Zone 2 Heart Rate Training: Promote Endurance and Longevity

Take your fitness low and slow—Zone 2 heart rate training unlocks endurance gains and may hold the key to a longer life.

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By Staff Writer
Jovan Mijailovic
Edited by Jovan Mijailovic

Updated April 8, 2024.

A group of people doing zone 2 heart rate training on a treadmill.

Zone 2 heart rate training has recently gained attention from athletes and world-renowned scientists as a way to improve endurance and longevity. With many wearable devices featuring this metric, tracking your training is easier than ever.

InsideTracker scientists recently reviewed the studies behind zone 2 heart rate training and found evidence that supports the use of this technique for improving aerobic capacity. But we didn't stop there. 

Here’s what you need to know about zone 2 heart rate training and how to incorporate it into your exercise regimen today.

What is zone 2 heart rate training? 

Zone 2 heart rate training is a low-intensity workout at which breathing is still easy and comfortable. Ideally, you should be able to hold a conversation while doing it and maintain the pace for hours. This has a more significant impact on higher-intensity endurance performance. 

Note: Zone-based training is a methodology that classifies workouts based on aerobic intensity.

Dr. Howard Luks—a top orthopedic surgeon and sports medicine specialist— promotes zone 2 as a way to improve performance and longevity. He says that “most elite athletes spend 90% of their training in low [heart rate] zones.”

But people won't easily follow this type of workout. He notices that “most average runners run too fast on their slow days and too slow on their fast days.” This can lead the body to inefficiently use energy reserves, resulting in a minimal improvement in performance.

A quote from Dr. Howard Luks about Zone 2 heart rate training.

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How does zone 2 training work?

Zone-based heart rate training is typically broken down into five intensities, depending on the percentage of maximum heart rate you're supposed to achieve.

ZonePercentage of max heart rate
Zone 1 (Active recovery)50–60%
Zone 2 (Endurance)60–70%
Zone 3 (Tempo)70–80%
Zone 4 (Threshold)80–90%
Zone 5 (VO2 max)90% and above

But before starting zone 2 heart rate training, you should understand the three critical physiological concepts: substrate use, aerobic threshold, and mitochondrial function. [1]

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1. Substrate use: Convert fat to fuel the body

One key difference between zone 2 heart rate training and higher levels is the substrate—or substance—the body uses for energy. In general, the body oxidizes—or breaks down—carbohydrates and fats, using them for energy. When exercising below your aerobic threshold, you rely more on this process.

As the intensity increases above your aerobic threshold, the body switches to breaking down stored carbohydrates—or glycogen. They stay in the muscle cells, and your liver converts them to glucose to get more energy.

But, there is a finite amount of glycogen in the body at a given time. That's why staying in zone 2 means you are exercising at the point of maximal oxidation, improving the body’s ability to use fat as fuel,

Zone 2 training will improve your ability to go longer, use energy sources more efficiently, have a lower recovery demand, lower cardiac strain, and less risk of injury, says Dr. Luks.

2. Aerobic threshold: Adapt high-intensity exercise 

The aerobic threshold is the limit of what the body can handle before it stops breaking down fat and turns to oxidizing carbohydrates. The first sign of this transition will be the buildup of lactic acid in the muscles, which arises when they start relying on glucose for energy.

Note: Rising lactic acid is responsible for the burning sensation you feel at higher-intensity exercise. It is a sign that you're fatigued and will likely burn out soon. For this reason, the aerobic threshold is also called the lactic acid threshold. 

In zone 2 heart rate training, lactic acid buildup doesn’t happen, which is why people feel comfortable maintaining this steady-state exercise longer. But your aerobic threshold isn’t static. Over time, you will increase it, which will help you maintain a higher intensity in that fat-burning state.

Inigo San Milan, Ph.D., is an elite cycling coach and supporter of zone 2 heart rate training. He found that endurance athletes can exercise much longer at higher intensities and remain below the aerobic threshold, supported by low blood lactate levels and higher fat oxidation. [2]

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3. Mitochondrial function: Reduce strain on muscles and heart 

You've probably heard that mitochondria are the powerhouse of the cell. But what does that mean?

