What Is VO₂ Max? How to Train Your Body to Handle Heart Rate Zones 4 and 5 Longer

At Kingdom FIT, we talk a lot about strength, conditioning, heart-rate training, and learning how to push ourselves with purpose. Strength is easy to recognize. If you used to squat 135 pounds and now you can squat 185, you know you've gotten stronger. Cardiovascular fitness can be harder to see because many of the most important improvements are happening inside your body.

You may notice that you can run longer before needing to slow down. You may recover faster after a hard round of Boxing. Maybe you can move from the rower to another DEKA station without feeling completely exhausted. Or perhaps your Myzone monitor shows that you can spend more time in the yellow and red zones than you could several months ago.

One of the reasons those improvements happen is an increase in your body's ability to deliver and use oxygen. That's where VO₂ max comes in.

VO₂ max sounds like something only runners, cyclists, or elite athletes need to understand, but the basic concept is actually pretty simple. Think of VO₂ max as the size of your aerobic engine. The better that engine becomes, the better your body can take oxygen in, transport it to your muscles, and use it to produce energy during exercise.

And yes—you can train that engine.

The goal, however, isn't simply to get your heart rate as high as possible or to see who can stay in the red the longest. To improve cardiovascular fitness, we need to understand what VO₂ max means, what Heart Rate Zones 4 and 5 actually tell us, and how to train those higher intensities without turning every workout into an all-out test.

What Is VO₂ Max and Why Should You Care?

VO₂ max is the maximum amount of oxygen your body can use during very hard exercise. The "V" refers to volume, while O₂ means oxygen. When exercise physiologists talk about VO₂, they're essentially talking about oxygen consumption.

Think about what happens when you start exercising. Your muscles need energy to contract. As the workout gets harder, your muscles need more energy. Your breathing increases because your body needs more oxygen, and your heart begins beating faster because it needs to move oxygen-rich blood throughout your body.

The process is pretty straightforward when we strip away the scientific terminology. You breathe oxygen into your lungs. That oxygen enters your bloodstream. Your heart pumps the oxygen-rich blood throughout your body. The blood reaches your working muscles, and those muscles use oxygen to help produce the energy necessary to keep moving.

As exercise gets harder, your body's demand for oxygen keeps increasing. Eventually, you approach the greatest amount of oxygen your body is capable of using. That's your VO₂ max.

VO₂ max is usually expressed in milliliters of oxygen per kilogram of body weight per minute, or mL/kg/min. A true VO₂-max test typically requires specialized equipment that measures the oxygen you're breathing in and out while exercise intensity gradually increases. Some watches and fitness trackers estimate VO₂ max, which can be useful for following trends, but an estimate from a wearable isn't exactly the same thing as a laboratory measurement.

The easiest way to understand all of this is to think about a car engine. Imagine two cars traveling 60 miles per hour. One has a small engine and another has a much larger engine. Both cars are moving at the same speed, but the smaller engine may need to operate much closer to its maximum capacity to maintain that speed.

People are similar.

Two people at Kingdom FIT could run at the exact same treadmill speed, use the same rower pace, or perform the same Bootcamp workout and have completely different experiences. One person's heart rate may be 145 beats per minute while the other person's is 175. One may still be able to speak in sentences while the other can barely get a few words out.

They're doing the same external work, but internally their bodies are operating at different percentages of their available capacity.

That's why increasing cardiovascular fitness can make such a noticeable difference in everyday training. The workout itself hasn't necessarily gotten easier. Your body has become more capable of handling it.

VO₂ max and cardiorespiratory fitness also matter for reasons that go far beyond athletic performance. A large 2022 meta-analysis looked at 37 studies involving 2,258,029 people and 108,613 deaths. Researchers found that people in the highest third of cardiorespiratory fitness had about a 45% lower relative risk of death from all causes compared with people in the lowest third. They also found that each additional 1-MET increase in cardiorespiratory fitness was associated with about an 11% lower relative risk of all-cause mortality. These are observational associations, so they don't mean improving your fitness by one MET guarantees that your personal risk drops by exactly 11%, but the relationship between higher fitness and better long-term health is remarkably strong.

