Exercise & Brain Energetics in PD

Inclusion Criteria: 1. Idiopathic PD according to UK Brain Bank criteria (modified to permit inclusion of subjects with a family history) 2. Mild to moderate Parkinsonism (Hoehn \& Yahr stages I-III) 3. Currently exercise less than 120 minutes per week (PD non-exercisers) General exclusion criteria: 1. atypical Parkinson syndrome (progressive supranuclear palsy, multiple system atrophy, drug-induced etc.); 2. significant osteoporosis or arthritis; 3. history of cancer within 5 years of study participation; 4. high dose of radiation from other procedures within the year; 5. not able to tolerate being off PD medication for up to 24 hours; 6. a female subject who is breast-feeding or pregnant; 7. current or past substance use problems; 8. serious head injury with loss of consciousness for ≥ 5 minutes; 9. people with other chronic diseases such as diabetes, kidney problems, high blood pressure, other known neurological disorders, or heart diseases; 10. people who report having a significant respiratory disorder (e.g. asthma, chronic obstructive pulmonary disease (COPD)) or using home oxygen will be excluded to avoid discomfort or breathing difficulties from respiratory manipulations. 11. current or past neurological disorder (e.g. strokes, seizures, neurodegenerative disease other than Parkinson's disease); 12. current or past psychiatric disorders (e.g. depression, anxiety disorders); 13. severe claustrophobia (a fear of closed in spaces); 14. weight of more than 158kg (347 lbs); 15. inability to lay still for up to 90 minutes; 16. are not medically cleared to exercise; 17. certain medications or conditions may impact participant's ability to participate and these will be discussed on a case by case basis with the participant and the study team. MRI-specific exclusion criteria: 1. Machinist or metalworkers; 2. cardiac pacemakers, wires, or defibrillator; 3. past injury where a piece of metal lodged in your eye or orbit; 4. ferromagnetic aneurysm clip; 5. artificial heart valve; 6. electrical stimulator for nerves or bones; 7. ear or eye implant; 8. implanted drug infusion pump; 9. coil, catheter, or filter in any blood vessel; 10. orthopedic hardware (artificial joint, plate, screws); 11. other metallic prostheses; 12. shrapnel, bullets, or other metal fragments; 13. dentures, braces, or retainer; 14. surgery (brain or otherwise), tattoos, or injection into a joint in the last 6 weeks; 15. pierced body parts (other than earrings) or tattooed eyeliner; 16. non-MRI safe intrauterine device (IUD). Subjects may be excluded following study enrollment if they meet any of the following exclusion criteria: 1. significant cognitive impairment or depression; 2. significant or unstable cardiovascular or respiratory disease - all subjects will undergo a screening aerobic exercise test; 3. failure to comply with the exercise intervention program by not completing at least 60 of the 72 exercise classes.