Zone 2 – A Comprehensive Look

NOTE: The article is a follow up to Evokecast episode #63 with additional information.

When I wrote Training for the New Alpinism and Training for the Uphill Athlete, the fitness industry and popular media weren’t talking much about Zone 2 training. The spotlight was firmly focused on high-intensity interval training (HIIT). Now, attention has shifted to Zone 2 training, or aerobic base training, as I prefer to call it, for reasons I will discuss later. However, misrepresentations, inaccuracies, and oversimplifications are running rampant. Each type of training has an intended effect and a specific way to apply it to the individual athlete. Understanding these nuances will produce more effective training and, ultimately, better performance.  In this article, we’ll take a comprehensive look at Zone 2 training.

How Has Public Perception of Endurance Training Changed in Recent Decades?

I’ve witnessed several big shifts in perception since I started coaching. Let me first say that the concepts of aerobic base training have been around for a very long time. My expertise is built on knowledge that has been passed down through endurance training circles for generations. I have added my own ideas to it and learned how to apply it to mountain environments, but the core concepts are nothing new.

The Traditional Methods

When I first got into endurance sports, many coaches and athletes were well aware of the importance of low-intensity aerobic base training. It was not called Zone 2 training at the time because heart rate zones were not as easy to gauge or analyze. Instead of heart rate (HR),  pace, breathing rate, and perceived exertion were often used to scale intensity.

Nevertheless, the effectiveness of executing a high volume of relatively easy work was well documented. Intervals and higher-intensity workouts were used, too, but people were generally aware that most of your training needed to be performed at slower paces to produce aerobic adaptations in the muscles and metabolic system.

With new science and more powerful instruments, our understanding of how these methods affect our bodies is better than ever. It’s also easier to apply them and assess the outcome. We will discuss these methods in much greater detail later.

The “All HIIT, All The Time” Fad

In the early 2000s, CrossFit and other high-intensity interval training captured the public’s attention. The foundation of this revolution was the misinterpretation or misrepresentation of a few studies on the effects of HIIT that were cleverly marketed in ways to lure the ignorant with the promise of a shortcut to fitness.

The quick, high-intensity workouts that left adherents bathed in sweat and flooded with feel-good hormones were just what the time-crunched fitness junkie craved. Why would anyone want to do “long slow distance” when they could run their fastest marathon time after only two months of doing HIIT sessions three times per week? Low-intensity aerobic base training was a waste of time. That may seem like hyperbole, but that was how HIIT was being marketed, and an entire industry was spending billions of dollars doing so. Internet copycats and the information age played a role, too. HIIT became an overnight sensation.

To the uninitiated, it seemed like a revolution in endurance training had taken place. The old ideas could be relegated to the dustbin of history. The pendulum swung toward a myopic focus on HIIT and seemed to be lodged there for the next 20 years.

However, human metabolic evolution didn’t get the memo when it came to this 21st-century training fad. Just because this approach fit into a busy life and left participants feeling like they had worked out didn’t mean it was the best way to adapt human physiology for optimal endurance. If CrossFit could be used to get you running your best-ever marathon in a matter of a few months, why is it that no elite marathoner or endurance athlete from other sports used this type of training?

I’ve been beating up on the fitness industry for years so I want to be clear that HIIT has a very important place in endurance training, but it is just one small tool that plays a specific role in the pantheon of training methods. It should be thought of as a supplement to, not a replacement for, those old-school methods we are going to be discussing soon.

Zone 2 Goes Viral

Perhaps you’ve noticed the explosion on the internet of Zone 2 training information over the past few years. Influencers and self-labeled authorities are parroting each other in YouTube videos and social media posts. These folks are a bit late (by like 30 years) to the party. But hey, as they say, “Better late than never.”

I’m glad people are talking about the importance of aerobic base training, which has been underrated for far too long. But I notice glaring omissions and a lack of nuance in a lot of the information out there. The myopic focus on HIIT has given way to a myopic focus on Zone 2.

I am going to fill in the gaps so that you, the athlete, can maximize your training outcomes.

What Are Heart Rate Zones, and What Do They Represent?

Since definitions matter if we want to be able to have a shared understanding and thus communicate effectively, let’s start with a dissection of HR zones.

