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Is It Possible to Do Too Much HIIT? A New Study Says Yes

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When exercise scientists first began establishing the benefits of high-intensity interval training—aka HIIT—it felt like we'd unearthed the holy grail of workouts. Higher fat-burning efficiency and muscle-building power in a fraction of the time? Yes, please. (Check out some of the health benefits of HIIT here.)

But according to a new study, it may be possible to have too much of a good thing.

While the benefits of a single HIIT workout are well studied, there hasn't been a lot of research done on whether the benefits of your butt-kicking workout dwindle if you do it too often, according to the most recent Physical Activity Guidelines released by the government's Office of Disease Prevention and Health Promotion.

"There are countless types and formats of high-intensity training available without tested recommendations on how much is too much," says Jinger Gottschall, Ph.D., an associate professor of kinesiology at Penn State University. After collecting data on thousands of HIIT devotees through her research, she started to notice a trend: "Individuals with a high volume of HIIT training were unable to reach their maximum heart rate regularly and complained of symptoms related to overtraining," she says.

To study the risks of overtraining with HIIT (specifically, workouts in which you perform short bursts of activity that push your heart rate above 85 percent of your max), Gottschall teamed up with Les Mills, a creator of research-based group fitness classes, including HIIT classes, taught around the world. "We wanted to ask: 'What is the optimal time per week to train in the 90 to 100 percent max heart-rate zone to maximize physiological and psychological benefits while minimizing overreaching or overtraining?'" she explains. Basically, how much HIIT is too much?

In the study, the researchers had 35 fit adults (28 of whom were women) record their heart rate during every workout and track their mood over the course of three weeks. After they established a baseline according to their normal workout routines, the researchers had the participants do double duty and complete two 30-minute HIIT classes four hours apart. Gottschall wanted to test how the HIIT workouts affected the participants' stress response. They collected saliva samples 30 minutes before each sweat session, immediately after, and 30 minutes post cool-down to measure cortisol and testosterone levels.

"I was surprised by the obvious difference between doing 30 to 40 minutes [of HIIT] and doing more than 45 minutes," Gottschall says. "The difference in performance, stress-related feelings, and sleep quality was significant." More than 40 minutes of truly high-intensity exercise each week can up your risk for injury and lead to overtraining (which is one of the major fitness mistakes people make). Overtraining can manifest itself in a variety of ways: "A decrease in performance, injuries, pain that won't go away, sleep disturbances, missed menstrual periods (which is associated with bone loss), depression. and anxiety," says Alissa Rumsey, C.S.C.S., a fitness and nutrition expert in New York. (Here are seven warning signs of overtraining.)

So How Often Should You Do HIIT Workouts?

Only 30 minutes of HIIT each week seems crazy short—especially when every other workout class suddenly has HIIT in the title (HIIT Yoga, anyone?). But that's more than enough to see serious benefits, says Rumsey (who wasn't involved with the study). "Studies show that 15 minutes of HIIT training can show similar performance benefits to longer, lower-intensity workouts," she says. "This means that you can get similar exercise benefits in a much shorter amount of time." (Remember Tabata, the killer four-minute workout?)

Before you start cutting classes, figure out how many of your workouts really qualify as HIIT: "A true HIIT workout contains intervals of intensity during which it would be nearly impossible to speak or to maintain the output for longer than two minutes," Gottschall explains.

This translates to capping your HIIT sessions at two 30-minute classes per week—keeping in mind that, in a 30-minute class, typically only 10 to 15 minutes of the workout are spent in the max heart-rate zone, she says. When you're not HIITing it, balance your workouts with lower-intensity cardio (a jog where you can comfortably talk) and recovery days to make sure your bod is hitting its highest potential. (This guide to a perfectly balanced week of workouts will help.)

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