I wasn’t a cardio guy for most of my life.
I just didn’t enjoy it.
I knew I was supposed to do cardio because it’s good for my heart and insulin resistance and whatnot. So when I did do cardio, I found ways to make the experience as not sucky as possible.
The first prong of my “make cardio less sucky” strategy was to get it done quickly by doing short sessions of high-intensity training. I’d go hard — sprinting, “cycling” on the Airdyne, or doing burpees and calisthenics — for 15 to 20 minutes and call it a day. But I wasn’t that consistent with my HIIT. I’d aim to do it at the end of my weightlifting sessions, but when the time came, I often found an excuse to skip this planned cardio cap-off.
The other prong of my “make cardio less sucky” strategy was to ruck. I enjoy rucking. But honestly, I tend to sandbag it — metaphorically — when I ruck. I’m just taking a leisurely stroll with 40 lbs on my back. My heart rate hardly gets above 100 beats per minute. I never feel like I’m really “exercising.” It’s better than doing nothing, certainly, but probably leaves a lot of the benefits of cardio on the table.
So, for all intents and purposes, the cardio portion of my exercise routine had been pretty non-existent.
But after my conversation with hybrid athlete Fergus Crawley on the AoM podcast, I’ve started to make cardio — in particular, slow and steady cardio — a bigger part of my life. And it’s all because of what he said about “Zone 2 cardio” and the benefits he’s seen it give him in his own life in terms of both his mental health and his strength-training performance.
Below I share what I’ve learned about Zone 2 cardio since that show. We’ll get into what Zone 2 cardio is, what it does for you, and how to make it part of your life.
What Is Zone 2 Cardio?
To understand what Zone 2 cardio means, you first have to understand how your body uses the food you eat to power your existence.
All cells in your body are fueled by adenosine triphosphate or ATP. When you walk, you’re using ATP. Deadlifting? Powered by ATP. Reading this article? ATP.
ATP can be produced in three ways:
- Through oxygen-dependent metabolism that utilizes fatty acids (oxidization). This is how most of the ATP you use throughout the day is created. When you breathe, oxidation turns fatty acids into ATP. Oxidation creates a lot of ATP. You get a lot of bang for your buck. Oxidation occurs within your cells’ mitochondria. This is an essential point for Zone 2 cardio.
- Through non-oxygen-dependent glucose metabolism (glycolysis). If you’re doing an intense exercise like sprinting or lifting weights, your body switches from oxidizing fatty cells to produce ATP to burning glycogen/carbs to replenish ATP stores. Glycolysis produces large amounts of ATP, but not as much as oxidation. Glycolysis doesn’t occur in your mitochondria but rather in your cells’ cytosol.
- Through the recycling of previously-stored ATP. When ATP transfers energy to cells, it breaks off one of its phosphates and becomes adenosine diphosphate (ADP). Creatine then comes along and says, “Hey, ADP, you can have my phosphate,” turning it back into ATP to once more be utilized as energy. Creatine supplementation can help this process.
Note: we can also get ATP from lactate. Basically, our body can convert lactate into glucose to be used in glycolysis. It’s fascinating but pretty technical. You can learn more about this process here.
Over the years, exercise scientists have developed “training zones” to help athletes and coaches understand which energy systems and muscle fibers they’re using at a given exercise intensity level.
One such system divides training intensity into six zones. Zone 1 is the lowest intensity (sitting down, walking around your house, etc.), and Zone 6 is the highest intensity (all-out sprinting, deadlifting a heavy set of five, etc.).
- Zone 1
- Energy source mainly used: fat
- Zone 2
- Energy source mainly used: fat trending into carbohydrates
- Zone 3
- Energy source mainly used: fat trending into carbohydrates
- Zone 4
- Energy source mainly used: carbohydrates
- Zone 5
- Energy source mainly used: carbohydrates
- Zone 6
- Energy source mainly used: carbohydrates trending into creatine phosphate
As you can see, the type of fuel your body uses to create ATP changes as you go up in intensity. In zones 1 and 2, you’re using primarily fat. As you shift to Zone 3, you start using carbohydrates. When you reach zone 6, you use creatine phosphate to create ATP.