For over 100 years, coaches and exercise scientists have understood that training at different intensities has different effects on the body. By intensity, we mean the rate of work you are doing. For example, running fast is a higher-intensity form of exercise than walking slowly. Another way to think about it is that even though they are both running, sprinters train very differently than marathoners because the training effect they are targeting is different.

What is relatively new in the history of training is categorizing intensity into heart rate zones. Heart rate zones are designations of exercise intensity as measured by heart rate. Heart rate is a useful, if imperfect, proxy for exercise intensity because heart rate increases along with exercise intensity.

The History of Determining Intensity

 With modern heart rate monitors, we now have real-time heart rate measurements available on our watch, but that was not possible until the early 1980s.

Road and track runners, as well as swimmers, have always been able to use recent race pace to establish current training intensities. For a well-trained athlete, training below 85-90% of race pace (depending on the athlete and race distance) would be considered aerobic base (or what we now call Zone 2) training. That intensity is the aerobic support for the training done at 95-100% of race pace, which would be considered specific endurance training. This could be Zone 3 or 4 in today’s parlance, depending on the athlete and race distance. Training at 100-105% of race pace would be considered specific speed endurance training. From 105-110% of race pace would be used for developing pure speed, which is neuromuscular support for the specific speed endurance training I just mentioned.

When it comes to sports that involve undulating and irregular terrain, pace can no longer be used effectively to control intensity. Traditional sports like cross-country skiing, cross-country running, and cycling fall into this category. However, the genre of new non-traditional sports, like trail and mountain running, ski mountaineering, and mountaineering, also presents this same problem.

Athletes in these sports have traditionally used much less accurate methods to control the intensity of their training. Breathing rate was popular for runners in the 60s. 3 steps of inhaling and 4 steps of exhaling would put you in Zone 2 intensity.

The upper limit of nose breathing has been used with limited success to help identify the aerobic base-building intensity. Similarly, being able to maintain a reasonable conversation would ensure an aerobic intensity.

In the 70s, I trained with some elite Norwegian cross-country skiers whose coach, when he wanted them to do an aerobic base training session, would have them wear their warm-ups and tell them not to sweat. Crude but effective.

In those days, athletes could only measure their heart rate during training by stopping and manually counting the number of beats they could feel in their neck or wrist for a short time.  Commonly, this was 6 seconds, and then they only had to multiply the number of beats by 10. The lack of real time measurements meant using heart rate for controlling training intensity was inconvenient and subject to error.

Then an upstart Finnish company called Polar introduced the first portable heart rate monitor for athletes in the early 1980s. For what is probably the only time in my life I was one of the early adopters of that new technology and began using it in about 1983. They were big wrist-worn devices that used a chest strap with electrodes to pick up the electrical activity of the heart.

Chest straps still remain the best and most accurate way to measure heart rate, but in the intervening 40 years, a lot has happened to the wrist-worn gadgets that now record and display heart rates and so much more. The whole endurance sports world owes Polar a big debt of gratitude for getting this technology off the ground and in common usage.

How Did HR Zones Come to Be?

Physiologists certainly understood the proxy relationship between heart rate and exercise intensity, or more accurately, the metabolic relationship to exercise intensity. But coaches and athletes, even if they understood this relationship, had no way to implement it into their training until these Polar heart rate monitors became available.

Once Polar developed this technology, they had to explain how to use it. As part of their marketing of this new gadget, Polar came up with heart rate zones to relate heart rate to the metabolism supporting the intensity of the exercise. They called Zone 1 (actually, they called it Level 1) the intensity/heart rate that would normally elicit a blood lactate concentration of about 1mMol/L. Zone 2 is equated to a lactate of about 2mMol/L. Zone 3 would be an intensity that would give, for most people, a lactate of 3mMol/L. Zone 4 was meant to be 4mMol/L or higher and was the highest aerobic intensity zone. So they had a nice, simple 4-zone system that came pretty close to relating blood lactate concentrations to the intensity of the exercise by way of the heart rate. It was ingenious!  For more on blood lactate please checkout this article.

It’s hard to overstate the impact this seemingly simple system had on the ability of endurance athletes to monitor and control their training intensity in real-time. It was a massive breakthrough.

