Ask a bunch of people why they don't get enough physical activity and you're likely to hear: "I have no time."
No wonder there's been so much hullabaloo about interval training, which alternates short bouts of relatively intense efforts with periods of recovery. Research suggests it can provide some of the benefits of longer, moderate-intensity workouts in less time. A survey by the American College of Sports Medicine just ranked it as the third biggest fitness trend for 2017, behind wearable technology and body-weight training.
Interval training, which has been used by athletes for years, may be a great way for you to make workouts more time-efficient. You may even find it fun. But it's not a panacea, says one of the leading researchers in the field, Martin Gibala, a professor and chair of the department of kinesiology at McMaster University in Hamilton, Ontario.
There are umpteen variations on interval training, but basically, you can break it down into two types, according to Gibala, author of the forthcoming book The One-Minute Workout.
The first, high intensity interval training or HIIT, mixes rest periods with intense bouts that get your heart pumping to at least 80 percent of your maximal rate.
That sounds hard, but sprint interval training, or SIT, is even harder: Those intense bouts are all-out efforts, like how you'd run if a bear were chasing you.
Lacking the bear, though, it can be hard for non-athletes to motivate themselves to go that hard. HIIT is better suited to the general, non-athlete population, and has also been more widely studied, even in people who have medical conditions like congestive heart failure, Type 2 diabetes and obesity, says Gibala.
In general, you just want your hard periods to be out of your comfort zone, says Jordan Metzl, a sports medicine physician at the Hospital for Special Surgery in New York and author of the forthcoming Dr. Jordan Metzl's Workout Prescription. "It's a mix of common sense and hard effort," he says. (It goes without saying that before you start any new exercise routine, you should check with your doctor, particularly if you have any underlying health conditions.)
So how does it stack up against, say, jogging or cycling at a more moderate pace? If you compare a similar dose of exercise, say calories burned or total workload, achieved with HIIT and with moderate steady exercise, the HIIT produces superior benefits in outcomes including cardiorespiratory fitness and blood glucose control, says Gibala.
But that doesn't address the question most of us really want to have answered: whether you can get similar or superior benefits from a relatively smaller dose of HIIT training compared to a bigger dose of moderate aerobic training. Alas, the research on that point is more limited.
There is some evidence that short SIT sessions can provide similar cardiorespiratory fitness benefits compared to longer steady exercise sessions. But there is more limited evidence for clinical outcomes like insulin sensitivity, and it's also not clear whether the more moderate HIIT in relatively small doses offers a similar benefit, says Gibala.
In short, "there's no free lunch," he adds.
Metzl says that based on the evidence he's seen, the sweet spot for benefits including cardiopulmonary fitness and health are workouts that total 20 to 30 minutes, including hard and easy periods. (The Centers for Disease Control and Prevention recommends at least 75 minutes of intense exercise per week or 150 minutes at a more moderate pace.)
Of course, many people work out not only to improve fitness and health, but to lose some weight. And the research on fat loss is mixed, according to Shelley Keating, a research fellow in the University of Queensland's Center for Research on Exercise, Physical Activity and Health in Australia. She and her colleagues published a study in 2014 showing that a program of continuous cycling, but not all-out sprint intervals, reduced body fat in overweight adults.
"I think where the evidence currently stands is that one is not consistently better — and it is the volume of exercise that matters for fat loss," she wrote in an email. Interval training burns more calories per minute, but because it's generally done for shorter periods of time, the total exercise volume may be lower, said Keating. She also said that while your body does tend to burn more calories after an interval workout than after a steady one, the total amount of calories that equates to is "minimal."
The takeaway is not that interval training is a magic bullet, says Gibala, but that even if you have 10 to 15 minutes of time to exercise, you can get something out of it. "You can fit exercise into your day," he says. Climbing stairs, for example, counts as "stealth interval training," he says.
This mindset has really changed my own view of exercise: If I only have a short period of time for exercise, rather than blowing it off, I will combine some circuit training and treadmill sprints that get my heart racing and take a grand total of 20 minutes.
I really enjoy intervals, and although others find HIIT more enjoyable than steady aerobic exercise, the truth is that some people find more intense exercise really un-fun. Still, they may be willing to add a few interval sessions to save time.
Metzl says training with a group can make it more enjoyable, and research also suggests that music can improve the experience.
But if you really prefer long, slow running, that's fine too. "The best type of exercise is the one you enjoy and will commit to in the long term," said Keating.
Katherine Hobson is a freelance health and science writer based in Brooklyn, N.Y. She's on Twitter: @katherinehobson.
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