Reviewed by Brian St. Pierre, MS, RD
“Cargo pants are back.”
This was the news that Brian St. Pierre, PN’s director of nutrition, broke when we met.
(St. Pierre, a father, found this out via his 12-year-old daughter.)
Why does this matter? Apparently, a certain type of exercise is running a parallel cycle: An old trend resurfacing as a new “it” thing.
Cardio’s back, baby.
Specifically, zone 2 cardio—also known as steady state cardio, low intensity steady state cardio (LISS), or what your treadmill may call the “fat burning zone” (more on this term later).
When I got into the fitness industry over a decade ago, cardio was at its peak of being disrespected.
“Are you trying to lose all your muscle??” the naysayers said.
St. Pierre—who’s coached top athletes in the NBA, NFL, MLB, and the NHL—remembered:
“You either did intervals, or you lifted. Maybe both. Steady state cardio was for endurance athletes only.”
Now, as steady state cardio makes its triumphant return, interval training seems to be getting tagged as overrated. (Lifting, of course, is as badass as ever.)
So, what’s the deal? Is there a “best” form of cardio? Is zone 2 exercise worthy of the hype?
In this article, you’ll find out. You’ll also learn:
- What zone 2 cardio is (and how to know when you’re “in it”)
- How much zone 2 cardio you should do per week to reap the health and fitness benefits
- How zone 2 cardio compares to other forms of exercise
- What the potential downsides of zone 2 cardio are—and how to mitigate them
Let’s get to it.
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What is “zone 2 cardio,” anyway?
St. Pierre struggled to give a simple answer to this question. Not because he didn’t know, but because it’s a trickier question than you might think.
In a nutshell though:
Zone 2 cardio is sub-maximal aerobic training—meaning, aerobic exercise that’s performed below your maximum effort.
But St. Pierre offers some caveats:
“Zone 2 training could mean different things in different contexts,” he says.
“How elite athletes measure and train zone 2 is going to be different from what my mother would be doing.”
Elite endurance athletes use precise (often expensive) tools to ensure they’re in zone 2 (such as lactate meters and power meters). They strive to improve zone 2 fitness to maximize performance.
Regular people, though, tend to train in zone 2 without using gadgets—just some simple body awareness cues—with the likely goal of improving overall health.
Zone 2 cardio examples
Any intentional physical activity that feels stimulating, but still relatively easy—like you could go for an hour, or even hours—counts as zone 2 cardio work.
For example:
- Casual cycling
- Using the elliptical machine
- Hiking or walking uphill
- Rucking (walking with a weighted backpack or vest)
- Slow jogging on flat terrain
- Rowing (using a machine, or if you’re lucky, a canoe on a calm lake)
Zone 2 cardio compared to other cardiovascular “zones”
Different levels of exertion—as measured by heart rate—are categorized into five different “zones.”
(This is a common model, but here are other zone models that have seven zones. And models that are based on power rather than heart rate. See how quickly this can get complicated?)
Each zone will use different energy sources at different rates, and will have unique benefits, as the table below shows.
| Zone | % of Max Heart Rate | Main Energy source* | Feels like… | Examples | Benefits |
|---|---|---|---|---|---|
| 1 | <60% | Fat | Comfortable; can nasal breath easily and hold a conversation | Walking or light household activities | Increases overall activity, improves blood flow |
| 2 | 60-70% | Fat | Can maintain nasal breathing, but not comfortably | Light jogging, hiking, cycling, elliptical | Improves aerobic base without impeding recovery |
| 3 | 70-80% | Fat and carbs | Tough to maintain a conversation; will need to start breathing heavier | Jogging or cycling at faster pace (but not sprinting) | Improves aerobic and anaerobic fitness, and lactate threshold |
| 4 | 80-90% | Carbs | Huffing and puffing; might be able to get out a few words | Running, cycling or using a machine for for 1-4 minute sprints | Improves power output, VO2 max, lactate threshold, and overall athletic performance |
| 5 | >90% | Carbs | Near or at maximal effort; heart pounding and talking is impossible | Maximal sprinting | Improves VO2 max, heart rate max, and fitness at maximal levels |
*Exercise intensity is the most important determinant of which energy source is used during exercise. However, the proportion of energy sources used is affected by several factors, including exercise duration, age, sex, body composition, training status, and diet.
Why zone 2 cardio is sometimes called “the fat burning zone”
Zone 2 cardio is a form of aerobic exercise.
Aerobic means “with oxygen,” which means the body needs oxygen in order to produce ATP—our primary source of energy—to power this kind of activity.
Anaerobic exercise—like sprinting, intense cycling, or heavy weightlifting—doesn’t require oxygen to produce the energy (ATP) needed to fuel it.
Instead, anaerobic activities use readily-available sources of energy—primarily carbohydrates stored in the muscles and liver. These energy sources can be accessed rapidly, but run out quickly. And, it takes time—and possibly a big sandwich—to replenish them.
Meanwhile, aerobic exercise taps into energy reserves gradually, primarily burning body fat for fuel. This process is slower, but the energy reserve is much larger (even among lean individuals) and thus can sustain activity for longer.
