man running on the treadmill like it's effortless. Must be nice.

Zone 2 Training: Why Less HIIT May Mean More Gains

Zone 2 training boasts major performance benefits that deserve a spot in your workout routine.

You’re on the grind. Digging into HIIT workouts, putting in the high effort, and consistently riding the top of the leaderboard. Major props. Studies show HIIT training can improve your VO2 max (1), boost your immune system (2), and protect cell health (3).

But, according to orthopedic surgeon and sports medicine specialist, Dr. Howard Luks, MD, if the sprints you were crushing a few weeks ago are starting to feel lackluster, you’re probably overtraining.

“Most people run too slow on their fast days, and too fast on their easy days,” says Luks. “They can’t hit their VO2 max because they can’t run fast enough. They can’t run fast enough because they haven’t built their aerobic base in lower heart rate zones—zone 2. Instead, they double down and push harder. It’s a perpetual downward cycle.”

This is what longevity specialist, Dr. Peter Attia, MD, calls the “garbage zone”. To avoid it, and get more out of your cardio workouts, Luks and Attia recommend zone 2 training—steady-state sustained cardio.

In his book, Longevity Simplified, Luks explains the fascinating link between zone 2 training, performance, a better healthspan, and a longer lifespan.

“Zone 2 is important for longevity in sport, and longevity as a human. Everything will improve from building your aerobic base with zone 2 training,” says Luks. Consider us sold. What exactly is zone 2 cardio and how do you use it for optimal results?

What is Zone 2 Training?

Zone 2 training, also referred to as base or aerobic training, involves cardio performed at a relatively low intensity for an extended period of time. According to Luks, the intensity should be somewhere between 65 to 75 percent of your maximum heart rate, and the duration should be anywhere from 45 minutes to 3 hours.

“I sometimes do this on a treadmill. Usually, a 15 percent incline at 3 to 3.4 miles per hour will get me there,” Attia said on The Drive. “Some people choose to do this on an elliptical, I’m not a fan of that personally, but there are multiple ways to skin this cat.”

Cycling, swimming, running, rowing, walking, hiking, or the elliptical all qualify—so long as it’s an activity you can maintain for well over 45 minutes.

If you’re thinking zone 2 sounds an awful lot like steady state cardio, it is. But it’s a little more technical than mindlessly slogging away on the elliptical. “Zone 2 is basically the highest level of exertion that is effectively pure mitochondrial phosphorylation before you start to net accumulate lactate,” explains applied physiologist, Dr. Iñigo San-Millán, PhD to Attia on The Drive.

In other words, as the intensity of your workout increases, lactate will start to build up in your muscles. Zone 2 should be just under a lactate threshold of 2 millimole (mmol)—which is where you’ll shift from zone 2 to zone 3 exercise, and from burning fat for energy (which occurs in the mitochondria of your cells) to glucose (which occurs outside of the mitochondria).

This distinction is important because to get the most benefits out of zone 2 cardio, you want to stimulate your mitochondria as much as possible by working in the higher end of your zone 2 range. Work too hard and you’ll shift into zone 3—where you tap into glucose for energy, your blood lactate rapidly increases, and you stop stimulating your mitochondria. 

The TL;DR: zone 2’s secret sauce is linked to its unique ability to stimulate the mitochondria, work too hard and you’ll miss out on the benefits. 

How Zone 2 Training Works Scientifically

All the cells in your body are fueled by adenosine triphosphate (ATP). Your body has various systems that break down fuel—fat, glucose, creatine phosphate, and sometimes lactate—to make that ATP.

Fat is broken down slowly in your mitochondria (the powerhouse of the cell) by oxidative phosphorylation, a process that uses oxygen to turn fat into energy. Whereas, glucose is processed faster because it doesn’t require oxygen to be broken down. It’s also located in the cytoplasm of the cell; thus, when it’s used for energy, your mitochondria aren’t stimulated.

Scientists have defined five training zones to help you understand which energy system you’re using at any given exercise intensity level. Zone 1 is the lowest intensity and involves activities like sitting down, and walking around your house. Zone 5 is the highest intensity and is assigned to activities like all-out sprinting, and squatting a heavy set of five reps.



