using breathing retraining (intervention group) versus no breathing retraining (control group) as part of a 12-week exercise program. The investigators are examining the long term effects of this training on exercise capacity and the ability to empty ones lungs during exercise. Patients will participate in a 12-week exercise program, with or without breathing retraining. They will then participate in a 42 week adherence program to maintain physical activity. Patient testing will be competed at baseline, 12 weeks, 6 months and 1 year. 250 patients with moderate to severe COPD will be enrolled.