High-intensity interval training has great gains — and pain

Every square inch of my body is sweating. I’ve been working out for only 15 minutes, and I’m not sure my legs can take any more. They are shaking uncontrollably. Later, I sit down to dinner, and my arms are so exhausted I can’t seem to pick up my spoon.

I had just suffered my way through a session of high-intensity interval training, or HIIT.

The regimen can be over and done in 25 minutes, warmup and cooldown included: Quick bursts of extreme physical exertion — 10 seconds to four minutes — are followed by rest periods two to five times the length of the intense parts. Studies show that HIIT increases cardiovascular fitness and can promote healthy blood glucose levels. The short workouts also increase endurance, and a recent study shows why HIIT has more endurance benefits for couch potatoes than it does for pre-trained athletes.

But while HIIT may hit health in the right direction, it does little for our waistlines. And some psychologists question whether a workout that’s so uncomfortable should be promoted in public health campaigns. Will people work through the pain? Or will promotion of such difficult workouts just make people give up in frustration? In the end, whether you pick up a workout plan and stick with it may have less to do with how much time you have, and more to do with where you priorities — and your willingness to deal with pain for gain — really lie.

What constitutes a HIIT workout can be a little difficult to define. Go through one bout of extreme work, and one bout of rest. Then repeat, usually three to six times in a given workout. A series of 20-second sprints and 40-second rests for 10 minutes is a HIIT workout. The seven-minute workout qualifies as high-intensity interval training, too, if interspersed with rest periods. Some types of CrossFit qualify, as well as some treadmill workouts and cycling bouts.

Workout intensity varies as well. “It’s about relative intensity to the individual, not absolute intensity,” explains Kathryn Weston, an exercise scientist at Teesside University in Middlesbrough, England. “For an older person, going up a hill would be HIIT, but for an athlete, they might need to go to sprint training.”

The most important thing, says Charlotte Jelleyman, an exercise physiologist at the University of Leicester in England, is “it should feel hard. For people who are more used to it, it can be all out.” And when Jelleyman says all out, she means it. “Your legs hurt, your lungs hurt, you absolutely cannot go on anymore once you’re finished,” she explains. She’s not kidding. Every time I go through a session of HIIT, I feel like I never want to do it again. The pain might be over and done with relatively quickly, but the muscle exhaustion feels eternal. The soreness can last for a week.

In sickness and in health

All that pain is worth it for the health gains, studies have shown. In young to middle-aged healthy adults, HIIT produced better improvements than endurance training in the maximum amount of oxygen that a person could consume — a commonly used measure of cardiovascular health, Zoran Milaović and colleagues at the University of Nis in Serbia reported in a meta-analysis August 5 in Sports Medicine.

In theory, these gains in maximal oxygen, called VO₂ max, should mean better health. Weston says her group is especially interested in how that translates to everyday life. “It’s great VO₂ max is improving,” Weston says. “But does it mean they are able to carry out daily tasks better? Does it translate over to real life, or is it just in the lab?”

HIIT may also reduce the risk of type II diabetes. “What … HIIT does very well is basically prevent the accumulation or worsening of insulin resistance, and therefore is a very good way of preventing type II diabetes,” says Jelleyman. “It helps keep the blood glucose within a healthy range.” In a meta-analysis of 50 studies, Jelleyman and her colleagues showed that blood glucose is lower following HIIT than it is following normal continuous exercise or no exercise at all. The meta-analysis was published  October 20 in Obesity Reviews.

Making muscles go the distance

One of the most dramatic effects of HIIT is how quickly it increases muscle endurance. “HIIT is much more time-efficient than normal endurance exercise,” notes Håkan Westerblad, a muscle and exercise physiologist at the Karolinska Institute in Stockholm.  “For some groups you get better faster with HIIT than with normal endurance exercise.”

It’s a short workout that produces results quickly. “Some studies have shown effects in as little as two weeks,” Jelleyman notes, if the person is working out at their highest intensity. “But usually you expect at least two months for long-term changes.”