Heart Rate 

Energy source

Zone 1 

Very Light 

50 to 60% HR max 


Zone 2 


60 to 70% HR max 


Zone 3 


70 to 80% HR max 


Zone 4 


80 to 90% HR max 


Zone 5 


90 to 100% HR max 

Glucose/Creatine Phosphate  

In zone 2, your intensity is at a level where you’re stimulating your cells mitochondrial function the most. You can meet your body’s need for ATP using only fat and oxygen in your mitochondria. If you went harder, you’d shift to using primarily glucose for energy; producing lactate.

If you’re looking at the chart and thinking “I thought Dr. Luks said zone 2 was 65 to 75 percent?” 60 to 70 percent is a good starting range. Because untrained individuals tend to rely on more glucose for energy, and aren’t able to use and clear lactate at higher efforts, they have a lower zone 2 ceiling.

However, “Well-trained individuals have more metabolic flexibility and are better able to process fat for fuel, and efficiently clear lactate,” says Luks. This allows you to workout harder and longer without shifting into zone 3.

It’s important to note that while training in zone 4 and 5 can improve your ability to work at maximal efforts, it doesn’t improve your zone 2 ceiling, because it occurs in the cytoplasm of the cell. Thus, the simple concept of training slow to go fast was born. 

Benefits of Zone 2 Training

The benefits of zone 2 training extend well beyond fitness. The big ones, below.

Boost metabolic health

“The healthier your mitochondria are, the healthier you will be,” says Luks. Poor mitochondrial function is linked to heart disease (4), dementia (5), type 2 diabetes (6), metabolic syndrome (7), cancer (8), and insulin resistance (9).

A study published in Oxidative Medicine and Cellular Longevity found that zone 2 training increases mitochondrial size, number, and function (10). Which means more efficient workouts, and better metabolic health.

Increase endurance

When you train in zone 2, you directly build mitochondrial density in your skeletal muscle (10), specifically in type 1 muscle fibers—the fibers your body depends on during endurance activities. More mitchondria means you can process fat more efficiently, pushing harder and longer without having tap into glucose for energy, or fight through lactate build-up. 

“The only way to improve your lactate threshold is by building a strong aerobic base. The only way to build a strong aerobic base is with months of base training in zone 2,” says Luks. Why? Those who don’t have mitochondrial flexibility rely on glucose to fuel their workouts, they produce more lactate, and have poor lactate clearance.

Lactate build-up is accompanied with hydrogen ion (H+) production—the reason you feel fatigued. This build-up changes the environment around your cells, reducing performance and muscle power. A study published in the Journal of Applied Physiology found that regular endurance training improves lactate production, disposal, and clearance (11).

Improve performance

Zone 2 training requires your heart, lungs, and blood vessels to work in unison to deliver oxygenated blood to your muscles. As your heart gets stronger, it becomes more efficient. Regular zone 2 training can contribute to a reduced heart rate (at rest and during exercise) and an increased VO2 max (12).

Prevent injury and aid recovery

Zone 2 puts very little stress on the body in comparison to high-intensity activities like HIIT or heavy strength training. It enables you to add volume to your training while lowering your risk of injury, overtraining, or fatigue. Plus, it delivers oxygenated blood and nutrients to your muscles to help them bounce back from harder workouts.

How To Know When You’re in Zone 2

You want to be working hard enough, but not too hard.

There are multiple ways to test this. Using the talk test, tracking your heart rate, and measuring your lactate are the top three. Attia subscribes to all. Here’s how he does it.

Try the talk test

The easiest (and surprisingly accurate) way to determine if you’re in zone 2 is the “talk test”. According to Attia, in zone 2 you should be able to breathe completely with your mouth shut if you’re not speaking. If you are speaking, it shouldn’t be pleasant. You can speak, but you don’t want to.

“If you can easily speak full sentences without having to pause you’re probably in zone 1 or 2. If you can speak or sing but need to pause to breathe occasionally you’re in zone 2. If you need to pause every few words you’re in zone 3,” explains Luks.

