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Crank Up Your Cardio

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When Cardio Was King

When trainer Jim Karas's book The Cardio-Free Diet was published in 2007, he was excited to give readers a new way of thinking about exercise. Based on his 20-plus years in the fitness business, he had come up with a workout strategy that would free people from hours spent jogging or striding away on the elliptical. Karas believed that all that cardio, while improving health, wasn't delivering weight loss. Worse, he felt it was leading to injury, boredom, and even overeating. But then came the backlash — from the media, exercise experts and the public. "I got blasted," he recalls. "People are addicted to cardio. They think more of it is better, and that's just not true."

Karas, whose latest book is The Ultimate Diet Revolution, may be getting the last laugh. His cardio-free plan involves doing strength work with weights and high-intensity interval training, commonly known as HIIT, in which you alternate challenging bouts with easier recovery periods. The acronym has rapidly become the fitness buzzword. For the last several years, there has been a frenzy of research on high-intensity workouts, even supershort ones (four minutes, anyone?). The aggregate effect of these studies is doing to steady-state cardio what the advent of the Atkins and Zone diets in the 1990s did to carbs: giving it a bad rap. Is cardio — in the form that we've known and loved — dead? Is cardio keeping us fat?

It may be hard to believe in this era of all-out war with the expanding American waistline, but regular workouts haven't always been must-dos. The fitness movement didn't really pick up steam until the late 1970s, early 1980s thanks in part to the man who coined the term aerobics, Kenneth Cooper, MD, the founder and chairman of Cooper Aerobics and the Cooper Institute in Dallas. Through his (and his colleagues') extensive research, he made physicians, health experts and the public realize that exercise is good for you, whether you're recovering from a heart attack or trying to lose weight. He and his team have tracked more than 110,000 patients for up to 40 years and published 600 papers on the health benefits of moderate-intensity aerobic exercise. "It's a gold mine of data, all based on good, solid scientific fact," Dr. Cooper says. "Aerobics is not dead; it's more alive than ever. These other things [like HIIT] are just fads."

Researchers typically compare HIIT routines with longer, steady moderate-intensity exercise (what I'll refer to as cardio throughout this article) because there are thousands of studies about the benefits of cardio, as opposed to mere hundreds for HIIT. And the cardio studies are large. For example, a landmark 1989 study of more than 13,000 men and women, which was published in the Journal of the American Medical Association, found that higher fitness levels resulted in a lower risk of death from cardiovascular disease and cancer. It suggested that even moderate exercise — walking (at about three miles per hour) for 30 to 60 minutes a day — can reduce mortality risk by almost 50 percent. HIIT studies, on the other hand, tend to be smaller and shorter. After all, it's hard to find people willing to gut it out on a treadmill or stationary bike — while breathing through a tube and being poked with a needle — for more than a few weeks.

So if moderate exercise is the holy grail of longevity and disease prevention, then why are so many people bolting from the bandwagon? As Karas explained, people get bored or injured or don't see the weight loss they were hoping for, and exercise, like any healthy habit, can be hard to stick with. But the real problem is time. Once everybody started getting the picture that cardio was really good for you, experts began developing guidelines for how much is needed to deliver benefits. The Centers for Disease Control and Prevention and the American College of Sports Medicine initially recommended a minimum of 30 minutes a day. The Institutes of Medicine advocated at least 60 minutes daily. The current benchmark for health benefits is a minimum of 150 minutes of moderate-intensity aerobic activity every week — even more if you're trying to lose weight.

"These public health guidelines are based on very good science," says Martin Gibala, PhD, the chairman of the Department of Kinesiology at McMaster University in Hamilton, Ontario, who has been studying HIIT for the past 10 years. "But lack of time is the number-one cited barrier to exercise. People want alternatives that are shorter but just as effective." Gibala's 2008 study of exercisers illustrated how HIIT can save time. He found that 20 to 30 minutes of HIIT three times a week, with a weekly total of only six to nine minutes of "work" (the all-out bursts in between the recovery breaks), boosted fitness levels comparably to exercising at a moderate pace continually for 40 to 60 minutes five days a week (with 200 to 300 total minutes of work time). The government must have heard the rumblings, because in 2008 the Department of Health and Human Services decided that 75 minutes of vigorous-intensity exercise is comparable to 150 minutes of moderate aerobic exercise. By that time, HIIT was hitting its stride.

