The Messy Truth About Heart-Rate Zones (Including Zone 2)

This is one of two companion pieces to our three-part NiaHealth series on exercise intensity (Part 1: defining intensity, Part 2: why it matters, Part 3: how to measure it). This post asks what your watch's "zones" — including the famous Zone 2 — actually mean. The other companion post asks how accurate the underlying numbers on your wrist really are.

You’ve just finished a run, and your watch says you spent 32 minutes in Zone 2. But what does that actually tell you about the intensity you were working at? And did you get any specific health or performance benefits from that time in Zone 2? The short answers: heart-rate zones are less clear-cut than people make them out to be, and the benefits of Zone 2 in particular aren't quite as impressive as some enthusiasts would have you believe.

First, some context. Training zones have been a fixture in the athletic performance world for years, usually defined by your heart rate — either expressed as a percentage of your maximum heart rate (%HRmax) or of your heart rate reserve (HRR). And as wrist-worn wearables have taken off, that data has become far easier to get, which has made training by heart rate easier too. Most wearables now hand you ready-made zones. One in particular — Zone 2 — has captured a lot of attention.

In this post, we’ll walk through the five-zone classification system used by most wearables and athletic performance experts, highlighting where experts disagree. We'll also zoom in on Zone 2 — and whether there's any evidence to back up the hype. 

The heart-rate-zone problem

As we saw in Part 1 of our exercise intensity series, training zones grew out of the athletic performance world, and they're built around your metabolic thresholds. The original model had just three zones: Zone 1 (below MT1), Zone 2 (between MT1 and MT2), and Zone 3 (above MT2). Over time, dozens of variations appeared. The most popular today is a five-zone model, sometimes called the Norwegian 5-zone scale,¹ which splits effort into Zones 1 through 5 running from well below MT1 up to above your VO₂ max.

Here's the key thing, though: those zones are defined by your thresholds, but in daily use they're rarely measured that way. Instead, they lean on heart rate to determine which zone you're in. That's why "training zones" and "heart rate zones" are often used interchangeably. And it's exactly where the trouble begins, because turning a heart rate into a zone is far less precise than it sounds.

So, how are heart-rate zones set? It turns out, there's a lot of variation. The table below shows how three groups — the American College of Sports Medicine (ACSM), common wearable makers, and a panel of sport scientists — divide heart rate into five zones, based on your percentage of maximum heart rate:

Heart rate (HR) intensity zones used by the ACSM, common consumer wearables, and an expert panel of sport scientists and coaches (percentages are of maximum HR)^1,2

ACSM = American College of Sports Medicine.

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You might notice that the ACSM zones and the zones provided by two common consumer wearables (Garmin and Oura) are somewhat similar. But the consensus from sport scientists and high-performance coaches leans towards much higher percentages of maximum heart rate for a given zone.

So, which of these breakdowns is the “right” one? The short answer: we don't know. These numbers come mostly from expert opinion rather than hard evidence. And the research we do have suggests that none of the zone models shown above lines up neatly with how an exercise is stressing your physiology (e.g. where it places you in terms of your metabolic thresholds - we covered these in Part 1 of the exercise intensity series).

For example, in one study, 350 adults (mostly men) did graded exercise tests so researchers could compare their thresholds with their heart rates.³(Anselmi et al. 2021) At the first metabolic threshold (MT1), the average was 70% of max heart rate in healthy sedentary adults and 69% in athletes. That suggests the top of light intensity (aka Zone 2) should sit around 70% of max heart rate. At the second threshold (MT2), the average was 87% in sedentary adults and 89% in athletes — suggesting the top of moderate intensity (aka Zone 3) should be around 87%.

But the studies don't all agree. An older one found that when people exercised at 80% of their max heart rate, about half were above MT1 and half were below⁴ — which would put MT1 closer to 80% of max heart rate, higher than the ~70% figure above. All of this just shows how rough heart rate zones really are.

One pattern does hold up: athletes tend to reach their thresholds at a higher percentage of maximum heart rate than people who are sedentary or exercise casually. For example, an elite athlete might hit MT2 at 90% of max, while a beginner might hit it at 75–80%.¹ In practice, that means the sport-science zones in the right-hand column may fit athletes best, while the wearable ranges are probably a reasonable middle ground for the average person.

What about heart rate reserve (HRR)?

Some devices sort intensity by percentage of heart rate reserve (%HRR) instead. As we mentioned in Part 3 of the exercise intensity series, HRR takes into account resting heart rate and tends to be more accurate than maximum heart rate, especially in athletes. Here’s the zone breakdown for %HRR (note that the sport-science panel didn't provide a consensus on %HRR zones):

Heart rate (HR) intensity zones used by the ACSM, common consumer wearables, and an expert panel of sport scientists and coaches (percentages are of heart rate reserve [HRR])

ACSM = American College of Sports Medicine. (American College of Sports Medicine, 2022)

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Something to note here: for a given HR, the corresponding %HRR will usually be lower than the corresponding %HRmax. When we compare the ACSM’s %HRmax to their %HRR ranges, the %HRR ranges are lower, as expected. However, when we look at Apple watch and Whoop–both of which report %HRR ranges as a default, the ranges are the same as Garmin and Oura, who default to %HRmax. Again, we don’t have research to tell us which ranges are “better,” but this discrepancy highlights the variation between devices.

