CHAPTER FOUR

Beyond Simple Fitness

By now you get that interval training is effective. But so far we’ve mainly focused on training regimens that increase athletic performance. This chapter is about the effect of intervals on health.

I think this is the most exciting chapter in the book, and here’s why. If you ask most people why they don’t exercise, they come back at you with a couple of standard excuses. Absolutely by far the most common excuse is “I don’t have time.” Getting fit just seems so overwhelming to them. They envision fitness as this thing that exists on the other side of a vast chasm. We’re talking the Grand Canyon here. The people are on one side, feeling tired and out of shape. And on the other side is fitness, and all the things that come with it, like more energy, a brighter outlook, a healthier body, and a longer life.

To cross that chasm—to get fit—people have all sorts of perceptions about how much they think they’re going to have to work out. They visualize months and months of exercising for hours at a time each week. The thought of even starting to get fit is overwhelming.

But they’re thinking about the traditional kind of exercise. The slow-and-steady approach. They don’t realize that a new and scientifically proven method exists for them. Thanks to interval training, we know that you can boost your fitness fast. As few as six sessions of interval training over two weeks triggers physiological adaptations that can double your endurance capacity. Six sessions, totaling a single hour’s worth of hard exercise, can lower blood sugar in people with diabetes. Six sessions can get you feeling like you’re in shape. And if you can tolerate it, a minute’s worth of maximal exercise, in the form of three all-out sprints for twenty seconds each can change your physiology as much as fifty minutes of cycling at a moderate pace. Studies from my lab have demonstrated all these things.

The naysayers warn that high-intensity intervals are only for people who are really fit and really motivated. But those naysayers are wrong. Listen: Some people shouldn’t perform interval training. But it’s a rather limited group, and many more—even those with chronic diseases—can benefit from an interval-based approach to fitness.

The Risk-Benefit Analysis

Before we get to the health benefits, I want to discuss something that invariably comes up when I talk about interval training, whether it be at conferences or social gatherings. “Wait a second,” people will say to me. “Interval training sounds great. But isn’t it risky?”

Here’s what I say. First, I point out that not all forms of interval training involve super-intense exercise. For the vast majority of people, any exercise is safer than doing nothing. “The biggest risk is not getting off your couch,” says my colleague Maureen MacDonald.

Everything involves some risk. Leaving your house in the morning is incrementally riskier than staying inside. Getting on a plane to head off on a week’s vacation is incrementally riskier than staying at home. And heading out on that sailboat is incrementally riskier than sitting inside your room at the resort all day. We do all those things because they have benefits that justify the incrementally increased risk. It’s the same for exercise.

According to a 2015 position paper put out by the American College of Sports Medicine, “vigorous-intensity exercise does have a small but measurable acute risk of [cardiovascular disease] complications.” That means that for a short period during and soon after the vigorous exercise, there is an elevated risk of a cardiac event, like a heart attack.

In 2016 the American College of Cardiology’s Sports and Exercise Leadership Council cited a study that showed the risk for a sudden cardiac death related to vigorous exercise was low, at one per 1.42 million hours of exercise—but nonetheless, that’s 17 percent higher than during periods of low or no physical activity. The cardiologists explained that conducting more bouts of vigorous exercise decreased the risk of experiencing a heart attack during that vigorous exercise. Whether people did interval exercise or steady-state endurance-based training, those who pushed the intensity into vigorous levels five times a week had a “markedly lower” risk of a vigorous-exercise-caused heart attack than those who didn’t do any vigorous exercise at all. “Such results,” the cardiologists noted, “demonstrate that vigorous physical activity transiently increases the risk for acute cardiac events, but reduces the overall risk.”

That added emphasis is mine. This is what’s so tricky about understanding the risks associated with interval training, and in particular the most intense forms like sprint training. In older people, the risk of a sudden cardiac death during or soon after exercise goes up the more vigorously one exercises. But vigorous exercise makes you more fit, which decreases your overall risk of dying.

To get a better sense of the risk, I talked to one of the authors of that cardiology paper—Paul Thompson, the director of cardiology at Hartford Hospital in Connecticut and a member of the Sports and Exercise Leadership Council. Thompson is a big believer in exercise. He ran to and from work every day for years and has completed twenty-seven Boston Marathons. “If you’re totally healthy, there’s no increased risk from the high-intensity stuff,” he says. “The problem is, if you’re in your forties, fifties, and sixties, you may not know whether you have things like atherosclerosis. You don’t know if you have cholesterol in your coronary artery. Most sudden heart attacks happen in people with no symptoms.”

