The High Intensity Cardio Debate

Interval training for muscle growth

In today’s gym, the wildly popular and hyperintense cardio workout has many faces: that guy sprinting full-tilt on the treadmill, the girl whipping ropes into the floor like a child throwing a tantrum. It also encompasses kettlebell swings, kickboxing classes, burpees—and basically anything else that involves short bursts of brutally intense exercise followed by periods of lighter activity or rest.

High-Intensity Interval Training (HIIT) has been the leading movement in cardiovascular exercise for fat loss and conditioning for more than a decade—ranked No. 1, in fact, on the American College of Sports Medicine’s survey of fitness trends for 2014. Fueled by claims of faster fat loss, shorter workouts, and less monotony (along with exclamations such as “It kicked my ass” or “Made me puke”), HIIT has largely supplanted traditional aerobic training—of the just-go-out-and-jog variety—as the preferred conditioning method of gym-goers everywhere. But old fashioned roadwork, the kind that Muhammad Ali and dozens of other champion athletes utilized, isn’t obsolete. In fact, it may actually be the more important style of cardio exercise for anyone looking to be in better, more well-rounded shape.

“The biggest reason aerobic training has fallen out of favor in the fitness industry is the flashy headlines,” says Joel Jamieson, author of Ultimate MMA Conditioning (available at 8weeksout .com) and strength coach to MMA fighters, pro football players, and other athletes. “We like the idea of being able to lose weight in four minutes versus 40 minutes, but it’s not the right approach.” He points out that the research HIIT enthusiasts frequently cite for support is often very short-term and flawed.

In 1996, a study was published that has perhaps done more to buoy the current HIIT movement than any other. Japanese researchers, led by Izumi Tabata, published the now-famous Tabata study, which showed that well-trained young men improved anaerobic endurance more in six weeks with interval training than a control group did by performing aerobic exercise. The experimental group performed intervals for only four minutes at a blistering intensity—20 seconds on, 10 seconds off. Because of the remarkably short workout time, the study has been hailed as proof that HIIT is vastly more efficient than aerobic training. However, the findings have since been greatly exaggerated to suit the HIIT agenda, and experts argue that the study simply doesn’t apply to the regular gym-goer.

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For one thing, results for the interval group began to level off after the third week. For another, the interval group performed some aerobic training (30 minutes’ worth) in addition to the intervals, so it wasn’t a pure test of HIIT. Furthermore, the “moderately trained” experimental group performed its intervals at a staggering 170% of VO2 max, the maximum amount of oxygen the body can use during exercise. Considering that 100% of VO2 max is enough to exhaust most people, you get a sense of just how fit these “moderate” subjects were.

Now consider a 2008 study from Medicine & Science in Sports & Exercise, in which eight obese men trained for four weeks at a moderate, constant pace (also known as “steady state”). Data were collected, and the men then performed another four weeks of intense interval training. On both programs, the subjects’ diets were set to avoid weight loss to allow for a pure test of the exercise protocols.

The workout length on both programs was the same, and so was the subjects’ average energy expenditure and body composition (remember, they never cut calories from the diet). Still, the men burned more fat on the steady-state protocol than with the intervals. In fact, the aerobic training yielded a jump of 44% in the amount of fat burned during exercise, while the intervals caused zero.

So what does all this science really mean? Well, two things. The main one is that cardio, whether steady state or intervals, doesn’t do much for fat loss. (Though the subjects burned more calories from fat during aerobic exercise, they didn’t lose weight overall.) And second, interval training isn’t necessarily any more effective than aerobic work, in any time frame. In fact, to truly maximize your performance and minimize fat, you need to be doing a combination of the two.

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Interval training for muscle growth

The Heart of the Mattter

To be clear: There are Two forms of cardiovascular exercise. Aerobic training includes longer activities such as jogging, swimming, and cycling, and occurs at relatively low intensities (60–70% of your maximum heart rate, or approximately 120–150 beats per minute). Anaerobic training, meanwhile, includes lifting weights, sprinting, martial-arts training, and any other exercise characterized by short intervals of hard work followed by light activity or complete rest. (Here, you’re raising your heart rate above 150 beats per minute and sometimes as high as 90% of your maximum heart rate.)

Aerobic workouts are fueled by oxygen, which provides energy for a steady rate of activity but no explosive power. Anaerobic training runs on phosphocreatine (hence the popularity of the supplement creatine for refueling it) and carbohydrate, which can supply quick energy for intense activity but peters out fast. Aerobic workouts tend to be long (up to an hour or more), while anaerobic sessions can last just a few minutes. Because most sports and exercise habits fall under the umbrella of anaerobic, many believe that anaerobic cardio workouts are all most people need to be in shape.

The truth, however, is more complicated. While the aerobic system is less active during exercise lasting less than 60 seconds, it never shuts off completely, and its involvement increases rapidly as the activity goes on. Even during highly intense work lasting a minute (such as punching a heavy bag), the aerobic system provides nearly 50% of the total energy. According to Jamieson, after about 90 seconds, the aerobic system provides the majority of energy—even if you’re still working intensely.

In other words, the fitter your aerobic system, the better your anaerobic performance will be. “Lower-intensity work develops the vascular system—the blood vessels and supply network that deliver oxygen to the working muscles,” says Jamieson. “It improves recovery and work capacity, and helps you oxidize more fat. We can do more work without overtraining when we develop the aerobic system.”

Want to know why you breathe hard after running down the block to catch a bus? That’s your body trying to replace oxygen to refuel the aerobic system—even though your bus sprint itself was technically an anaerobic burst.

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Interval training for muscle growth

The Best of Both Worlds 

This isn’t meant to be an indictment of interval training. As Jamieson points out, “It increases the endurance of the faster- twitch muscle fibers,” the big ones you use to lift heavy weights or run fast. Intervals can play a critical role in preparing you for any quick-burst sports you engage in. HIIT also burns calories and makes for fun workouts you can do when short on time. But because it’s so intense by nature, you have to use it sparingly. “With lower-intensity training,” says Jamieson, “we can do more work without being worried about what it might do to recovery. High-intensity training has to be managed properly because it’s very easy to overtrain.”

So, to maximize your cardiovascular fitness, he advises that you cap your HIIT sessions at 20–30 minutes, and spread them out—do intervals on Monday and Thursday, for example. Aerobic training, on the other hand, can be done virtually every day, although you need only three to four sessions of 20–30 minutes per week—building up to 90 minutes over time—to see results. The right balance of aerobic and anaerobic cardio conditions the body to perform any activity better—and does as much as cardio can to help you burn fat. 

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