What Is VO₂ Max? A Coach's Explanation (Not a Doctor's)

Here's what usually happens. A client walks in holding their phone, showing us their Apple Watch screen. "It says my VO₂ Max is 38. Is that good?"

And that question, right there, tells us almost everything about what's wrong with how VO₂ Max gets talked about. Not because the number doesn't matter. It does. But because "is that good?" is the wrong first question. The right first question is: what is this number actually measuring, and what does it tell me about how I should train?

We've stood next to hundreds of clients during lab-measured VO₂ Max tests here at Mavericks. We've watched people hit the wall. We've watched the data print out in real time. And what we've learned is that the score everyone fixates on is the least interesting thing about the test. The interesting stuff is hiding inside the curve.

Let's start from the beginning.

What VO₂ Max actually means — three parts, not one

VO₂ Max gets thrown around like it's a single idea. It's not. It's three ideas packed into one metric, and understanding each one matters.

V stands for volume. We're measuring how much of something your body uses. Not a rate, not a peak — a volume per unit of time.

O₂ is oxygen. The thing your muscles need to produce energy aerobically. When you breathe in, your lungs pull oxygen from the air, your blood carries it to working muscles, and those muscles use it to break down fuel. The more oxygen your body can extract and use, the more aerobic work it can sustain.

Max means maximum. This isn't your resting oxygen consumption or your comfortable jogging oxygen consumption. It's the absolute ceiling — the most oxygen your body can take in, transport, and use when you're working as hard as you possibly can.

Put those together and you get: the maximum volume of oxygen your body can consume per minute during maximal exertion. It's expressed in milliliters of oxygen per kilogram of bodyweight per minute (ml/kg/min). That per-kilogram part matters because it lets us compare a 140-pound endurance runner with a 210-pound former linebacker. Raw oxygen consumption favors bigger bodies. Relative VO₂ Max levels the playing field.

So when someone says "my VO₂ Max is 42," what they're really saying is: at the absolute hardest I can possibly work, my body consumes 42 milliliters of oxygen for every kilogram I weigh, every minute. That's a real, physiological measurement. And getting to that number requires pushing your body to a genuine limit.

What the mask actually measures

This is where most explanations go wrong. People think the test measures how much oxygen you breathe in. It doesn't. It measures the difference between what you breathe in and what you breathe out.

During a VO₂ Max test, you wear a mask or mouthpiece connected to a metabolic analyzer. That device tracks two things continuously: the concentration of oxygen in the air you inhale, and the concentration of oxygen and carbon dioxide in the air you exhale.

Room air is about 20.9% oxygen and 0.04% carbon dioxide. When you exhale during rest, the oxygen drops to maybe 16% and CO₂ rises to about 4%. Your body used some oxygen and produced some carbon dioxide as a byproduct of metabolism. Straightforward.

Now start working harder. Get on a bike or a treadmill, and begin ramping up the intensity every few minutes. As the workload increases, your muscles demand more oxygen. Your heart rate climbs. Your breathing rate increases. The analyzer watches that exhaled air shift in real time.

At low intensities, your body handles the work almost entirely with aerobic metabolism — oxygen in, carbon dioxide out, nice and efficient. As intensity climbs, something changes. Your body starts recruiting anaerobic energy systems to supplement. Those systems produce lactate as a byproduct, and your body buffers that lactate by producing extra CO₂. The ratio of CO₂ produced to O₂ consumed — called the respiratory exchange ratio, or RER — starts climbing.

When that ratio hits 1.0 and keeps rising, it means your body is producing more carbon dioxide than can be explained by aerobic metabolism alone. You're redlining. And at some point, no matter how hard you push, your oxygen consumption plateaus. It doesn't go up anymore even though the workload does. That plateau is your VO₂ Max.

We watch this happen in real time during testing. There's a moment where the numbers on the screen stop climbing even though the client is clearly working harder. Their legs are burning, their breathing is ragged, and the oxygen consumption line just flattens. That's the ceiling. That's the number.

