Head to the gym after work, and you’ll see them—the people dutifully cycling or stepping on the elliptical at a constant clip for 45 minutes.
While we applaud them for doing something—any workout is better than none—we hope they’re reading this. A growing pile of research has shown that steady moderate exercise isn’t the best use of your precious workout time. Truth is, intervals of higher-intensity efforts are just as effective as long endurance workouts at improving health and fitness in people just starting a workout program—and they take a fraction of the time.
For instance, in a pair of new studies just published in the Journal of Physiology, 6 weeks of three 30-minute interval workouts—totaling 1.5 hours a week—produced the same benefits in the way the body uses insulin as about 5 hours per week of steady pedaling. “It is a massive time savings, and you still achieve effects that are totally comparable,” says study author Anton Wagenmakers, Ph.D., of Liverpool John Moores University.
To understand why, it helps to know what happens in your body after any cardio session. Your muscle cells adapt to the challenge over time by forming new mitochondria, powerhouses that generate most of the energy your hamstrings and biceps use to flex and move during aerobic exercise. New blood vessels also form to shuttle fuel—along with the oxygen required to burn it—more efficiently.
As a result of these changes, you can work out harder and longer without getting tired.
Why Intervals Work
When you’re exercising at a steady rate, research shows you need about 5 hours a week to generate these effects. But when you mix in brief bouts of more intensity, these adaptations happen much more quickly—and the new Journal of Physiology studies shed light on exactly how.
In large part, it seems to be because doing intervals recruits more muscles. Intervals recruit both your fast-twitch and slow-twitch muscles, while steady-state exercise mainly uses your slow-twitch muscles. While fast-twitch muscles are typically thought of as “power” muscles for explosive exercise, they also can be aerobically trained, which increases their mitochondria count.
In addition, intervals may be just as good as steady endurance workouts for warding off type 2 diabetes. One of the studies found that both types of exercise boosted levels of proteins your body uses to burn triglycerides (blood fats). When triglycerides get out of control, they interfere with your body’s ability to use insulin, which is the hormone that regulates your body’s blood sugar.
As for your blood vessels, the shear stress of blood pushing against vessel stimulates the growth of new veins and arteries. Since sprints get your blood pumping with more force, your body gets the message to sprout new blood vessels more quickly, the other study found.
The Super-Fast Workout Anyone Can Do
So just how hard do you need to go to reap these benefits? Study volunteers did 30-second all-out sprints with 4.5 minutes of rest on specially designed cycles, so it’s not easy to steal their exact workout.
However, Wagenmakers says short bursts of any cardiovascular exercise that gets your heart rate up to 80 percent of your maximum will work, so you don’t have to do an all-out sprint. “A good way to gauge the effort would be for people to be able to talk, but only in very brief phrases, by the end of each interval,” says coauthor Sam Sheperd, Ph.D.
Sheperd’s advice: Three times a week, hop on a bike, treadmill, or rowing machine. After an easy 5- to 10-minute warmup, start with 30 seconds of hard exercise followed by 2 to 4 minutes easy (as long as it takes you to be able to say a full sentence). Start with four sets and work up to six.
Even better: Load up your intervals with the weight of your body or dumbbells, as I have done with my athletes for years. Resistance-training intervals offer cardio benefits while boosting your afterburn, the calorie-torching metabolism boost that lasts long after your last set.