A broader 2024 review of meta-analyses reached a similar conclusion. Looking across research representing more than 20.9 million observations from 199 unique cohort studies, researchers found cardiorespiratory fitness to be a strong and consistent predictor of health outcomes. Across included meta-analyses, every 1-MET higher level of fitness was associated with roughly an 11% to 17% lower risk of all-cause mortality.

So improving your aerobic engine isn't simply about performing better in FORGED, Boxing, Bootcamp, Cardio XP, DEKA, HYROX, or whatever challenge comes next. Cardiovascular fitness is an important part of overall health.

What Do Heart Rate Zones 4 and 5 Actually Mean?

Now let's connect VO₂ max to something many Kingdom FIT members can see in real time: heart-rate zones.

Heart-rate training divides exercise intensity into different zones. The exact percentages depend on the system being used and how maximum heart rate is determined, so there isn't one universal definition of every zone. However, in a common five-zone model, Zone 4 is roughly around 80% to 90% of maximum heart rate, while Zone 5 is roughly around 90% to 100%.

At lower intensities, your breathing is relatively comfortable and you can continue exercising for a long time. As your heart rate climbs, breathing becomes heavier and maintaining the workload becomes progressively more difficult.

By the time you're working in Zone 4, you know you're exercising. Conversation becomes more difficult, breathing is noticeably heavier, and the effort requires concentration. As you move toward Zone 5, you're approaching the upper end of your cardiovascular capacity. The effort becomes very difficult to sustain for long periods.

This is where we need to clear up one of the biggest misunderstandings surrounding heart-rate training: Heart Rate Zone 5 and VO₂ max are not the same thing.

Heart rate tells us how fast your heart is beating. VO₂ tells us how much oxygen your body is actually consuming. They're related, but they don't perfectly match.

This matters when you're looking at Myzone. If your tile turns red and says you're at 92% of your maximum heart rate, that does not necessarily mean you're currently exercising at 92% of your VO₂ max. Likewise, spending ten minutes in your heart-rate Zone 5 doesn't automatically mean you spent ten minutes at VO₂ max.

A major 2026 systematic review helps illustrate why this distinction matters. Researchers analyzed 86 studies containing 239 different high-intensity interval-training protocols. They found considerable variation in how much time different workouts actually produced near VO₂ max. Of the 239 protocols examined, 133 produced at least five minutes near VO₂ max, while only 48 exceeded ten minutes.

Heart rate is still extremely useful. We just need to understand what it's telling us.

Your heart-rate zones are also individual. Suppose two Kingdom FIT members are doing exactly the same workout. One person's Myzone monitor says 92% while the person next to them says 84%. Who is working harder?

We don't necessarily know.

One person may naturally have a higher maximum heart rate. One may be more conditioned for that particular exercise. Sleep, stress, caffeine, hydration, heat, medications, accumulated fatigue, and even the type of exercise can influence heart-rate response.

This is why I don't want people looking across the Myzone board thinking, "I'm at 94% and they're only at 84%, so I'm working harder."

Maybe. Maybe not.

Your monitor is there to help you understand your body, not to beat the person beside you.

There is also an important reason you shouldn't try to live in Zone 5. True high-intensity work is difficult precisely because it cannot be sustained indefinitely. If you can comfortably perform an exercise for 45 minutes, you're probably not operating at your true maximum capacity.

That's why cardiovascular training uses different zones for different purposes.

And it's why high-intensity training uses intervals.

How Do You Train Your Body to Handle Zones 4 and 5 Longer?

This is usually the question people really want answered: How do I get better at working at a high heart rate without immediately feeling exhausted?

The answer isn't simply to go harder.

The answer is to progressively train your cardiovascular system.

Think about strength training. If someone can squat 100 pounds, we wouldn't put 300 pounds on the bar every week and tell them to keep trying until eventually they become strong enough to lift it. We would gradually increase the challenge. Maybe they squat 105, then 110, then 115. Their body adapts to the workload and becomes capable of doing more.

