- Zone 2 heart rate training refers to completing aerobic exercise below your aerobic threshold.
- Exercising in zone 2 encourages your body to rely on stored fat sources for fuel rather than carbohydrates.
- Zone 2 heart rate training can also promote metabolic flexibility, which has healthspan-related benefits.
- Fitness trackers, like those InsideTracker integrates with (Apple Watch, Garmin, Oura Ring, and Fitbit), can track your heart rate zones among other metrics of fitness and sleep.
- Low-intensity training like zone 2 should make up the majority of your exercise schedule, but strength training and higher-intensity exercise should also be included.
Zone 2 heart rate training has garnered attention recently from athletes to world-renowned scientists alike as a way to improve endurance and enhance longevity. And with many wearable devices adding heart rate zones to their product insights, tracking zone-based training is now easier than ever. InsideTracker scientists recently reviewed the studies behind zone 2 heart rate training and sure enough, there’s evidence that supports the use of this technique for improving aerobic capacity. But the benefits don’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-based training is a methodology that classifies workouts based on aerobic intensity. Zone 2 is essentially a low-intensity, steady-state workout at which breathing is still easy and comfortable. It’s a pace and intensity you should be able to maintain for hours. If you’ve ever heard the phrase, train slow to run fast, that is the same idea as zone 2 heart rate training. You should be able to hold a conversation while in zone 2. Training at a very low intensity can have a great impact on higher-intensity endurance performance.
Dr. Howard Luks, a top orthopedic surgeon and sports medicine specialist, promotes zone 2 training’s role in both performance and longevity. He says that, “most elite athletes spend 90% of their training in low [heart rate] zones.” This type of training, however, is not easily followed. In his practice, he notices that, “most average runners run too fast on their slow days and too slow on their fast days.” This can lead to inefficient energy use by the body and minimal progression in performance—leaving you feeling like you’re in a rut.
Zone 2 heart rate training is a more efficient way to build your aerobic base and see greater performance improvements.
The science behind zone 2 heart rate training
Zone-based heart rate training is typically broken down into 5 zones, with zone 1 being the lowest intensity and zone 5 being the highest intensity. The exercises you can do in each intensity zone depend on how aerobically (or cardiovascularly) fit you are at baseline. For an average person, zone 1 is a light walk, zone 3 is a steady run, and zone 5 is an all-out sprint.
It’s important to understand the three key physiological concepts behind zone 2 training; substrate utilization, aerobic threshold, and mitochondrial function. 
1. Substrate utilization: Use fat as fuel
One of the key distinctions between zone 2 heart rate training and higher zones is the substrate—or substance—utilized by the body for energy. In general, carbohydrates, and fats can both be oxidized, or broken down, and used for energy. When exercising below your aerobic threshold, your body is more reliant on fat oxidation for energy. As your exercise intensity increases above your aerobic threshold, your body switches to breaking down stored carbohydrates (glycogen), which is found in muscle cells and the liver, to glucose for energy. However, there is a finite amount of glycogen in the body at a given time.
Staying in zone 2 means you are exercising at the point of maximal fat oxidation, therefore improving your body’s ability to use fat as fuel, “which will improve their 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. “Considering we have a limited amount of glycogen available, and an infinite number of fat calories available, the advantage [of zone 2 training] is clear.
2. Aerobic threshold: Adapt to a higher-intensity exercise
So the aerobic threshold is the uppermost limit of what your body can handle before fat oxidation switches to carbohydrate oxidation. The hallmark of this transition is the buildup of lactic acid in muscles. Lactic acid is created when muscles start to rely on glucose for energy. It’s responsible for that burning sensation you feel at higher-intensity exercise and is a sign you're getting fatigued and will likely burn out soon. The aerobic threshold is also called the lactic acid threshold for this reason.
In zone 2 heart rate training, lactic acid buildup doesn’t occur, which is why people feel comfortable maintaining this steady-state exercise for long periods. But your aerobic threshold isn’t static.
Over time, low-intensity training will increase your aerobic threshold—meaning you’ll be able to maintain a higher intensity of exercise in that fat-burning state.
Inigo San Milan, Ph.D. an elite cycling coach and supporter of zone 2 heart rate training, found that endurance athletes can exercise much longer at higher intensities and remain below the aerobic threshold, as signified by low blood lactate levels and higher fat oxidation. 