This is why the treadmill at your gym may call zone 2 cardio “the fat burning zone”.
When engaged in steady, moderately paced aerobic work, your body uses fat as its primary energy source.
If the term “fat burning” perked your ears, just keep the following in mind:
- Other forms of cardio—such as high intensity interval training (HIIT)—are equally effective in terms of fat loss, on average.1 2
- Exercise alone doesn’t tend to yield significant changes in body fat. Meaning: There’s nothing “magical” about zone 2 cardio’s ability to burn fat as it relates to losing fat and body weight.
Truly, the most effective exercise—in terms of fat loss and overall health benefits—will be the kind(s) you enjoy, and are able to do most consistently.
Three big benefits of zone 2 cardio
So, why might you incorporate zone 2 cardio work into your routine (or suggest your clients do the same)?
Let’s discuss three strong arguments for jumping on this trend.
Benefit #1: It builds your aerobic base.
St. Pierre offers this analogy:
“Imagine your overall cardio fitness is a pyramid: The base is your aerobic fitness, and the top is your peak anaerobic fitness,” he says.
“If you only train the peak, the structure is top heavy; it’s not built to last.”
This is one of the biggest assets of zone 2. Training at the peaks may be fun (in a masochistic way), but it’s not the best way to build your base.
To see how this works, let’s use an example with St. Pierre’s sport of choice: Hockey
On the ice, you’ll be mostly fueled by the anaerobic system.
With a strong aerobic base, you’ll recover quickly between “sprints” on the ice while resting on the bench.
Without a good aerobic base, your body may actually stay in an anaerobic state while you’re bench-warming. This not only inhibits recovery; it also drains precious energy reserves.
(And if you burn through your reserves in the first period, those second and third periods are going to suck.)
This ability to adapt to changes in physical demands is called metabolic flexibility4—and zone 2 cardio is particularly good at enhancing it.
With good metabolic flexibility, your body can toggle between energy sources as needed (instead of using mostly glucose or mostly fat all the time) to power activity, leading to better endurance, power, and performance.
Benefit #2: It’s exercise that gives more than it takes.
Intense workouts are both mentally and physically draining. They also “cost” a fair bit, from a recovery perspective.
Not zone 2 exercise.
“Zone 2 cardio may even help your recovery in between sessions,” says St. Pierre. “At worst, it’s going to be recovery neutral.”
Cycling on a bike at a relatively low intensity for 45 minutes might not be the most fun, but it’ll improve your overall fitness without adding much stress or demanding recovery in the same way intervals would.
If you have time to train five hours per week, but only have the energy to train all out for two to three hours, that still leaves you with time to train—just at a lower intensity.
Many folks have an “all or nothing” mindset and get mad at themselves for not being able to train like a beast for all five hours. But you don’t need to. Three hours of intense training and two hours of low intensity training is amazing.
Benefit #3: It boosts mitochondrial health—which might help you live better, longer.
One of the promises of zone 2 is that it can improve mitochondrial health.
Better mitochondrial health means a lowered risk of many diseases, such as type 2 diabetes, cardiovascular disease, metabolic syndrome, and cancer.5
Zone 2 cardio might be the most effective form of exercise to maximize mitochondrial health6 (though the research supporting this has participants doing many hours of zone 2 work per week).
Fortunately, all physical activity—including interval and resistance training—supports and improves mitochondrial health.7 8 9
Plus, effectiveness is a spectrum. Obsessing over having “the best mitochondrial health possible” is pointless if you can’t consistently perform the amount of exercise it takes to get there.
“How do I know if I’m in zone 2?”
Understandably, many people (including your clients) will ask. There are several ways to assess if you’re in zone 2, ranging from “fancy and high-tech” to “luddite-approved.”
Tracking method #1: Gadgets
If you’re a high level endurance athlete fixated on tracking hard data, a lactate meter will be your most accurate measurement tool.
If you’re just looking to achieve better overall health and aerobic fitness, you can use a heart monitor. (Try a chest strap or a wearable wrist watch that tracks heart rate.10)
Tracking method #2: Math
If you want to use your heart rate to calculate if you’re in zone 2—which is about 60 to 70 percent of your heart rate max—you first have to figure out your max heart rate.
The simplest way to estimate your heart rate max is to take 220 and subtract your age. Calculate 60 to 70 percent of that number, and you’ll get your target zone 2 heart rate range.
For example, if you’re 42 years old:
220 – 42 = 178 beats per minute is your maximum heart rate
0.6 (or 60%) x 178 = 106.8
0.7 (or 70%) x 178 = 124.6
So, if your heart rate is between 107 and 125 beats per minute, that puts you in zone 2.
(Another common approach: Take 180, subtract your age, and that’ll give you the top of your zone 2 range.)
Of course, if you’re in that range and can’t talk, nasal breath, or focus on anything other than just… keep… going, you’ll know you’re not in zone 2.