Cardiovascular training works on the same basic principle.

If you currently struggle to maintain high-intensity exercise for longer than a minute, you don't need to immediately perform four-minute Zone 5 intervals. You might start with one minute of hard work followed by two minutes of easy recovery. As your conditioning improves, those hard intervals can become two minutes. Eventually they might become three minutes and then four.

This is where the famous 4 × 4 interval workout comes into the conversation. The idea is simple: perform four minutes of hard but controlled exercise, recover for roughly three minutes, and repeat the process four times. That gives you 16 total minutes of high-intensity work, but you don't have to perform all 16 minutes continuously.

The recovery periods are not wasted time. They're what allow you to produce another quality interval.

The American College of Sports Medicine describes HIIT as repeated high-intensity efforts separated by recovery periods. ACSM notes that intense work intervals can last anywhere from approximately five seconds to eight minutes and are commonly performed at roughly 80% to 95% of estimated maximum heart rate. ACSM also describes the effort as feeling "hard" to "very hard," with conversation becoming difficult.

One of the biggest mistakes people make during this type of training is confusing hard with all-out.

If I tell you to work hard for four minutes, that doesn't mean sprint as fast as possible for the first 30 seconds. If you do that, there's a good chance you'll spend the remaining three and a half minutes watching your performance fall apart.

A better approach is what I call controlled aggression. You're working hard. Your breathing should eventually become heavy. Conversation should be difficult. You should know that you couldn't maintain the pace indefinitely. But you should still be able to complete the prescribed workout.

Think hard and repeatable, not maximal and destructive.

Recent research supports the value of longer intervals when the goal is spending meaningful time near VO₂ max. The 2026 review of 239 HIIT protocols found that work intervals lasting two minutes or longer produced significantly more accumulated time near VO₂ max than intervals lasting 30 seconds or less or intervals between 30 seconds and two minutes. The researchers also found that as the accumulated duration of longer intervals increased, so did the amount of time participants spent at or above 90% and 95% of VO₂ max.

That doesn't mean longer is always better. Another 2026 network meta-analysis involving 240 trained athletes found no single HIIT protocol structure to be clearly superior for accumulating time at or above 90% VO₂ max. The researchers specifically concluded that protocol selection should consider the athlete, feasibility, and workload rather than assuming there is one universally best interval design.

That's important because fitness isn't one-size-fits-all.

For one person, four-minute intervals might be appropriate. Another person may need to begin with two-minute intervals. Someone else may respond well to shorter intervals. The point isn't finding a magical workout. The point is applying progressive overload to the cardiovascular system.

There is also something important to understand when you're watching your heart-rate monitor: your heart rate takes time to catch up.

Suppose you begin a hard interval at 130 beats per minute. Your muscles immediately begin demanding more energy, but your monitor isn't going to instantly jump from 130 to 175. Your heart rate rises progressively.

You might spend the beginning of the interval in green. Then you move into yellow. Later you reach red.

That is normal.

Don't suddenly start sprinting because you're 45 seconds into the workout and thinking, "I'm supposed to be in Zone 5!"

Give your cardiovascular system time to respond.

That's also why a proper warm-up matters. Before a serious VO₂ workout, spend approximately 10–15 minutes gradually increasing intensity. Start easy, progress toward moderate work, and include a few short accelerations toward the end. Then begin your primary intervals.

The goal isn't to chase the zone.

The goal is to build the capacity that eventually allows you to handle the zone.

If You Want a Better Zone 5, You Still Need Zones 2, 3 and 4

This may be the most important concept in the entire article: you don't become better at Zone 5 by trying to live in Zone 5.

High-intensity cardiovascular fitness sits on top of an aerobic foundation.

Think about building a house. Zone 5 is near the roof. Your lower-intensity aerobic work helps build the foundation underneath it. If the foundation is weak, simply adding more work to the top isn't necessarily going to produce a stronger structure.

This is why every workout should not become a competition to see how long you can stay in the red.