The technology has never stopped advancing, and the heart rate zone system of training intensity has become a cornerstone of endurance training theory and practice. While the original 4-zone system that Polar came up with is still probably the most popular, there are many other zone systems used today that go from an even simpler 3-zone system all the way to 7- and 9-zone systems.

How Do I Determine Heart Rate Zones?

 The point of these zones is to help you individualize your training based on your body’s response to exercise. As you can understand, being able to accurately identify these zones is the only way to meaningfully use them in your training. This is one of the areas that is largely overlooked in most of the Zone 2 material I have seen on the internet.

At Evoke we anchor each athlete’s zone system to two important metabolic events. The first of those is called the Aerobic Threshold (AeT), which demarcates the top of Zone 2. The second is called by various names: anaerobic threshold or lactate threshold are the most common. This marks the top of Zone 3. Knowing these two points allows the athlete to personalize the zone system so that it represents their metabolic response to exercising at different intensities.

These points can be identified with a couple of very simple, self-administered, free tests. For a complete discussion of the recommended methods I would refer people to Chapter 5 in Training for the Uphill Athlete. Here is a link to an article about determining Zone 2 with the HR Drift Test .

I do not recommend using a chart that gives zones by percentage of max heart rate. There is a very slim chance of that actually identifying your individual metabolic response to exercise. Before someone brings it up I will admit that in my first book, Training for the New Alpinism, I did recommend the percentage of max heart rate method. I debated long and hard about how complex to get in the presentation of training theory in that book. In those days, there were virtually no climbers using any sort of science-based training methods. I was afraid that if I went too far into the weeds I would lose an audience that was already going to find structured training a hard pill to swallow. We got quite a lot of push back from the climbing community when that book came out. As a rule, climbers didn’t want to be told that they needed to engage in structured training like athletes in conventional sports. My concern was vindicated.

However, now there are many climbers using these training methods who are open to a deeper discussion on the topic, which is why I provided that information in the second book and continue to discuss it.

What Does Zone 2 Training Mean?

As I mentioned earlier, Zone 2 as Polar originally defined it, had its upper limit set at a blood lactate concentration of 2mMol/L. That is one way, albeit a rather arbitrary and blanket way, of defining the maximum capacity of the aerobic metabolic pathway to provide the energy for exercise before the anaerobic pathway begins to dominate energy production.

I’m not going to get too deep into the science of metabolism here and recommend you read Chapter 2 of Training for the Uphill Athlete for a deeper but nontechnical discussion on metabolism. Suffice to say, that 2mMol/L marker is but one of several contentious ways of defining the upper limit of a person’s aerobic metabolic capacity.

Exercising continuously for 30 minutes to several hours at an intensity resulting in a lactate concentration at and below 2mMol/L gives the biggest stimulus for improving an athlete’s aerobic capacity. The reason it does this is that the primary energy production at this intensity is coming from the aerobic metabolism occurring in the mitochondria of the slow twitch muscle fibers.

Those slow twitch muscle fibers are the main fibers that propel you at lower intensities. However, they are still involved in propulsion at higher speeds. It is through long durations of this training stimulus that an athlete can maximize aerobic capacity.

Coaches without any scientific understanding of metabolism had figured this out long ago.  Back in the day, coaches called this sort of training “aerobic base training,” or simply “base training.” The low intensity allows the athlete to accumulate a high volume of training and thus provides the biggest training stimulus, since duration is the biggest multiplier in any sort of training.

Just as with getting good at anything, as long as you do it well and carefully: the more you practice, the better you get. The more you train at this Zone 2 intensity, the better the mitochondria in those slow twitch fibers get at producing energy aerobically.

Why is Zone 2 (Aerobic Base) Training Important?

I prefer the term aerobic base training for several reasons. For one, it is more descriptive of what you are trying to achieve with the training. The word base says it all. The aerobic metabolic pathway provides a base of support for all endurance events.  We’ll get into the underling metabolism shortly.

Imagine that your endurance fitness is a pyramid with the aerobic base sitting at the bottom of that pyramid. The bigger that pyramid’s base, the taller the endurance pyramid can be built. In other words, the base supports all the other layers on top of it. I will discuss why that is the case here soon.

The other reason I think base training is a better term is because highly trained endurance athletes may need to shift their training into lower intensities such as Zone 1 to avoid injury and overtraining, and to keep making the adaptations that lead to increased endurance. This is one of the main nuances that is completely left out of the information being presented in popular media today. I will discuss this in more detail after we first look at why base training is so important.