Why the score alone is incomplete

Here's the part that surprises most people who come in for testing.

We'll run the test, the client will cool down, and we'll sit down with the results. They want to know their number. And we'll give it to them. But then we'll point at two spots on the curve that came before the max, and say: this is where coaching actually lives.

Those two spots are your ventilatory thresholds. VT1 and VT2. And they matter more than the peak number for almost everyone who isn't an elite endurance athlete.

VT1: the first threshold

As intensity ramps up during the test, there's a point where your breathing pattern changes for the first time. Below this point, breathing is comfortable. You could hold a conversation. Above it, breathing becomes noticeably harder. You can still talk, but in shorter sentences. You're aware of your breathing in a way you weren't before.

This is VT1. Physiologically, it's roughly where lactate begins to accumulate faster than your body clears it at rest. It corresponds loosely to what some people call the "aerobic threshold." For most of our clients, VT1 happens at somewhere between 50% and 75% of their VO₂ Max.

Why does this matter? Because VT1 tells us the upper boundary of what we'd call true base-building intensity. Training below VT1 develops your aerobic engine without accumulating systemic fatigue. It's the intensity where you build the foundation that everything else sits on. And the vast majority of people — including people who think they train hard — have never spent serious time developing this zone.

VT2: the second threshold

Keep pushing past VT1 and eventually you hit a second inflection point. Breathing becomes labored. Full sentences are impossible. You're working in short bursts of speech, maybe a word or two between breaths. Lactate accumulation is now outpacing your body's ability to buffer it. You're on borrowed time.

This is VT2, sometimes called the "anaerobic threshold" or "lactate threshold" (though those terms aren't perfectly interchangeable — the physiology nerds can fight about that). VT2 typically occurs between 75% and 90% of VO₂ Max.

VT2 tells us something critical: how close to your ceiling you can sustain hard work. A well-trained endurance athlete might have a VT2 at 85-90% of their VO₂ Max. An undertrained person might hit VT2 at 70%. Both might have the same VO₂ Max score. But the athlete can sustain a much higher percentage of their max before crossing into unsustainable territory.

This is what we mean when we say the score alone is incomplete. VO₂ Max gives you the height of the building. The thresholds tell you which floors you can actually live on.

What a "good" number looks like vs. what a useful number looks like

We get this question constantly. "Where should I be?" People want a benchmark. So let's put some context around the numbers, and then explain why context changes everything.

For general population health, research has consistently linked higher VO₂ Max values with lower all-cause mortality. A landmark study using data from the Cleveland Clinic found that cardiorespiratory fitness was the single strongest predictor of long-term survival — stronger than smoking, diabetes, or hypertension. People in the bottom 25% of fitness had roughly five times the mortality risk of the most fit group. That's a staggering number, and it's why VO₂ Max has earned a place in conversations about longevity.

Here's a rough breakdown by age for general population norms. These are approximations — different studies use different percentiles — but they give a working frame of reference:

Men (ml/kg/min):

• Age 30-39: Below average is under 35, average is 35-44, above average is 45-50, excellent is above 50
• Age 40-49: Below average is under 33, average is 33-42, above average is 43-48, excellent is above 48
• Age 50-59: Below average is under 30, average is 30-38, above average is 39-44, excellent is above 44

Women (ml/kg/min):

• Age 30-39: Below average is under 28, average is 28-36, above average is 37-44, excellent is above 44
• Age 40-49: Below average is under 26, average is 26-33, above average is 34-40, excellent is above 40
• Age 50-59: Below average is under 24, average is 24-30, above average is 31-37, excellent is above 37

Those numbers are useful for a first conversation. They tell you roughly where you sit relative to other people your age. But they don't tell you what to do about it, and they definitely don't tell you how to train.

A useful number is one that changes what happens next. And for that, you need the full picture: the peak value, the two thresholds, the heart rates at which those thresholds occur, and the percentage of max at which they happen. That's what a real test gives you. It's also why wearable estimates are a rough starting point but not a training plan. Your watch can guess the peak. It can't find the thresholds.

