4×4 interval training – 6 common sources of confusion

As you may have guessed from our video posted before the weekend, CERG is quite the proponent of 4×4 (“four by four”) aerobic interval training. We’ve compared it to other types of exercise, and found it to be very effective compared to continuous moderate exercise in multiple studies. The video provides a very good introduction, but we’d like to address six common sources of confusion:

1. It’s too exhausting
If you’re not using a heart rate monitor, the rule of thumb is that during the intense parts of interval training, you should be unable to maintain a conversation comfortably. But what does this actually mean? That you should exhaust yourself completely? No. A common pitfall is pushing too hard during the high-intensity intervals, so the four minutes exhaust you. The intensity should get your heart rate up, but you should feel capable of continuing another minute after your four are up, and doing another interval after you’ve completed 4×4. Trial and error may be good enough to figure it out, but using a heart rate monitor can help you from tiring yourself out. Ideally, you should feel roughly the same after every interval – something that may be especially important for the sake of motivation.

2. It’s not intense enough
Conversely, perhaps you’re confused after hardly being out of breath and lacking lactic acid buildup from completing your intervals at 90% of maximum heart rate, per the measurements from your heart rate monitor. Isn’t this interval stuff supposed to be high-intensity training? In fact, unless you’re incredibly fit, an issue like this probably stems from “90%” not actually being 90% of your maximum heart rate. While there are formulas for calculating your max, the variation between people can be so great that you’re better off actually testing your maximum heart rate rather than just calculating it. Which brings us to…

3. Heart rates
So there’s this mention of heart rates, but what’s that all about? 4×4 interval training can be done by almost all people, but since fitness varies from person to person, it is useful to define the method by heart rates relative to the individual. To find your maximum heart rate with a heart rate monitor, warm up thoroughly, and do two 4-minute intervals with active breaks. Then, on what would have been your third interval, run as fast as you can, until you’re unable to continue. When you’re at exhaustion, the highest pulse you’ve logged will be your maximum heart rate. This is the number from which you can then calculate your percentages (e.g. maximum heart rate*0.85 will give you 85% of max). 4×4 interval training focuses on exercising the heart since cardiac health and overall fitness are so closely related.

4. Is it safe for me?
I have an atrial flutter/ I’m recovering from a stroke/ I have some other type of cardiovascular disease – is this training appropriate for me? While in most cases the answer to this question likely is yes – very broadly speaking exercise is good for you – we recommend that if there’s doubt, you should ask your doctor for a referral so you can do an exercise test at a hospital. That way, professionals can give an unequivocal answer that pertains specifically to you.

A related question is whether pregnant women can do interval training without detriment to the fetus. While not much research has been done on high-intensity exercise specifically, moderate-intensity exercise is recommended for pregnant women in international guidelines. In our experience, exercising at up to 90% of maximum heart rate during pregnancy is fine, so you should be able to do interval training.

5. How often?
How often should I be doing 4×4 interval training? Once a week? Every day? The answer to this question depends on the individual, and your exercise goals. Generally, it looks like 1 set of intervals per week is enough to maintain your current shape, and 2-3 sets are recommended if you are untrained and want your physical fitness to improve. For fit people the situation is more complex, but if you exercise more than four times a week, it’s appropriate to spend one third to half of the time on interval training. We performed a study where people did 5-8 intervals per week, but found that it was hard to maintain motivation at that frequency, and near 8 times a week, test subjects were overspent and no longer got the desired physical adaptions.

6. Will 4×4 help me lose weight?
While there’s nothing magical about it, 4×4 interval training can be used to lose weight. We performed a study on obese teenagers where they did 4×4 training twice a week for 12 weeks under professional supervision, followed by regular check-ups 1-2 times a month for a year. Aside from a 30-minute talk on nutrition before the program began, we did not emphasize dietary change, yet many successfully lost weight.

A common misunderstanding is that high-intensity training is bad for burning fat. This is incorrect. The misunderstanding arises from the fact that the metabolism of fat provides ~50% of all the energy during low-intensity muscular work, vs. 20-30% during high-intensity work. Therefore, low-intensity work is better to lose fat, right? Wrong. What this thinking misses, is that the total energy spent matters far more than the percentage per se. During high-intensity exercise, the energy expenditure per unit of time is greater, and more fat is metabolized in total, even if a higher percentage of the energy comes from carbohydrates.

Hopefully that clears up some frequent questions about interval training. So get out there and get fit! Conversely, if this hasn’t addressed your question about 4×4 interval training, please ask in the comments, or submit a question on our website.

Compiled by Hanna Sofie Ellingsen at CERG.

This entry was posted in 4x4, Exercise, In English, Obesity, Pregnancy, Public health, Research by CERG. Bookmark the permalink.

About CERG

The Cardiac Exercise Research Group (CERG) at the Norwegian University of Science and Technology (NTNU) seeks to identify the key mechanisms underlying the beneficial effects of physical on cardiac health in the context of disease prevention and treatment. Named the K.G. Jebsen Center for Exercise in Medicine under Professor Ulrik Wisløff's leadership in 2011, CERG uses both top-down and bottom-up approaches to combat lifestyle-related disease.

7 thoughts on “4×4 interval training – 6 common sources of confusion

  1. I am following the 4×4 protocol ( usually on a treadmill). I also like long steady state cardio sessions ( jump rope and elliptical). Are the long cardio sessions counter productive for weight loss? Is it advisable to follow intervals with long cardio sessions ?

    Thanks for posting it is nice to get solid fitness advice that is not part of a sales pitch.

    • Glad you enjoyed it!

      Essentially, the balance between 4×4 training and other training will depend on how much you exercise overall. If you exercise many hours of week or daily, 4×4 every day is probably too much, and will leave you feeling burned out. That’s not good! A comfortable balance with long cardio sessions and strength training is probably a good idea. But if you’re only exercising a little, 4×4 is one of the more effective and time-efficient ways of seeing significant improvement over not too long of a time span. So there’s no answer that fits everyone…

      With respect to weight loss, it’s also complicated, and naturally will depend on the balance between your energy intake vs. how active you are. If you’re consuming more calories than you burn, you’ll find difficulty in losing weight. In this sense, high-intensity exercise has an advantage compared to moderate-intensity exercise, because you burn more calories per unit of time. But long cardio sessions are much better than just sitting on the couch!

  2. During intervals there is a lag between the increased effort and the increase of heart rate. Should I start counting the for minutes only when the heart rate reaches the target?

  3. Hi,
    you should start counting the four minutes when you increase the effort. It will take a little time for you to reach 85-95 % of your maximum heart rate, and that’s the reason why we want you to do the intervals for as long as four minutes.

    Good luck with your exercise!

  4. Hi

    This might be a silly question but why is the duration of exercise 4 mins? Why not 5 mins or 3 mins? Why is 4 mins the magic number?
    Also, why not aim for a 100% of maximum heart rate I find that if I do 4 x 400m sprint in say 1.20s min with 2 mins recovery between it is a very tough work out or alternatively do swim 4 x 100m medley with 3 mins rest I hit maximum heart rate and am totally spent by the end because it is about giving your maximum each time.


    • Scientists tried with lots of different combinations. 4×4 was found the mest effective for increasting O2 intake. Don’t have the source here, unfortunately.

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