Sometimes, the body knows best. Which brings us to…
Tracking method #3: Body awareness
Without gadgets or formulas, can tell if you’re in zone 2 if:
- You’re doing a form of cardio that requires effort—but also feel like you could perform it for an extended period of time
- You can breathe through your nose
- You can talk (but perhaps not sing very well)
- You could pay attention to a podcast, movie, or have a thoughtful conversation
As St. Pierre eloquently put it:
“When you’re done with your session, you should be able to say you could do it again if not for time and boredom.”
TL;DR: Zone 2 work shouldn’t crush you.
How much zone 2 cardio should you do per week?
The shortest (and most practical) answer: Whatever you can fit in.
If you have more time and want some specificity, the WHO and the CDC suggest between 150-300 minutes of moderate-intensity aerobic activity per week. (Or 75-150 minutes of vigorous intensity aerobic activity. Or even a combination of the two.)11 12
For example, let’s say you did a 30 minute interval session. This is equivalent to 60 minutes of moderate (Zone 2) exercise. So you would only need another 90-240 minutes of moderate aerobic exercise to reach the recommendations.
Weekly, that could look like three 30 minute-sessions, two 45 minute-sessions, or one longer 90-minute session.
But don’t get bogged down by specifics. Any cardio is great if you haven’t been doing any.
How long should zone 2 cardio sessions be?
You’ll commonly hear sessions need to be at least 45 minutes.
Your aerobic system doesn’t fatigue easily during zone 2 work, so duration is somewhat important if you’re aiming to maximize adaptations.
If you go with the WHO and CDC’s guidelines, two to three 45 to 75 minute sessions of zone 2 cardio per week is pretty ideal. (Note: If you’re a competitive athlete, you’ll probably need more.)
But if you can only fit in 25 or 30 minutes a week total, it’s not pointless.
“Any amount of activity improves health, so while yes, more is better, anything you can get in will make a difference for your wellbeing,” says St. Pierre.
You may not get the maximum benefits by doing less, but you’ll experience many amazing health improvements by getting in some cardio.
Factor in your goals.
Don’t get so fixated on zone 2 that you dedicate all your workout time to it and lose the benefits you can get from other kinds of training.
And, consider your fitness priorities.
If you want to build muscle and strength, resistance training should be the focus of your training. (In other words: Don’t cut your strength workouts in half just to squeeze in ideal zone 2 training targets.)
“I hate cardio” and other barriers to zone 2 work
Have you ever seen the show Suits? It’s not the Sopranos, but it’s entertaining and full of tea.
St. Pierre hadn’t seen it—that is until he started watching it while doing zone 2 work on his bike at home.
Zone 2 training isn’t his favorite way to train. With this, he found a way to make it enjoyable.
If you hate cardio, find ways to turn down the suck.
Here are a few ways to do that.
Consume that sweet, sweet content.
Whether it’s watching a fun TV show or listening to an audiobook or podcast, you can offset the tedium of zone 2 cardio with something you enjoy.
Play.
You can also get zone 2 work with sports or various leisure activities.
Personally, I like to shoot around the basketball with my heart rate monitor on. (Yes, there will be times I’m at a higher heart rate zone than what is truly zone 2, but that’s okay. I’m not an elite endurance athlete, so precision isn’t crucial.)
Grab your frisbee, pickleball racket, or ball-of-choice, and have fun.
Make it work with your schedule—and life.
“I just don’t have the time.”
If this is your primary obstacle, incorporate zone 2 cardio in a way that supports your life.
Some examples:
- If you can, bike to commute to work. Especially in busier cities with lots of traffic, this can actually be more time efficient than driving or taking transit.
- Run your errands with a purpose. Walk briskly to the store (or around the mall), and carry your groceries if you can.
- Do domestic chores like you mean it. More laborious house work such as cutting the grass, shoveling the snow, or vacuuming—anything that takes a while and takes some effort—counts.
- Run around with other animals. Your kids and your pets are hard to keep up with, right? Make their week and chase after them at the park or local rec center. Alternatively, pull them in a wagon or take a brisk walk pushing the stroller.
If these activities don’t keep you in zone 2 the entire time, that’s okay. These are just ideas for those who simply don’t have the time for more structured cardio.
Start with less.
If 45 minutes of anything still sounds like too much, just start with 10 minutes. You can always build up from there.
Ignore what’s optimal, and integrate what’s practical.
Something is truly better than nothing. (If you’re strapped for time, remember that line.)
Another tool in the kit
Trends in the fitness industry are cyclical. (Kind of like trends in pants-with-pockets.)
Training styles will come and go. When one comes back in, remember this and temper your response. Nothing—no food, exercise, or supplement—is a magic bullet.
The zone 2 cardio trend has been awesome for re-inspiring folks (including myself and St. Pierre) to do more cardio.
It’s also been confusing to some, leaving them even more stressed about how to train “the optimal way.”
At PN, we’re less fussed about what’s theoretically optimal than what’s practically optimal. Do the best you can. Find activities you enjoy. And do those consistently.
References
Click here to view the information sources referenced in this article.
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