Training Your Metabolism

An endurance event is anything lasting longer than about 2 minutes. That may sound awfully short but the reality is that if you exercise at your maximum sustainable intensity for any duration longer than 2 minutes, the majority of the energy needed for that exercise will be coming from the aerobic metabolic pathway. Obviously, as the duration of exercise lengthens, the average intensity will have to drop and the aerobic metabolism will play an even bigger role in providing the needed energy.

For example, a well-trained runner moving at top speed for 1 kilometer will be getting the needed energy from about 70-80% aerobic carbohydrate metabolism plus another 20-30% from anaerobic carbohydrate metabolism. The same runner going as fast as they can for 100 kilometers will rely 90-95% on fat to fuel the aerobic pathway, 5-10% on the aerobic metabolism of carbohydrates, and only a tiny fraction coming from the anaerobic metabolism of carbohydrates.

The gist as it relates to your training is that the goal of aerobic base training is to enhance the capacity of the aerobic metabolic pathway to churn out the energy required to move your body for extended durations.

How Our Bodies Adapt to Training

In 1946, Hans Selye described what he called General Adaptation Syndrome. This theory describes the way complex organisms respond to stress. It applies to physical, emotional and psychological stress. It kind of goes like this: if you repeatedly apply a stress to an organism, in this case humans, that is beyond the organism’s ability to handle, said organism will begin to adapt to that stress so that it is better able to handle it in the future.

Famed running coach Renato Canova put it this way (paraphrasing): You embarrass the body when you try to get it to do something it is not able to do. It makes adaptations so as not to be embarrassed the next time.

Aerobic exercise triggers a cascade of follow on events that are called signaling pathways. These pathways start with the increase of various enzymes that have effects reaching all the way down to the level of individual genes that are responsible for the adaptations, primarily in muscle fibers that are doing the work, whose end result is increased aerobic work capacity.

Zone 2 Training is All About Your Mitochondria:  

We’ve established that different intensities produce different training effects in the body and that each HR zone is associated with a different intensity. So, what effects does Zone 2 training produce?

Mitochondria

The primary adaptations are increases in the muscle cells’ mitochondrial density, increases in aerobic enzymes, and an increase in the capillary density in the muscle fibers being trained. Mitochondria are the cells’ aerobic powerhouses. They are responsible for ALL the aerobic energy production in every cell of your body.  Increasing the mitochondrial mass in a muscle cell has the direct result of increasing that muscle cell’s aerobic work capacity (its ability to do more work/second aerobically).  This in turn endows that muscle cell with more endurance.  This increase in aerobic capacity is the direct result of Zone 2 training.

This graphic gives an idea of the myriad complex processes taking place inside the mitochondria.  Luckily, you don’t need to understand all these in order to grasp the basics of the metabolic function that keeps us alive and kicking (literally).

All these adaptations contribute to increasing the aerobic capacity and, consequently, allowing you to move faster for longer, which is the very definition of endurance.

Training at higher intensities than Zone 2 has important beneficial effects as well. It’s just that the sum of those effects is different than those accrued from training in Zone 2 or below.

The Lactate Shuttle

I’ve said low-intensity training can improve high-intensity performance. How is this possible? One important process to understand is the lactate shuttle.

I already mentioned this substance called lactate and how Polar used the concentrations of it in an athlete’s blood to anchor their zone system. They did that because lactate production increases right along with exercise intensity.

Because of this correlation, increased levels of lactate above a certain concentration have long been associated with fatigue. Fatigue is an extremely complex subject with many causes, but for decades, the accumulation of lactate, or its precursor, lactic acid, was thought to be the cause of fatigue during high-intensity exercise. Lactic acid and lactate were viewed as harmful waste products of glycolytic metabolism, which is the breakdown of carbohydrates for energy.

The strong correlation between fatigue and high lactate levels is important for our current discussion. While it is an oversimplification, for our purposes, we can think of lactate accumulation above a certain low level as acting as a proxy for fatigue. Lower lactate goes hand in hand with less fatigue. Higher lactate accompanies fatigue.

What if there were a way to keep lactate from accumulating? Either by reducing its production or increasing the rate at which it could be disposed of? Wouldn’t that reduce fatigue and thereby improve endurance performance?  It turns out the answer to both those questions is yes!

