The objective of this study is to assess by what physiological mechanisms patients with heart failure benefit from exercise. Effects of an exercise intervention will be assessed for both central (heart and lungs) and peripheral (muscle fiber and mitochondria) factors.

A randomized, non-blinded prospective intervention trial. Patients randomized to exercise intervention will undergo supervised aerobic training on ergometer cycle for 12 weeks whilst control group receives standard care. Before and after the intervention, all participants will undergo cardiac magnetic resonance imaging (cardiac MR) examinations, Ventilation/Perfusion Single Photon Emission Computed Tomography (V/P SPECT), Cardiopulmonary exercise tests, muscle biopsies and blood sampling.

Inclusion Criteria:

1. Signs and symptoms of heart failure
2. Age ≥50 years
3. Stable chronic heart failure (no hospitalizations within past month, no new medications past month)
4. Able to understand and follow exercise prescription
5. Currently physically inactive (less than 150 minutes/week moderate intensity exercise)

Exclusion criteria:

1. Currently or within the past 6 months been involved in cardiac rehab with exercise
2. Devices or implants not compatible with magnetic resonance imaging or exercise testing
3. Any orthopedic or medical condition affecting participation in testing and training
4. Signs of acute ischemia during baseline cardiopulmonary exercise testing
5. Participation in other clinical trials that may affect results
6. Moderate or severe valvular disease
7. Atrial fibrillation
8. Moderate or severe chronic obstructive pulmonary disease
9. NYHA class IV