High-Intensity Interval Training: How Intense Does It Have to Be?

image of Cathe, Jenn,Brenda,Jai and Nicole in Fit Split Low Imapct & Metabolic Conditioning doing a high-intensity interval training workout


High-Intensity interval training is all the rage these days. In fact, this training approach tops the list of fitness trends for 2018, as it has for the past few years. It’s a trendy way of working out that shows no sign of slowing. There’s a reason HIIT training is so popular. It’s a way to get a workout without spending hours per week doing monotonous exercise that doesn’t keep you motivated. In other words, HIIT is short, sweet, and effective. In fact, HIIT training is the most time-expedient way to improve your general fitness.

The MOST important reason to do high-intensity interval training is that it works. As Todd Astorino, a professor of kinesiology at California State University points out, high-intensity exercise has similar health benefits to longer periods of moderate-intensity aerobic exercise – and this is supported by ten years of research. So, HIIT training is backed by science, unlike some exercise trends that capture the fancy of the public for a few months and fade away. In fact, some research suggests that more intense exercise is superior to moderate-intensity exercise for cardiovascular health. High-intensity training also improves insulin sensitivity, making it an ally in the battle against metabolic syndrome and type 2 diabetes. All good things, right?

How Intense Must High-Intensity Interval Training Be?

But what defines the term high intensity? We hear the words HIIT tossed around a lot. But, have you ever wondered how intense a HIIT workout must be to qualify as high intensity? We know that high-intensity interval training workouts leave us feeling tired and even a bit breathless but how hard do you really have to push to get the full benefits? Some sources define “vigorous” exercise as an exercise that increases heart rate to 70% of maximum heart rate or greater. You can roughly calculate your maximum heart rate by subtracting your age from 220. Keep in mind, there are some problems with this formula but for general purposes, it’s close enough.

For example, if you’re 50 years old, your maximum heart rate would be 170 beats per minute. Take 70% off that and you get 119. Although this is the definition of “vigorous” exercise, true interval training kicks the intensity up higher – to 90 to 95% of maximum heart rate. This means if you’re doing it properly, in the above example, your heart rate would hit 153 or higher during the active intervals. You can check your own maximum heart rate using this formula and see how close you’re coming to 90 % of your maximum heart rate during the active intervals.

One mistake people make with high-intensity interval training is they don’t push hard enough. The key to getting the most benefits out of HIIT training is to work very hard during the active intervals and partially recover during the rest intervals. With true HIIT training, you push hard enough to build up lactic acid and fatigue your muscles during the active intervals, unlike moderate-intensity aerobic workouts where lactic acid doesn’t accumulate as the intensity isn’t high enough. During the rest intervals is when you clear some of that lactic acid and your muscles partially recover so you can do another round. If you don’t push hard enough, you don’t get the anaerobic benefits that high-intensity interval training offers. One advantage of HIIT training over moderate-intensity aerobic is it enhances your aerobic AND anaerobic energy systems.

Plus, due to the intensity of the workout, you don’t have to exercise as long to get the full benefits. Studies show that you can get benefits from as short as a 10-minute, HIIT session. But, if you’re doing high-intensity interval training, you need to work harder than you do during a moderate-intensity workout. You’re giving up time for intensity.

Monitoring the Intensity

So, how do you make sure you ARE working hard enough? If you wear a heart rate monitor, that will give you an indication of how hard you’re working. Is your heart rate getting up around the 90 to 95% of maximum heart rate we discussed? If you don’t have a heart rate monitor, the talk test will give you a rough idea. If you can say more than a few words during an active HIIT interval, you’re not pushing hard enough. HIIT training sucks enough wind out of you that you can’t speak in complete sentences. You can also use a perceived exertion chart to roughly monitor how hard you’re working. For high-intensity interval training, a minimum rate of perceived exertion would be at least 7 on the perceived exertion scale, which starts at 1 and goes to 10. However, if you’re just starting out, don’t try to push it beyond 7. There’s plenty of time for that once your fitness level improves.

A Versatile Way to Work out

The beauty of high-intensity interval training is you can customize it to meet your needs and goals. The exercises you do during the active intervals run the gamut from plyometric moves to sprints. The only requirement is that the exercise is intense enough that it qualifies as a 7 or greater on the perceived exertion scale. The best exercises are those that use the large muscle groups in your lower body. For example, during the active intervals, you could sprint, swing a kettlebell intensely, perform a challenging bodyweight exercise, do burpees, or even pedal an exercise bike at a high intensity.  The type of exercise isn’t as important as the intensity.

Don’t Overdo It

As much as you may enjoy high-intensity interval training and the benefits it offers, don’t overdo it. Doing so is stressful to your nervous system and, in extreme cases can lead to a rise in the stress hormone cortisol. Two times per week is enough to get the benefits without overstressing your body. Don’t forget about the importance of adequate recovery between sessions. Overdoing it won’t get you, fitter, just more fatigued and, possibly, injured. Excessive high-intensity exercise can also suppress your immune system and increase the risk of catching viral infections. Remember, you don’t need to do it every day for it to be effective. Enjoy the benefits without overdoing it.



Time.com. “Don’t Have Time to Exercise? Do This for 10 Minutes”
HIIT vs Continuous Endurance Training: Battle of the Aerobic Titans. Micah Zuhl, Ph.D. and Len Kravitz, Ph.D.
Science Daily. “HIIT helps combat high insulin resistance — a warning sign for diabetes”
Obes Rev. 2015 Nov;16(11):942-61. doi: 10.1111/obr.12317.


Related Articles by Cathe:

Can Music Make High-Intensity Interval Training More Effective?

How High-Intensity Interval Training Slows Aging at the Cellular Level

Top Fitness Trends for 2018: What’s Trending?


Related Cathe Friedrich Workout DVDs:

HiiT, Metabolic, and Interval Workout DVDs


Hi, I'm Cathe

I want to help you get in the best shape of your life and stay healthy with my workout videos, DVDs and Free Weekly Newsletter. Here are several ways you can watch and work out to my exercise videos and purchase my fitness products:

Get Your Free Weekly Cathe Friedrich Newsletter

Get free weekly tips on Fitness, Health, Weight Loss and Nutrition delivered directly to your email inbox. Plus get Special Cathe Product Offers and learn about What’s New at Cathe Dot Com.

Enter your email address below to start receiving my free weekly updates. Don’t worry…I guarantee 100% privacy. Your information will not be shared and you can easily unsubscribe whenever you like. Our Privacy Policy