They are the only place in your body where fat oxidation happens. Type 1—or slow-twitch muscle fibers—have a higher mitochondrial density than others, which is why the body recruits them during low-intensity exercise.

Zone 2 heart rate training can increase mitochondrial density in type 1 muscle fibers and help fuel your body with fat during longer workouts. It also reduces the recovery time and decreases the cardiac strain of exercise.

a line graph showing how high intensity can be

Benefits of zone 2 heart rate training 

Endurance performance

A 2014 prospective cohort study of athletes training for an Ironman competition found that the percentage of workout time spent below the aerobic threshold led to better performance. [3]

Additionally, a 2022 review of 10 studies found that elite distance runners spend about 75% of training time below their aerobic threshold to optimize performance. [4]

Note: Based on the current research evaluated by InsideTracker scientists, zone 2 heart rate training is now a recommendation for customers who select the endurance goal to guide their action plan.

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Metabolic flexibility 

During exercise, your body demands a quick shift into high gear. Metabolic flexibility helps the body efficiently switch fuel sources, tapping into stored reserves to create the energy you need to power through your workout.

Studies even suggest that metabolic flexibility is linked to a lower risk of insulin resistance and certain age-related diseases. [6] The results align with

Dr. Luks sees zone 2 training as a critical tool to promote metabolic flexibility. He emphasizes the importance of training our bodies to burn fat for fuel over longer durations. This metabolic flexibility enables us to perform more work before our bodies switch to glycolysis.

Many untrained runners and cyclists make that switch far too early, whereas elite athletes can maintain fat oxidation well into their upper zones, Dr. Luks notes. 

So, what benefits do metabolic flexibility and fat oxidation have on health? According to Dr. Luks, “Overall improvements in mitochondrial flexibility improve our fitness and metabolic health.

For those who exercise, zone 2 training will improve their performance in all those above it. It will also enhance their energy partitioning and energy use patterns, diminish the risk of injury, and decrease the recovery burden.”

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The benefits of metabolic flexibility extend to longevity. Dr. Luks says, “It ties into longevity because most sedentary folks have very poor metabolic flexibility. They will kick into glycolysis with walking, which is the pattern seen in [those with] insulin resistance and type 2 diabetes.”

He also notes that "Exercising at an intensity below your aerobic threshold “improves mitochondrial fitness, efficiency, and flexibility. This diminishes the consequences of poor metabolism, improves the breakdown of glucose, enhances insulin resistance, and decreases the risks associated with various chronic diseases,” notes Dr. Luks.

Note: Zone 2 heart rate training may also reduce injury risk, but this is highly dependent on the type of exercise you do and individual factors like age and fitness level.

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How to tell if you're in zone 2 when training

Determining your heart rate zone can be tricky. Each wearable device—Garmin Connect, Apple Watch, Fitbit—uses different methods to calculate your location, and it is not always clear how they do it.

The most accurate way to determine if you are training in zone 2 is to use lactic acid strips or go to a performance lab for exercise testing with a metabolic cart.

But there are several other methods you can try to get an approximate measurement, such as:

Heart rate method

Dr. Luks says you can get a rough estimate of your heart rate cutoff for zone 2 if you know your max heart rate (max HR). Subtracting your age in years from 220 gives you a ballpark estimate of your max HR.

Zone 2 is around 60-70% of your max HR. This is just an estimate, and many factors can affect both heart rate and wearable estimates, so use this method with a grain of salt. 

Talk test

Simply listening to your body while exercising may give an even more accurate estimate of whether you’re in zone 2.

The talk test is an easy yet effective method of determining whether you are training below your aerobic threshold. While training in zone 2, you should be able to hold a conversation without feeling out of breath.

For example, if you were speaking on the phone during the workout, the person on the other line could tell that you’re exercising but not breathless.

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Nose breathing

Successfully and solely relying on nose breathing while exercising signals that you are likely in Zone 2. But this may only be the case for some, as other people may have a difficult time with nose breathing in general.

