It seems like the ideal workout: You go on a run and gradually increase your speed until you hit your sweet spot—a challenging pace that makes you pant but not gasp for air. Once there, you work hard to maintain your stride until, 30 minutes later, you lope back into your driveway, exhausted but invigorated. The next day, you do it again.
Many exercisers—especially those who try to maximize their limited time to work out—fall into this no-pain, no-gain routine, says Stephen Seiler, PhD, a sports-science professor at the University of Agder in Kristiansand, Norway. But pushing yourself all the time can backfire. “It puts too much stress on the body,” he says. “The next time you go out, you’re not fully rested, which means that you tire sooner and become more prone to injury and stagnation.”
If moving at a steady but challenging clip wears out the body, what works better? Surprisingly, Seiler has found that to get fitter faster, you have to slow down. First, think about the effort you expend during aerobic workouts in terms of three zones—zone one (easy), zone two (medium) and zone three (hard). Most cardiovascular exercise, Seiler says, should be done at a low intensity in zone one for longer periods of time.
Positive adaptations happen with any cardiovascular exercise: Your lung capacity increases, your heart pumps more oxygen-rich blood to muscles, and your muscles develop additional networks of capillaries and mitochondria (the power centers of cells), which increases your body’s ability to burn fat. The news is that the most efficient way to develop an Energizer Bunny–like capacity to go faster and longer is to exercise most often in zone one. Why? Because you can work out longer without as much risk of injury or fatigue, says Carl Foster, PhD, an exercise physiologist at the University of Wisconsin–La Crosse. A 2007 study that Seiler coauthored points out the effectiveness of this strategy: A group of runners who did 80 percent of their training in the slow zone for five months improved their 6.5-mile race times by an average of 36 seconds more than runners who did only 65 percent at an easy pace.
MIXING UP THE ZONES
Still, you won’t get faster by always taking it easy. A small portion of your exercise time should be reserved for zone three—an almost all-out effort that really taxes your heart. Zone three develops strength and speed, but it also takes the longest to recover from, which is why the adaptations you develop while exercising in zone one are critical. Typically, zone-three training is incorporated into workouts by alternating intervals of high-intensity exercise with periods of easier, zone-one efforts. Think of it this way, says Seiler: “Longer, low-intensity exercise develops your motor. Intervals are for tweaking. It’s like putting turbo on an already good engine.”
However, the majority of exercisers do most of their cardio in zone two (working hard but not at their max) for shorter periods of time. Seiler calls this zone “the black hole” because it’s easy to get sucked into. It begins when your body shifts the kind of fuel it’s using to power your muscles and creates lactic acid, a by-product that can lead to muscle burn and fatigue. “In zone two, you’re getting tired, but you’re not going to be able to hold that effort long enough to get as many muscular adaptations as you would in zone one,” says Foster. “You’re also not exerting yourself enough to strengthen your heart and lungs as much as you would in zone three.” It’s an inefficient way to get fit.
THE MAGIC FORMULA
In a review of dozens of studies, Seiler found that working out in zone one 80 percent of the time and zone three (via high-intensity intervals) 20 percent of the time yields peak performance. Elite German rowers who adopted this easy-hard approach as high school juniors achieved international success in their twenties (world championships and Olympics finals), according to a study in the International Journal of Sports Physiology and Performance. Their peers, who trained less in zone one and more in zone two, were successful only at a national level.