The Rise of HIIT

The whole concept of interval training isn't new. "The Swedes coined the term fartlek, or 'speed play,' 80 or so years ago," Gibala says. And athletes haven't been the only ones realizing the benefits. About 30 years ago, European researchers were successfully using interval training for cardiac patients. Then there's Izumi Tabata, a Japanese researcher who did some of the original modern HIIT research in 1996. He found that going all-out for 20 seconds and resting for 10 (repeated for just four minutes) increased anaerobic capacity — essentially your body's ability to work really hard — better than 60 minutes of moderate-intensity training did. (Both routines delivered similar increases in aerobic fitness.) His ultra-intense paradigm is now known as the Tabata protocol and is still being tested in labs — and co-opted by gyms — today. One recent study clocked subjects burning 15 calories a minute, which is comparable to what you would melt running up stairs.

"Nobody's suggesting that HIIT will replace cardio," says Glenn Gaesser, PhD, a professor of exercise physiology at Arizona State University in Phoenix. "But essentially, it lets you accomplish many of the same things that cardio does in less time." Most of the research has centered on four areas where HIIT appears to be at least as good as if not better than moderate cardio: improving VO2 max (a measure of your body's ability to take in and utilize oxygen and a key indicator of fitness), increasing glucose uptake (your body's ability to send glucose into the muscles, where you want it), and boosting anaerobic performance and fat burning. HIIT seems to prompt your body to release fat stores after exercise in much the same way you might toss your wallet at a mugger: Ack! Take everything I have! Let's get this over with already! "It's simple: To get a stronger heart and greater aerobic fitness, you have to challenge your system," says David Swain, PhD, a professor of exercise science at Old Dominion University in Norfolk, Virginia. "You don't do that by exercising at a low level. You do that with something like HIIT."

"It's also good because it recruits both slow- and fast-twitch muscles, while conventional cardio really uses only slow twitch, so essentially about half the muscle fibers in your legs don't even get trained," says Carl Foster, PhD, the director of the Human Performance Laboratory at the University of Wisconsin-La Crosse. "The more muscle fibers you use, the greater the metabolic effect." That means you burn more calories 24-7.

So why not do it all the time and skip the moderate stuff completely? Because it's freakin' hard! Those 20-second Tabata bursts? They're supposed to be done at 170 percent of VO2 max! "I've yet to see anyone do this correctly," says Brett Klika, the 2013 IDEA personal trainer of the year. "At that level, you shouldn't be able to get off the floor when you're done." Thankfully, most HIIT workouts aren't as tough, but they are challenging, both physically and mentally.

"We don't have enough scientific evidence about the long-term adaptations for HIIT," Gibala says. Experts don't know what impact month after month of intervals will have on the body or even what type of interval training is ideal. "If you want a better body in two weeks, HIIT is the way to go," Gaesser says. "But if you want a lifelong better body, the jury is still out. A mix of HIIT and cardio is probably the best plan."

Your New Exercise Diet

As for a recommended weekly dosage of HIIT, most experts I spoke with suggest aiming for two or three high-intensity sessions a week. "Any more than that and you set yourself up for overuse injuries," says Geralyn Coopersmith, the director of Nike SPARQ Performance Training in Portland, Oregon. "Plus it's on your off or light days that all those good changes happen in your heart, lungs, and skeletal muscles." Your body scrambles immediately after HIIT to get all of its various processes back to normal, repair itself and remove metabolic waste. That large internal effort expends calories, which results in the coveted "afterburn" effect.

Confounding the matter of determining the "best" HIIT workout — the one that's long on afterburn but short on recouping time — is that there are limitless ways to set up your work-to-rest pattern.

To examine the pain versus gain of HIIT firsthand, I headed to Foster's lab, where he, John Porcari, PhD, the director of the school's Clinical Exercise Physiology graduate program, and their team are comparing Tabata with steady-state cardio and the classic one-minute-on, one-minute-off Meyer intervals. By 4:00 p.m. — having already knocked out a couple of fitness tests in a breathing mask in the morning and completing a series of 13 submax speed bursts at noon for the Meyer intervals — I was back on the special tricked-out stationary bike that we had used for testing all day. It was time to do the mother of all HIIT sessions — the Tabata protocol, pedaling at absolute max intensity up a steep hill's worth of resistance for 20 seconds with 10 seconds rest, eight times in a row. Ten seconds isn't nearly enough time to recover from going full tilt. I barely made it through the last interval; my legs and lungs felt like burning lead. When I finished, the student assistants stepped in to prick my finger and take my blood pressure.

"Based on previous research, we expect to find that intervals will increase VO2 max as well or better than steady-state cardio, and we think Tabata will do it better than Meyer intervals," Porcari says. "But we're also looking at blood pressure, blood lactate levels and the enjoyment factor of each workout."

And that's really the point of all this research — to help people figure out how to work intervals into a routine and whether the benefits are worth the extra effort and discomfort. (While it will be months before Porcari and his staff finish crunching the numbers from the ongoing testing at the lab, I've already vetoed Tabata in my rotation.)

