Your heart is an incredibly powerful organ. Every day it beats about 100,000 times. Imagine the energy takes it to squeeze a tennis ball – that’s not far off the force your heart is using each time it beats to pump blood around your body. So, with that many reps each day, why doesn’t it burn out?

While your heart is a muscle, it’s not quite the same as your skeletal muscles – such as the biceps and quads – that are attached to your bones. This is primarily because the heart is made of cardiac muscle, consisting of special cells called cardiomyocytes. Unlike other muscle cells in the body, cardiomyocytes are highly resistant to fatigue. True, cardiomyocytes are primarily powered by mitochondria (the energy house of the cell), similar to your other muscles. However, cardiomyocytes have as much as 10 times the density of mitochondria, skyrocketing their energy output.

The cardiomyocytes have also evolved to have an enhanced blood supply, being better than ordinary muscle cells at extracting oxygen from the blood. Plus, the heart has another secret weapon: it’s flexible in terms of fuel, able to consume glucose, free fatty acids and lactate.

So, why aren’t all muscles made of cardiomyocytes? For all their benefits, cardiomyocytes contract without nerve supply, making them incapable of voluntary and purposeful movements. But when it comes to beating constantly and without tiring, your heart has evolved to do its job brilliantly – it has no time for DOMS (delayed onset muscle fatigue) after a hard workout. Which is fortunate, because, when it comes to your cardiac muscles, rest days are not an option.

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Asked by: Ben Morris, York

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Authors

Dr Nish Manek

Dr Nish Manek is a GP in London. She completed her medical degree at Imperial College and was runner-up in the University of London Gold Medal. Manek has also developed teaching courses for Oxford Medical School, and has penned articles for The Guardian and Pulse magazine.

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