After all, in the past decade, research has consistently shown that minute-per-minute, going hard results in bigger calorie burns and greater effects on metabolic health, including insulin sensitivity and VO2 max. And who isn’t crunched for time? “Plus, people find repeats and intervals inherently more interesting than going out and jogging at a super slow speed for 45 minutes,” explains running coach Matt Fitzgerald, author of the new book 80/20 Triathlon.
But, at least in the endurance realm, a focus on high-intensity exercise may could very well be running its course, with physiologists and run coaches campaigning hard for a reduction in exercise intensity.
Low Intensity Cardio Isn’t Necessarily Easier
“I’m not sure that I like the false equivalency between intensity and quality,” says Janet Hamilton, C.S.C.S., an exercise physiologist at Running Strong in Atlanta. She explains that every exercise intensity has its benefits, and where you spend the majority of your training time should depend first and foremost on your goal.
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If that’s improving your performance at any race that takes more than 1 minute to complete (think: anything longer than a 400-meter dash), that intensity is going to be low. Likely, much lower than where you’re currently training on even your so-called “recovery” days.
“There’s a common misunderstanding that low intensity and high intensity exercise do the same thing, but high-intensity does it better,” Fitzgerald says. “There is overlap, but they also do unique things. Low intensity cardio has some benefits that high intensity does not—and vice versa.”
Exercise Intensity, Defined
The gold standard for determining how hard you’re working during any given sweat sesh revolves around the two ventilatory thresholds. The first ventilatory threshold, VT1, is the intensity at which lactate rises above resting levels, and typically occurs around 78 percent of one’s maximum heart rate, per Fitzgerald. However, in people who are newer to endurance exercise, it can be at a much lower heart rate.
Once you cross VT1, you’re no longer able to chat comfortably, but can still string together a few words and short sentences, explains Carl Foster, Ph.D., director of the Human Performance Laboratory at the University of Wisconsin – La Crosse, who has extensively studied the “talk test” over the past 30 years.
At VT2, also called the second ventilatory threshold, anaerobic threshold, or lactate threshold begins to accumulate in the blood much more quickly and breathing becomes rapid as anaerobic metabolism takes an exponentially greater role in keeping you moving. This typically occurs around 85 to 95 percent of max heart rate. But again, the more aerobically trained you are, the higher than heart rate will be when you do cross VT2. Once you do, talking is close to impossible and exercise duration will decrease—your body just can’t sustain it very long, Foster says.
The Unique Benefits of Low Intensity Steady State Cardio
By definition, if you’re in it to win it (or PR), your race pace is going to be high-intensity. A shorter race like a 5K might have you at 95-plus percent of your max heart rate. A 10K at about 90 to 92, and a marathon roughly or just over 85 percent, Hamilton explains.
That raises the question: why train at a lower intensity?
“If you don’t have adequate aerobic infrastructure built, you’ll end up relying more and more on anaerobic energy production during what should be aerobic activity and you’ll burn through that available fuel quickly,” she says. Aerobic infrastructure refers to enzymes that allow your body to break down carbs and fat into energy through aerobic metabolism, strength in the slow-twitch muscles that allow for sustained efforts, capacity of the heart’s left ventricle, and the size and number of your body’s capillaries and mitochondria.
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This last part is huge: “The ability for heart to pump more blood can improve by about 10 to 15 percent, but capillaries and mitochondria have enormous, almost infinite, adaptability,” Foster says. Capillaries allow you to effectively transport oxygen, while mitochondria are the microscopic power plants that put it to use. And, no, high-intensity or even moderate-intensity exercise isn’t going to zero in on them the way low intensity cardio will.
“Once you’ve built your aerobic infrastructure, you can then utilize it more efficiently. You run faster, you need more fuel, you need more oxygen to make that fuel… But, hey, you’ve got the ability to deliver and utilize it, so you’re good. Speed up and you’re not dipping so easily into the anaerobic realm. You are staying aerobic at faster paces.”
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What’s more, as Foster explains, your ability to store glycogen (carbs in the liver and muscles) increases the more often you deplete what you’ve got. “So if you are doing a long run every two to three days, your fuel tank gets bigger,” he says. And then, once it does run out of glycogen, it more efficiently turns to fat for fuel to keep you from hitting the wall. “You can utilize more fat for fuel at higher intensities; you become a better butter burner,” Hamilton says.
Not to be overlooked, however, are the mental benefits of low-intensity, long-duration exercise. “The brain adapts to exercise more than any other organ in the body does,” Fitzgerald says. “Studies shown that inhibitory control [your ability to focus despite distractions] increases response to endurance training. If you just sit a bunch of people in front of computers, their ability to stay on task is highly predictive of their endurance performance.” After all, at a certain point, endurance is as much (if not more) mental than it is physical.
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Figure Out What Low Intensity Cardio Is for You
Most coaches recommend that endurance athletes, no matter their level of competition, should perform 70 to 80 percent of their training below their VT1, 5 to 10 percent between VT1 and VT2, and a max of 20 percent of above VT2, says Foster, explaining that the split is largely the result of work from Stephen Seiler, Ph.D., the kinesiologist who first found that the world’s top-performing endurance athletes all use a similar training setup. His subsequent research has found that a making the bulk of training low-intensity, long-duration, with fewer, highly intensive bouts is complimentary for optimizing the exercise adaptations needed for endurance performance.
Most runners are training way too hard. According to Fitzgerald, the average runner spends about half of their weekly training time at a moderate intensity. They naturally gravitate to a pace that they find seriously challenging—but not too uncomfortable. The result, according to Frank, is that a lot of miles ran end up being “junk miles” and don’t allow runners to properly recover from their workouts or build their aerobic infrastructure.
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The first step in finding a better balance is to determine what high-, moderate-, and low intensity cardio look like for you. To calculate your max heart rate—and your target heart rate zones—the ‘ole “220 minus your age” equation will give you a rough idea.
But for a more precise measure, perform some field testing. Fitzgerald recommends warming up and then running a 30-minute race on a flat, smooth course. Your average heart rate for the final 20 minutes of those 30 represents your max. Pretty similarly, Hamilton says that you can take the greatest sustained heart rate in a 5K as a pretty accurate guide of max heart rate. “For example, in a 5K, the athlete saw a peak HR of 198, but a sustained high HR of 188 for the last 5 minutes of the race, I’ll use the 188 rather than the 198,” she says.
Take the Talk Test
A less complicated measure, which Foster says is “idiot proof,” is to simply gauge your ability to talk during a run. When talking during exercise is “unequivocally comfortable” during a run, that means you’re operating below VT1 at a low intensity. Between VT1 and VT1, you should be able to speak in short sentences and you won’t really want to engage in deep conversation with your running buddies. “After VT2, you can only say two to three words at a time, and they are going to be things that your momma wouldn’t want you saying,” he says.
Then, devote roughly 80 percent of your overall total running mileage or time to that comfy exercise, he says. The rest should be broken up between moderate- and high-intensity, with the majority skewing to that puddle-inducing level of exertion you already know and love.