Heart rate when running: when to go easy, and when hard

Your heart rate when running is a good measure, but it isn't as precise as your watch makes it look. It responds to much more than your pace, your watch measures it worst exactly when you run fast, and the formulas for maximum heart rate can be off by many beats. Here's what your heart rate actually tells you, what the research says about watches and the talk test, three ways to set your pace, and when heart rate is a sign to stop.

Your heart rate responds to more than your pace

Run the same loop at the same pace two days in a row and your heart rate can differ noticeably. That's not strange. Your heart rate responds to everything your body is dealing with: heat and humidity, how much you've slept, stress from work, the coffee you drank, a cold coming on. It also rises by itself during a long run, even if the pace stays the same.

That makes heart rate a measure of how hard your body is working today, not of how fast you are. A day when your heart rate is unusually high for a pace that usually feels easy is often a day when your body already has a lot on its plate. It's a signal to run easier, not to push on.

The formulas for maximum heart rate, such as subtracting your age from a fixed number, give an average across many people. For any one person, the real maximum can be far off. Zones calculated from a formula can therefore be wrong from the start.

Your watch is least accurate when you run fast

Most heart rate watches shine light through the skin at your wrist. In 2020, Bent and colleagues tested several watches and wristbands, both consumer and research devices, at rest and during different activities. They found no clear difference in accuracy between skin tones, but clear differences between the watches and between the activities. During activity, the error was on average about a third larger than at rest.

The watches were reasonably accurate at rest and when heart rate stayed high for a longer stretch. They differed when heart rate changed quickly. For running, that means your watch works best on steady, easy runs and worst in intervals, where your heart rate jumps up and down. There, a chest strap usually tracks the changes better.

The talk test holds up

The oldest way to set your pace is also one of the best studied. In 2014, Reed and Pipe reviewed the research on the talk test and found it's a valid and reliable way to guide how hard you train, for athletes, regular exercisers and heart patients alike. If you can talk comfortably, you're usually below the point where your breathing starts to take over. If you can't, you're above it. The test gave the same results whether people were walking, jogging or cycling, but it's less practical for short, very hard intervals.

So the talk test measures the same thing heart rate zones try to measure, but directly in your own body, today, without a watch.

Three ways to set your pace

No single method is best for everyone. The best one is whichever tells you something on the run you're doing right now.

The talk test. For you if you want to keep it simple, or you're just starting to run. On easy runs, run so you can talk in full sentences. If you can sing, you can speed up. If you can only get out the odd word, it's a hard run. First step: say a full sentence out loud every other kilometer on your next run, and slow down if it won't come out.

Heart rate zones. For you if you like data and want to follow your fitness over time. Build your zones on a measured maximum heart rate or your watch's own test, not on a formula, and use a chest strap for intervals. Check your zones against the talk test: if you can't talk in the zone that's supposed to be easy, that zone is wrong for you. First step: run your next easy run in your watch's easiest zone and see if you can talk the whole way.

Feel on a scale. For you if you run without a watch, or in heat and on hills where your heart rate runs away from you. Give each run a number from one to ten for how hard it felt. Easy runs sit around three or four, hard ones around seven or eight. First step: right after your next run, write down a number for how hard it felt in your training log.

Easy most of the time, and when heart rate says stop

The most common mistake is running every run at the same in-between pace: too hard to be easy and too easy to be hard. A proven approach is to run most of your runs easy and one run a week harder, for example intervals or hill repeats. If you want more of a challenge, add a second hard run only once the first feels routine, and never do two hard days in a row.

It's normal for your heart rate to go up when you run and come down when you slow down. What isn't normal should be taken seriously. Stop and contact a doctor if you get:

• chest pain, pressure or discomfort in the chest
• dizziness, feeling faint, or your vision going dark
• palpitations or an irregular pulse that doesn't go away when you stop
• shortness of breath out of proportion to how hard you've run

If it's an emergency, call your local emergency number. If you have a heart condition or high blood pressure, or take medication that affects your heart rate, talk to a doctor before you start running or plan your training around heart rate zones.

A first step today: run your next outing entirely at talk-test pace. If you can't do it without walking in between, walk in between.

If you want your running to last, the next part of the series is about strength training for runners.

This is the understanding and a pace to start at. Atlas is where it becomes a habit. The brisk walk is built on the same talk test and is a good start if you haven't run in a long time, and you can add your own run as an exercise of your own. You log every time and see the weeks fill up, not a single fast day. Try Atlas free for 7 days, and cancel whenever you want.