You may have heard about all the health benefits of high-intensity interval training [1], or HIIT. It's more efficient since you're working to your max, so you burn more calories in less time. You can also burn more fat, and give your metabolism a boost with the afterburn effect [2].
But while HIIT seems like the best way to get in an effective workout, it is possible to have too much of a good thing. In fact Jinger Gottschall, PhD, associate professor of kinesiology at Penn State University, presented research at the American College of Sports Medicine's annual meeting in June that suggested people shouldn't do more than 30 to 40 minutes of HIIT exercise per week.
The researchers said any more than that can reduce performance and potentially increase risk of injury [4]. Plus, it doesn't actually help achieve the positive effects of HIIT; odds are, if you are able to do 30 minutes or more of HIIT per week, you aren't actually working to your max capacity.
"Currently there are no guidelines concerning the greatest amount of HIIT people should do in a week for the optimal training effect," Dr. Gottschall said in a press release. "Given the extreme intensity involved in this kind of exercise, it's imperative that maximum guidelines are provided in the same way that minimum guidelines have been in the past. We hope this study will be instrumental in helping make these recommendations official."
She also said it's important to let your body recover properly [5] after a HIIT session, because you're likely to perform better on your next HIIT workout and get the health benefits from it.
In the research, high-intensity interval training is defined by periods of extreme exhaustion where you push your heart rate to greater than 85 percent of your maximum capacity, followed by periods of rest. The study evaluated 35 active individuals (28 women and seven men) who exercise more than eight hours per week. They recorded their heart rate during training and answered questions about their mood for three weeks. Thenl on an experimental day (at the end of their typical three-week training period), the participants did two high-intensity workouts, four hours apart.
Researchers evaluated their cortisol [6] and testosterone levels, which tracked each person's stress response, and monitored their heart rate. The results showed a correlation between weekly time spent training at an intensity higher than 90 percent max heart rate and the symptoms of overreaching (or overtraining).
Incorporating HIIT into your workout routine may help you reach your fitness and weight-loss goals, but be careful not to overdo it. You can get an efficient HIIT workout in just a few minutes [7]. You can also do HIIT-style training for longer periods of time, just be careful not to push yourself to your actual max (85 to 90 percent) the entire workout.