Like most fit guys, you’re probably addicted to numbers.

Chances are you know your max bench and squat, and you might have a pretty good fix on your body mass index, too. If you’re super-hardcore, you might even know your basal metabolic rate (for the uninitiated, that’s the amount of energy your body churns through when you’re at rest). And no doubt if you’re an endurance guy, you can list your PRs in everything from the 5K to a Spartan Race.

But before you get too confident in the story that these numbers tell, especially as they pertain to your long-term health, Ulrik Wisløff, Ph.D., a professor of physiology at the Norwegian University of Science and Technology, has an important question for you: What is your fitness age?

Wait, you don’t know? Well, according to Wisløff, a 48-year-old former semipro soccer player who is also one of the world’s top exercise scientists, that’s deeply unfortunate. Because your fitness age—even more than your real age—is the key to providing confirmation of your physical prowess or exposing a gaping void at the center of what you thought was a solid training program.

What’s more: Paying special attention to your fitness age, which you can maintain with a very targeted HIIT training regimen, might just save your life years down the road.

Fitness age, defined

Fitness age, which Wisløff introduced to the world in a 2014 study, is rooted in your body’s level of cardiorespiratory fitness (CRF)—its ability to disperse and consume oxygen. In fact, having great CRF—not to be confused with cardiovascular fitness, which refers only to the heart and blood but not the body’s breathing apparatus—is such an important factor to your longevity and your long-term health that a recent scientific statement from the American Heart Association described it as a “potentially stronger predictor of mortality than established risk factors such as smoking, hypertension, high cholesterol, and type-2 diabetes mellitus.”

But as Wisløff knows all too well, CRF is difficult to measure—and even more difficult to make sense of once you have it.

The surest way of gauging CRF is to calculate your VO2 max, the maximum amount of oxygen you can process during an activity. (The average person has a VO2 max of 30 to 60, with some elite athletes, such as pro cyclists, reaching the 90s.) Since the Nobel Prize–winning physiologist A.V. Hill introduced the concept in 1923, the only reliable way to measure VO2 max has been with an exercise test, which asks subjects to push their bodies to exhaustion on a treadmill or a stationary bike while breathing into an ergospirometry system. Even if you endured the process, the larger question remained: What does it even mean? If you’re, say, a 34-year-old guy with VO2 max of 52, how does that inform your health and your training? “When we started this [research] many years ago,” Wisløff says, “we always told people that they had a VO2 max of 30 or 40 or 50, and then they’d always look at us and ask, ‘OK, well, what is that?

So Wisløff set off to find a way to do two things simultaneously: 1) easily and accurately calculate VO2 max without the hassle of equipment, and 2) translate the findings into something the average athlete can understand and use to his advantage.

Enter fitness age. In 2006 he and his colleagues began conducting an enormous study of cardiorespiratory fitness and other health indicators in 4,637 Norwegian men and women, and devised a proprietary formula, which you fill out on his website, that assigns you a fitness age, essentially defined as the average VO2 max of healthy people at any given age.

That 34-year-old with a VO2 max of 52? According to Wisløff’s calculations, he’s in fine shape. Generally speaking, the average healthy guy in his 30s has a VO2 max of roughly 49, so the 34-year-old’s fitness age is close to his real age. But he could be doing better, and with the right training regimen, he could easily bring his fitness age down to something on par with a healthy man in his 20s. (Twenty-something males have an average VO2 max of 54.) But if that same 34-year-old found out that he had a VO2 max of 39? Well, he’d have the same fitness age of your typical 60-year-old. He’d be out of shape, with a dangerously elevated risk of developing cardiovascular disease and, according to some studies, cancer and Alzheimer’s.

But I know what you’re thinking. “I work out. I run. I lift. Surely my fitness age is super-young!” Well, not necessarily.

"Many guys who looked fit—and worked out a ton—turned out to have geriatric fitness ages."

When Wisløff began to measure the fitness ages of his test subjects, he encountered many people who looked fit and worked out but had practically geriatric fitness ages. One group of bodybuilders were lean and muscular, but “their fitness in terms of peak VO2 was scary low,” Wisløff says.

When he tested amateur endurance athletes—many of whom trained up to 10 hours per week—he also found unexpectedly high fitness ages. That’s because, as Wisløff has consistently found, great CRF is achieved through high-intensity exercise, not long, slow jogging.

This has not gone unnoticed by Wisløff ’s peers, who believe his greatest accomplishment might not be in creating the fitness age algorithm—a simple way to estimate VO2 max—but in devising an easy, efficient way to dramatically improve it. Carl “Chip” Lavie, M.D., a leading cardiologist and the author of The Obesity Paradox, told me that he revered Wisløff for expanding “our knowledge of the importance of higher-intensity exercise and its impact on improving fitness and reducing the risk of cardiovascular disease.” When Wisløff pioneered fitness age, he didn’t just create a diagnostic tool; he laid the groundwork for developing what might just be the world’s most useful exercise cure.