“If you gave me a thousand people and had one group doing high-intensity exercise and one moderate, the high-intensity group would be more likely to have a sudden heart attack during exercise than the moderate-intensity group,” says Thompson.

The key there is during exercise. Because things change if you’re considering one’s risk of heart attack in general. “Both the high intensity and the moderate group are going to live longer and have fewer heart attacks than the sedentary folks,” he says.

Do you see the difference? Work out hard and you’ll have a slightly higher risk of experiencing a cardiovascular event while you’re exercising. But all the rest of the time, that hard exercise is lowering your overall risk of developing heart disease. There’s a slightly increased short-term risk, as well as a long-term increased benefit. That trade-off is worth it in my eyes, and the time-efficiency of intervals is why I have trained that way for years. However, Thompson points out that the risk of a sudden cardiovascular event increases with age. “If you said to me, ‘I’m either going to do nothing or HIIT,’” says Thompson, “I’d say, do HIIT. If you said to me, ‘I’m either going to do moderate or HIIT and I’m sixty-five and time isn’t an issue,’ then I’d say, do moderate.”

In the decision to perform interval training, as with many things in life, we’re managing risk. The takeaway from all this is that your risk depends on your age. Hard exercise is generally safe for the young and the healthy. But you’re going to reach an age at which you’ll have to ask yourself whether the benefits of time-effective exercise are worth it. There may be an age when you start to ease off on the really vigorous stuff. And if you’ve been including interval training in your workout regimen, the age that you have to make that decision will likely be later than it is for those who haven’t done any interval training.

Interval Training and Health

Now let’s get back to the irrefutable fact that exercise boosts health. First, we’re going to discuss in general terms how we know exercise boosts health, and then we’ll focus on interval training and the evidence that suggests it has some peculiarly potent health-boosting effects.

The physiological changes that come with exercise and enable us to run faster and harder in the short term, also help us live longer and more active lives—with less chronic disease. This is something that many had suspected: Hippocrates, the ancient Greek physician credited with founding the profession of medicine, for example, said that “eating alone will not keep a man well; he must also take exercise.” But it was well into the twentieth century before the link between physical activity and health was scientifically established. In the late 1940s, the British physician Jeremy Morris set out to study the relative health of the approximately thirty-one thousand transit workers who staffed London’s bus network. Two broad job classes existed: the drivers piloted the crowded double-decker buses around London’s congested streets, while the conductors moved among the vehicle’s passengers, going up and down the stairs to take tickets and maintain order.

The drivers sat for 90 percent of the time they were working. In contrast, the conductors climbed an average of six hundred stairs each shift. In what he called the London Transport Workers Study, Morris examined the employees’ medical records and tallied up incidences of heart disease among them. And he discovered a remarkable difference. The more physically active conductors experienced fewer than half the heart attacks than the drivers did and contracted heart disease much less frequently. And when they did experience a cardiac event, the conductors were much more likely to survive it than were the more sedentary drivers.

In 1953 Morris published a classic epidemiological study in the Lancet that was one of the first attempts to link activity level to disease risk in a large population. Since then, dozens of epidemiological studies have established that exercise is associated with a reduced risk of developing cardiovascular and many other diseases. It also reduces mortality or the risk of dying from all causes. Today we know that exercising regularly is probably the single most effective thing we can do to prolong life and improve health. Which is why we have guidelines from agencies like the American College of Sports Medicine and the World Health Organization that advocate 150 minutes of weekly moderate-intensity aerobic physical activity to promote health.

Unlike some other public health agencies, the ACSM and the WHO acknowledge a potential role for exercise intensity—their guidelines state that people can opt for 75 minutes of vigorous-intensity activity in lieu of 150 minutes of moderate-intensity exercise. Studies out of my lab and others suggest that if you crank up the intensity past vigorous to near maximum or all-out, you can boost health in even less time.

Here, then, are the most important ways that interval training has been shown to increase the quality and quantity of our time on this planet.

Intervals Fight Cardiovascular Disease

Cardiorespiratory fitness is an umbrella term that refers to the health of the heart, lungs, and blood vessels. As I noted earlier, the gold standard measurement of cardiorespiratory fitness is maximal oxygen uptake, or VO2max, which is the maximum rate at which oxygen is used by the body during heavy exercise. (It’s sometimes called peak oxygen uptake, or VO2peak. While there are technical differences between the two terms, for most purposes they’re synonymous.)

Scientists have realized that VO2max also turns out to be an important indicator of a person’s overall health—a kind of “magic number” that goes a long way toward predicting how long you’ll live and your risk of developing many chronic diseases. It’s probably the most important metric you have.

