Low-intensity cardio—sometimes called LISS or “zone 2”—is a previously-underrated form of exercise. It’s finally coming back into fashion after spending years on the sidelines while HIIT and lifting-only routines ran the field. But what is zone 2 cardio, exactly? How can you make sure you’re doing it right? And is it really so bad if your heart rate drifts into zone 3?
What is zone 2 training?
Zone 2 training is cardio done at a steady, low intensity. It has benefits for aerobic training (so you can run faster), anaerobic training (helping you to recover faster between reps and sets, or between intervals), and it’s also an efficient, low-fatigue way to burn calories.
“Zone 2” is a term drawn from the five-zone system of heart rate training, and the same concept is also known by other names and metrics. To runners, it is “easy pace” or “long slow distance pace.” To cyclists, it corresponds to zones 2 and 3 of a seven-zone system of mechanical power. In other sports, like swimming and rowing, it’s often just called “easy” or “low-intensity” exercise.
The hallmark of zone 2 training is that it’s relatively slow, and it’s done at a steady pace. You’ll be aiming for a heart rate of around 70%, but that doesn’t mean doing intervals at 90% and then resting at 50%. It means holding that 70% level the whole time you’re exercising. You can do zone 2 cardio with any steady, repetitive movement. Popular methods are running, cycling, swimming, rowing, brisk walking, fast hiking, or churning away on the elliptical. (Walking can count if you’re new to exercise; as you get fitter, you’ll probably need to choose a slightly harder form of exercise.)
To be clear, it’s still work—it’s not the same as sitting around resting—but it should feel like you could keep going forever if you wanted to. You don’t stop a zone 2 session when you get tired, you stop it because you set out for a 30- or 45- or 60-minute session, and your time is up.
Zone 2 work builds your aerobic base. When you do cardio at this kind of easy pace, your body adapts by growing more capillaries (tiny blood vessels) to get oxygen and nutrients to your muscles more efficiently. You get more mitochondria to power each muscle cell—you have thousands of mitochondria per cell—and you produce more of the enzymes that turn food into usable energy. Your heart and your lungs get more efficient at taking in oxygen and moving blood to your muscles, and your muscles become able to store more carbohydrates (in a form called glycogen) so that more of the carbs you eat are at the ready when you start a run or ride.
For runners, your zone 2 work, often called “easy pace,” should make up most (some say 80%) of your weekly mileage. The fitness you build doing easy running is what allows you to benefit from, and recover from, harder efforts like interval training.
For people whose main sport is more strength oriented (meatheads like myself), zone 2 cardio strengthens the same body systems that help us recover between sets, and even between reps. It increases our work capacity, letting us get more work done in total. As long as you’re eating enough, adding cardio into your routine should make you able to do more work in the gym, not less. (And no, cardio does not kill your gains.)
Importantly, zone 2 training is very low fatigue. More time-efficient forms of cardio and conditioning like HIIT, hill sprints, or CrossFit WODs are great for your aerobic fitness, but those workouts have to be high intensity to work, and you’ll be pooped afterward. They’re great in small doses, but they kind of have to be in small doses.
By contrast, easy pace work is basically free. You can work up to doing an easy zone 2 session every day, in addition to your regular training, and feel fine. Once you’re used to it, you can do zone 2 work on your “rest” days without defeating the purpose of a rest day. As long as you have the time for it, it’s like a cheat code for aerobic gains.
How do you know when you’re in zone 2?
The best way to know if you’re in zone 2 is to pay attention to your effort, and how it feels. Even though “zone 2” is defined in terms of heart rate, I don’t recommend that beginners calculate it from heart rate percentages at first. Unless you’ve actually gotten your heart rate up to a maximum effort level, and know what that number is, you don’t really know your max heart rate.
Instead, let’s talk about what it should feel like. On a scale from 0 to 10, where 0 is nothing and 10 is all-out sprinting, low intensity cardio is about a 3. You’re working, but not very hard.
This pace is conversational, and it’s sometimes described with a “talk test.” If you could tell your friend about your day while you’re exercising, you’re probably around the right level. That doesn’t mean spitting out a few words here and there, but speaking in full sentences without feeling out of breath. If you find yourself stopping to catch your breath or if you feel like you need to take a break, you’re going too hard.
That said, it shouldn’t feel too easy—Zone 2 is more like a brisk walk or easy jog than a leisurely stroll. If you stop, you should feel like you took a break from something. It should take a little work and focus to keep up the pace.
Running
Runners often have trouble with this, because the dividing line between walking and running is sometimes too high to maintain an easy pace while you’re running, if you’re a beginner. The next best thing is to jog as slow as you can, and when you start to feel out of breath, switch to a brisk walk. For some people, a brisk walk for the whole session is a perfect zone 2 workout. As you get more fit, easy jogging will become possible.
Here’s a real world example of how to keep a zone 2 pace: if I’m heading out for an easy run, I start at a nice chill jogging pace. But I live in a hilly neighborhood, so pretty soon I’ll be chugging up a hill and notice my breathing getting heavier. At this point, I switch to a power walk. Jogging the flats and walking the uphills is a great way to keep your effort level steady, instead of accidentally turning your easy jog into a hill sprint workout.
Cyclists often measure their training in a seven-zone system that is based on how much power, in a physics sense, you are putting into the gears of your bike. These are called Coggan power zones, and bike training systems like Zwift and Peloton use the same basic idea. To calibrate these zones, you’ll take an FTP (functional threshold power) test—basically, a 20-minute race against yourself. From there, you (or your training app) calculates wattage numbers for your zones.
Zones 2 and 3 in this system usually correspond to our “zone 2” cardio. (Your heart rate at the end of an FTP test will be pretty close to your max heart rate, by the way, if you really went all-out.) If you train with both a heart rate monitor and a bike power meter, don’t worry if they don’t match exactly. A ride in power zones 2 and 3 will meet your low-intensity cardio needs, even if your heart rate starts in zone 1 and spikes into zone 3 by the end.
For other forms of cardio, go by feel, or by heart rate if you truly know your heart rate percentages. When you’re on the elliptical or the rower or the airbike, you should feel like you could carry on a conversation easily, and stop because time is up rather than because you’re running out of gas.
By 
Nermeen Nabil Khear