Zone 2: what it is and why it isn't the same number for everyone

One label, several definitions

As you gradually increase the intensity of exercise, your body passes through two turning points that can be measured in a laboratory, from blood lactate or from the gases you breathe. The first one, the first threshold, marks — according to the authors of one study — the shift to a greater anaerobic contribution, with more use of carbohydrates relative to fat.⁴ Above the second, lactate keeps rising without leveling off, and a steady state can no longer be reached.³ Training zones are built around these two points, but not all in the same way.²

The most striking confusion is in the research itself. Studies of polarized training use the three-zone model, and in them “zone 2” is precisely the band where athletes spend the least time: less than 10%.³ What is popularized today as “zone 2” falls, in that model, within zone 1.²

So before comparing a plan, a study or your watch screen, the first question is which zone 2 is being talked about.

Why a formula can't find your zone 2

Outside the lab, it's common to use a percentage of maximum heart rate,⁴ and that maximum is usually estimated from age.⁵ That stacks two errors on top of each other.

The error in the maximum. “220 minus age” and the formulas that have corrected it estimate the average maximum of people your age, not yours.⁵ In a study of 3,320 healthy adults in Norway who did a maximal test, even the age-based formula that best fit that sample (211 minus 0.64 times age) had a typical error of 10.8 beats per minute, and the earlier formulas fell short from age 30 onward.⁶

The error in the percentage. Even if you knew your exact maximum, the same percentage doesn't land at the same point for everyone. In 100 women and men who did laboratory tests, the first lactate threshold appeared at between 60% and 90% of their maximum heart rate, depending on the person; the authors conclude that fixed percentages don't do a good job of controlling the metabolic stress of exercise.⁷

The authors point out that a difference of just 5% of maximum can mean about 10 beats per minute, and they advise against using fixed percentages instead of a physiological measurement.⁴

How to approximate it without a lab

The reference is an exercise test that measures blood lactate or respiratory gases to locate the first threshold. The authors of the cyclist study propose basing zone 2 on markers measured this way rather than on fixed percentages.⁴

Without a test, there are two simple tools that have support behind them. According to one review, the talk test and the perceived exertion scale are valid measures of the ventilatory threshold: the point at which talking starts to get harder — you still can, but not quite comfortably — corresponds to that threshold, and perceived exertion reflects it too.⁸ If zone 2 is defined as just below the first threshold,¹ it would be at a pace where you can still talk comfortably, close to the point where it starts to get harder. This is our deduction from these two sources, not something that has been measured this way. How to use these tests day to day is covered in how to tell moderate from vigorous intensity without a device.

If you use your watch's heart rate, keep two things in mind. If its zones are based on a maximum estimated from your age, they carry the formula's error. And the optical sensor is less accurate during movement: in a study with 53 adults and six devices, the absolute error was on average 30% higher when walking than at rest, although all of them were reasonably accurate with heart rate at rest or high and sustained.⁹ Which watch data deserve more trust is summarized in the wearables reliability table.

What's known about its benefits

According to a critical review, the popularity of zone 2 comes mainly from observing elite endurance athletes, who train many hours at low intensity and have a large mitochondrial capacity and a large capacity to use fat.¹⁰ That the best athletes train this way doesn't prove that this intensity is the cause, or that it's the best for other people.

In athletes. A meta-analysis of 17 studies with 437 participants compared polarized training (75–80% of the time below the first threshold, 15–20% above the second and less than 10% in between) with other ways of distributing intensity. It improved maximal oxygen uptake slightly more (a small effect), but that advantage only appeared in programs shorter than 12 weeks and in highly trained athletes — two things that went together in those studies — and it gave similar results on the other performance measures.³ These are studies of athletic performance, not health.

Mitochondria. A review with meta-regression that pooled data from 5,973 participants found that the mitochondrial content of muscle increased similarly with continuous low- or moderate-intensity training (23%), with high-intensity training, whether interval or continuous (27%), and with sprints (27%). Per hour of exercise, higher intensities were more efficient, and the improvements were larger in people who started out less fit. The authors conclude that the best predictor is total load: volume times intensity.¹¹ In that review, “low or moderate” includes everything below the second threshold, not just zone 2.

The 2025 critical review concludes that current evidence doesn't support zone 2 as the optimal intensity for improving mitochondrial capacity or the capacity to oxidize fat, and that, especially with low training volumes, higher intensities appear to be important for cardiometabolic benefits.¹⁰ It's a narrative review, not a systematic one. And the expert panel that proposed defining zone 2 as just below the first threshold also cautions that the adaptations they expect from it may not be exclusive to zone 2.¹

If you're not training to compete

The WHO recommendations for adults don't talk about zones: they propose at least 150 to 300 minutes a week of moderate aerobic activity, or 75 to 150 of vigorous activity, or a combination of both, plus strength exercises two or more days a week; and they point out that some activity is better than none.¹² On a personal scale from 0 to 10, the WHO places moderate intensity at a 5 or 6, and vigorous at a 7 or 8.¹²

Seen this way — and this is our deduction, not an equivalence the guidelines make — zone 2, properly understood, is one more way of doing continuous aerobic activity, generally at moderate effort. It can work for you if you enjoy it and it helps you move more, but it isn't a requirement and it doesn't replace strength training, which has its own role; we explain it in why strength matters for independence.

If you're short on time, some higher-intensity work gives you more per minute, at least for mitochondrial content,¹¹ and the WHO notes that people who already do moderate activity regularly can gradually move up to vigorous.¹² Which minimums count when your week is tight is covered in habits when you're short on time.