Cocoa to Maximize Exercise Training in Older Adults - The COMET Trial

Inclusion Criteria: * Age 55 years and older * Slow walking speed of \< 1m/s * Willingness to be randomized to either treatment group * Willingness to participate in all study procedures (muscle biopsy will be optional) Exclusion Criteria: * Failure to provide informed consent; * Regular consumption of flavanol and/or cocoa supplements * Current involvement in supervised rehabilitation/exercise training program * Absolute contraindication(s) to exercise training according to American College of Sports Medicine guidelines \[17\] * Daytime average of systolic blood pressure ≥ 180/100mm Hg. * Refusal to stop blood thinners such as baby aspirin dose. * Peripheral vascular disease; peripheral neuropathy; retinopathy * Severe cardiac disease, including NYHA Class III or IV congestive heart failure, clinically significant aortic stenosis, history of cardiac arrest, use of a cardiac defibrillator, or uncontrolled angina; * Myocardial infarction or stroke within past year * Significant cognitive impairment, including known dementia diagnosis or a Mini-Mental State Examination exam score \< 24 * Progressive, degenerative neurologic disease, e.g., Parkinson's Disease, multiple sclerosis; * Severe rheumatologic or orthopedic diseases, e.g., awaiting joint replacement, active inflammatory disease; * Severe pulmonary disease, requiring either steroid pills or injections or the use of supplemental oxygen; * Hip fracture, hip or knee replacement, or spinal surgery within past 4 months; * Other significant co-morbid conditions that would impair ability to participate in the exercise-based intervention * Simultaneous participation in another interventional trial