Zone 2 Cardio: Definition, Benefits & How To Know When You're In It
The best kind of exercise is the one that you'll actually do. And you're more likely to do a morning session that won't wipe you out before work or an evening workout that you can handle after a long shift. Enter: Zone 2 cardio training.
Slowing things down to train in zone 2 will benefit your cardiovascular system, mitochondrial health, mood, and more. Here's a primer on why zone 2 is so beneficial and how to incorporate it into your training—whether you're an elite athlete or an occasional fitness class junkie.
What is zone 2 cardio?
Zone 2 cardio is an exercise performed within a heart rate zone that represents 60% to 70% of the maximum heart rate (MHR). Your heart rate is typically split into five zones, so zone 2 will feel like a light exercise that you can continue for a prolonged period of time. Some reasons to incorporate zone 2 into your training are increased cardiorespiratory endurance, improved VO2 max, and more benefits that we'll get into later.
The 5 zones of training.
Lisa Niren, CPT, trainer and vice president of content at Fiture Fitness, explains that "Your heart rate is one of the best indicators of how hard your body is working during a workout."
Typically, Niren explains, your heart rate can be divided into the following five zones. These are also the zones that most fitness companies like Polar, MyZone, and Garmin use to program their devices:
- Zone 1: Very Light - 50-60% HR max
- Zone 2: Light - 60-70% HR max
- Zone 3: Moderate - 70-80% HR max
- Zone 4: Hard - 80-90% HR max
- Zone 5: Very Hard - 90-100% HR max
Each zone also correlates to a different rating of perceived exertion (RPE), which is a numeric scale from 1 to 10 that the person exercising can use to represent how hard they feel they are working. Kara Kilian Lazauskas, M.S., CSCS, a sports performance coach, states that zone 2 cardio has an RPE of 4 or 5.
So to determine whether you're in zone 2, you can go off perceived exertion or calculate your maximum heart rate and work backward from there.
How to calculate your maximum heart rate.
When discussing heart rate zones, it's best to start with how to find your maximum heart rate—and there are three ways to do so: a stress test1 with a medical professional, a field test, or an age-based prediction equation. While a stress test might not be an option for the average fitness enthusiast, field tests and equations are.
Field test (with wearable):
Doing a field test using a heart rate monitor is one way to measure your maximum heart rate. Polar released the world's first wireless chest strap heart rate monitor, and it's more accurate than wrist-based heart rate monitors, according to a 2019 study in Cardiovascular Diagnosis and Therapy2.
With that said, a 2020 JMIR Health study found that devices like MiBand and Garmin "generally produce accurate heart rate readings" compared to the Polar H7 chest strap, so it's largely a matter of preference.
Once you've got your heart rate measuring device, you can identify your MHR using a field test—a practical way to perform an all-out, maximal-endurance effort that will get you to your highest possible heart rate.
Lazauskas says that depending on your fitness level, the Cooper Run Test (run as far as you can in 12 minutes), the George Jog Test (a shorter jog), and the Rockport Walk Test (walk as fast as you can for 1 mile) are "great MHR field tests due to their minimal use of equipment."
You can also use a simple equation to get a picture of your ideal heart rate.
Most doctors will tell you that your maximum heart rate is found using the equation: 220 - your age = MHR.
So, the average 30-year-old would have an MHR of 190 beats per minute (BPM).
However, a newer formula used among trainers is 208 - (0.7 x your age) = MHR
This would put a 30-year-old at a MHR of 187.
A recent study of recreational marathon runners3 compared both of these equations to the Polar chest strap and found that, for women, both equations overestimate MHR by about 5 BPM. For men, the 220 - age equation underestimates MHR by about 3 BPM, but the 208 - 70% of age equation is similar to the chest strap.
The science behind zone 2.
When we train in zone 2, we use oxygen to maintain a low-intensity cardio workout. Transport of oxygen throughout the body requires the coordinated functions of the heart, lungs, blood vessels, and blood.
The heart is our main pumping mechanism, circulating oxygenated blood throughout the body to the working tissues. As the body begins to exercise, the heart must pump more blood to meet this increased demand.
Stroke volume is the volume of blood being pumped out with each beat. The heart pumps out approximately 70 ml of blood each beat. Stroke volume and heart rate together determine cardiac output, which is the volume of blood being pumped through the heart in a given time.
The training effect of zone 2 is that stroke volume increases while exercise heart rate decreases, signaling greater cardiorespiratory endurance. That's the long-term magic of taking things slow in your training.
"Zone 2 cardio makes your heart stronger, and it will require fewer pumps to pump blood, making it more efficient," says Niren. "Your body also expands its vascular system, better enabling it to deliver oxygenated blood to different parts of the body."
A note on energy systems.
While exercising in zone 2, we are primarily using the oxidative or aerobic energy system. Here is a refresher on the three energy systems used during exercise. In most activities, both aerobic and anaerobic systems function simultaneously, but the degree to which they are involved is determined by the intensity and duration of the workout.
