Heart Rate Zones: What the Research Actually Shows About Training by Heart Rate

Key Takeaways: Heart Rate Zones

• Heart rate zones translate training intensity into a number you can follow during exercise, but each zone is only as accurate as the anchor it is calculated from.
• Age-based maximal heart rate formulas showed a standard error of 11.4–12.4 bpm in a large study of sedentary adults, and that error is likely to carry over into zones calculated from them.
• Even with a measured HRmax, fixed percentages place different people in different physiological intensity domains: in one study of 100 adults, the lactate threshold occurred anywhere between 60% and 90% of HRmax.
• Zones anchored to individually measured thresholds (lactate or ventilatory) have better support for separating easy from harder work than generic percentages.
• “Zone 2” is not one universally defined intensity, and current evidence does not show that it is superior to harder training for mitochondrial or fat-oxidative capacity.
• Heart rate drifts upward during longer sessions and in the heat; holding a fixed heart rate target then means the workload has to fall.
• In one comparison study, a Polar H7 chest strap tracked ECG-measured heart rate more closely than the tested optical wrist- and forearm-worn devices, whose accuracy varied with the type of exercise.
• In my clinical experience, heart rate zones are rarely discussed among doctors, and they may not apply in the usual way to patients taking beta-blockers, whose heart rate does not rise in the same way during exercise.

Introduction: Heart Rate Zones

Almost every sports watch now divides a workout into colour-coded heart rate zones. The appeal is obvious: a single number on the wrist seems to tell you whether a session is easy, moderate or hard. The research behind those percentages, however, is less tidy than the colours suggest. In a study of 100 women and men who had their maximal heart rate measured directly, the lactate threshold occurred anywhere from 60% to 90% of HRmax, and the maximal lactate steady state anywhere from 75% to 97% of HRmax.[1] The authors concluded that prescribing intensity as a fixed percentage of a maximum fits poorly with the underlying physiological intensity domains and does not reliably control the metabolic stimulus.[1]

In practice, this means that the same zone may be genuinely easy for one person and already relatively demanding for another.

When I was training, before my medical education, I judged my own sessions largely by how they felt. A workout hardly felt like training unless my heart rate rose and I was clearly out of breath; in practice, breathlessness was my main gauge of a good session. That is an intuitive way to judge effort, but it tends to undervalue easier work. Only later did I come to recognise the importance of low-intensity aerobic training and of varying the type of training, even though easy sessions still did not feel like training in quite the same way.

This article covers how heart rate zones are calculated, why age-based and percentage-based zones often miss the mark, what threshold-based zones add, what the evidence says about “Zone 2”, and how heat, session length and the measuring device itself affect the number you see.

What Are Heart Rate Zones?

Heart rate zones divide the exercise intensity spectrum into bands, each defined by a heart rate range. Meixner and colleagues describe three-zone, five-zone and seven-zone models as the most commonly used frameworks, depending on the sport and discipline.[2] The boundaries of those bands are usually set in one of three ways:

• Percentage of maximal heart rate (%HRmax). Each zone is a fixed percentage of an estimated or measured maximum.
• Percentage of heart rate reserve (%HRR, the Karvonen method). Heart rate reserve is the difference between maximal and resting heart rate; a target is calculated as resting heart rate plus a chosen percentage of that reserve. Because it uses resting heart rate as well, this method also depends on an accurate resting heart rate.
• Physiological thresholds. Heart rates at the first and second ventilatory or lactate thresholds, measured in a graded exercise test, are used as zone boundaries. In the classic three-zone model, the first and second ventilatory thresholds delineate Zone 1 (below the first threshold), Zone 2 (between the thresholds) and Zone 3 (above the second threshold).[3]

The American College of Sports Medicine (ACSM) position stand gives an approximate classification for general exercise prescription: moderate intensity corresponds to 64–76% of HRmax or 40–59% of HRR, and vigorous intensity to 77–95% of HRmax or 60–89% of HRR.[4] The same position stand notes that no studies have compared all intensity methods simultaneously, so one method cannot be assumed to be equivalent to another.[4]

There is also a terminology trap. In the three-zone model, “Zone 2” lies between the two thresholds,[3] whereas in a five-zone model, Zone 2 refers to low-intensity training.[2] The same label can therefore describe two physiologically different intensities.