Rate of perceived exertion 

Rate of perceived exertion (RPE) is another method to determine what zone you are in. On an RPE scale of 1-10, staying below a four signifies that you are below your aerobic threshold. Aim for an RPE of 3-4 to remain in zone 2. [1]

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How to start zone 2 heart rate training 

It can be mentally and physically challenging to exercise at a low intensity for more extended periods, but the benefits you'll get will be worth it.

Here are some tips for incorporating zone 2 heart rate training into your current workout regimen:

  • First, determine your baseline endurance level. For some people, zone 2 training may be walking on a treadmill, and for others, it may be jogging.
  • Using a treadmill with an incline option or a stationary bike are both excellent choices when getting started because you can easily adjust your output based on how your body is feeling. 
  • Begin with a minimum of 45-minute sessions. Gradually increase the duration of your sessions up to 90 minutes as you start to see improvements in your aerobic base. Aim for at least two to three sessions per week.
  • Remember to include HIITstrength, and sport-specific training.

Is zone 2 heart rate training better than exercising in other zones?

It depends on your goal. Zone 2 training is practical for optimizing endurance, but time spent in the higher ones can be crucial for performance. 

Including high-intensity interval training (HIIT) is one of the best ways to improve your VO2 max, which is an essential marker of cardiovascular health.

A good rule of thumb is to spend 75-80% of your training time in zone 2 (low intensity) and 20-25% of your time doing high-intensity, strength, and sport-specific workouts. 

Zone 2 heart training: Start the grind and boost your fitness

Zone 2 heart rate training is like a simmering pot on the fitness stove. It might not boil over with immediate results, but it promises a long and slow cook that builds a foundation of impressive endurance and potentially even better health.

Studies are promising, but more research is needed to solidify the connection between zone 2 and longevity. Ultimately, this type of training offers a compellingly simple and potentially transformative approach.

InsideTracker integrates with various fitness trackers, such as Apple Watch, Garmin, and Fitbit. By syncing your fitness tracker, you can monitor your performance during workouts to see if you're staying in the zone 2 heart rate.

Disclaimer: InsideTracker doesn't diagnose or treat medical conditions. Consult a physician for any health concerns.


[1] “Heart Rate Acquisition and Threshold-Based Training Increases oxygen uptake at metabolic threshold in triathletes: a pilot study,” PubMed, Jan. 01, 2019. Available:

[2] I. San-Millán and G. A. Brooks, “Assessment of metabolic flexibility by means of measuring blood lactate, fat, and carbohydrate oxidation responses to exercise in professional endurance athletes and Less-Fit individuals,” Sports Medicine, vol. 48, no. 2, pp. 467–479, Jun. 2017, doi: 10.1007/s40279-017-0751-x. Available:

[3] I. Muñoz, R. Cejuela, S. Seiler, E. Larumbe, and J. Esteve-Lanao, “Training-Intensity distribution during an Ironman season: relationship with competition performance,” International Journal of Sports Physiology and Performance, vol. 9, no. 2, pp. 332–339, Mar. 2014, doi: 10.1123/ijspp.2012-0352. Available:

[4] A. Casado, F. González-Mohíno, J. M. González-Ravé, and C. Foster, “Training Periodization, Methods, Intensity distribution, and Volume in Highly Trained and elite distance runners: A Systematic review,” International Journal of Sports Physiology and Performance, vol. 17, no. 6, pp. 820–833, Jun. 2022, doi: 10.1123/ijspp.2021-0435. Available:

[5] V. Manzi, A. Bovenzi, C. Castagna, P. S. Salimei, M. Volterrani, and F. Iellamo, “Training-Load distribution in Endurance runners: objective versus subjective assessment,” International Journal of Sports Physiology and Performance, vol. 10, no. 8, pp. 1023–1028, Nov. 2015, doi: 10.1123/ijspp.2014-0557. Available:

[6] B. H. Goodpaster and L. M. Sparks, “Metabolic flexibility in health and disease,” Cell Metabolism, vol. 25, no. 5, pp. 1027–1036, May 2017, doi: 10.1016/j.cmet.2017.04.015. Available: