5 Times HIIT Could Be Dangerous for You

High-intensity interval training (HIIT) is one of the most popular types of workouts. And for good reason: HIIT is a great way to improve your fitness level and burn loads of calories in a relatively short amount of time, making it a popular choice for those looking to lose weight and get in shape.

And consider these results: Women with overweight and obesity who did three weekly HIIT sessions lost more than seven pounds by the end of 15 weeks, according to a 2008 study in the International Journal of Obesity.

But while HIIT is a great, efficient workout option, it may not be right for every person — or every workout. Here are five times to play it safe.

For better and sometimes worse, exercise stresses your heart. "Exercise increases the need for the heart and the cardiorespiratory system to deliver oxygen to your working muscles," says HIIT researcher Yuri Feito, Ph.D., associate professor of exercise science at Kennesaw State University. With high-intensity exercise, that need for oxygen only increases.

Generally, this stress only causes your heart to become stronger. However, in rare cases, exercise can cause the heart to work harder than it should, ultimately leading to a cardiac event (ex. heart attack).

The risk increases if you smoke or have high blood pressure, heart disease, high cholesterol, high blood sugar or a family history of heart disease, says Shelley Keating, Ph.D., a researcher at the School of Human Movement and Nutrition Sciences at The University of Queensland, Australia.

However, it's worth noting that a 2018 study published in the Journal of the American Heart Association found that HIIT has a low rate of cardiac events, even in patients with coronary disease or heart failure. But just to be safe, you should consult your doctor before trying HIIT if you fall under any of the categories mentioned.

Newer exercisers beware: If you jump straight into the deep end of HIIT right off the bat, you run the risk of injury and pulled muscles, as well as burnout from doing too much too soon, says exercise physiologist Tom Holland, author of The Marathon Method.

Why? Not only are your muscles and other tissues not strong enough to handle that intensity just yet, but chances are you don't know your body well enough to recognize when it's time to dial back or stop altogether. That kind of insight only comes as you gain more experience with exercise.

So spend at least two months building up a base of strength and aerobic fitness before trying HIIT, Holland says. Or incorporate interval training but at a lower intensity (think jogging instead of sprinting) before you give the all-out effort of HIIT a shot.

‌Read more:‌ HIIT for Beginners: 7 Tips to Jump-Start Your Workouts

Due to the intensity and high-impact nature of some of the exercises, HIIT places a lot of stress on your muscles, joints, tendons and ligaments and may increase your risk of injury or re-injury, Keating says. If you're dealing with any kind of injury that causes you pain, you may want to stick with gentler forms of exercise while you recover.

That said, "not exercising is probably going to be worse for me than starting an exercise program, regardless of the intensity," Feito says. So, if HIIT doesn't cause you pain (and you cleared it with your doctor or physical therapist first), feel free to modify the exercise modality to accommodate any limitations you might have.

As Keating notes, she once worked with an individual who had osteoarthritis in his knee, and he was still able to perform HIIT workouts (using a punching bag while seated).

‌Read more:‌ 6 Low-Impact HIIT Workouts That Won't Hurt Your Back, Knees or Ankles

HIIT is meant to be intense — hence the "high-intensity" part. As such, it's not something you'll want to do for days in a row. In fact, most people shouldn't be able to do HIIT often. "If you can do it every day, you're not doing it right," Holland says.

In general, it's a good idea to follow a hard workout with one or two easier ones, Holland says. So, if you did a HIIT workout on Tuesday, go for a recovery jog on Wednesday, and maybe do a cross-training session on Thursday.

By taking a couple of days off from HIIT, you'll give your joints, muscles and tendons the chance to recover from your last session — so they can come back stronger for the next one. By the end of the week, you should have no more than three HIIT workouts under your belt.

If you're still not sure whether HIIT is right for you, check out this pre-screening test from Exercise is Medicine Australia. You may also want to meet with an exercise physiologist for more guidance. Assuming you're good to go, this person can develop a HIIT protocol that will suit your goals, preferences and fitness level, Keating says.

OK, so it's not technically dangerous to do HIIT if you hate it, but it can be detrimental to your commitment to working out if you force yourself to always do a workout you dread.

And while it's true that most people don't enjoy HIIT workouts (except for maybe masochists), some people truly hate them. "I have worked with a few individuals — who, in my experience, are definitely a minority — who strongly dislike HIIT workouts," Keating says.

For these people, HIIT would not be a great option, as it's unlikely that they would keep doing it often enough to see benefits, she says. If this sounds like you, you may want to look to other forms of exercise.

That said, it could be that you just haven't found the right flavor of HIIT. For example, if you hate using the treadmill, you may prefer doing intervals on a bike. Or, maybe you'd rather do body-weight exercises like jump squats or push-ups, instead of traditional cardio. "Just keep trying different forms of it," says Holland.

And if, after all your experimentation, you find that you really don't like HIIT, then don't do it — just like with any other workout.

‌Read more:‌ How to Find a Workout You'll Actually Stick With