Inclusion Criteria - Heart Failure Group:  Adult 18 years and older. English speaking. Has a qualifying heart failure indication for center-based cardiac rehabilitation.  Inclusion Criteria - Healthy Control Group:  Adult 18 years and older. English speaking. Absence of pulmonary, cardiovascular, musculoskeletal, neurologic, or orthopedic diseases. Controls will be matched for age and sex.  Exclusion Criteria:  HF patients who are unable to engage in a regularly structured exercise training program as part of a clinically indicated center-based outpatient cardiac rehabilitation program. Participants unable/unwilling to provide informed consent. Uremia, history of allergy to iodides. Impaired renal function. Creatinine value greater than 1.3 mg/dL (via clinical record within the past 6 months). Diagnosis of liver disease.