At its normal resting rate of 60 to 100 beats per minute, the heart must contract and relax more than 100,000 times a day without stopping or tiring. To achieve this extraordinary feat, electrical signals generated by the heart’s pacemaker cells trigger the synchronized contractions of heart muscle cells (myocardiocytes) that pump blood into systemic circulation. Myocardiocytes require a steady supply of energy to sustain their continuous contractions. As such, they contain numerous mitochondria that produce energy in the form of adenosine triphosphate (ATP). Mitochondrial respiration, a series of metabolic reactions known as the Krebs cycle, converts nutrients into ATP within mitochondria. Heart muscle health correlates directly with the structural and functional health of cardiac mitochondria. Oligomeric proanthocyanidins (OPCs) and flavonoids, phenolic compounds found in hawthorn leaf, flower, and berry, strengthen heart muscle contractions by slowing the sodium-potassium pump, a complex enzyme that sustains cardiac electrophysiology. As free radical scavengers, OPCs and flavonoids protect cell membranes and mitochondrial structures from oxidative damage while boosting the natural production of antioxidant enzymes such as superoxide dismutase. Flavonoids enhance the uptake of oxygen by mitochondria—increasing their overall ATP productivity.