A lot of factors dictate your VO2max, some of which you can control (like your activity level and whether you smoke), and others that you can’t (such as your age and sex). The single most important factor that determines individual differences in VO2max is the amount of blood your heart can pump.

Scientists figured this out in part through comparative physiology—studying physiological differences in other animals besides humans. Especially when comparing animals of similar size but with different aerobic capacities, they found out that the bigger an animal’s heart, the higher its VO2max. For example, a dog has more than twice the VO2max of a goat—and a heart that’s twice the size. Similarly, a racehorse has more than twice the VO2max of its mammalian cousin, the steer—and again, the racehorse’s heart pumps twice as much blood as that of a steer.

The human heart is pretty remarkable. In a typical human at rest, the heart can pump about ten pints of blood every minute, regardless of whether the person is fit or a couch potato. Think about that—that’s five 32-ounce Big Gulps. Going through your heart each minute.

But it’s when a person really gets active that fit hearts distinguish themselves. Thanks to my interval training, I’m in decent shape for a guy my age—hardly an elite athlete, but on the upper end of the spectrum. When I’m really cranking—mashing the pedals of my bike as I climb up the Niagara Escarpment near my house, for example—my heart pumps about five times as much blood as it does when I’m at rest. And for highly trained endurance athletes, like the US Olympic swimming champion Michael Phelps? When they’re exercising, their hearts can pump up to eight times as much blood per minute as they do when at rest—a total of about ten gallons of blood per minute. That’s the equivalent of forty Big Gulps. Pumped through the heart. In a single minute.

Here’s another way to think about it. Some smaller cars have fuel tanks that hold about ten gallons of gas. At the filling station, it takes more than a minute to fill up these tanks. So the amount of blood that can come out of the human heart during exercise is greater than what comes out of a gas station fuel pump. It’s just one more indication of how remarkable the human body is.

So what’s the most time-effective way to boost the capacity of the heart to pump blood, which in turn boosts VO2max and reduces the risk of cardiovascular disease and overall mortality? Studies from my lab, and a substantial body of evidence from academics across the world, suggest that it’s the more intense forms of interval training, like high-intensity interval training and sprint interval training. “It appears that HIIT improves cardiorespiratory fitness more than [moderate-intensity continuous training does] across a broad range of populations, including healthy sedentary and heart failure patients,” states a recent overview in the academic journal Heart.

We’ll get to those heart-failure patients in a bit, but first let’s talk about the way an increase in heart and lung health translates into longer lifespans. The author of the Heart article, Matthew Wilson, cited a 2013 study published in the European Heart Journal that tracked the health of all French cyclists who had participated in the Tour de France cycling race between 1947 and 2012. The study compared the mortality risk of those 786 cyclists with that of the general population of French adult males. The cyclists had a 41 percent reduction in all-cause mortality risk compared with the community’s average risk. Put another way, those French cyclists tended to live 6.3 years longer than the average French male. A similar study comparing runners with an average control group in the United States, published in 2008 in the Archives of Internal Medicine, demonstrated even greater effects due to exercise, with 15 percent of the runners having died nineteen years after the study began, compared with 34 percent of the average control group.

Why is that? What is it about exercise that so extended the athletes’ lives?

Wilson points out that exercise can decrease the resting heart rate by 20 beats per minute and increase the volume of blood the heart can pump by about 20 percent. That’s because, under the influence of physical activity, in normal people the heart’s four pumping chambers get bigger. The heart muscle itself gets stronger and better able to pump. The blood vessels also change as a person becomes more fit. There’s an improvement in something scientists refer to as “endothelial function.” That means the walls of the vessels become more pliable. When we transition from rest to some sort of physical activity, the interior diameter of our blood vessels increases, which permits a greater flow of blood. A 2015 review of scientific studies out of the University of Queensland in Australia revealed that one of the most-used HIIT protocols was able to improve a key measure of endothelial function by twice as much as traditional moderate continuous exercise. In the short term, these sorts of changes mean the heart doesn’t have to work as hard to get blood to the parts of the body that require it. Over the long term, it decreases the likelihood of the sort of events that tend to kill us, such as heart attacks and strokes.

About the very long term, though: Getting older tends to change the cardiovascular system in the opposite way that becoming fit does. Over time, the heart grows less able to pump, and the vessels grow stiffer, diminishing their capacity to carry blood. The nice thing is that we can substantially slow, and in some cases even reverse, this age-related decline. The way to do this is through exercise. But what sort?