Track your heart rate

Per Luks, zone 2 should be somewhere in the range of 65 to 75 percent of your maximal heart rate (HR). By determining your max heart rate, you can calculate 65 and 75 percent of your max to get a rough estimate of your zone 2 bounds.

You can get an idea of your HR max by using this equation: 208 – (0.7 x age)—which is a more precise formula (when compared to the classic 220 minus your age) that’s adjusted for people over the age of 40. Then multiply your max HR by .65 and .75 to determine your bounds.

For example, my age is 29. I plug it into the first equation (208 – (0.7 x 29)) to get my max HR of 188 beats per minute (bpm). Then I multiply that by .65 to get 65 percent of my max HR (188 x .65) which comes out to 122 bpm. And again by .75 to get 75 percent of my max HR (188 x .75) which is 141 bpm. Now I know to stay in zone 2 I roughly want to aim between 122 to 141 bpm during my zone 2 workouts.

Measure your lactate

Measuring your lactate threshold in a performance lab is the most accurate way to determine exactly when you transition in and out of zone 2. Between 1.7 and 1.9 mmol is when you’re in zone 2. Cross into 2 mmol and you’re in zone 3.

You can do it at home. “Sometimes I use a lactate meter to make sure I’m not pushing too hard but pushing hard enough,” says Attia. The catch? Lactate meters are expensive (the test strips are two bucks a pop). And it’s a blood test. A prick might be worth it, however, if you’re trying to get the clearest picture of your zone 2 threshold.


How Long Should You Train in Zone 2?

According to Luks, 60 to 90 minutes per session is ideal. “If you’re trying to do one to one-and-a-half hours [of zone 2] you’re on target,” agrees San-Millán.

For those new to zone 2, Luks and San-Millán recommend working up from 30 minutes if necessary. “Eventually you’ll need more to see adaptations,” says San-Millán. Make it your goal to build up to an hour plus. “As you start to improve, you should be putting in fewer but longer sessions,” says Luks. “Once your fitness level is there, your shortest aerobic session should be 60 to 90 minutes.”

What About Cardiac Drift?

Cardiac drift will occur for most people during the end of a long zone 2 workout. That means your heart rate starts trending upward despite holding the same effort (speed, incline, resistance) throughout the workout.

“Cardiac drift is often due to metabolic stress,” says Luks. “You are leaving zone 2 and shifting into more anaerobic work. The lactate buildup causes your muscles to be less effective and work harder to maintain the same effort.” This can impact the length of time your body is able to stay in zone 2.

If your heart rate is drifting, Luks recommends adjusting your effort to maintain the same heart rate. As you continue to train your mitochondrial efficiency and ability to clear lactate will improve. As your training and fitness level improves, the drift time will improve as well.

How Often Should You Do Zone 2 Training?

There is no clear consensus, but experts agree any amount of zone 2 cardio is better than none. “This type of training is designed to be done three to four times a week,” says San-Millán. “Ideally five or six days a week, but not everyone has this many days a week to train.”

He points out that strength training and high-intensity training will need to fit into your routine, too—which can make time a limiting factor.

According to Luks, zone 2 training should make up about 80 percent of your overall cardio training, with around 20 percent reserved for sprints or higher-intensity workouts. “Elite athletes use polarized training (the 80/20 rule) to optimize aerobic and performance gains, and achieve optimal recovery,” says Luks. Polarized training is just a fancy way of saying the majority of your training load occurs at a low intensity.

One study published in the Scandinavian Journal of Medicine in Science and Sports found that elite athletes who performed 75 percent of sessions below a lactate threshold of 2 mmol, 5 to 10 percent of sessions in between 2 and 4 mmol, and 15 to 20 percent of sessions above 4 mmol optimized both performance and recovery (13).

Attia agrees that training in both modes really is the perfect blend. “Zone 1, zone 2, and zone 5 is where we spend the most time in everyday life,” he says. “By training zone 2 and zone 5 we’re really teeing ourselves up to do these things structurally and metabolically.”

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