It is also worth being clear about what heart rate zones are. In my experience, heart rate zones are not really a medical concept: they are rarely discussed among doctors, they are not part of a physician’s everyday vocabulary, and in a basic primary care practice they have little clinical significance. The concept is used mainly in sports circles, coaching, sports watches and popular media, and in my practice it comes up mostly when patients raise it themselves. A heart rate zone is not in itself a measure of performance. In my experience, when doctors discuss exercise intensity, the focus is usually on what happens in metabolism at the cellular level: the intensity at which the body shifts from relying mainly on fatty acids toward carbohydrate, the range in which carbohydrate is still used aerobically, and the threshold at which anaerobic energy production begins to rise. These physiological intensity thresholds can be assessed with blood lactate responses or with ventilatory and gas-exchange measurements during exercise testing, and fat oxidation can also be estimated from gas-exchange measurements. From a clinical perspective, a heart rate zone is therefore best understood as an indirect stand-in for these thresholds.

Where Heart Rate Zones Start: The Max Heart Rate Problem

One common way to calculate heart rate zones is from an age-predicted HRmax. In a meta-analysis by Tanaka and colleagues, HRmax was strongly related to age (r = −0.90), producing the equation 208 − 0.7 × age; the regression did not differ between men and women and was not affected by wide variations in habitual physical activity.[5]

A strong population-level relationship does not, however, mean an accurate individual estimate. In sedentary Black and White adults from the HERITAGE Family Study, the standard error of estimate was 12.4 bpm for the traditional 220 − age formula and 11.4 bpm for the Tanaka formula, and the authors concluded that these prevailing age-based equations do not precisely predict an individual’s measured HRmax.[6]

To illustrate with simple arithmetic (not a figure reported in the study): an 11.4 bpm error in HRmax shifts a zone boundary set at 70% of HRmax by about 8 bpm. Every zone above and below moves with it. The ACSM position stand makes the same point more formally — the accuracy of both %HRmax and HRR methods may be influenced by how maximal heart rate is estimated, and direct measurement of heart rate and oxygen uptake is recommended for greater accuracy when individualizing exercise prescription, with estimation considered acceptable when that is not feasible.[4]

The relationship between age and HRmax, and why a high or low maximum is not itself a fitness score, is covered in more detail in Max Heart Rate and Age.

Medication is another reason why heart rate zones do not apply to everyone. In my clinical practice, some patients use beta-blockers, for example for atrial fibrillation or to calm extra heartbeats (ectopic beats). Patients with hyperthyroidism are often on propranolol, and in my experience a beta-blocker is also commonly on the medication list of patients who have had a myocardial infarction or have coronary artery disease. In these patients, heart rate does not rise in the same way during exercise, so standard heart rate zones may not be applicable. Many of my patients describe exercising on a beta-blocker as feeling as if the handbrake were on: when walking uphill, the power just is not there, and they cannot get everything out of their body. In practice, exercise is generally still possible with a beta-blocker, but maximal heart rate is limited, and zones calculated from age-based formulas may not hold for these patients.

Why Percentage-Based Heart Rate Zones Often Miss the Mark

Measuring HRmax directly solves only part of the problem. In the study described above, HRmax was measured in all 100 participants, and the percentages of HRmax at which the lactate threshold and the maximal lactate steady state occurred still varied widely; both also occurred at a higher %HRmax in women than in men.[1]

A 2020 review in Sports Medicine reached a similar conclusion across methods. Despite their common use, fixed percentages of maximal anchors — including VO₂max and HRmax — were judged to have little merit for producing distinct, domain-specific physiological responses (the moderate, heavy and severe intensity domains).[7]

