Zone 2 Training, Part 1: The Basics

Zone 2 Training, Part 1: The Basics

It's time to talk about Zone 2 training, the simple idea of exercise at (relatively) low heart rates. It is everywhere lately, from medical research, to coaching, to Netflix's new Blue Zone series about longevity. But people over-complicate it, caricature it, and generally misunderstand why Zone 2 training can be useful, even if it's no magic fitness elixir.

So, we're going to dig in with a four-part series on Zone 2 training. This first installment is free, but the rest will be a mix of our usual free and subscribers-only. We will make all four parts available afterward as a downloadable package.

Here is what we have planned:

  • Part 1: Introduction and basics

What is Zone 2? Why do we care? What do we need to know? What benefits should I expect? What claimed benefits should I not expect? Do I only need to do Zone training?

  • Part 2: Zone 2 myths

There are many claims made about Zone 2 training. Which ones are accurate? Which ones aren't? Do heart rate zones even exist? Do pro athletes do this? Does it matter if they do? And lots more.

  • Part 3: The science underlying low-intensity training

Why low-intensity training works on a physiological and metabolic level. Why it isn't magic and is based on the nature of energy substrates, mitochondrial health, fat oxidation, and human adaptation to load.

  • Part 4: Gear recommendations and techniques

What tools, trackers, and other devices do I need? What just clutters things up? What are the best ways to figure out my zones?


Called by many names, like low-intensity or as a component of polarized training, Zone 2 is fundamental to our views about simplifying longevity. We are going to be writing about it a lot, plus suggesting gear, launching an app, and doing much more over time, so think of this as a taster.

Let's clear up some stuff first. Here is a non-exhaustive list of things Zone 2 training will not (likely) do for you:

  • Make you skinny
  • Make you smarter
  • Cook you dinner
  • Make your kids say 'please" or clean up after dinner
  • Let you live forever

Granted, not all these claims are made for Zone 2 training, but it doesn't take long reading the non-academic literature before you start to feel like Zone 2 training is a fitness magic trick. And it isn't.

Now, here is a fairly exhaustive list of things Zone 2 training can do for you:

  • Improve your "aerobic" (fat oxidation) fitness
  • Help you avoid injury and the hellscape of detraining

These may not sound like much, but, as we will go into, they are everything. (Granted, it would still be nice if kids cleaned up after dinner, but we can't have everything.)

What is Zone 2 Training?

In the simplest terms, Zone 2 training is the act of getting your heart rate up enough and for long enough to build aerobic fitness, but not so much that you start panting like a crazy person. That is pretty much it. Enough to make a difference, but not so much that you slobber on strangers.

And higher fitness is a good idea because there are reams of research showing that higher levels of aerobic fitness have myriad benefits, from metabolic health, to longevity. Remember one of our tenets: Movement is a miracle drug.

Movement is a miracle drug

Loosely, exercise physiologists and trainers think about heart rate during activity in terms of zones. Lower heart rates correspond to lower zones, like zones 1 and 2, and then you go up from there. There can be as many zones as you want, although we rarely see more than seven. (We will save why you want more zones, or less, for another post.) Zone 2 is, as the name suggests, the second heart rate zone from the lowest.

Defining Zone 2

You might think it would be easy to say where Zone 2 is, given all the attention and interest. And it is, really, but people complicate the picture unnecessarily. Here it is: Zone 2 starts at the top of Zone 1, and ends at your aerobic threshold.

Of course, this just opens up two more questions:

  • Where is Zone 1?
  • What is my aerobic threshold?

We will get into this in more specifics ahead, but here are some practical definitions. Zone 1 is a warmup heart rate, the heart rate that a normal person (not a trained athlete) touches when you walk quickly, hike at a moderate pace, bike at a steady pace, jog slowly, etc. You are at the top of Zone 1 and entering Zone 2 when you go past what feels like light exertion.

Zone 2 is a level of effort you could do for some time, but not forever

Zone 2 starts at the top of Zone 1, and extends to your aerobic threshold, so you need to stay at least a few beats below that. The latter is the level of effort at which your breathing changes perceptively. One loose proxy is your aerobic threshold is the heart rate around which you go from speaking comfortably in paragraphs to only being able to speak in short sentences. You're still comfortable, and could likely continue doing it for some time, but you are working.

You will find endless calculators across the Interweb for this stuff. Go Google your brains out. And they're useful, but they're by no means definitive. Studies show immense zone variation across we bipedal apes, even trained humans/apes who are the same age and gender. So, calculators are a decent starting point, but they are only that, a starting point. We will go into some other ways of figuring out your zones in Part 4 of this series.