That’s the question German cardiovascular physiologist Katharina Meyer asked early in the 1990s. Meyer’s academic lineage goes back to the very beginning of research into interval training’s effects on the human body. She has been the lead author on studies supervised by Helmut Roskamm, the researcher who coauthored the very first paper on interval training published in an academic journal, back in 1959.

In the late 1990s Meyer conducted a pioneering series of interval-training studies on patients with severe cardiovascular issues—everything from heart failure to those who had undergone bypass surgery. The experiments she conducted were way ahead of their time. “Katharina Meyer was doing things back then that nobody else was doing,” recalls Jeff Christle, a Stanford University physiologist who specializes in exercise.

Several of Meyer’s most innovative experiments applied interval training to patients who had chronic heart failure, a condition in which the heart does not pump blood as well as it should. The condition can develop after the heart becomes damaged or weakened by other conditions or diseases, including heart attacks, and it can leave patients winded by actions that most people wouldn’t consider taxing, such as walking up a slight incline. The poor pumping capacity of the heart often causes fluid to back up in other tissues, including the lungs, leaving people feeling tired and out of breath. In one study, published in 1997, Meyer showed that three weeks of an interval-training protocol improved cardiac function in patients with heart failure, such that cardiorespiratory fitness increased by about 20 percent.

People thought she was crazy. Back then, intense exercise was thought to be dangerous for anyone with any sort of heart issue. Norwegian physiologist Ulrik Wisløff recently led an effort to compile a short history of cardiac rehabilitation. According to Wisløff and his coauthors, well into the middle of the twentieth century, cardiologists prescribed patients who had survived major cardiac events, such as heart attacks, as close to complete bed rest as possible. Essentially, the patients were supposed to lie flat on their backs, on a bed, for thirty days. No activity whatsoever was allowed because the rest was thought to heal the scarring caused by their heart attacks.

Then, in 1952, a pair named Samuel Levine and Bernard Lown suggested that maybe patients would do better if they weren’t completely immobilized. If, say, the recovering patients sat up in an armchair. Soon, other cardiologists and scientists were suggesting the patients might be able to do even more activity—that actually, up to a point, the more active the cardiac patients were, the better.

Remember the early buzz around high-intensity exercise to boost athletic performance propagated by people like Ed Fox in the 1970s? Well, it turns out the world of cardiac rehab experienced a similar phenomenon. Beginning in the ’70s, some cardiologists and physiologists felt that the low-intensity exercise they were prescribing their patients wasn’t strenuous enough. They felt that cardiac rehab patients could safely engage in activity that was a lot more strenuous. Not only that—the patients should work themselves harder. It would be good for them, because the higher activity would assist with the healing of the heart.

Some of this activity included interval training. Scientists who conducted early studies on interval training’s effects on cardiac patients included Vojin Smodlaka in New York and Roy Shephard in Toronto. The first of them, Smodlaka, asked some patients to cycle at high intensities for sixty seconds at a time, followed by thirty seconds of rest—and repeat. Afterward, Smodlaka discovered that the cardiac patients who had completed the interval-training program could exercise for twice as long as those who had engaged in continuous training.

A Toronto cardiologist named Terry Kavanagh even convinced eight of his patients, all of whom had experienced heart attacks anywhere from one to four years previously, to train to run the 1973 Boston Marathon. Their training included several different flavors of interval training, and seven of the eight heart patients completed the race.

Still, even in the late 1990s, many scientists were alarmed by the studies Katharina Meyer was conducting on cardiac rehab patients.

“When I first saw Meyer’s work, I shuddered,” recalls Carl Foster, the University of Wisconsin physiologist and former ACSM president, who was running the cardiac rehab program at Milwaukee Heart Institute at the time. Foster introduced himself to the German scientist after a seminar she gave at a conference both attended. “Nice talk, Dr. Meyer,” Foster said. “How many people have you killed?”

In fact, Foster would come around to introducing interval training to patients in cardiac rehab. One of the key moments in his conversion happened after he wonderingly mentioned Meyer’s results to a nurse who had years of experience working with heart patients. “Oh, Carl. You’re so stupid,” the nurse said, and patted the physiologist on the cheek. Then she took him to a hospital window that overlooked the parking lot, where a rehab patient was making his way to his car. “Watch how this guy walks, Carl,” she said, and Foster noticed that the patient could walk only for a short distance before getting winded and having to rest. “They do interval training themselves—because they have to,” the nurse told Foster, who has since integrated intervals into the cardiac rehab programs he’s managed.