Training status adds another layer. In a large cross-sectional treadmill study, the percentage of HRmax at which the first and second ventilatory thresholds occurred was higher in people with a higher training status, while HRmax itself did not differ between training-status groups.[8] The authors concluded that setting intensity as a fixed percentage of maximum may not match intensity across individuals with different training status.[8]

The ACSM position stand notes that the HRR method may be preferable because %HRmax can underestimate or overestimate intensity, although this advantage has not been supported by all studies.[4] It also lists factors that change the relationship between heart rate and actual exercise intensity, including the test protocol, exercise mode, resting heart rate, fitness level, age and body composition.[4]

In my view, a high heart rate is generally a rather poor measure of how hard a training session is. I noticed this myself when I wanted a standardised measure of interval intensity: wearing a chest-strap heart rate monitor, I performed each interval hard enough for my heart rate to reach about 170 bpm. At the time, chest straps were the main option: wrist-based monitors did not work well, and counting the pulse by hand in the middle of an effort was not possible. Today there are more devices that measure heart rate from different parts of the body. Over the following weeks, reaching that number became steadily harder, both physically and subjectively, because my fitness was improving in the meantime. At least in my own case, high heart rates were easier to reach in poorer condition. A fixed heart rate target therefore did not measure training intensity objectively, and in hindsight lactate would probably have been the better tool for standardising how hard the sessions were.

Threshold-Based Heart Rate Zones

The alternative is to anchor zones to the individual’s own physiology. The 2020 Sports Medicine review found some evidence supporting the first lactate threshold, the gas exchange threshold and the ventilatory threshold as markers of the boundary between the moderate and heavy intensity domains.[7] For the boundary between the heavy and severe domains, the evidence was weaker: apart from critical power and critical speed, there was little support for the validity of the commonly used methods.[7] The authors also emphasized that acute responses do not always predict long-term adaptation, so training studies are still needed.[7]

Threshold-based heart rate zones have a long history in endurance research. Seiler and Kjerland tested 11 well-trained male junior cross-country skiers, used the first and second ventilatory thresholds to define three zones, and quantified their training with continuous heart rate registration. Across 318 endurance sessions, heart-rate-based analysis placed 75 ± 3% of sessions in Zone 1, 8 ± 3% in Zone 2 and 17 ± 4% in Zone 3, a distribution similar to that obtained with session ratings of perceived exertion.[3] The authors noted that elite endurance athletes appear to train surprisingly little at the lactate threshold intensity.[3] How this “polarized” pattern compares with other training distributions is discussed in Polarized Training.

In my view, it is important to do varied training. Training that is too one-sided is generally not ideal, not least for recovery. Training at high intensity all the time can also be problematic: the neuroendocrine system needs its own time to recover, and under constant high-intensity load it may become fatigued. For that reason, the rule of thumb I tend to use is that much, and perhaps even most, training is lower-intensity aerobic work, with roughly one or two hard, interval-type sessions per week. How often athletes train at high intensity is discussed in more detail in How Often Should Athletes Train at High Intensity?.

This broadly matches how I trained during my competitive years. In the pre-competition phase, I sparred one or two times per week, and the rest of the training was light. Base-building periods were different: more low-intensity training and strength training, and simply a lot of it. When the high-intensity phases came, training volume decreased, but the work became more intense.

Threshold-based zones are not permanent either. In the cross-sectional treadmill study, heart rate at the ventilatory thresholds was generally higher in individuals with higher training status (although in women, heart rate at the second threshold did not differ significantly between training-status groups), but the authors cautioned that this was a cross-sectional finding and that earlier longitudinal studies had not shown significant changes in threshold heart rates over a single season.[8] How often thresholds need re-testing therefore remains an open question. What lactate testing measures, and how the two thresholds are defined, is explained in Lactate Testing for Athletes.

VO₂max adds a different piece of information: it reflects maximal oxygen uptake capacity, which is not the same thing as the lactate thresholds. VO₂max testing is often the more practical option. In my experience, though, if the aim is to be truly precise, lactate measurement may be one of the most informative ways to characterise an athlete’s individual physiological thresholds. It can also give an idea of an athlete’s physiological profile: how the lactate response changes as exercise intensity rises and the contribution of anaerobic metabolism grows.