How a 42 and a 58 can both be undertrained

This is the concept that changes how people think about their fitness once they hear it.

Client A walks in with a VO₂ Max of 42. Decent. Slightly above average for a 45-year-old man. He runs three times a week, usually at the same pace — a steady six-mile loop at a moderate effort. He's been doing this for years. He feels fine. He's not getting faster, but he's maintaining.

We run his test. VO₂ Max: 42. VT1 occurs at a heart rate of 138, which corresponds to about 60% of his max. VT2 occurs at 158, about 80% of his max.

Here's the problem. His usual running heart rate? 152. Every single run, he's hovering just below VT2. He's too hard for base building and not hard enough for threshold work. He's in no-man's land — the intensity zone that feels productive but drives the least adaptation. He's been pouring effort into the zone that gives the least return for years.

Client B walks in with a VO₂ Max of 58. Impressive by any chart. She's a 34-year-old former college swimmer who now does CrossFit four times a week and runs occasional 10Ks. She's fit. She looks fit. Her number confirms she's fit.

We run her test. VO₂ Max: 58. VT1 at 62% of max. VT2 at 74% of max.

That VT2 number should jump out. At 74% of a high ceiling, her thresholds are low relative to her capacity. She has a big engine but she's only using the bottom gears. Her CrossFit workouts blast past VT2 into max effort territory — great for power and grit, but they're not pushing her thresholds up. Her occasional runs are too slow to challenge VT1. She's fit, but she's leaving a huge performance gap on the table.

Both clients are undertrained. Not because they're lazy. Because they don't have the data to train at the right intensities. One needs to slow down dramatically and rebuild his base. The other needs structured threshold work in the gap between her VT1 and VT2 — sustained efforts that feel uncomfortable but not devastating.

Without the thresholds, both would just see a number and either feel good or bad about it. With the thresholds, they have a training plan.

What this changes about how we coach

We run VO₂ Max tests at Mavericks because the data fundamentally changes the conversation we have with clients about their training.

Before the test, the conversation is usually about feel. "I feel like I'm working hard." "I feel like my cardio is decent." "I feel like I should be running more." Feel is unreliable. Everyone's internal effort gauge is calibrated differently. Some people think they're training hard when they're barely touching VT1. Others think they're going easy when they're grinding above VT2 every single session.

After the test, we can anchor every intensity prescription to a real physiological marker. We can say: your easy days should keep your heart rate below 135 because that's your VT1. Your hard days should target 155-162 because that's where your VT2 lives and that's the intensity that will push it higher. Everything else is either recovery or race-specific work.

This is precision. Not the fake precision of a fancy program template. Real precision based on what's actually happening inside a specific person's body.

And here's the thing people don't expect: the prescription often feels wrong at first. Clients who've been training in no-man's land for years are shocked when we tell them their easy days need to be genuinely easy. Slower than feels productive. Slower than they'd ever choose on their own. And their hard days need to be genuinely hard — harder than the moderate grind they've gotten comfortable with.

That polarization — truly easy and truly hard, with very little time in the middle — is what drives adaptation. And you can't polarize your training without knowing where the poles are. The thresholds are the poles.

Where to go from here

If you've read this far, you're probably one of two people. Either you're someone who just got a VO₂ Max number from a wearable and wanted to understand what it means, or you're someone who senses that the way they're training could be smarter but doesn't have the data to prove it.

For the first group: your watch gives you a ballpark. It's not worthless. But it's a single number without the internal architecture. If you want to understand what a full test result actually looks like and how to read it, the next post in this series breaks that down.

For the second group: this is exactly why we offer the test. Not as a novelty. Not as a bragging-rights number. As a coaching tool that tells us — and you — exactly where to aim every training session. If you want to get tested, our lab-measured VO₂ Max test takes about 20 minutes and gives you a complete metabolic picture. We'll sit down with you after and actually explain what it means for your training.

VO₂ Max isn't a grade. It's a map. And like any map, it's only useful if you know how to read it.