When You Do Too Many HIIT Workouts, This Is What Happens

HIIT, or high-intensity interval training, has grown into one of the most popular workouts in the nation. It's a system of working out that alternates short periods of hard work with longer periods of moderate work or rest. It can be done using weights or aerobic activity, and the length of the high-intensity work portion can be adjusted for fitness level, goals, and type of exercise.

But researchers are discovering that, just like anything else, HIIT is best done in moderation. That's because too much of it can actually begin to have detrimental effects on your performance, metabolism, and overall health (according to Insider).

In a small study published in the journal Cell Metabolism, researchers in Sweden had participants do HIIT sessions on a stationary bike with maximum effort for four to eight minutes at a time, with three minutes of rest. They completed five rounds of intervals. Over a two-week period, their performance improved, and there were positive changes, on a cellular level, to their mitochondria.

Things changed, however, when researchers had the participants increase both the frequency of their workouts and the length of their high-intensity intervals. At this point they were exercising for about 152 minutes per week, as opposed to 90 minutes per week previously. Testing now revealed that they were experiencing oxidative stress, a cellular level of damage that can be caused by intense exercise, and showing damage to mitochondria. Their metabolic rates — the rate at which they burn calories — was also decreasing.

While workouts like HIIT are generally good for the body, they also bring on a surge of stress. According to Men's Health, the body can only deal with so much stress, for so long before we begin to see breakdown.

Thankfully, the fix is simple: just scale back the frequency of HIIT workouts. Enjoy their positive effects, without pushing the body to the brink, and you'll see the expected benefits.