How Much HIIT Is Too Much HIIT?

By Jane Hanisch

Interval-based workouts are popping up everywhere. Popular chains like Orangetheory, personal trainers, and fitness influencers have all embraced the superrrr sweaty style of training. In fact, fitness pros voted HIIT (yep, that’s high-intensity interval training) one of the top fitness trends for 2020, according to a recent survey by the American College of Sports Medicine.

If you haven’t heard, HIIT workouts generally combine bursts of intense exercise (lasting from a few seconds to a full minute) with rest or less intense exercise for the same amount of time. An entire HIIT workout can be as short as 20 minutes, making it a great option for those who don’t have much time to exercise. Importantly, HIIT also doesn’t require any exercise equipment, making it ideal for those who want to break a sweat outside or at home. Jump squats, burpees, and plyo lunges all make regular appearances in a HIIT workout. If that sounds tiring, it is!

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Any other benefits of HIIT?

Uh-huh, plenty. HIIT workouts have been shown to improve heart health and VO2 max, along with blood pressure, blood sugar, and cholesterol levels. And all that hard work leads to a quick calorie burn in a shorter time frame. Studies are still being conducted to truly understand how much HIIT can help in reducing body fat, but HIIT has shown to burn body fat similar to long endurance exercise.

Great. What’s the problem?

While there are guidelines for vigorous exercise, there are no guidelines for HIIT. A recent study from Les Mills set out to find the limits of HIIT in order to prevent overtraining, and the findings might surprise you.

During the study, 35 people who exercise eight or more hours per week recorded their heart rate and mood for three weeks. At the end of the three-week training period, they performed two high-intensity workouts four hours apart. Researchers monitored their heart rates and took saliva samples prior to each workout, immediately after, and 30 minutes post-exercise to assess cortisol and testosterone levels.

The findings: Researchers saw a correlation between high-intensity training and the variables associated with overtraining (fatigue and reduced athletic performance) and concluded that people should limit themselves to 30 to 40 minutes of HIIT each week.

The NL bottom line on HIIT

First off, this is a very small study, so we need more research to confirm these findings. And you should also be honest with yourself about your effort; you might not be working out as hard as they did in the trial. (No shade—just the truth! People in the study worked at 90 percent of their max heart rate.) To estimate your max heart rate, subtract your age from 220. Then, use that number to establish your 90-percent max, monitor your fitness tracker, and dial it back accordingly. 

On a similar note, study participants did these workouts just four hours apart. Typically, people have at least a 24-hour window between sweat sessions, and adequate recovery is super important if you want to perform—and feel—your best.

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So, if you truly love high-intensity workouts like bootcamp and boxing, just make sure to listen to your body (injuries are still a factor!), take rest days regularly, and keep an eye on your heart rate to prevent overtraining.

Oh, and if you hate HIIT? Go ahead and consider this your excuse to say “nope, thanks” to all those burpees. Anyone up for yoga?

(Photo: Shutterstock)

About Jane Hanisch
Jane is a personal trainer, yoga instructor, nutritionist, and model living in Charlotte, NC. She holds her Bachelors in Exercise Science and is a certified Personal Trainer and Exercise Physiologist with the American College of Sports Medicine. She recently completed her 500 hr yoga teacher certification and Masters Level 2 with TNS.

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