How slow should you go?

Stop wasting time. The magic of interval training is in perfectly pairing your intensities.

Life (and marriage) may be a marathon, but evidence suggests it's best to cardio train in fits and starts. The benefits of interval training have been studied in labs, made national headlines and continue to be evangelized by every hard-body glossy on the shelves. And for good reason: It works.

According to a study in the Journal of Physiology, 20 minutes of interval training (30 second bursts followed by 4 minutes of recovery) produced the same physiological adaptations in the body and improvements in performance as 90 minutes of continuous cardio at a moderate pace. Another study published in the American Journal of Clinical Nutrition found that those who practiced high intensity interval training for 15 weeks had a reduction in subcutaneaous fat that was nine times greater than those who performed steady-state cardio workouts for 20 weeks. And the list goes on and on.

So what if you've tried bouts of high-intensity intervals but aren't seeing results? While eating up the interval training philosophy, you could be falling short on the execution by failing to properly prepare the meal. The key ingredient, says Maria Pagano, RD, Tier 4 personal training manager, is hitting the right heart rate intensity.

"The two most common mistakes I see people making with intervals are not going hard enough on the effort and not going easy enough during recovery, which is really the crux of the whole thing," Pagano says, "It's the juxtaposition of near maximum and minimal effort that produces the desired physiological response."

After a perfectly executed interval routine, your body experiences EPOC, or excess post-exercise oxygen consumption. "EPOC is essentially your body trying to catch up with the oxygen demand put on it during the session," says Pagano. "It's trying to return to homeostasis, and the harder it has to work to return, the more calories your body burns to get you there."

As a general rule, you should aim to work between 85 and 90 percent of your maximum heart rate on intense bursts — and you'll need a heart rate monitor to eliminate the guesswork. (To calculate your max heart rate use 220 minus your age if you're a man and 206 minus 88 percent of your age if you're a woman). Then recover for as long as it takes you to get back down to 60-70 percent of your max. "The recovery is just as important as the effort because that's where the cardiovascular adaptations take place," Pagano says, "and you need to fully recover in order to allow your body to expend a close to maximal effort on the next burst."

Somewhat counterintuitively, your interval training could also be suffering from a lack of old-fashioned steady-state cardio. "It's important to have a solid aerobic foundation so you know your joints can tolerate the intensity of interval training and your body can call on the appropriate fuel to burn during exercise like fatty acids," says Pagano. "Plus, having a solid base will ultimately help you go at higher intensities for longer while buffering more lactic acid, which means you'll get much more from your interval training." Pagano's prescription: do two or three 30-45 minute steady-state cardio sessions per week and two to three interval sessions.

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