Nobody Can Agree on What 'Zone 2' Cardio Is

This post is part of Find Your Fit Tech, Lifehacker's fitness wearables buying guide. I'm asking the tough questions about whether wearables can really improve your health, how to find the right one for you, and how to make the most of the data wearables can offer.

“Zone 2” is the term the fitness world has (mostly) agreed upon to describe the low intensity cardio most of us should be doing regularly. When you’re in zone 2, you’re working hard enough that you start breathing more heavily, but easy enough you could hold a conversation doing it. You stop a zone 2 session because your workout time is up, not because you’re too exhausted to continue.

But what heart rate should you expect to see on your fitness smartwatch when you’re in zone 2? That’s where people disagree.

What is zone 2 training?

As I’ve explained before, the name “zone 2” comes from heart rate training. In most of the popular systems, there are five zones. Zone 1 is your resting or recovery zone; zone 2 is low intensity cardio; and zones 3, 4, and 5 are for harder efforts. (I have a guide to the zone system here.)

The zones are usually defined as percentages of your maximum heart rate. So when I set my Apple Watch to keep me in zone 2 during my runs, it wants my heart rate to be between 60% and 70% of maximum. Even at an easy effort, I found I was commonly exceeding that limit. On the other hand, when I hop on a Peloton bike, my heart rate is often still in zone 1 when I could swear I’m riding riding at a zone 2 effort. It turns out that system defines zone 2 as 65% to 75% of my max.

Who is right? Well, everybody. “Zone 2” isn’t a term with a scientific definition. Anybody can split up heart rate zones any way they like. (Stay tuned for my patented eight-zone system, coming as soon as I can find a way to monetize it!) If you train with more than one gadget, or if you find yourself discussing heart rate training with a friend who uses a different system than you do, it’s worth knowing the differences.

What heart rate percentage counts as zone 2?

Let’s take a tour of some of the more popular wearables and fitness systems that measure heart rate in a five-zone system, or something like it.

First, it’s important to know that most (not all) of these percentages are based on your max heart rate. To know your max heart rate, you need to do a real-world test, not just calculate it from a formula. For example, one formula calculates my max heart rate as 178 beats per minute, and another says it should be 169; in reality, I’ve seen as high as 207 when I’m running, and 198 on a spin bike. (And yes, your max can differ for different types of cardio. My heart rate while I’m swimming would probably be lower still; when your body is horizontal your heart has an easier time moving your blood around.)

There are other systems to consider too. “Heart rate reserve” (HRR) means that you take the difference between your max and your resting heart rate (instead of between your max and zero) and calculate from there. Some gadgets will estimate a different benchmark, like your lactate threshold, and use that as a basis for the zones.

So, here are the zone 2 percentages from a variety of popular wearables, along with what they are percentages of:

• Apple Watch: Zone 2 is 60-70% of your heart rate reserve, with your “resting” heart rate set to either 72 or a number the watch has picked up automatically, and your maximum calculated with the 220-age formula. (You can choose to set the zones manually, instead.)

• Garmin: Depends on the model. Some set zone 2 to 60-70% of your max or HRR, but other devices will measure your lactate threshold and set the zone to 80-89% of your threshold heart rate. You can also set the zones manually, and some models allow you to set different heart rate zones for different sports such as swimming, cycling, and running.

• Fitbit: instead of “zone 2,” Fitbit devices have a “fat burn” zone that they define as 50% to 69% of your max heart rate on some devices, and 40% to 69% of HRR on others. With either, your max is calculated according to the 220-age formula. You can set your max and your zones manually if you prefer.

Some other fitness platforms have defined heart rate zones to be used with your training. To name a few:

• Orangetheory gets its name from the “orange” zone they want you to be in during workouts. Their equivalent of zone 2 would be the “blue” zone, at 61% to 70% of max heart rate. They use an “industry standard formula” to determine your max, which Self reports is 208 minus 0.7 times your age. After you’ve taken 20 classes, an algorithm will pick out a new max heart rate for you.
  

• Peloton defines heart rate zone 2 (no relation to Power Zone 2) as 65% to 75% of your max heart rate. Max heart rate is 220-age unless you adjust it manually in your settings.

• The American College of Sports Medicine defines “light” training, arguably their version of zone 2, as 57% to 63% of maximum heart rate. “Moderate” is 64% to 76%.

How do you know which benchmark to use?

Rather than obsessing over numbers, think about the big picture and decide what training effect you are trying to achieve with your workouts.

If you want to build your endurance with low-intensity cardio, or if you want to rack up minutes in this zone to help with weight loss, it doesn’t matter exactly what your heart rate works out to be. What matters is that you can do the exercise for a long time without fatiguing, but that you’re also not sandbagging so badly that you’re barely doing any work at all.

In other words, you can use your gadget’s heart rate numbers as a guide, but keep them honest with a reality check based on what fitness professionals call “perceived exertion.” If you want a number to focus on, you can rate your exertion on a scale of 1 to 10—called RPE for “rating of perceived exertion”—and aim for an RPE of about 3 to 4.

Over time, you’ll start to notice what heart rate tends to show on your watch when you’re at that level. I know that if my heart rate is in the 140’s to low 150’s, I’m doing a good job of keeping my jogging to a “zone 2" sort of effort. If it pokes up into the 160’s at the beginning of a run, that’s probably harder than I’m going for—but if it hits 160 at the end of a long run on a hot day, that’s fine. (Heart rate changes with the temperature and the length of your workout, a phenomenon called cardiac drift.)

Ultimately, this is probably the most accurate way of using heart rate to determine exercise intensity: figure out the intensity you want first, and use heart rate as a guide to be able to hit that same intensity on a consistent basis. After all, if there were one correct number that was easy to determine, the different gadgets and platforms would have all gotten on board with it by now. So trust your body more than your watch.