In Zone 2, your exercise intensity is at a level in which you are stimulating your cells’ mitochondrial function the most. You can meet your body’s demand for ATP using only fat and oxygen in your mitochondria. If you were to go a bit harder, your body would start using more carbs in your cells’ cytosol to create ATP through the process of glycolysis.
According to Dr. Iñigo San Millán, an expert in exercise physiology and sports medicine, most recreational athletes don’t spend a lot of time in Zone 2. Instead, they usually skip to zones 3, 4, and 5, where they primarily use carbs as their fuel source. He attributes this to the emphasis on high-intensity training over the past decade. When a lot of people start working out, they’ll do something like P90X or CrossFit — if not those exact programs, then a workout that includes the same kind of form and intensity. We have this idea that if exercise doesn’t feel painful and uncomfortable, it’s not doing anything.
We also jump right past Zone 2 because it’s simply really easy to do, unless you intentionally try to dial back your effort. When you go out for a jog, you probably run at a pace that isn’t a sprint, but does leave you huffing and puffing. This feels like a “normal” pace to you, but you’re almost assuredly in Zone 3 or higher.
Unlike the way most people typically do their cardio, Zone 2 doesn’t feel painful or uncomfortable. In fact, you could probably move at that intensity for hours at a time.
This is why people skip over Zone 2. It’s deceptively easy.
But according to a lot of research, you’re missing out on many health and performance benefits by giving Zone 2 cardio the short shrift.
The Benefits of Zone 2 Cardio
Increases the number and efficiency of mitochondria in your body. Zone 2 is the level at which you’re stimulating your mitochondria the most to create ATP. As you spend more time in Zone 2, your body will respond by creating more mitochondria to power your Zone 2 activity. The more mitochondria you have, the more you’re able to create ATP through fat oxidation. And remember, your mitochondria can make a lot of ATP.
Zone 2 training not only increases the number of your mitochondria, but improves their efficiency as well. The more you exercise in Zone 2, the better your body gets at burning fat for energy. The body’s inefficiency at using fat for fuel contributes to many health problems like insulin resistance, dementia, and even cancer. Basically, if you want to move better, have more energy, and live longer, you want to exercise a lot in Zone 2.
Improves your performance as an endurance athlete. If you’re a runner or cyclist, spending more time in Zone 2 will allow you to go faster for longer. Zone 2 cardio has been used for decades by elite athletes. As I discussed in my podcast with running coach Matt Fitzgerald, one of the things that separates professional runners from amateurs is that the latter spend about 80% of their workout time in Zone 2. In contrast, the weekend warrior runner will spend most of his time in zones 3 and 4. Pros go slow (relative to their fitness, of course).
What’s counterintuitive about spending more time in Zone 2 during your runs and rides is that while it doesn’t feel like you’re going that hard, it does make you a better athlete. You are able to go progressively faster and longer, while your heart rate remains low. As you train your body’s ability to use fat for fuel, you’ll be able to increase your pace without shifting over to carb-burning. And because you’re using fat, you’ll be able to go faster for longer because you have a nearly unlimited supply of fat.
Kate has been doing Zone 2 cardio for six years now, ever since hearing primal-living guru Mark Sisson talk about the idea on the podcast. She’s noticed that even though the bulk of her running workouts are “easy,” her finish times in races are significantly better than they ever were, even in her younger years. That’s the paradox of Level 2 cardio: you go slow, to go fast.
Improves cardiovascular health. Besides exercising your mitochondria, Zone 2 cardio also gives your heart and blood system a workout. Your heart will get stronger and will require fewer pumps to pump blood; it will become more efficient. Your body also expands its vascular system, better enabling it to deliver oxygenated blood to the different parts of your body. This improvement in cardiovascular fitness will result in better performance when exercising as well as a lower resting heart rate. An average adult’s resting heart rate is 60-100 bpm. Kate’s resting heart rate is around 40 bpm. Mine is in the low 60s (that’s what I get for being a late adopter!). I’m hoping it will decrease further the longer I stick with the Zone 2 habit.