Work done in the early 80s by George Brooks showed that lactate was moved from where it was produced in fast twitch muscle fibers to nearby slow twitch muscle fibers where it could enter the mitochondria to be used as a fuel in aerobic metabolism. Slow-twitch muscle cells are predominantly powered by aerobic metabolism because they have the highest concentration of mitochondria, and, as it turns out, they love to use lactate as fuel. Brooks termed this process the lactate shuttle.

The slow twitch fibers act as a reservoir or repository for the lactate being produced by glycolytic metabolism fueling higher-intensity exercise. The lactate can be taken up and put to use as fuel rather than accumulating and gumming up the works. The bigger that repository, the more lactate that can be utilized for useful work. The size of that repository is, by definition, the aerobic base we’ve been talking about! So, the bigger the base, the longer an athlete can sustain a higher work rate before fatiguing.  Why is that…..?

Aerobic Glycolysis

Glycolysis is an anaerobic process.  So what gives with the title to this section?  How can there be aerobic glycolysis?  This is a case where the term is not technically correct but the idea it conveys is correct.  Glycolysis is the process used to power Fast Twitch muscle fibers required for high intensity exercise because it can turn over ATP at a much faster rate than can the breakdown of fat.   Pyruvate and lactate are the end products of the glycolytic process.  Both those molecules contain energy that can be used aerobically in the mitochondria.

You must rely on glycolysis to run fast.  If the aerobic capacity of the Slow Twitch muscle fibers (that results from a high volume of Zone 2 training) is high enough to take up and productively utilize the inevitable lactate produced by that glycolysis the athlete will be able to sustain a high speed for longer while maintaining low lactates.  That, dear reader is the very definition of endurance.   This is another way of emphasizing the importance of the aerobic base/Zone 2 training for endurance.  I hope you can see why the term aerobic glycolysis is appropriate to describe the process.

Who Should Use Zone 2 Training?

By now it should be pretty apparent that the aerobic base, which is improved by doing a lot of Zone 2 training, is the single most important quality that every endurance athlete needs to develop to his or her maximum genetic potential. Unfortunately, this quality of aerobic capacity takes months and years of consistent Zone 2 training to maximize.  You can’t rush these adaptations.

Without that base, the top of the metaphorical pyramid will never be very high. You can do all the sexy interval training you want, but if the aerobic base is underdeveloped, those intervals will not produce their best effect.  I can unequivocally say that every endurance athlete should be doing Zone 2 training. But there is a big asterisk that goes with that statement.

Zone 2 Training for Elite Athletes

For endurance athletes with a long training history, there is some danger of doing too much Zone 2 training. I know that sounds like the opposite of what I’ve been saying up to this point, and that may be why many others have left this little tidbit out of their Zone 2 conversations. I am going to assume our audience is capable of understanding this if I do a good job of explaining it.

The best example I can think of to illustrate this point is a world-class male marathoner who is able to run 26 consecutive miles at a blistering pace of 4:40/mile. For these athletes, the marathon is competed at the top end of their Zone 2, and their blood lactate hovers around 2.5mMol/L, which, as you might recall, is close to how Polar defined Zone 2. As hard as it might be to believe, from a metabolic standpoint, this pace is very aerobic for these extraordinary athletes.

However, the neuromuscular load at these tremendous speeds is extremely high and takes a huge toll. If you ever wonder why world-class marathoners race so little, that is your reason. It doesn’t take much imagination to grasp what would happen if these runners trained at a high volume in Zone 2. Day after day of running 10-20 miles at 4:40/mile would have them overtrained and injured despite their phenomenal fitness.

I became aware of this problem many years ago when I was coaching a few nationally ranked cross-country skiers. I had them doing a lot of Zone 2 training, and we saw major gains in their speed at this aerobic intensity. At that time, I was ignorant of the problems I just explained. All I saw was their improved speed in Zone 2, so I kept piling on the Zone 2 volume.

The gains continued until the wheels came off for all three athletes at the same time,  just before the major competition season. I had to face the music of having badly overtrained people who had put their faith in me. I was devastated but learned a very valuable lesson that I impart to all the Evoke coaches: first, do no harm.