As far as supporting evidence goes, one recent study found that exercising at 40% of HRR — the bottom of the ACSM's moderate range — lined up well with MT1.⁵ But in the same study, 59% of HRR — the top of the moderate range — left people about 11 beats per minute below MT2. In other words, the point where the average person crosses into vigorous territory might be closer to 70% of HRR. So, whether you're using the ACSM %HRR zones or the zones provided by an Apple Watch or Whoop, chances are you may be working harder than your watch says at higher intensities.

The bottom line on heart-rate zones

Heart rate zones are a rough guide, and they don’t reliably tell you how a workout is changing your body. That's one reason the ACSM/ESSA joint statement advises against treating heart rate as the definitive measure of intensity.

That said, a general sense of where your heart rate sits relative to your max, or your reserve, is still useful. Paired with how hard the effort feels, it can give you a decent read on how hard you're working. It's a tool in the toolbox — just not the only one.

A closer look at Zone 2

No discussion of training zones would be complete without Zone 2, which has exploded in popularity. You might have seen it praised online as the secret to better metabolism and mitochondrial health (the mitochondria are the tiny energy generators inside your cells).

Here's the evidence-informed view. First, there's actually no single agreed-upon definition of Zone 2, although a recent expert panel of 14 sport scientists and coaches landed on a general consensus: Zone 2 is the intensity range just below your first metabolic threshold (MT1).⁶ As far as public health guidelines go, it’s at the top of the “light-intensity” range. At this effort, your heart rate stays fairly steady, your blood lactate rises only slightly, and you could keep going for a long time. The expert panel agreed that heart rate in Zone 2 tends to fall in the ballpark of 72-82% of maximum (in athletes), but noted that this varies by sport and geographic location–this range is a ballpark, and it’s not evidence-based.

So, we have a general idea of where Zone 2 lies. But why all the buzz? It comes back to the work of scientists Iñigo San-Millán and George Brooks,⁷ who proposed that training at an intensity where blood lactate is just above baseline but holding steady might give a boost to the mitochondria and improve how your body burns fat. Add in the observation that many top endurance athletes do 75–85% of their training just below MT1 (which fits with Zone 2), and interest took off. In theory, Zone 2 lets you pile up training time with less risk of injury and quicker recovery than harder efforts. Interestingly, the original peer-reviewed research by San-Millán and Brooks didn't actually use the term "Zone 2." The label got attached later.

So, is Zone 2 magic? Not quite. A recent review found a lack of evidence that Zone 2 training specifically improves mitochondrial function, leads to better health outcomes, or lowers the risk of death.⁸ And, the expert panel of 14 sport scientists and coaches concluded that, while Zone 2 training may have benefits, you could get similar results training a little harder or a little easier.⁶

There's one more catch that ties back to everything above: because Zone 2 sits right around your first threshold — and that threshold lands at very different heart rates for different people — the "Zone 2" your watch shows you may not be your true Zone 2 at all. If you’re aiming to spend time in Zone 2, balancing your heart rate data with your perceived exertion might help: an effort of around 4 out of 10 where conversation is comfortable tells you that you’re in the right place.

The bottom line: although training at Zone 2 might provide unique health benefits, there’s surprisingly little evidence to support these claims. This is a scenario where more research is needed and might genuinely change recommendations. Based on what we know now, training in Zone 2 appears to be a relatively comfortable, low-injury way to build a base of fitness–but we don’t have evidence that it’s the one true path to health and fitness. If you are looking to train in Zone 2, relying on a combination of heart rate and perceived exertion might help get you there.

The bottom line: although Zone 2 is often said to have unique benefits — especially for metabolism and mitochondrial health — there's surprisingly little evidence to back that up. This is a case where more research is genuinely needed, and could change the recommendations. Based on what we know now, Zone 2 looks like a (relatively) comfortable, low-injury way to build a base of fitness — but there's no evidence it's uniquely powerful compared with training a little harder or a little easier. If you do want to train in Zone 2, a combination of heart rate and perceived exertion is your best bet for finding it. 

The takeaway

So what do your watch's heart-rate zones actually mean for you? Less than the tidy labels might suggest. When it comes to giving you an accurate measure of exercise intensity: the zones your device shows come largely from expert opinion, they vary from brand to brand, and none of them line up cleanly with your personal metabolic thresholds — so a reading like "32 minutes in Zone 2" is a rough estimate, not a precise measure. When it comes to Zone 2 specifically: there's surprisingly little evidence it's uniquely powerful compared with training a little harder or a little easier — and your watch's Zone 2 may not even be your true Zone 2.

None of this means zones are useless. Treat them as a ballpark, not the truth: use your heart-rate zone as a general gauge, and cross-check it against how hard the effort feels. 

(And if you're wondering how accurate the underlying heart-rate number is in the first place, that's the subject of our companion post on wearable accuracy.)

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This article is for general education and isn't a substitute for personalized medical advice. Talk to a qualified health professional before starting a new exercise program, especially if you have an existing health condition.

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