Zone 2 Heart Rate: What the Evidence Does and Doesn’t Support

“Zone 2” has become the most discussed of all heart rate zones. A 2025 study by Meixner and colleagues examined the variability of commonly used Zone 2 intensity markers in 50 experienced cyclists (30 men and 20 women).[2] The coefficients of variation of the different Zone 2 markers ranged from 6% to 29%. In the abstract, the authors reported strong alignment between the first ventilatory threshold (VT1) and maximal fat oxidation (FatMax), whereas fixed percentages of HRmax and blood lactate thresholds showed wide individual differences.[2] In the full results, however, FatMax was generally about 25% lower than VT1, and the authors described the two markers as representing distinct intensity domains.[2] The authors suggested that individualized Zone 2 prescription based on physiological measurements such as the first ventilatory threshold and FatMax may describe intensity more accurately.[2]

The benefits attributed to Zone 2 have also been questioned. A 2025 narrative review in Sports Medicine by Storoschuk and colleagues did not find substantive evidence that Zone 2 is superior to higher exercise intensities for improving mitochondrial and fat oxidative capacity, and the authors challenged the broad endorsement of Zone 2 training for the general public.[9] This does not mean low-intensity training is useless; it means the claim that Zone 2 is a uniquely effective intensity is not supported by the evidence reviewed.

When Heart Rate Zones Drift: Heat and Session Length

Even a perfectly calibrated zone is a moving target during exercise. Cardiovascular drift is a progressive rise in heart rate over time despite an unchanged work rate; when it occurs, holding a target heart rate requires lowering the work rate, which can reduce the training stimulus.[10] The same authors note that cardiovascular drift typically appears after 10–15 minutes of exercise.[10] During continuous exercise in the heat, this progressive rise in heart rate is accompanied by a decrease in stroke volume,[10] a topic covered in Stroke Volume.

A 2025 study of 8 adults performing interval training at 35 °C illustrates the trade-off. When intensity was guided by target heart rate, work rate fell by 46 ± 29 W from the first to the fifth work interval; when it was guided by perceived exertion, it fell by 30 ± 28 W. The decline was therefore about 53% larger with heart rate guidance, although this difference did not reach statistical significance.[10] The price of the RPE-guided approach was greater cardiovascular strain: heart rate rose by 12 beats per minute from the first to the fifth work interval, and VO₂max measured after the session fell by 15.6% compared with the shorter RPE trial, whereas the 6.5% decline in the heart-rate-guided trials was not statistically significant.[10]

In other words, in this small study, holding a fixed heart rate target in the heat limited the rise in heart rate but required a larger reduction in work rate, while effort-based pacing was associated with a numerically smaller drop in work rate and more cardiovascular drift; the difference in work rate between the two approaches was not statistically significant. Which matters more is likely to depend on the purpose of the session.

Wrist or Chest: How Accurate Is the Number in Your Heart Rate Zone?

Zones also depend on the device. In a study comparing wearable monitors against an electrocardiogram across several types of aerobic exercise, the chest strap (Polar H7) showed the best agreement with ECG (concordance correlation coefficient rc = 0.996), followed by the Apple Watch (rc = 0.92), TomTom Spark (rc = 0.83) and Garmin Forerunner (rc = 0.81); the Scosche Rhythm+ and Fitbit Blaze were less accurate (rc = 0.75 and rc = 0.67).[11] Optical monitors performed best on the treadmill and worst on the elliptical trainer, where none of the devices was accurate when the arm levers were used.[11] The authors concluded that electrode-containing chest monitors should be used when accurate heart rate measurement is essential.[11]

These devices were tested in 2017, and current models may perform differently, but the principle remains: the measurement method affects which zone the screen reports.