Improves work capacity. This is of interest to me as a strength athlete. Zone 2 cardio can help you recover more quickly between lifting sets which in turn can allow you to do more work in a training session. I’ve noticed that I’ve gotten less winded during my barbell sessions as I’ve added Zone 2 cardio to my routine. I just feel like I can get everything I need done during my one-hour workout.
Prevents injury and aids recovery. Zone 2 stresses the body very little. It thus enables you to add volume to your training routine without getting sidelined by injuries or fatigue. Even after doing a long session of Zone 2 cardio one day, you’ll be ready for another the next.
Zone 2 cardio also serves as great “active recovery exercise.” When you’re in Zone 2, you get the blood pumping, which delivers nutrients to your muscles to help them bounce back from a weightlifting session. My physical therapist recommended that I do some Zone 2 training to help my recovery from tennis elbow and an issue with my knee. “It helps get some blood there to help heal things,” he told me, and it indeed seems to work.
Boosts mood. Fergus mentioned how he’s noticed that his mood has gotten better as he’s incorporated Zone 2 cardio into his fitness routine. I’ve noticed these mood-boosting benefits as well. Probably because you’re getting the muscles moving and blood flowing, but aren’t severely stressing the body, Zone 2 cardio generates plenty of feel-good endorphins. It just feels fantastic.
How to Determine If You’re in Zone 2
To exercise at optimal Zone 2 intensity, you want to be exerting yourself at a level where you’re primarily using fat oxidation to power your movement and maxing out mitochondrial function, without switching into burning carbs; you want to butt right up against the boundary to Zone 3, without crossing over into it.
How do you know if you’re residing in this cardiovascular sweet spot?
There are several different ways to figure it out:
Heart Rate Method
The most common way people determine whether they’re in Zone 2 is by tracking their heart rate while exercising.
The idea is that our heart beats at a certain percentage of our maximum heart rate, depending on what zone of intensity we’re in.
When you’re in Zone 2, your heart beats at 70-80% of your maximum heart rate, more or less.
So if your maximum heart rate is 180, your Zone 2 heart rate would have a minimum threshold of 126 and maximum threshold of 144; the more you can stay around the 144 mark without exceeding it, the better.
But how do you figure out your maximum heart rate?
Do a max heart rate test. The best way to determine your maximum heart rate is to test it by running on a treadmill or cycling on a stationary bike. You start running and cycling and increase the speed every 2 minutes until you can’t sustain the pace any longer. Your heart rate at that moment is your maximum heart rate.
Estimate based on your age. If you don’t want to do a max heart rate test, you can estimate your maximum heart rate using your age. The formula is 220 minus your age.
So if you’re 30 years old, your maximum heart rate would be 190 (220 – 30 = 190).
70-80% of 190 would give you a Zone 2 heart rate range of 133-152, and you would want to stay as close to that 152 number as possible, without going over.
This method isn’t very accurate. Your max heart rate might be more or less depending on your fitness level, but this will at least get you in the general ballpark.
Athletic performance coach Phil Maffetone was a pioneer of the “train slow to go fast” idea, having advocated this approach back in the 1980s. While he had plenty of critics over the decades, a lot of folks have now come around to, if not the exact formula he uses to get your Zone 2 heart rate, his general philosophy.
That formula, like the age-based equation described above, isn’t very accurate, but will get you in the Zone 2 ballpark.
The number the Maffetone formula gives you is the heart rate that will put you at the upper limit of Zone 2. The formula is as follows:
Subtract your age from 180, then modify based on the categories below:
- If you have or are recovering from a major illness (heart disease, any operation or hospital stay, etc.), are in rehabilitation, are on any regular medication, or are in Stage 3 (chronic) overtraining (burnout), subtract an additional 10.