Meyer’s work established something counterintuitive about interval training’s relationship to the heart. The short, intense spurts of interval training placed a great deal of stress on the working skeletal muscles, like the thighs that power an exercise bike. But because the activity happened for only a short time, the heart muscle didn’t have to work as hard as it did during the steady-state exercise typically being prescribed to heart patients. In one study, Meyer compared three different interval-training protocols with the sort of moderate, continuous exercise more commonly prescribed to patients with heart failure. She found that the interval training protocols stressed the heart by an amount between 12 and 17 percent less than did the traditional aerobic workout. Meyer’s research suggested that so long as a doctor had concluded the patients’ cardiovascular disease was stable, the new technique actually was significantly safer for them than steady-state aerobic training.

Scientists working out of St. Olav’s University Hospital in Trondheim, Norway, extended Meyer’s research the following decade. In 2004 they conducted a comparative study that gathered a small group of patients with cardiovascular disease—patients who’d had things like heart attacks, surgery to insert artery-opening stents, or an arterial blockage so severe it required bypass surgery—and asked them to conduct exercise on an inclined treadmill. The volunteers were separated into two groups. Over the course of ten weeks, both groups conducted three training sessions a week, with half conducting regular steady-state exercise, and the other half conducting interval training. The interval-training group exercised for thirty-three-minute sessions, including five minutes of an easy warm-up and then four reps of four-minute sessions separated by three-minute intervals, with a maximum intensity of 85 to 95 percent of their VO2peak. To match the work done by the interval group, the steady-state group conducted forty-one minutes per session at 50 to 60 percent of VO2peak.

After the ten-week intervention, the steady-state patients boosted their fitness by a little—and the interval-training group boosted their fitness by a lot. In fact, the interval-training group more than doubled their improvement in cardiorespiratory fitness compared with that of the steady-state group. “The results of this randomized controlled study demonstrate that high intensity aerobic exercise is superior compared to moderate intensity exercise for increasing [cardiorespiratory fitness] in stable [coronary artery disease] patients,” the study authors concluded.

The results were so promising that St. Olav’s University Hospital acceded to a bigger study, this one a randomized clinical trial, which was published in the prestigious journal Circulation in 2007 by Ulrik Wisløff and his team. It compared subjects who performed intervals at high intensities with those who performed continuous exercise at moderate intensities. The subjects were patients with heart failure, most of whom tended to be excluded from scientific studies like this because it was considered too dangerous to test them. Previous to Wisløff’s, most other studies excluded subjects who were older than seventy. In contrast, the average age of Wisløff’s twenty-seven subjects was seventy-five. They were stable, but pretty out of shape, with one measure of their cardiorespiratory fitness coming in at just 13 milliliters per kilogram per minute. That’s incredibly low. To put that figure into context, it allows a maximal effort that’s only three times the resting-energy expenditure. The average sedentary person can perform maximally at ten times the resting-energy expenditure, and endurance athletes at more than twenty times that. The group that Wisløff studied would have been completely winded taking a single flight of stairs.

Some of the subjects conducted moderate-intensity training three times a week, which entailed forty-seven minutes of incline walking on a treadmill at 70 to 75 percent of their peak heart rate. The interval group warmed up for ten minutes and then conducted four repetitions of four-minute intervals that were hard enough to get the heart rate up to 90 to 95 percent of peak heart rate, with each rep separated by three-minute rests. Including the cool-down, the interval group exercised for nine minutes less than the continuous group, at thirty-eight minutes a session.

So how did they compare? The interval group’s cardiorespiratory fitness, that all-important indicator of mortality risk, increased three times as much as did the continuous group’s. The boost was a remarkable 46 percent for the interval group, compared with 14 percent for the continuous group. That’s an enormous increase in cardiorespiratory fitness for patients in cardiac rehab.

But the remarkable thing about Wisløff’s study was the way interval training changed the patients’ hearts. It suggested that you could actually reverse some of the damage that had occurred in people with heart failure. The end result was an improved ejection fraction, or the amount of blood that gets pumped from the heart when it contracts. Recall that in heart failure, the heart does not pump blood efficiently. Often the heart muscle gets enlarged, like an overstretched balloon. The goal of an intervention is to return the heart’s pumping capacity—restoring the basic function of the organ so that the muscle walls can squeeze tight and effectively pump blood through the rest of the body.

In the interval-training group, the amount of blood the heart was able to pump improved by 17 percent; in the continuous group, there wasn’t any improvement. In fact, by numerous indicators, intense exercise conducted in an interval format helped to reverse the damage of heart failure far more effectively than continuous training, according to the findings of Wisløff and his team.