The American College of Sports Medicine emphasizes that a well-rounded program includes both aerobic exercise and strength training and notes that regular physical activity provides more health benefit than occasional high-intensity workouts.

Your easier aerobic work allows you to accumulate substantially more exercise without creating the same level of fatigue as Zone 5 training. Moderate conditioning has a purpose. Threshold work has a purpose. Strength training has a purpose. Recovery has a purpose. High-intensity intervals have a purpose.

The key is putting them together.

For example, someone training several days each week might strength train one day, perform a dedicated VO₂ interval workout another day, perform easier aerobic work or strength on another, use a moderate conditioning session later in the week, and participate in a mixed conditioning workout such as FORGED, Boxing, Bootcamp, Cardio XP, or DEKA-style training on another day. The exact schedule will depend on the person's goals, experience, health, and recovery capacity.

The important part isn't which workout happens Tuesday versus Thursday. The important part is understanding that not every day should be a maximum-intensity day.

A simple VO₂-focused workout for an appropriately conditioned person could begin with a 10–12 minute progressive warm-up. After that, perform four minutes of hard, controlled exercise followed by approximately three minutes of easy recovery. Repeat that four times and finish with five to ten minutes of easy cooldown.

You could do the workout on a treadmill, rower, bike, or another modality that allows you to maintain a relatively consistent effort.

If four minutes is currently too difficult, don't force it. Start with two minutes. When you can perform those intervals consistently, progress toward three. Eventually work toward four.

That's not backing down from the workout.

That's training.

And this is where Myzone can become incredibly useful.

During your first interval, you might begin in green, gradually move into yellow, and eventually reach red. During recovery, your heart rate drops. When the second interval begins, you're starting from a higher cardiovascular baseline. You may reach the upper zones sooner. By intervals three and four, you might accumulate considerably more time in Zones 4 and 5.

Over time, however, don't measure progress only by how many red minutes you accumulated.

Look at what you actually accomplished.

Maybe you can row farther during each four-minute interval. Maybe you can hold a faster treadmill pace. Maybe you can maintain higher watts on the bike. Perhaps your heart rate recovers more quickly during those three-minute recovery periods. Maybe the same pace that used to push you immediately into the red now keeps you in yellow.

That last example is especially important.

Sometimes getting fitter means you actually spend less time in the red during a workout that previously put you there.

Why?

Because your body has become more efficient.

The workout didn't get easier.

You got better.

Stop Chasing the Red Zone and Start Building the Engine

VO₂ max gives us a way to describe the upper capacity of our aerobic system. Heart-rate zones give us a practical way to estimate exercise intensity. Myzone gives Kingdom FIT members a visual representation of what their cardiovascular system is doing during a workout.

Used together, these can be extremely valuable tools.

But they have to be interpreted correctly.

A red tile does not automatically mean you're at VO₂ max. A higher heart rate does not automatically mean you had a better workout. Spending more time in Zone 5 doesn't automatically mean you're fitter than the person beside you. And accumulating the most red-zone minutes isn't the goal of cardiovascular training.

The goal is capacity.

Can you perform more work?

Can you sustain it longer?

Can you recover faster?

Can you repeat it?

Can you maintain better movement quality while you're tired?

Can you move from the run to the next DEKA station and still perform?

Can you finish a hard Boxing round, recover during the break, and come back capable of producing another quality round?

Can you get through Bootcamp without your performance falling apart halfway through?

Can you row harder without immediately needing to stop?

Those are meaningful improvements.

A 2026 systematic review of short-bout HIIT illustrates why heart rate alone doesn't tell the whole story. Across 139 data samples from 46 studies, average heart rate during short-bout HIIT was about 88% of maximum, while peak heart rate averaged about 94% of maximum. Yet participants averaged only about 259 seconds above 90% VO₂ max and about 125 seconds above 95% VO₂ max. In other words, very high heart rates did not mean the athletes spent the entire workout at equally high percentages of VO₂ max.

That is exactly why we shouldn't look at the Myzone screen and assume that red automatically equals VO₂ max.

The bigger picture matters.

