Photography: Dylan Coultier
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 hardcore, you might even know your basal metabolic rate (the amount of energy your body churns through when you’re at rest). And if you’re a runner, no doubt you can list your PBs in everything from the 5K to a Spartan Race.
But before you get too confident in the story these numbers tell about your long-term health, Professor Ulrik Wisløff, a physiologist at the Norwegian University of Science and Technology, has an important question for you: what’s your fitness age?
If you don’t know, says Wisløff – a 50-year-old former semi-pro footballer who is also one of the world’s top exercise scientists – that’s deeply unfortunate. Because even more than your real age, your fitness age is the key to knowing your true physical prowess or exposing the holes in your training programme.
What’s more, paying special attention to your fitness age, which you can maintain with a targeted HIIT training regimen, just might save your life years down the road.
What is fitness age?
The concept of a fitness age, which Wisløff introduced in a 2014 study, is rooted in your body’s level of cardiorespiratory fitness (CRF) – its ability to disperse and consume oxygen. In fact, CRF (not to be confused with cardiovascular fitness, which refers to the heart and blood but not the body’s breathing apparatus) is such an important factor to your longevity and long-term health that a 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”. Unfortunately, as Wisløff admits, 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 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 AV Hill introduced the concept in 1923, the only reliable way to measure VO2 max has been with an exercise test, which requires 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 out to find a way to do two things: 1) easily and accurately calculate VO2 max and 2) translate the findings into something the average athlete can understand and use to their advantage. Enter fitness age.
The fit formula
In 2006 Wisløff and his colleagues began conducting a study of cardiorespiratory fitness and other health indicators in 4,637 Norwegian men and women. They devised a formula 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 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, he could easily bring his fitness age down to something on par with a healthy man in his 20s (they have an average VO2 max of 54).
If that same 34-year-old found out that he had a VO2 max of 39, he’d have the same fitness age of a 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.
As old as you feel
We know what you’re thinking. “I work out. I run. I lift. Surely my fitness age is super-young!” Not necessarily.
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 for up to ten 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 jogs.
Wisløff’s peers believe his greatest accomplishment might not be in creating an algorithm to find people’s fitness age – ie a simple way to estimate VO2 max – but in devising an easy, efficient way to dramatically improve it. Dr Carl “Chip” Lavie, a leading cardiologist and the author of The Obesity Paradox, says he reveres 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.
Do you want to live longer?
To calculate your fitness age, visit worldfitnesslevel.org and fill out Wisløff’s detailed online questionnaire (skip to “How to calculate your fitness age” for our expert analysis). Once you’ve got your fitness age, you can supplement your training programme with a scientifically proven fitness-age-reducing 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: five-a-side football, distance running, or improving those max bench and squat numbers. 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 and reduce fitness age, he considered one fundamental question: what limits the body’s ability to consume 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 amount of blood it can pump in a given amount of time directly increases the body’s ability to take in 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. (The best formula is calculate yours is 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, 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 part is keeping your heart rate and stroke volume at that rate. The key to sustaining that kind of workload, Wisløff says, is to use interval training.
“It is obvious that one cannot exercise for very long periods 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 don’t give your heart enough sustained work at its maximum stroke volume. Tabata training – 20-second high-intensity intervals followed by ten seconds of rest – can work, but 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, it’s better to keep your heart pumping consistently at 85% of its maximum rate than to be yoyoing from 75-100% of max rate throughout your active workout time.
So how long is the ideal stroke volume-maximising interval? In theory, 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 long enough to be effective, and it’s sustainable for untrained individuals – it’s even beneficial to elite athletes looking to boost their already excellent CRF.
Wisløff’s recommended programme is simple: a ten-minute warm-up, 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 programme, 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 programme online,” Wisløff says. “That story is the most visited story in that newspaper’s history. There are training groups and training centres 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 marathon runners, 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.
“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 periodisation of 4×4 intervals or even 3×3 intervals.”
Wisløff himself is a runner – he takes regular 45-minute runs through the forest near his home in Trondheim. When he does, he makes sure that he is giving his heart extended periods of time above the 85% threshold by including 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 heart-healthy labels on foods, and you’ve heard that “eating clean” is good for your health. So can you eat your way to a lower fitness age?
In a word: no. “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 [other] 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 a hypoxic mask? After all, the top endurance athletes run in the mountains – wouldn’t just living at altitude boost your VO2 max and reduce your fitness age?
