The heart is made up of cardiac muscle. Cardiac muscle has the longest resting period. It never ceases. Normally, when metabolic waste (lactic acid) accumulates in a cell, it is exhausted. But because the cardiac muscle has a long resting period, metabolic wastes are released more easily. In addition, the heart muscle directly uses lactic acid as a nutrient. Therefore, waste cannot accumulate in the heart muscle. So the heart is sometimes exhausted.