Heart Rate Zones vs. Perceived Effort and the Talk Test

Perceived exertion and the talk test are low-cost alternatives or complements to heart rate zones. The ACSM position stand states that the Borg RPE scale, the Talk Test and related tools can be used to modulate or refine prescribed intensity, but that the evidence is insufficient to support them as a primary method of exercise prescription; the Talk Test was described as moderately associated with cardiorespiratory exercise intensity measures such as ventilatory threshold, heart rate, oxygen uptake and blood lactate.[4]

A 2018 review in Kinesiology took a more favourable view, reporting that the Talk Test appears able to identify intensities closely associated with the ventilatory and respiratory compensation thresholds without a maximal exercise test, and that it appears to work across populations ranging from college students and healthy adults to elite athletes and patients with chronic diseases.[12]

As the heat study above shows, effort-based and heart-rate-based guidance are not interchangeable: under heat stress they were associated with different heart rate responses and numerically different reductions in work rate.[10]

Heart Rate Zones in Practice

Taken together, the research suggests that zones are most informative when their limitations are understood:

• Generic zones are estimates. Zones built on age-predicted HRmax inherit the formula’s error, and fixed percentages of HRmax place different people in different physiological domains even when HRmax is measured.[1][6]
• Threshold-based zones are better supported for the boundary between moderate and heavier work, while the evidence for locating the upper boundary is weaker.[7]
• “Zone 2” needs a definition. Its meaning differs between three-zone and five-zone models, and percentage-based Zone 2 markers vary widely between individuals.[2][3]
• Heart rate rises during long or hot sessions even when the workload stays the same.[10]
• The device matters. In one study, the tested optical wrist- and forearm-worn sensors were less accurate than a Polar H7 chest strap during several types of exercise.[11]
• Effort-based tools can complement heart rate but are not considered sufficient on their own as a primary prescription method in the ACSM position stand.[4]

Despite these limitations, I see heart rate zones as a fairly handy construct for non-specialists. From a clinical perspective, however, I would not fixate on any single zone; varied training across different heart rate zones makes more sense to me. Interval-type athletes cannot train at interval intensity all the time and may benefit from lighter, lower-intensity aerobic work alongside a couple of hard interval sessions per week, as described above. Likewise, endurance athletes may benefit from occasionally taking their heart rate above the aerobic level.

Conclusion: Heart Rate Zones

Heart rate zones are a practical way to give structure to training, but the percentages on a watch are approximations rather than physiological boundaries. Age-based HRmax formulas had a standard error of 11.4–12.4 bpm in a large study,[6] and even a measured HRmax did not prevent the lactate threshold from falling anywhere between 60% and 90% of HRmax.[1] Zones anchored to individually measured thresholds have better support, Zone 2 is less clearly defined and less uniquely effective than its popularity suggests, and heart rate itself drifts with heat and duration.

This matches what I see in practice: heart rate zones are a handy construct for non-specialists, but the more meaningful markers in medicine are the metabolic thresholds measured with lactate, and in patients on beta-blockers the usual zones may not hold. From my own competitive years, I also know how misleading a fixed heart rate target can become as fitness improves.

In my view, used with these limits in mind, heart rate zones remain a useful tool — not as fixed truths about intensity, but as a starting point that becomes more meaningful the more closely it is tied to the individual’s own physiology.

References

1. https://doi.org/10.1249/MSS.0000000000002147
2. https://doi.org/10.1155/tsm2/2008291
3. https://doi.org/10.1111/j.1600-0838.2004.00418.x
4. https://doi.org/10.1249/MSS.0b013e318213fefb
5. https://doi.org/10.1016/s0735-1097%2800%2901054-8
6. https://doi.org/10.1002/ajhb.22431
7. https://doi.org/10.1007/s40279-020-01322-8
8. https://doi.org/10.1007/s00421-024-05622-z
9. https://doi.org/10.1007/s40279-025-02261-y
10. https://doi.org/10.3389/fphys.2025.1506325
11. https://doi.org/10.1249/MSS.0000000000001284
12. https://ojs.srce.hr/kinesiology/article/view/6349