So do you want to live forever?

To calculate your fitness age, visit worldfitnesslevel.org to fill out Wisløff ’s detailed online questionnaire (also be sure to read How to calculate your fitness age—and why you should care for our analysis of the questions and the 7-week workout plan to add years to your life). Once you’ve got your fitness age, you can supplement your training program with a scientifically proven fitness-agereducing intervention.

Even if you don’t calculate your exact fitness age, you can still follow these six tips to boost your body’s cardiorespiratory fitness, bulletproofing your health while leaving plenty of time to do all the activities you love: pick-up hoops, distance running, or improving those max bench and squat numbers inside the power rack. Whatever your goals are, here are the six ways to keep your body young.

Stay young tip #1: Supercharge your heart

When Wisløff began to design a training program that could boost VO2 max andreduce fitness age, he asked himself one fundamental question: What limits the body’s ability to uptake oxygen?

Wisløff knew skeletal muscles weren’t the principal problem—they can handle more blood than they can possibly get. He also knew that the lungs, while crucial, couldn’t be dramatically altered with training. But the heart is highly trainable, and increasing the heart’s pumping capacity—the amount of blood it can pump in a given amount of time—directly increases the body’s ability to uptake and distribute oxygen. In other words, a more efficient, more powerful heart leads directly to a higher VO2 max.

But how exactly do you train your heart to be more efficient and powerful? Two factors govern pumping capacity: maximal heart rate and stroke volume. Your maximal heart rate is inborn. (What’s the best formula? 211 minus your age multiplied by 0.64.) No matter how hard you train, that number will tick down throughout your life. But you can do a lot to increase the stroke volume of your heart. “The heart is like any other muscle,” Wisløff says. “It must be loaded to get trained. And the only healthy way to challenge the heart’s pumping capacity is to fill it with maximal amounts of blood for long periods of time.”

The heart achieves maximum stroke volume when it’s pumping at 85–95% of its maximum beats per minute. (For most people, the 85–90% range is sufficient.) So if you want to boost your VO2 max, Wisløff says, you’ll want to work out within that range of cardiorespiratory intensity for as long as you possibly can. If you do it right, you’ll end up with an “athlete’s heart,” one that’s bigger, contracts more forcefully, and relaxes quicker. As Wisløff puts it: “You’ll have a better motor.”

Stay young tip #2: Four minutes is your interval sweet spot

So how exactly do you get your heart rate to the 85% threshold, and how long can you (and should you) keep it there?

It usually takes more than a minute of vigorous exercise before you reach maximum stroke volume. That’s easy enough to do—try running, cycling, or rowing really hard for 60 seconds—but the trickier and more exhausting part is keeping your heart rate and stroke volume locked at that rate. The key to mentally and physically sustaining that kind of workload, Wisløff says, is to use interval training.

“It's obvious that one cannot exercise for very long periods of time at 85–95% of maximal heart rate,” he says. “But intervals get you up to that needed intensity” and give you enough rest in between “to get rid of lactic acid that builds up during the interval.”

But not all interval training is equal. Sprint intervals of one minute or less can get your heart rate past the 85% threshold, but they just don’t give your heart enough sustained work at its maximum stroke volume. Tabata training with 20-second maximum intensity intervals followed by 10 seconds of rest can work, but be aware that your heart rate drops as soon as you stop moving. (And the more fit you are, the faster your heart rate plummets.) If your goal is to improve VO2 max, then it’s better to keep your heart pumping consistently at 85% of its maximum rate than for it to be yo-yoing from 75–100% of its max rate throughout your active workout time.

How long is the ideal stroke-volume maximizing interval? In theory, make it as long as possible. (If you can push out 30-minute intervals at 90% of your max heart rate, go ahead and do it. Also, congratulations, your VO2 max is almost certainly spectacular.) Wisløff and his colleagues found that four minutes is a length most can manage. It lets your heart pump at its maximum stroke capacity for an extended time, and it’s sustainable for untrained individuals and beneficial to elite athletes looking to boost their already excellent CRF.

Wisløff’s recommended program is simple: A 10-minute warmup, followed by four four-minute intervals of large muscle mass exercise (running, cycling, rowing, swimming, cross-country skiing) broken up by three minutes of active rest (a very low-intensity version of whatever you’re doing). The results can be dramatic. After the seven-week program, Wisløff has seen spikes in VO2 max and benefits that go beyond CRF into weight-loss and lean muscle gain. In Norway the response has been ecstatic.

“The biggest newspaper here [Verdens Gang] presented this program on their online version,” Wisløff says. “That story is the most visited story in that newspaper’s history. Now there are even training groups and training centers around Norway that are using this. It’s used a lot.”

