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You might have thought during an intense workout that your heart couldn’t possibly beat any faster. I mean, you were going all out.

But you probably didn’t reach your maximum heart rate, the fastest you can make your heart go, unless you really overexerted. And even then, technically your heart probably could beat faster. You just wouldn’t want it to.

While you’re exercising, muscles need more oxygen so they can keep up with their energy demands. They can get that oxygen if your heart either pumps more blood or pumps more often, which is why your heart rate jumps as you work out. But no matter how hard you push things, you’ll eventually hit an upper limit based on the physics of your heart.

Before you start, your heart beats just a little bit faster than its resting rate (the minimum speed needed to keep cells humming along while you sleep), which is roughly 60-100 beats per minute in adults. Then, as you work harder and harder, the part of your nervous system that works unconsciously tells your heart to pump faster, anticipating the needs of your body. You’ll get closer and closer to what doctors call your maximum heart rate: the max speed seen when you push yourself really hard.

That’s around 200 beats per minute for a 20-year-old, give or take a dozen beats or so. And you can subtract about a beat per minute for each year after that, because your heart muscle becomes less responsive to those nervous system signals as you age. The thing is, technically, your heart muscle could go a bit faster than it does in practice.

Its contraction speed is ultimately limited by how long it takes to reset after each contraction signal. That signal is an electric stimulus called an action potential, and it’s created by the movement of ions across cellular membranes through a series of channels and pumps. So after one signal is received, the muscle can’t be signaled again until enough of those ions are returned to where they started.

For heart muscle, that period of time is between 200 and 250 milliseconds, so hypothetically, your maximum possible heart rate could be as high as 240 or even 300 beats per minute. But the body stops short of that because the heart needs to pause briefly to actually fill with blood. If your heart muscle were to beat at top speed, it wouldn’t pump as much blood, and therefore wouldn’t deliver the oxygen you need to your muscles, pretty much defeating the whole dang purpose of going faster.

That can actually be pretty dangerous, and it’s usually a bad sign. Luckily, you don’t need to push your heart fully to get the health benefits of exercise, which is why doctors usually recommend shooting for a target heart rate zone of only 50-85% of your max. You can let your smart treadmill or app figure that pace out, or do a back-of-the-envelope calculation assuming your max rate is 220 beats per minute minus your age.

It’s not a perfect estimate, but it’ll give you a ballpark figure to work with. Thanks for asking, and thanks as always to our patrons on Patreon! If you’d like to pose questions like this, vote on questions we’ll answer, or just get some nifty rewards, you can head over to patreon.com/scishow. [ ♪ OUTTRO ].