The notion that cardiac rehab should include physical activity is now taken as a given. Heart centers even have their own workout areas, and there are exercise guidelines for cardiac patients. Most of the exercise prescribed to patients is continuous moderate exercise—steady and sustained exertion on an exercise bike or a treadmill. The problem is, many people don’t do it. According to Maureen MacDonald, only 25 percent of cardiac patients actually engage in any exercise-based rehab program. Even more troubling? Only 6 percent ever complete their programs, according to that 2015 paper by the American College of Cardiology’s Sports Leadership Council. Those who don’t engage in the rehab programs cite a lack of time as the main reason. So is there a better way? For example, what about more time-efficient cardiac rehab—with intervals?

MacDonald was the senior author on a study that compared a typical endurance-based cardiac rehab program with an interval-based approach that required a lot less time. Published in 2013, the study involved men and women in their sixties who were overweight and who suffered from cardiovascular disease so serious that they’d experienced a major event, such as bypass surgery or a heart attack. Twice a week, the endurance group pedaled an exercise cycle at moderate intensity, for thirty minutes at a time at the study’s beginning and fifty minutes by the study’s end. The interval group also conducted their exercise twice a week, except they performed an interval protocol that involved pedaling hard on an exercise bike for one minute and then pedaling at an easy pace for the following minute, repeating this minute-on, minute-off format ten times. By the end, the interval training was taking about half the time of the endurance training, including warm-up and cool-down. And yet, the interval trainers experienced similar benefits, such as improvements in VO2max and arterial health.

Afterward the researchers asked the subjects what they thought of the interval workout. “They called it a ‘really good workout,’” MacDonald says. “They said, ‘I could do it. It was hard, but I could do it.’”

Then MacDonald laid out an argument popular with physiologists to justify the incorporation of interval training into more cardiac rehab programs. If “lack of time” is the biggest excuse people provide when asked why they don’t participate in cardiac rehab programs, she said, perhaps “time-efficient” exercise programs like high-intensity interval training could provide the benefits of exercise to more people.

Stanford’s Jeff Christle says that if he had a relative with heart failure, he would suggest that they undergo some sort of cardiopulmonary testing to ensure that intense exercise is safe for them. And then, he says, “I would want them to exercise at the highest intensity they could.”

How safe are intervals for cardiac rehab patients? Ulrik Wisløff conducted a multicenter study that examined the potential risk of intense exercise in cardiac rehabilitation patients—a group that would be considered at high risk to begin with. Three centers across Norway enrolled patients into programs that incorporated what I’ve come to call the Norwegian protocol. The patients walked on treadmills set to an incline, with the speeds set to elicit certain heart-rate targets. For example, during ten minutes of warm-up, the heart rate target amounted to 60 to 70 percent of the peak heart rate. The main phase consisted of four intervals of four-minute-long duration, with the intent to get the heart rate up to at least 85 percent, and no more than 95 percent, of peak heart rate.

After seven years of tracking, from 2004 to 2011, the research team had data on a group of 4,846 patients with an average age of fifty-eight. Before the exercise intervention, all of them had experienced a diverse range of heart problems, including heart attacks and heart failure. All the patients engaged in both high-intensity and moderate-intensity exercise, with HIIT forming about a third of the total number of workouts. The moderate-intensity exercise was the same incline-treadmill walking used for HIIT, conducted at a constant speed that kept heart rate under or equal to 70 percent of the peak.

So how many experienced adverse cardiac effects as a result of the exercise they conducted? Over the course of seven years, the researchers tracked 175,820 exercise sessions of about an hour each. During or immediately after 129,456 hours of moderate exercise, one patient had a fatal cardiac arrest. And during 46,364 hours of high-intensity interval training, two patients experienced nonfatal cardiac arrests. The essential conclusion from the study was that the risk of a cardiovascular event is low after both high-intensity exercise and moderate-intensity exercise in a supervised cardiac-rehabilitation setting.

Interval Training Fights Diabetes

The less active you are, and the more extra weight you’re carrying, the less your body is able to control your blood sugar levels. Eventually things can get so bad that the lack of activity and the extra weight can push sedentary and overweight people into a condition called insulin resistance, or prediabetes, which, if left unchecked, can then lead to full-blown diabetes. Diabetes really impacts a person’s quality of life. Wounds don’t heal as quickly, and there’s fatigue, a feeling of perpetual thirst, frequent urination, tingling in the extremities, and blurry vision. Eventually, you can go blind, and in extreme cases limbs need to be amputated. Diabetes also puts you at greater risk for all sorts of unpleasant, dangerous conditions. For example, those with type 2 diabetes are 50 percent more likely to have a stroke than those in the general population. Also, managing diabetes can be tricky. There are drugs that reduce blood sugar levels, but they come with side effects. In extreme cases, those with type 2 diabetes will even have to give themselves regular injections of insulin.