The bottom line: Play around and figure out what works for you. Beginners should give themselves at least as much rest as work time and usually more. As you get fitter, you can increase the intensity of each interval and reduce the amount of recovery. "It may take several workouts to realize what feels best," Gaesser says. "If you finish and think you can't possibly do another interval, you're working harder than necessary to reap the benefits of interval training." And that's the point of doing intervals in the first place — you're trying to work out smarter, not harder or longer than you have to.

What Counts As Cardio Now?

For Your Health

  • 5 x 30 minutes of moderate exercise weekly, like walking at 3 to 4.5 miles per hour
  • 3 x 25 minutes of vigorous exercise weekly, like HIIT workouts, jogging or running at 5 mph or faster, cycling at more than 10 mph


For Weight Loss

  • A minimum of 5 x 30 to 50 minutes of moderate exercise weekly. Aim to burn 300 calories a session (an hour-long power walk at 4 mph or 35 minutes of HICT, or High Intensity Circuit Training).*
    *Based on a 140-pound woman


The New Fat-Burning Zone

One of the enduring myths about cardio is that it's the best way to get lean, because at low to moderate intensities, your body prefers to use fat, as opposed to carbs, for fuel. But weight loss is a numbers game: the more calories you burn, the more you lose. HIIT seems to offer the best of both worlds. Even though it taps more carbs than fat for fuel, it smokes a ton of calories and still targets fat post-workout (afterburn) at least as well if not better than moderate-intensity exercise. A study in the International Journal of Obesity compared steady-state and high-intensity training in women during a 15-week period. Both groups burned a similar number of calories over the test period, but the HIIT exercisers — whose workouts were half as long — lost more weight, abdominal fat and total fat than the steady-state group, who actually gained ab fat. If you're trying to get lean, interval training adds another weapon to your arsenal, and if you can burn 200 calories in 20 minutes instead of 40 minutes and blast more fat, why wouldn't you?


High-intensity circuit training (HICT) is the strength-training version of HIIT. It's the formula — back-to-back exercises with a little rest thrown in here and there — behind all those supertough classes at your local gym. Check out the hottest HICT classes around the country.

The Club: Crunch Gyms, nationwide (
The Class: Powerwave Battle Roping
You'll alternate high-intensity rounds of using thick, heavy ropes with slightly easier recovery periods of sculpting moves.

The Club: 24 Hour Fitness, select clubs in Dallas, Los Angeles, San Francisco, and New York City (
The Class: Nike Training Club
Pro athletes use these routines that follow an interval format with 30 seconds to two minutes of high-intensity cardio followed by strength moves with hand weights.

The Club: Equinox Fitness Clubs, select clubs in Los Angeles (
The Class: 4x4
One-minute rounds of upper-body, lower-body, core, and cardio moves make up each four-minute block, which progress in difficulty during the 45-minute class. Tools include weights, tubing, kettlebells, and more.

The Club: The Fhitting Room, New York City (
The Class: The Signature Fhix
Small group sessions (12 clients, two trainers) at this high-intensity-only boutique studio, which often has a waiting list, combine rowing machines with body-weight, dumbbell, kettlebell, and medicine ball moves, among others. Intervals range from 20 seconds to seven minutes.

The Club: East Bank Club, Chicago (
The Class: Lunch Break
This sweat-fest incorporates sprinting, ropes, the TRX Suspension Trainer, sports drills, and medicine ball moves into one fast-paced workout.

The Club: Life Time Fitness, Chanhassen, Minnesota (
The Class: TCX Total Conditioning
Each circuit progresses from stationary strength exercises using dumbbells or tubing to strength-with-movement exercises to plyometric exercises that spike the heart rate.

The Club: Multnomah Athletic Club, Portland, Oregon (
The Class: Maxed Out
The club's most intense class is just 30 minutes long, not including warm-up and cool-down. It alternates four to five minutes of strength work using body weight with a minute of recovery — if you can call it that.

The Club: Sports Club/LA, Miami (
The Class: Circuit Strength
Adopting a boot camp-style approach, this class involves doing as many rounds as possible in five minutes of moves like jumping rope, kettlebell swings, lunges, or squats — and that's just the first circuit!

The Club: Colorado Athletic Club-Monaco, Denver (
The Class: H.I.T.
The club's H.I.T. class incorporates weights, the ViPR, sandbells, a sled, rowing machines, tires, plyo moves — whatever they can drag into the studio. Every three months, members compete with themselves, their classmates, or peers from other clubs in squats, jumping, rowing, pull-ups, and more.

Originally published in FITNESS magazine, April 2014.