- ATP/Phosphagen (anaerobic): Adenosine triphosphate (ATP) is the ultimate usable form of energy for muscular activity. When we eat carbohydrates, we break them down into glucose. ATP is produced in the muscle from glucose. During intense, short-burst activities like sprinting or heavy weightlifting, ATP is used to meet energy demands. Although the body stores about 80 to 100 grams of ATP at one time, this is not enough for exercise. So, the phosphagen system also kicks in to produce energy during intense training bursts.
- Glycolytic: After a few seconds (about six) of intense exercise, the body turns to glycogen (stored glucose) as an energy source. Glycogen is broken down into glucose, which is then metabolized into more ATP for muscle contractions without the need for oxygen. This breakdown of glycogen also produces a byproduct called lactic acid, which immediately dissociates to lactate, which leaks into the bloodstream.
- Oxidative: This system burns the lactate produced during glycolysis using oxygen, creating more ATP than the anaerobic ATP systems. Training to improve endurance helps get rid of lactic acid before it can build to the point where it contributes to muscular fatigue4. The oxidative system uses primarily carbohydrates and fat as substrates. While at rest, 70% of ATP is derived from fats and 30% from carbohydrates. Once you begin to exercise, there is a shift from fats to carbohydrates. If the exercise is high-intensity, nearly all energy is derived from carbohydrates. However, during prolonged, submaximal cardio (like zone 2 training), there is a gradual shift from carbohydrates back to fats. Protein is also used during long-term starvation and long bouts of exercise (greater than 90 minutes).
Benefits of zone 2 cardio.
There are many benefits to slowing things down and spending time in zone 2 cardio, including:
Increased cardiovascular health.
As previously stated, training in zone 2 requires the heart, lungs, and blood vessels to work in unison to deliver oxygenated blood throughout the body, improving cardiorespiratory fitness. This can contribute to a reduced heart rate (during rest and exercise) and increased VO2 max.
VO2 max is the greatest rate at which oxygen can be taken in and used during exercise. Normal VO2 max for most college athletes would be 45 to 60 mL/kg/min whereas a world-class marathon runner would be 70 to 80 mL/kg/min.
During zone 2 training, we use fat for fuel, so prolonged low-intensity aerobic training can help promote weight loss5.
Mitochondria are organelles in cells that are responsible for aerobically producing ATP via oxidation of glycogen and free fatty acids. Zone 2 cardio increases the size, number, and function of mitochondria within cells6.
"Zone 2 is the level at which you're stimulating your mitochondria the most to create ATP," says Niren. "Zone 2 training improves mitochondrial efficiency since the more you train in this zone, the better your body gets at burning fat for energy."
Niren suggests that zone 2 cardio generates plenty of feel-good endorphins because it gets the muscles moving and blood flowing but doesn't severely stress the body.
People recovering from musculoskeletal injuries can use zone 2 cardio to say in aerobic shape while they are unable to push their workouts into more intense zones. Working out in zone 2 is unlikely to cause injury due to the low amount of force it generates.
How often to do zone 2 training.
As for the frequency of zone 2 training, Lazauskas recommends dedicating at least 45 minutes to it two to three times per week.
"The minimum effective dose of zone 2 cardio to get the mitochondrial benefits seems to be 45 minutes," says Niren. She recommends 80% of all cardiovascular exercise to be in zone 2, with a small amount of high-intensity interval training thrown in, maybe one session per week.
Though, Lazauskas says, there is no clear consensus on the amount of time one should spend on one zone versus the others. Everyone is different, so find what works for you.
It's worth noting that chronic zone 2 training without resistance exercise can lead to muscle loss7. This is why variety in training is essential for people looking to maintain strength and muscle mass too. Check out this guide to resistance training to get started.
Working it into your routine.
One easy way to gauge if you're in zone 2 during your favorite exercise is to do the talk test: You should be able to hold a conversation in this zone, and it shouldn't feel painful or uncomfortable. Instead, you should feel as if it's possible to move at this intensity for hours at a time.
"To make my zone 2 cardio something I look forward to rather than avoid, I watch movies and TV shows while I work out," Niren adds.
Here are some light cardio workouts that can help you get into zone 2 and reap all the benefits of staying there:
Zone 2 cardio isn't flashy. It's not the headline of your latest fitness magazine or the name of the latest trendy boutique fitness studio. Instead, zone 2 cardio is an investment in your long-term health. And since it's a conversational way to work out, it's also ideal for training with a friend.
Mark Barroso, MS, LAT, ATC, CSCS, NSCA-CPT is an athletic trainer, strength and conditioning coach, and freelance writer based in New Jersey.
A former editor of Muscle & Fitness, Barroso earned a B.A. in Journalism/Professional Writing from The College of New Jersey and has been featured in several health and fitness publications as both an author and expert. Barroso would eventually trade editing in a cubicle for training clients at gyms including Snap Fitness, 24 Hour Fitness and Blink Fitness. Most recently, Barroso earned his Master of Science in Athletic Training at Montclair State University and looks to continue his career in Sports Medicine.