Stay young tip #3: Don't train for a marathon

Ask a random sampling of men and women to name the kind of athlete with the best cardiorespiratory fitness, and you’ll almost certainly get answers like marathoners, triathletes, and Tour de France cyclists. While this may be true at the elite level, it’s often not the case for weekend-warrior endurance athletes, and the reason is simple: Running, cycling, and swimming for long distances won’t push your heart to its maximal stroke volume, so they won’t do a lot to improve VO2 max if you are already in good shape and go hard for four minutes.

“I know a lot of endurance athletes on a really high level,” Wisløff says. “Even in those people we have been able to improve fitness a lot by exchanging two to three hours of running for periodization of 4x4 intervals or even 3x3 intervals.”

Wisløff himself is a runner. He likes to go on 45-minute runs through the forest near his home in Trondheim, Norway. When he does, he makes sure that he is giving his heart extended periods of time above the 85% threshold by working in long, steep uphills. “I would like to say that low intensity long distance is the best, because I like to do that,” he says. “But it’s surely not the best.”

Stay young tip #4: Forget beetroot juice and hypoxic masks

You’ve seen those heart-healthy labels at the supermarket, and you know that “eating clean” is a good thing for your health. So can you eat your way to a lower fitness age? Nope.

“Indirectly, it’s important to have a good diet, because if your diet is better, you adapt better to exercise,” Wisløff says. “There have been some reports that if you drink beetroot juice or stuff with a lot of nitric oxide in it, that may help your cardiorespiratory fitness—and that may be true with untrained people. But as you get fitter, that supplement doesn’t seem to work a lot.”

What about training at elevation or working out on the treadmill with one of those Predator–style hypoxic masks? After all, don’t all the top endurance athletes run high up in the mountains? Wouldn’t just living at altitude boost your VO2 max and reduce your fitness age?

Nope again. The science on hypoxic masks is thin. “Even though there are some believers out there, I know that world-class endurance athletes in, for instance, some cross-country skiing do not use them,” Wisløff says. While some world-class endurance athletes travel to high altitudes to train, the effect on performance is tiny. If you’re the third best half-miler in the world and you want to become the best half-miler in the world, then by all means move to La Paz, Bolivia (altitude: nearly 12,000 feet). But if you’re something other than an Olympian, you’re going to make the same gains if you do all your interval training in Miami.

Stay young tip #5: Save time for cross-training

You might expect that Wisløff would advise those looking to reduce their fitness age to do nothing but lung-busting sessions of 4x4 interval training. But he knows personally that such a course would be counterproductive.

“I can’t just do 4x4,” he says. “I think it’s totally boring to do just that.”

In his fitness-age-reducing fitness program, Wisløff reserves days for fun runs and 60-minute activities like five-a-side soccer, and he practices what he preaches. He performs 4x4 interval training only a couple of times per week. (One session is always a lab-wide workout in which he leads his 60-person staff in exercises.) The rest of the time, he works out like an outdoorsy and not especially fitness-obsessed man. He plays a weekly game of soccer. He kayaks. In November, he ended one email to me with “good cross-country skiing conditions here now!”

Wislø views the 4x4 training as a key fitness intervention, something everyone should and can integrate into the fitness routine that they’re already doing.

“When I stopped playing soccer, and I got kids, I became more inactive. But when I started to become active again, I would do interval training two times one week, then three times the next, and that’s a really good way to improve fitness quickly,” Wislø  says. “It’s an oxygen cure.”

Stay young tip #6: Choose your devices wisely

By now you’ve probably realized that many popular device-based approaches to improving fitness just don’t pass muster when you’re trying to reduce fitness age. Walk 10,000 steps per day? Why? Your heart rate is never going to get anywhere close to a range where you can lower your fitness age. Exercise for 150 minutes per week? Sure, that sounds good. But what’s your real output going to be? Heart rate is a better measure, but Wislø realized that on its own, it didn’t mean a whole lot.

“I’ve been struggling and trying to find how we can translate changes in heart rate into a meaningful index that actually tells me if I’m doing enough exercise per week to be protected against lifestyle-related diseases,” Wislø  says.

What he came up with was a new metric called Personalized Activity Intelligence (PAI), which is basically Wislø’s fitness-age calculator in weekly-exercise-plan app form. Your PAI goal is to maintain a weekly score of more than 100. That’s the point at which Wislø’s studies show that a man’s risk of cardiovascular disease gets reduced by 17%. (After that point, you’ll get fitter, but your risk of cardiovascular disease won’t significantly decrease.) A couple of exercises per week that in total raise your heart rate so you breathe heavily for about 40 minutes will give you 100 PAI. You can get it also by exercising at moderate intensity for a few hours. The higher the intensity the more PAI you earn. It absolutely can be achieved by low to moderate intensity as well.

Daily workouts are not required. “The data is so clear. You don’t need to exercise every day; you just need to have 100 PAI per week,” Wislø  says. So superintense workouts like 4x4 interval training can easily be spaced out with rest days or days of low-intensity workouts, and you’ll still be bulletprooffng your body and health. By that point, you might even be able to compete with Wislø . His fitness age, he told me, is below 20.