This is particularly a big problem in the United States. An estimated 60 to 75 million Americans are insulin-resistant and almost 30 million people have diabetes, with 90 to 95 percent of those having type 2 diabetes. And the vast majority of these cases are related to lifestyle factors, such as physical inactivity and a poor diet.

Two great ways to help manage insulin resistance and mitigate the effects of type 2 diabetes are exercise and weight loss. Current physical activity guidelines from the American Diabetes Association recommend that people with diabetes get at least 150 minutes a week of moderate-intensity exercise. The problem is that many people feel they don’t have the time to fit in this required amount of exercise. So do we have any indication there might be another way? A more time-effective method to improve blood sugar control?

Yes—and I bet you can guess what it is.

In 2010, my lab conducted a study on the effects of interval training in people with type 2 diabetes. One of the main problems for these individuals is that their muscles don’t properly suck up the sugars from the blood. We knew that interval training was particularly effective at rapidly improving the muscles’ ability to take up sugar. “We just thought, if your muscles aren’t good at taking up glucose,” recalls my then–graduate student Jon Little, now a professor of kinesiology at the University of British Columbia’s Okanagan campus, “then maybe through interval training we could engage more muscle fibers, which would use up our carbohydrate stores and then suck more glucose out of the blood.”

Our original study on type 2 diabetics was a small one: eight people. Their average age was sixty-three, and the average body-mass index was 32, or well into the obese category. Over two weeks we asked them to conduct a modified interval-training protocol we’d developed to try to make the benefits of the training more accessible and less scary to sedentary people. Similar to Katharina Meyer’s training program for patients with heart failure, our protocol involved hard (but not all-out) efforts that lasted one minute. Our subjects performed a total of ten repeats on an exercise bike, with one minute of rest in between, for a total of twenty minutes, not including warm-up or cool-down, a protocol that we subsequently dubbed the Ten by One. How hard are we talking? On average the resistance on the exercise bike elicited about 85 percent of the subject’s maximal heart rate during the intervals. By the end of the workouts, our subjects were sweating and out of breath—but they didn’t find the activity unbearable. In fact, on a 10-point rating scale, with 10 being the most intense effort possible, the subjects on average rated the first interval about a 5 and the last interval about an 8. We had them conduct this protocol six times over two weeks. Then we compared their ability to manage their blood sugar levels before and after the two weeks, to see whether the exercise had changed anything.

The training period was short. I mean, just six sessions over fourteen days. To quantify this in a different way, we were asking our subjects to conduct just an hour’s worth of hard exercise throughout the entire experiment. Regardless of it being such a small total dose of exercise, some people wondered whether the older diabetics would even be able to perform the protocol. Would the experiments be a bust altogether?

First of all, they needn’t have worried about our subjects handling the training. These people were champs. People have all sorts of misconceptions about the obese and the sedentary. Jon Little has more experience than almost anyone on earth studying the effects of interval training on people with type 2 diabetes. He’s noticed a couple of things. One, he tends to be impressed with how hard they work. Two, he’s found that many obese and sedentary people tend to find the Ten by One protocol easier than going out and walking for thirty or so minutes. Possibly because it doesn’t take much effort to get these people up to 85 or 90 percent of their maximum heart rate.

Back in 2010, once our subjects had gone through their two weeks of training, we inserted these little glucose-monitoring devices under the skin in their abdomens. The devices tracked glucose levels through the course of a full day, taking measurements every five minutes. They allowed us to analyze how well the subject’s bodies managed blood sugar levels over a twenty-four-hour period, including when they ate standardized meals.

Little was the researcher who conducted the analysis on the first subject. He was in the lab, saw the curve on the computer screen, and was so excited by what he’d seen that he sprinted upstairs to my office. “Holy smokes!” he exclaimed. “Look how much better we made her glucose after just two weeks of training!”

The results were much better than any of us anticipated. That hour’s worth of exercise turned out to have some remarkable effects. The average blood glucose concentration decreased by 13 percent.

If sustained over time, this kind of decrease could improve the body’s ability to manage blood sugar enough to actually reverse the onset of type 2 diabetes.

That’s how remarkable these results were.

Type 2 diabetes takes a long time to develop—between ten and fifteen years of consistently high levels of insulin resistance, or prediabetes. That means more than a decade of sedentary living and poor diet.