And the gains even seem to affect those who aren’t gym rats by nature. “It almost favors less fit people,” Weston says. “Our data has shown it’s the people who don’t exercise, they’re the ones that might get the most benefit from it.”

To look at how these big gains take place in such a short time, Westerblad and his colleagues examined 18 recreationally active men and male endurance athletes who did six rounds of 30-second bursts of high-intensity cycling followed by four minutes of rest.  After the workout, the scientists took biopsies of the working muscles in the participants’ legs.

When a muscle cell receives a signal to contract, tiny pumps called ryanodine receptors open, and calcium pours out of holding spaces within the cell into the cellular fluid. The high concentrations of calcium signal the muscle cell to contract. Scale this up across all muscle cells, and the whole muscle flexes.  

After a single HIIT workout, un-athletic guys showed fragmentation of the ryanodine receptor. Breakdown of the ryanodine receptor means calcium can leak out into the cell in a continuous drip. With only a little bit of calcium getting released, the muscle cells don’t contract. Instead, the calcium causes a little bit of stress to the cell. Cells react to this stress by increasing their endurance, making them better able to withstand the next bout of HIIT.

Endurance athletes, however, didn’t get the same benefits. It turns out that the breakdown of the ryanodine receptor is a consequence of the production of free radicals — highly reactive molecules — during exercise. Westerblad says that after prolonged endurance training, the muscles of endurance athletes have “a more effective antioxidant system,” something that the nonathletes will develop as their muscles get used to broken down ryanodine receptors. Endurance athletes, he argues, had no benefit because they had nothing left to improve — they were too fit for HIIT. Westerblad and his group published their results November 2 in the Proceedings of the National Academy of Sciences.

Great grain, but what about pain?

But while HIIT has proven to improve health and endurance, it may not hit us where it really counts — our weight. “The only thing it definitely doesn’t do is weight loss,” Weston says. “Generally, it has not been claimed that HIIT is effective for losing weight, per se. The energy expenditure isn’t great enough.” But, she says, some studies have shown decreases in waist circumference. “It doesn’t affect the scale but may affect how you look in the mirror.”

Even so, HIIT is still good for cardiovascular health and your blood sugar. What’s not to love? Well, it turns out, people don’t love the workout itself.

“I was constantly questioning whether people would want to do that kind of thing,” says Stuart Biddle, who studies psychology and active living at Victoria University in Melbourne, Australia. “We know that those much higher levels of intensity are experienced as unpleasant.”

In a debate with Alan Batterham of Teesside University published July 18 in the International Journal of Behavioral Nutrition and Physical Activity, Biddle argues that pushing HIIT for everyone might just doom most people to failure. “[HIIT] has physiological benefit, of that there’s no doubt,” he says. But he thinks the workouts themselves won’t make people come back for more. “It’s unpleasant, it’s hard to do,” he explains. “I don’t think people look forward to it.”

The compressed time means a HIIT workout could work for people who say they have no time to exercise, but the reality may be they don’t want to exercise at all. “When people say they don’t have time, they aren’t documenting it,” Biddle says. “It’s a statement to reflect they don’t want to spend their free time doing exercise.” To overcome this, Biddle believes that people don’t need workouts that are faster and more challenging, like HIIT. Instead, he promotes exercise that can be incorporated into daily routines. 

With a tough workout and no weight loss to show for it, a HIIT regimen needs to be something that you’d actually want to do. And time spent working hard doesn’t feel so bad if it’s spent doing something you love. Weston says that applying HIIT principles to the types of exercise you prefer might help you come back for another bout.  “People like different things,” Weston says. “I hate the treadmill, personally. There’s a misconception that [treadmill or cycling] is the only way [HIIT] can be done.” But that’s not true. Weston says it’s really just about getting yourself to work really hard. “It could be stair climbing or boxing drills, football drills, dance drills, gym equipment,” she says. “As long as you can get that cardiovascular response that shows you’re working hard, it doesn’t matter so much about the mode of exercise.”