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Muscles in the heart are continually undergoing cellular respiration, which requires oxygen, to provide heart muscles with energy (i.e. ATP) to pump. The heart also needs to continually pump so that other muscles in the body obtain their required source of oxygen. If the heart does not get its oxygen source, heart muscles will get damaged and eventually the heart will stop pumping all together. Muscles in your legs for instance, are able to rest whenever you're not running or walking, but the heart never rests, it's always beating until the moment of death.