- If you are injured, have regressed or not improved in training or competition, get more than two colds, flu, or other infections per year, have seasonal allergies or asthma, are overfat, are in Stage 1 or 2 of overtraining, or if you have been inconsistent, just starting, or just getting back into training, subtract an additional 5.
- If you have been training consistently (at least four times weekly) for up to two years without any of the problems mentioned in a) or b), no modification is necessary.
- If you have been training for more than two years without any of the problems listed above and improved competitively, and are without injury, add 5.
So if you’re 40 years old, the upper limit of your Zone 2 heart rate using this formula would be 140, more or less.
The easiest (and yet still surprisingly accurate) way to determine if you’re in Zone 2 is to do the “talking test.” When you’re exercising in Zone 2, you can comfortably have a conversation, but will still sound a little breathy; you’re not able to talk as clearly as when you’re at rest, but you’re still capable of talking.
Physician and health/performance guru Peter Attia has the best description of what this feels like. He says when he’s in Zone 2, he can comfortably have a conversation with someone on the phone for hours, but the person on the other end of the phone would notice that he was exercising.
Personally, how I determine if I’m in Zone 2 is a combo of the Maffetone formula and the talk test. I use the Maffetone formula to figure out roughly where my Zone 2 heart rate is. I’ll work up to that heart rate and then use the talk test to refine my intensity.
Measure Your Lactate
The most accurate way to determine if you’re in Zone 2 is to measure your lactate after your training session. If your lactate is between 1.7 and 1.9 mmol, then you were in Zone 2.
The downside of this method is that you have to stick yourself to get a drop of blood for the test. That’s annoying. Also, the test strips are expensive — like $4 a pop.
This method is for the people who really want to geek out on this.
How to Get Your Zone 2 Cardio
You can get your Zone 2 cardio however you want. If you like to run, run. If you like to bike, bike.
Regardless of what exercise modality you choose to get your Zone 2 cardio, the tricky part about this kind of training is consistently keeping your heart rate within its parameters. You want your heart rate to pass from Zone 1 and into the upper strata of Zone 2, but not rise into Zone 3 — which is an easy jump for it to make, even when you’re not pushing yourself very hard.
Let’s say you start jogging right now. Your heart rate will likely surge way above your Zone 2 target. You then have to start walking to bring it down. At which point it falls below your Zone 2 threshold. So you start jogging again. And your heart rate once more jumps past your Zone 2 upper limit. So you start walking again. Your workout ends up being a series of frustrating fits and starts.
For this reason, full-body, high-exertion exercises like running and swimming are hard ways to do Zone 2 cardio, at least when you’re first getting going with it. Look to do less taxing forms of cardio instead. Since cycling only uses half your body, it’s a good option. So is using cardio machines like the elliptical. Or crank a treadmill up to its highest incline and take a walk. In our experience, using a special incline treadmill, where you can set the incline all the way to 30-40 degrees (a standard treadmill tops out at 15 degrees) and then walk while holding onto handles, is especially good for getting your heart rate high-but-not-too-high, and offers the perfect, highly satisfying Zone 2 workout.
Whichever cardio machine you choose, the benefit of using one is that you can set a very steady pace/resistance/incline (without sudden heart-rate-spiking hills) which will allow you to consistently stay in Zone 2. Start each workout slowly, as your heart rate has a tendency to spike when you first initiate exercise, and, as long as you’re keeping your warm-up nice and easy, don’t worry too much if your heart rate spikes some during the first 5-10 minutes of your training session; it takes awhile for your heart to settle into a steady state rhythm, which is why, as we’ll discuss below, it’s recommended that every Zone 2 session be a minimum of 45 minutes long.
