The heart failure syndrome that occurs when the heart is too sick to properly do its job. One of the main symptoms is difficulty with exercise. One way to improve symptoms is to start patients in a 12 week exercise program called cardiac rehabilitation. Cardiac rehabilitation been shown to improve symptoms for heart failure patients. However, the investigators do not know exactly what exercise does to the molecules that make up the human body. If the investigators could answer this question, the investigators might find a whole new way to treat the symptoms of heart failure. Therefore the investigators want to know what molecules might be responsible for the benefits of exercise. The plan for this study is to measure the levels of thousands of proteins in blood samples which come from people with heart failure and see how those levels change after 12 weeks of cardiac rehabilitation, compared to the protein levels in patients whose cardiac rehabilitation is delayed until after the study period. If the investigators know the proteins that change with exercise, the investigators can then look to see if targeting these proteins with medicines can mimic the benefits of exercise. The long term goal of our work is to identify exercise-in-a-pill medicines that will help people with heart failure.