3. Mitochondrial function: Reduce strain on muscles and heart
The phrase, the mitochondria is the powerhouse of the cell, is a popular saying in high school biology classrooms. But what does it actually mean?
The mitochondria are the only place in your body where fat oxidation occurs. Type 1 muscle fibers—commonly known as slow-twitch muscle fibers—have a higher mitochondrial density than other muscle fiber types and are primarily recruited when you are doing low-intensity exercise. Zone 2 heart rate training can increase mitochondrial density in type 1 muscle fibers and improve our body’s ability to utilize fat as fuel for longer periods of exercise. This is not only a more effective way to use energy, but also reduces the recovery time needed and lessens the cardiac strain of exercise.
Benefits of zone 2 heart rate training
Any type of exercise is beneficial for your health and longevity, but there are certain benefits that are unique to zone-based training.
A 2014 prospective cohort study of athletes training for an Ironman found that the percentage of training time spent below the aerobic threshold was associated with better Ironman performance.  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. 
*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.
Metabolic flexibility refers to one's ability to adapt and thrive during shifts in metabolic demand. During exercise, the body needs to be metabolically flexible. The body demands a shift in energy in an active state, and metabolic flexibility helps to meet this demand by creating energy from its reserves. Studies show that metabolic flexibility is associated with a lower incidence of insulin resistance and several age-related diseases. 
For Dr. Luks, the biggest benefit of zone 2 training is its role in promoting metabolic flexibility—the ability to oxidize (or burn) fats for energy for longer periods instead of switching over to carbohydrate breakdown.
“By improving our ability to oxidize fat for longer during our training, we are improving our ability to do more work while remaining in fat oxidation vs. switching over to glycolysis [the breakdown of carbohydrates]. Many untrained runners and cyclists make that switch far too early whereas elite athletes can maintain fat oxidation well into their upper zones,” he notes.
So what benefit does metabolic flexibility and fat oxidation have on health? According to Dr. Luks, “Overall improvements in mitochondrial flexibility improve our overall fitness and metabolic health. For those who do exercise, zone 2 training will improve their performance in all zones above it. It will improve their energy partitioning and energy utilization patterns, diminish the risk of injury, and decrease recovery burden.”
The benefits of metabolic flexibility extend beyond performance 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.” Exercising at an intensity below your aerobic threshold, “improves mitochondrial fitness, efficiency, and flexibility. This diminishes the downstream consequences of poor metabolic health and improves glucose metabolism, improves insulin resistance, and diminishes the risks associated with various chronic diseases,” notes Dr. Luks.
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.
How to tell if you're in zone 2
Determining what heart rate zone you’re in can be tricky. Each wearable device (i.e. Garmin Connect, Apple Watch, Fitbit, etc.), uses different methods to calculate its heart rate zones, and it is not always clear how they’re calculated. The most accurate way to determine if you are training in Zone 2 is by using lactic acid strips or going to a performance lab and undergoing exercise testing with a metabolic cart. But, there are several other methods you can try to approximate what zone you are in.
Heart rate method
Dr. Luks says you can get a rough estimate of your own 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 65-75% of your max HR. This is just an estimate and many factors can affect both heart rate and wearable estimates of your heart rate, so take this method with a grain of salt.
Simply listening to your body while exercising may give an even more accurate estimate of whether you’re in zone 2 than the heart rate method mentioned. The talk test is an easy yet effective method to determine if you are training below your aerobic threshold. While training in zone 2, you should easily 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. If you can’t hold a conversation without strain or heavy breathing, you are likely in a higher-intensity zone.
Nose breathing is another method to ensure you are in zone 2. Successfully and solely relying on nose breathing while exercising signals that you are likely in zone 2. However, this may not be the case for everyone—as some 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 stay in Zone 2. 
Is zone 2 heart rate training better than training in other zones?
It depends on what your goal is. Zone-based training is effective for optimizing endurance performance. However, time spent in the higher zones is still crucial for performance.
Including high-intensity interval training (HIIT) is one of the best ways to improve your VO2max, which is an important 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/or sport-specific workouts.
How to start zone 2 heart rate training
It can be very difficult to exercise at a low intensity for a long time, both mentally and physically. However, it will be worth it to train low and slow. Here are some tips to incorporate 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 good 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 begin to see improvements in your aerobic base. Aim for at least two to three sessions per week. Finally, don’t forget to include HIIT, strength, and/or sport-specific training.