Iron deficiency may contribute to exercise intolerance by altering mitochondrial oxidative capacity in skeletal muscle. Functional iron deficiency is common in heart failure patients, but the relationship to exercise intolerance and mitochondrial oxidative capacity is unknown. This is a pilot study to determine the feasibility of the use of specialized 31P-MRS and MRI techniques for measurement of skeletal muscle bioenergetics in patients with heart failure with and without functional iron deficiency.