There are also many Sports Scientists who approach zones from a physiological perspective. Meaning–what is happening with energy partitioning (burning fat [fat oxidation] vs. glucose [glycolysis]) in our body, as well as our lactate levels. For dialing in the training of elite athletes, this level of granularity is useful. Perhaps you might find it useful too. In future posts, we will explore this aspect of zone calculations as well.

Think of the top heart rate of Zone 2 as a ceiling, not a goal

Why Zone 2 Matters

While some people make it sound like bioenergetics black magic, Zone 2 isn't. It's actually something that's been well-known in the training community for a long time, as well as in the related exercise physiology literature.

We need to go on a couple of side journeys at this point because there are various ways to think about what happens when you exercise, and why low-intensity training does certain useful things to your body.

Energy to move muscles comes from ATP, which is made within the mitochondrion.

Human exercise requires energy. Energy comes from ATP (adenosine triphosphate), an organic compound that is (mostly) made in the mitochondria. Among the important things about ATP is that we have virtually none stored; it's almost all made and used in real-time. A human at rest uses around 100 pounds of ATP every day but never has more than a few seconds worth of it stored, perhaps a gram.

You can begin to see why mitochondrial health is crucial. The more mitochondria you have, and the better they are at producing ATP, the better, all else equal, your body will be at producing energy, whether for digesting food, buying groceries, going for a hike, or, yes, running a marathon. Mitochondrial health is the real driver when it comes to improving human longevity. Poorly functioning, inefficient, and otherwise less effective mitochondria are turning out to be at the root of a huge number of human health problems.

You can improve your mitochondrial health through exercise

So, how is mitochondrial health improved? Through exercise. And the more of it, the better. The more time you spend forcing your body to produce more mitochondria and improve their efficiency, the better your body will function: less inflammation, more economy, and less overall metabolic and cardiovascular stress. (And more cardiovascular slack, which we've written about previously.)

This brings us back to Zone 2. While your body, within reason, doesn't care what you do with respect to exercise, it cares that you do something, do it regularly, and do enough of it for it to force metabolic and physiological changes. Those changes can be muscle growth, improved bone density, or, yes, improved mitochondrial health. Zone 2 makes it easier to get in enough "exercise area under the curve" to drive those improvements.

Low vs. High-Intensity Training

But Zone 2 isn't magic. Mitochondria are produced in response to all exercise loads. There is evidence, for example, that low-volume, high-intensity exercise (sometimes called HIIT) is more effective than low-intensity training in mitochondrial production on a per unit time basis. But that data is built around biomarkers of mitochondrial production circulating in our blood, not necessarily studies demonstrating a higher incidence of mitochondrial production.

There is another hitch, however. While HIIT is effective in the lab, in the real world results are more mixed. One reason is that most people don't take seriously enough the "intensity" part of HIIT: Exercising at high intensity is really hard. It's intense. You are likely pushing up against your maximum heart rate, and feeling like crap. Most people doing HIIT don't reach high enough heart rates, and don't do it for long enough to make a difference, thus defeating the purpose of HIIT.

As we say often, most people run too fast on their slow days and too slow on their fast days. Zone 2 is all about trying to get you to exercise easy enough on your easy days that you get in lots of easy days without injuries that derail you. Going hard all the time can drive metabolic volume, but it can also put you into physiotherapy for weeks. (And there are other more insidious effects that we will get into in Part 3.)

Zone 2 and HIIT can both work, so combine them

So, why Zone 2 vs. HIIT? The most honest answer is, Why choose? Do both. There is nothing wrong with mixing up high and low-intensity training. This is the essence of polarized training, after all: training at two poles, high and low intensity. We can and will debate how much time to spend at each intensity, but there is no need to be so binary.

What's Next

  • Part 2: Myths about Zone training
  • Part 3: The Science
  • Part 4: Gear & Practical Tips

This is the end of Part 1 of our look at Zone 2 training, and its merits. In the next installment, we will deal with many of the myths about Zone 2 training, which are endless, from people making it sound like a magic potion that can solve everything (it can't), to people calling it an unsupported fad (it isn't).

Next, in Part 3 we will do a deep dive into the underlying science, which we have alluded to in general terms. While it can get somewhat technical, it is important to understand, however, if you really want to know what your body is doing while you're plodding around in your over-cushioned trainers.

Finally, in Part 4 of this series we go into more detail about gear, as well as about better methods of figuring your various thresholds, in particular your aerobic threshold. While you can get by without these methods, if you really want to get on top of Zone 2 training, it helps immensely to know.

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