The Science of HIIT
Tons of social media accounts post ‘fat-blasting HIIT workouts’ you can do in the comfort of your own home, touting quick-and-easy workouts you can get done in no time at all. But how scientifically accurate are these fit-fluencer workouts?
To start, let’s break down some definitions that relate to “HIIT.”
HIIT: High intensity interval training is short periods of work at a very high effort, or around >80% of your maximum heart rate, followed by rest periods. Think sprinting up a very steep hill for 30 seconds, and then taking one minute to walk down and jog in place.
SIT: Sprint interval training is a more specific type of HIIT training, where the work is at an even higher effort, and the rest periods are usually much, much longer. Think and all-out 30 second sprint on a bike where you push yourself so hard you absolutely could not go for that 31st second, following by four minutes of rest to allow for complete recovery by time the next 30 second work period comes back around.
MISS, or MICT: Moderate Intensity Steady State, or Moderate Intensity Continuous Training is what most of us might be doing when we “go out for a jog.” This can be any cardio exercise that you can comfortably sustain for anywhere from 30-60 minutes. This might even include group fitness classes like Zumba or HIGH Fitness!
Most scientific studies show pretty similar results between HIIT/SIT and MICT training, with HIIT/SIT showing better effects for various outcomes such as total body fat mass reduction. HIIT/SIT protocols usually see a slighter higher fat-loss outcome likely due to higher post-exercise fat oxidation compared to MICT, meaning the ‘afterburn’ where you are burning more calories even after you are done with your workout. SIT training can produce greater skeletal oxidative capacity, which means that your muscles can more effectively use oxygen to work better and harder. However, it is very difficult to measure the effectiveness of different types of HIIT because there are so many protocols and time ratios that it is difficult to create consistency.
Since both HIIT/SIT and MICT both produce fairly similar results in terms of fat-loss and overall health, then how do I know which one to do? In terms of just hitting the Physical Activity Guidelines of 150 minutes per week of moderate cardiovascular activity or 75 minutes of vigorous intensity activity, it really just depends on your personal preference. In order to get to true HIIT training, you have to push yourself pretty hard and it is usually not fun while you are doing it. So, if doing all-out sprints on a treadmill does not sound fun to you, then perhaps try jogging outside with a friend for an hour! It might take more time, but if it is something you enjoy, you will likely stick with it more. Since the outcomes between different types of cardio training are slight and nuanced, exercise adherence and time is probably a better indicator of what would work best for you. If you have time to do an hour of Zumba 1-3x per week and you really enjoy it, then do that! If you are very busy and can push yourself to the max, then complete some running sprints in a 15-20 minute HIIT workout at the park.
In scientific literature, HIIT and SIT are almost always represented by traditional forms of cardio like running or cycling. Unfortunately that means that a lot of social media workouts that include light weights or resistance bands in a circuit-style format are usually not HIIT, since it does not allow you to get to a high enough work effort to get your heart rate high enough. Now, that definitely doesn’t mean weight circuits don’t have a place in your exercise routine! Incorporating full-body strength training 2-3x per week is very important for fat loss and overall health. The Physical Activity Guidelines for Americans recommends strength training for all your major muscle groups at least 2 times per week. However, it shouldn’t be misnamed as “HIIT.”
Expression of concern: Is interval training the magic bullet for fat loss? A systematic review and meta-analysis comparing moderate-intensity continuous training with high-intensity training (HIIT)
British Journal of Sports Medicine Published Online First: 27 June 2019. doi: 10.1136/bjsports-2018-099928eoc1