My first Orangetheory class started with several short bursts of running on a treadmill and ended in the same place, with a minute of hard running followed by another of all-out effort. In between heart-thumping intervals on the treadmill and a rowing machine, there were multiple rounds of squats, weights, side-crunches and more. After an hour, I was dripping in sweat and, according to my instructor, Natalie, poised to burn fat for hours afterward.
It’s an enticing claim that has become increasingly trendy thanks to a variety of apps, trainers and franchises including Orangetheory: Fitness gains from spending short chunks of time in severe discomfort can rival those from much longer periods of more-moderate exercise. Some programs tout workouts lasting seven minutes or less.
But does it work?
High-intensity interval training can enhance fitness, improve health and even aid recovery from heart disease, according to a growing body of compelling evidence. But, experts caution, intervals should not replace moderate exercise completely. Instead, the two types of activity can complement each other, offering more opportunities for getting fit and staying motivated.
“The more exercise options we can give to people is a good strategy,” says Martin Gibala, an exercise physiologist at McMaster University in Hamilton, Ontario. “Interval training is not a panacea.”
High-intensity interval training has long been standard practice among top runners and other elite athletes. Through short periods of hard effort that alternate with periods of less-strenuous recovery, the goal is to get the heart rate to surge repeatedly to the point where it can’t go much higher. (With Orangetheory, that means above 84 percent of maximum effort, but 90 percent is a common goal for other programs.)
And while it took some time for the science to catch up with early proponents of this approach, studies have demonstrated a variety of benefits after people do these kinds of workouts regularly for a few weeks.
In one seminal study, published in 1996, Japanese researcher Izumi Tabata and colleagues challenged seven active young men to pedal on an exercise bike at a moderate pace that kept them at 70 percent of their VO2 max for an hour, five days a week. As their fitness improved, intensity increased to maintain that level of exertion. A second group of participants did one moderate workout along with four sessions of seven or eight 20-second high-intensity intervals, followed by 10 seconds of active rest.
After six weeks, both groups showed a rise in VO2 max. (VO2 max gauges the body’s ability to turn oxygen into energy.) But the interval training led to a 28 percent increase in anaerobic capacity, a measure of the body’s ability to turn energy into power, while the steady cyclists got no anaerobic boost.
In one of the latest studies to compare intervals with less-grueling but more time-consuming exercise, Gibala and colleagues put nine sedentary men through three 10-minute interval sessions per week. After a warm-up and before a cool-down, the workout incorporated three all-out sprints on an exercise bike lasting just 20 seconds, with two minutes of easier cycling in between. Another group did 45 minutes of steady cycling at about 70 percent of their maximum heart rate. After 12 weeks, the team reported this spring, both groups had improved equally on measures of heart health and fitness, even though the interval group exercised for 30 minutes a week compared with the other group’s 135 minutes.
Dozens of studies have shown that interval training can be beneficial for patients with Type 2 diabetes, heart disease and other chronic diseases. According to a 2007 study of 27 heart-failure patients who were mostly in their 70s and 80s, intervals were three times better than more-moderate exercise at improving aerobic capacity.
“The literature is suggesting that the more intensely you are willing and able to work, the less total exercise you need to perform to reap benefits,” Gibala says. “Many more people than we typically think can perform interval-based exercises.”
One way the muscle cells respond to sprints, according to Gibala’s work, is by growing more mitochondria, which turn fuel into energy.
Mechanically, intervals also force the heart to pump more blood with each beat, adds Brian Kinslow, a physical therapy doctoral student at Northern Arizona University in Flagstaff. Over time, that makes the heart stronger and more efficient.
Intervals may provide a psychological boost, too, giving athletes confidence about their speed and adding variety to exercise routines. In a 2011 study of eight active men, participants rated a 50-minute treadmill workout that included intense running intervals as more enjoyable than a continuous 50-minute run.
One hope is that getting people hooked on intervals might help improve the dismal rates of adherence to public health guidelines, Gibala says, with few people meeting the weekly recommended 150 minutes of moderate exercise or 75 minutes of vigorous exercise.
As scientific support for intervals builds, plenty of caveats remain. Most interval studies have been small and short. None have tested performance outside the lab or beyond a few months. Studies have also used different interval lengths and ratios, making it impossible to offer universal recommendations. And although some people enjoy opportunities to test their physical limits, others can’t tolerate the suffering they need to endure to get benefits from intervals.
For those who want to give it a try, experts advise starting gradually: Even spurts of fast walking can do good for people just starting to exercise.
Based on the bulk of research, Kinslow recommends equal periods of intense effort and active recovery, with 30 seconds of each as a good starting point. Intervals can get longer as fitness improves. Five to 10 intervals in a workout are enough to make a difference. And two or three interval workouts a week are plenty. Beyond that, studies show that back-to-back interval workouts can harm sleep quality and stress muscles without boosting performance. Incorporating interval sessions into a varied workout plan, according to other research, is likely to deliver the best results.