Your heart muscle does something truly incredible-it expands and contracts, non-stop, every moment of every day of your entire life. By comparison, if you tried to squeeze and release the muscles in your hand, they would grow fatigued and need to rest probably within an hour. Yet the heart never rests-it can't let up beating for even one day of your life. How is this feat achieved?
One answer is that the "cardiac" muscle that comprises the heart is of a different kind than the "skeletal" muscle comprising the hand. Skeletal muscles are attached to bone structures and cannot stay long in a flexed position without depleting their energy reserves. Those energy reserves come from mitochondria: structures inside the cells that use the energy taken in from food. Thus the more mitochondria it has, the greater the available energy for the muscle.
Because it has not been necessary in the course of evolution for humans to be able to flex our skeletal muscles for prolonged periods of time, the total volume of skeletal muscle contains an average of only 1 to 2% mitochondria. This is an entirely sufficient energy source for such intermittent muscular tasks as walking or running. The total volume of the heart, by contrast, is between 30 and 35% mitochondria.
That massive amount of energy-generators means cardiac muscle, in a healthy state, need never rest: there is always some energy being transferred to the muscle at the same time that more energy is being derived from caloric intake. And always just in time for that next beat.