It’s possible and desirable to eventually do your Zone 2 runs out of doors. As Fergus told me, it’s the combination of Zone 2 cardio + nature that has most helped his mental health. With high intensity exercise, and its attendant spike in heart rate, your higher-level thinking starts to shut down. With Zone 2 training, the engaged-but-near-automatic movement pattern of your run keeps your body occupied, while your mind remains unstressed and clear; it’s a great way to find flow. Fergus says it’s his version of zen. Even though he’s out there for hours at a time, he trains without headphones, as he loves the opportunity to switch off and let his mind wander as he runs for mile after mile through fields and along rivers. In fact, he makes a list of things he wants to think about before he heads out, giving himself prompts as what he’d like to reflect on as he runs. You may find that your outdoor Zone 2 training sessions become your best brainstorming times. Kate often receives and refines Sunday Fireside ideas while she’s out trail running.
If you want to do outdoor Zone 2 runs, without having to run 15-minute miles and move in walk/jog fits and starts, first build up your Zone 2 fitness base by walking on an incline on the treadmill. This obviously isn’t the same as running, but it will improve your cardiovascular fitness using a similar movement pattern to running.
Next, start jogging on a flat/non-inclined treadmill. You should be able to stay in Zone 2 by running at a slow pace, like 5 miles per hour. As your training progresses, increase your pace while staying in Zone 2. You should be able to run progressively faster, while keeping your heart rate in the same place.
Once you’ve built a solid Zone 2 training base on the treadmill, take your running outside, starting by running a flat course (no hills). Once you can run a flat course while staying in Zone 2, you can start running a route that includes hills. No matter how fit you get, though, there may be hills you’re never able to ascend while staying in Zone 2 without walking up them.
How Much Zone 2 Cardio Do You Need?
There are a lot of numbers thrown around as to how much Zone 2 cardio you need each week.
Based on all the articles and podcasts I dived into, 150 to 180 minutes a week of Zone 2 cardio is a good number to shoot for as a minimum.
According to Dr. Iñigo San-Millán, to get the full benefits of Zone 2 cardio, you want your sessions to be at least 45 minutes long. That seems to be the minimum effective dose to get the mitochondrial benefits of Zone 2 training.
So if you’re aiming to do 180 minutes of Zone 2 cardio a week, you could do four 45-minute weekly sessions, or you could do your 180 minutes in one very long session. Find what works for you.
Personally, I’ve been doing three 60-minute sessions a week. That works for me.
60 minutes is a pretty long time to be slowly spinning along on an elliptical. To make my Zone 2 cardio something I look forward to, rather than avoid, I’ve done some temptation bundling with it and watch movies and TV shows while I work out. Right now, I’m rewatching 30 Rock during my Zone 2 sessions. Three episodes will get me through an hour of Zone 2 cardio, and as someone who doesn’t watch any television outside of exercise hours, this makes the workout feel like a treat.
While the above is ideal, San-Millán is quick to point out that any amount of Zone 2 cardio is better than none. If you can’t squeeze in 180 minutes a week, then get what you can. Don’t let the perfect be the enemy of the good!
Finally, while Zone 2 cardio should make up the majority of your cardiovascular exercise — 80% is a good proportion to aim for — doing a small amount of high intensity cardio is still important for health, fitness, and athletic performance. One weekly HIIT session, where you go all-out, should do it.
What you don’t want to be doing is spending all your time in what Attia calls the “garbage zone,” where you’re not exercising at a high enough intensity to improve your anaerobic fitness, and you’re not exercising at a low enough intensity to improve your aerobic fitness. Get out of this all-too-typical no-man’s-land of exercise! Really push yourself when you push yourself, and the rest of the time, take up residence in the slow-and-steady, health-wonder-working Zone 2.
Take a deep dive into the ins and outs of Zone 2 cardio in this podcast interview I did with Alex Viada:
Zone 2 Resources to Check Out
If you’d like to learn more about Zone 2 training, I highly recommend listening to these two podcasts with Peter Attia and Iñigo San-Millán. They take a deep dive into what’s happening in your body with Zone 2 training, the benefits, and how to measure it. They were a really useful resource for me as I learned about Zone 2 cardio. Really fascinating stuff!