No again. The science on the effect of hypoxic masks is thin. “Even though there are some believers out there, I know that world-class endurance athletes in, for instance, cross-country skiing do not use them,” Wisløff says. While some elite 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 (the world’s highest capital city at more than 3,500m above sea level). 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 Norfolk.
Stay young tip #5: Make time for cross-training
You might expect Wisløff to advise those looking to reduce their fitness age to do only lung-busting sessions of 4×4 interval training. But he knows personally that such a course would be counterproductive. “I can’t just do 4×4,” he says. “I think it’s totally boring to do just that.”
In his fitness age-reducing fitness programme, Wisløff reserves days for fun runs and 60-minute activities like five-a-side football, while still performing 4×4 interval training a couple of times a 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 football. He kayaks. Like many of his fellow Norwegians, when the conditions are right, he goes cross-country skiing.
Wisløff views the 4×4 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,” Wisløff says.
Stay young tip #6: Choose your devices wisely
One thing that the previous five tips should have made clear is 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 Wisløff 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,” Wisløff says.
What he came up with was a new metric called Personalized Activity Intelligence (PAI), which is basically Wisløff’s fitness age calculator in the form of a weekly exercise plan app. Your PAI goal is to maintain a weekly score of more than 100. That’s the point at which Wisløff’s studies show that a man’s risk of cardiovascular disease is reduced by 17%. After that point, you’ll get fitter, but your risk of cardiovascular disease won’t significantly decrease.
A couple of activity sessions each week that raise your heart rate so you breathe heavily for about 40 minutes in total 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 activity as well.
Most importantly, 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,” Wisløff says. So super-intense workouts like 4×4 interval training can easily be spaced out with rest days or days of low-intensity workouts, and you’ll still be bulletproofing your body and health. By that point, you might even be able to compete with Wisløff, pictured below. His fitness age is below 20.
You can find this out by completing the questionnaire devised by physiologist Ulrik Wisløff at worldfitnesslevel.org. Here Wisløff walks Men’s Fitness’s Michael Rodio (a casual lifter and former CrossFitter) through each question.
Step 1: What’s your sex, age, height and weight?
Wisløff says: “Height and weight are just for calculating BMI, which goes into the algorithm. But sex matters a lot – women’s values tend to be about 20% lower.”
Rodio’s take: I’m 26, 5ft 9in, 175lb – the site gives you the option of entering your stats in feet, inches and pounds or centimetres and kilos.
Step 2: What’s your maximum heart rate?
Wisløff says: “This is a common means of denoting intensity for endurance training.”
Rodio’s take: I had no idea, but the site computes it for me. It’s 196bpm, apparently.
Step 3: Exercise: how often, how long and how intense?
Wisløff says: “All these factors matter in a balanced way, but exercise intensity is the most indicative of fitness age.”
Rodio’s take: I pick “little hard breathing and sweating” because there’s no option for “vigorous swearing or crying”.
Step 4: What’s your waistline? What’s your resting pulse?
Wisløff says: “A low resting heartbeat is the sign of a fit heart – world-class endurance athletes use it to see if they’re ready for their next exercise session – but we do know that it’s not enough to predict fitness on its own. Hydration can sway it, for example, so make sure you’re hydrated when you take your measurement.”
Rodio’s take: My belt suggests a 31in (79cm) waistline. My Fitbit says 55bpm.
The takeaway I’m 26, with an expected VO2 max of 53, but I have the fitness age of someone under 20 years old, with an actual VO2 max of 60. “That’s not bad for a 26-year-old,” Wisløff says. “It’s about the same as mine.” Of course he’s 50…
You’re a 35-year-old desk jockey with the heart of a 60-something couch potato? You’re not alone – but there’s hope. Wisløff has created a seven-week programme that primes your heart for a better fitness age (and burns plenty of fat) in just minutes. It’s all built around Wisløff’s lab-tested 4×4 interval training workout: four minutes of high-intensity exercise, followed by three minutes of active recovery, repeated four times.
The core concept is to blast your heart to 85-95% of its maximum rate – not quite all-out, but intense enough that you’ll be able to say only a few four-letter words by the end. Then you downshift to a three-minute active-recovery phase at 70% capacity – still moving, but moderately enough to catch your breath and flush lactic acid from your muscles. After the fourth round, you should feel like you could have done another round.
The best part? You can do any kind of exercise you want – swimming, cycling, rowing and running are popular, but anything will work as long as you’re pushing your heart to the limit. With just two of these 4×4 workouts a week for seven weeks, Wisløff’s lab has made progress with everyone from untrained shlubs to elite athletes.
A version of this article first appeared in the US edition of Men’s Fitness