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Why Doesn’t Your Heart Get Tired?

Skeletal muscles are attached to bone structures and cannot stay long in a flexed position without depleting their energy reserves.

People running in marathon

Photo: Marcos Vasconcelos Photography (flickr)

Our hearts must keep beating even though our muscles get tired

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.

  • seshansundaram

    its not only the energy that makes the difference ………..
    fatigue is caused due to lactic acid accumulation……….heart muscles also produce huge amounts of lactic acid but this is removed / destroyed effectively.

  • seshansundaram

    its not only the energy that makes the difference ………..
    fatigue is caused due to lactic acid accumulation……….heart muscles also produce huge amounts of lactic acid but this is removed / destroyed effectively.

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  • http://pulse.yahoo.com/_44UKUQ5FVSHPE2Q5BBFRUMBWHQ ColinM

    No, lactic acid accumulation only happens during lactic acid fermentation, which is an anaerobic process, meaning it only happens when there is a lack of of oxygen to the mitochondria, impeding the process of the electron transport chain (with oxygen as the final recipient).  Because the heart has no problem obtaining sufficient oxygen to power the ETC in its mitochondria (being the direct recipient of oxygen rich blood from the pulmonary vein), heart muscle tissue never accumulates lactic acid. 

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