And the long-term health data give us another reason to build cardiovascular fitness rather than simply chase intense workouts. The 2022 meta-analysis involving more than 2.25 million people found that each additional MET of cardiorespiratory fitness was associated with approximately an 11% lower relative risk of all-cause mortality.  The 2024 overview of meta-analyses similarly found that each additional MET was associated with approximately an 11% to 17% lower risk of all-cause mortality across the included analyses.

Those are population-level associations rather than guarantees for any individual, but they reinforce something important: cardiovascular fitness matters.

That doesn't mean every workout needs to hurt.

Green isn't bad.

Yellow isn't mediocre.

And red isn't victory.

They're different intensities with different purposes.

There will be days when we're building strength. There will be days when we're building our aerobic base. There will be days when we're working at moderate intensity. There will be days when we're challenging threshold. There will be days when we're deliberately pushing into Zones 4 and 5. And there should be days when we're recovering from all of it.

Put enough of those days together consistently and something starts changing.

The pace that used to crush you becomes manageable. The one-minute interval becomes two minutes. Two becomes three. Three eventually becomes four. Your breathing settles faster during recovery. You can produce more work before fatigue takes over. Your technique doesn't fall apart as quickly. You recover from one station and become ready for the next.

That's cardiovascular fitness.

There are also times when high-intensity training requires extra caution. People with known cardiovascular disease, unexplained chest pain or pressure, fainting, unusual shortness of breath, uncontrolled hypertension, or other significant health concerns should get appropriate medical guidance before beginning vigorous interval training. Some medications can also change heart-rate response, which can make standard percentage-based heart-rate targets misleading. Concerning symptoms such as chest pressure, fainting, or severe dizziness during exercise are reasons to stop—not reasons to try to earn another minute in the red.

For everyone else, the message is still the same: progress rather than punishment.

Build your aerobic foundation. Strength train. Use moderate conditioning. Add structured intervals. Learn how to pace yourself. Allow yourself to recover. Watch your heart rate, but also pay attention to pace, power, breathing, perceived exertion, technique, and how quickly you recover.

The goal isn't simply to make your heart beat faster.

The goal is to make your entire cardiovascular system more capable.

At Kingdom FIT, that's how we should look at heart-rate training. Myzone isn't simply showing us how much we suffered during a workout. It's giving us information that can help us understand how our bodies are responding to the work.

Some days we build strength. Some days we build endurance. Some days we recover. And some days we challenge the upper limits of our cardiovascular system.

We're not just chasing the red zone.

We're building the engine. ❤️🔥💪🏾

Faith. Fitness. Purpose.

Every Body. Every Journey. One Kingdom.

References

The American College of Sports Medicine describes HIIT as repeated high-intensity work separated by recovery, with work periods commonly performed around 80–95% of estimated maximal heart rate and lasting approximately five seconds to eight minutes.

Schoenmakers and colleagues' 2026 systematic review and meta-analysis evaluated 86 studies and 239 HIIT protocols and found that intervals of at least two minutes generally produced more accumulated time near VO₂ max than shorter intervals, although considerable variation existed among protocols.

Held and colleagues' 2026 network meta-analysis included 240 trained athletes and found no single HIIT protocol structure clearly superior for accumulated time at or above 90% VO₂ max, supporting an individualized approach to interval selection.

Laukkanen and colleagues' meta-analysis included 37 cohort studies, 2,258,029 participants, and 108,613 deaths and found a strong inverse association between objectively measured cardiorespiratory fitness and all-cause mortality.

A 2024 overview of systematic reviews representing more than 20.9 million observations from 199 cohort studies found cardiorespiratory fitness to be a strong and consistent predictor of morbidity and mortality, with each additional MET associated with an approximately 11–17% lower risk of all-cause mortality across included meta-analyses.

A 2026 systematic review and meta-analysis of short-bout HIIT included 139 samples from 46 studies and reported an average heart rate of approximately 88% HRmax and peak heart rate of approximately 94% HRmax, while time above 90% and 95% VO₂ max was substantially shorter—another example of why heart rate and VO₂ should not be treated as identical measurements.