Many people find the prospect of 150 minutes a week of moderate-intensity exercise overwhelming. And that’s especially true of people who are so sedentary that they have insulin resistance. They know they should exercise, they might even want to do it, but they just don’t get around to it. Instead they resort to controlling their diabetes with drugs and, in extreme cases, insulin injections.

Our 2011 study and others since have shown that high-intensity interval training is a potent way to help people manage their blood sugar and decrease symptoms of insulin resistance and type 2 diabetes. It could be enough to nudge some people off the road they’re on—a road that leads to diabetes. “When you try to statistically tease out the causes [of diabetes], 70 percent of diabetes cases in the United States could be prevented through lifestyle [changes],” says Little. “It’s as simple as eating real foods and exercising.”

How Do Intervals Fit into the Guidelines?

How did we arrive at this guideline that we need 150 minutes of moderate-intensity exercise a week to promote and maintain health? The fact is, continuous moderate exercise—the long hours of walking, jogging, swimming, cycling, and their variants—get the emphasis from the public-health guidelines mainly because this type of physical activity has been most studied in the academic literature. The vast majority of this research is epidemiological in nature, where large groups of people are followed over time. In fact, no randomized clinical trials have directly tested whether physical activity extends your life or prevents you from getting cardiovascular disease. We have really good data to show that if you do a lot of long, slow steady-state exercise, you have a lower risk of dying and a lower risk of getting type 2 diabetes and cardiovascular disease, as well as some types of cancer. There are lots of studies about the long-and-slow method because that’s how most people get their physical activity.

We have much less data on the benefits of interval training. In fact, think of interval training as a recently developed drug—after all, its powerful and acute effects rival those of any drugs. Drugs must undergo a long and arduous process before government regulators approve them for medical use. The process typically begins with basic research, progresses through small-animal studies, moves on to small-group human studies, and culminates in long-term randomized clinical trials.

Dozens of such studies have found that moderate-intensity continuous training improves human health over the long term. Interval training is so cutting-edge that it’s still in the early stages of testing. Ulrik Wisløff is the senior author on the first randomized and controlled clinical trial on high-intensity interval training and human health. In progress since 2012 in Trondheim, Norway, the Generation 100 study hopes to wrap up in June 2018, having included more than 1,500 people in its exercise interventions over five years.

A professor of epidemiology at Harvard University, I-Min Lee is a dual MD and PhD who studies human health on a population level. She specializes in the way exercise affects the health of populations and helped create the US public-health guidelines on exercise.

She says the Generation 100 study is the sort of evidence that public health organizations want to see before they incorporate specific language about high-intensity and sprint interval training into their exercise guidelines.

That said, Lee believes the guidelines are robust enough, and general enough, to encompass interval training. (I’ll delve deeper into the various flavors of interval training, including examples of different workouts, in chapters six and seven.) “The guidelines don’t specifically mention these protocols, but sprint interval and high-intensity interval training do fall under the broad umbrella of vigorous exercise. . . . Individuals should fulfill [the guidelines] in whatever way they choose.”

So if interval training fits within the exercise guidelines, how do you “count” the workouts against the guidelines’ weekly totals? Recall that the American College of Sports Medicine calls for 150 minutes of moderate exercise per week or 75 minutes per week of vigorous exercise. And let’s say the interval workout we’re discussing is the Ten by One protocol employed in the studies I mentioned earlier on people with cardiovascular disease and diabetes. The protocol involves ten hard efforts of sixty seconds each, and lasts about twenty-five minutes when you factor in the warm-up, cool-down, and recovery periods between intervals. When determining how to “count” that against the guidelines, do you include just the ten minutes of hard effort? Or the workout’s total duration of twenty-five minutes—including the recovery periods?

As she notes, Lee believes high-intensity interval training belongs in the category of “vigorous” exercise. As to whether you count the sprint duration or the whole workout’s duration, Lee answers by bringing up other activities that qualify as vigorous, like singles tennis, basketball, and ice hockey. In those cases, the guidelines would allow you to count the total time that you were playing the sport—even though each game includes many moments of inactivity, such as the pauses between tennis serves or the time that basketball players spend standing around as someone takes a free throw.

Applying that same logic to high-intensity interval training, Lee says that you count against the guidelines the time required to conduct the entire workout. So if the whole Ten by One workout requires twenty-five minutes of your time, you would have to repeat the workout three times a week to fulfill the spirit of the guidelines, according to Lee. The point is, you don’t just count the time spent doing the hard sprints. The recovery periods count as well.

By now, whether we’re talking about varying your speed on a walk or all-out twenty-second sprints, I hope I’ve convinced you there’s likely an interval-training method that’s appropriate for you.