World-class marathoners tend not to do more than about 10-15% of their overall training volume in Zone 2. They do the bulk of their aerobic base training at much slower speeds and lower heart rates.

It should be noted that lower-impact sports like swimming, cycling, and even mountain running allow elites in those sports to do a higher volume of Zone 2 training with less neuromuscular fatigue.

Zone 2 Training for Recreational Athletes

Because most recreational athletes aren’t as aerobically fit as elite athletes, their Zone 2 pace is going to be lower, and they will not incur the same neuromuscular fatigue. As their fitness increases and the speed that they can maintain in Zone 2 becomes faster, they will need to pay attention to accumulated fatigue levels and be more willing to take very easy days where they train in Zone 1 or switch to another training modality like cycling if their legs are feeling beat up.

This is another point that is largely omitted in many social media posts about Zone 2 training. I’ve seen Zone 2 almost universally presented as “easy,” but Zone 2 can be very challenging for a fitter athlete.

On the other hand, some people suffer from what Phil Maffetone termed “Aerobic Deficiency Syndrome” (ADS). ADS simply means that the person, either by being too sedentary or having engaged in too much HIIT, has an underdeveloped aerobic metabolism and poor mitochondrial function.

Phil and I discussed ADS in detail in Evokecast episode #5, not only its effects on endurance performance but on general health. He has postulated that the epidemic of Type 2 diabetes is a result of a population-wide extreme case of ADS, which is a reflection of poor mitochondrial health.  Luckily, some of the medically trained influencers are making the case for Zone 2 training from a health and longevity standpoint.

For athletes with ADS, Zone 2 training is critical. Fixing the ADS problem must be the priority, and a healthy dose of Zone 2 training is the best way to accomplish that.

The Risk of Training Like a Pro

 One of the most common mistakes amateur and beginner endurance athletes make is to copy the training of an elite athlete, thinking that they can get similar results by following the same plan. The missing ingredient for these less well-trained athletes is the aerobic base, which takes years or decades to fully develop.

It frustrates me to see elite training posted on the internet without the “don’t try this at home” warning that should accompany it. I saw this not too long ago on Outside Online in an article about the Norwegian Ingebrigtsen brothers, who are world-class middle-distance runners. Their training schedule was posted with no context or explanation whatsoever.

An uninformed runner could be tempted to prescribe the same plan to themselves. But in fact, it’s their aerobic base that gives these brothers the ability to handle such an incredibly arduous training plan and benefit from it. Trying to replicate that sort of training without the base of aerobic support would likely, in very short order, wreck a lower-level athlete.

I was recently asked on Instagram by an elite Swedish mountain runner if there was a guideline I could give him for appropriate Zone 2 volume. Unfortunately I couldn’t give him a prescriptive response because it comes down to the specific athlete’s work capacity. This is going to be determined by their genetics and training history, among other factors.

How is HIIT Used for Endurance Sports?

Coming full circle back to HIIT, there is a place and time for intensities above Zone 2. I have used interval training to great effect with professional athletes I coach, such as Tom Evans and Ruth Croft. But I must reiterate that the demanding workouts I prescribe for them are only effective because of their already high aerobic capacities that have been developed over years of base training.

As you can now understand from my earlier explanation of the lactate shuttle, if you do not have a big repository for the lactate being produced when you train at those higher intensities, you will be limited in several ways.

First, you will be limited in the number of repetitions you can complete of this higher-speed work. Next, you will be limited in the overall volume of faster training you can do in a single workout. Finally, you will recover slower from each of these higher-intensity workouts, thereby limiting the frequency and overall volume you can achieve. It’s also worth a reminder that without a proper fitness foundation, the risk of injury increases during higher-intensity workouts.

As with any form of training, the duration, frequency, and overall volume of training are directly related to the benefits you get from it. If you are limited in the ways I just mentioned, you will see fewer improvements from high-intensity training.

Similarly, how to structure high-intensity training depends on what the individual athlete is able to tolerate. Smart athletes will start conservatively and increase reps, durations, and volumes as their tolerance increases.

In a well-crafted program, high intensity is a supplement to, not a replacement for, aerobic base training. If you structure your training properly, with a priority on increasing your base speed through a high volume of Zone 2 training, and then layer on a judicious amount of higher-intensity training, your endurance will be virtually guaranteed to improve.