Ultimate guide to cardio.

HIIT, LISS or none of the above?

Since I got one cardio related article in my mail and another jumped at me as a paid ad on the internet right after I decided to write this article. Both of those articles I got looked like they were based on scientific research, but apparently this article (CZ) was not enough and I still need to go over some information.

First, let's go over the basics of thermodynamics. It's something you cannot simply gloss over even though many trainers do jsut that. "Thermodynamics is the branch of physics that deals with the relationships between heat and other forms of energy. In particular, it describes how thermal energy is converted to and from other forms of energy and how it affects matter. “ (source


The 4 thermodynamis law (in short):

Energy cannot be destroyed, it must be transformed into some form (of which there are many). For us the most important are:

  • Chemical energy in food and human tissue
  • Kinetic energy needed to move things. For this we burn ↑ chemical energy.
  • Thermal energy is a type of energy we feel as heat. Made by ↑ chemical energy.

Which means  that even though so many trainers are trying to ignore this, for burning one need to be in energy deficit, for gaining in energy plus. But weight in itself can be influenced in other ways too. If you are gaining muscles very fast (which we can usually see in people who take steroids) it's possible to burn fat whilst on big caloric intake. Just as well there are studies that show it is possible to build muscle in calorie deficit. But change of weight does not only mean gain or loss of fats.

Based on this law we can arrive to a logical conclusion, that cardio is not important for weight loss. It doesn't matter if you're burning sugar or fat. For weigh loss calorie deficit is the msot important. That's why it doesn't matter if you're doing cardio exercises on an empty stomach or you do HIIT or low intensity steady-state cardio (LISS). So burning 400kcal during cardio or modifying your diet has the exact same effect from the point of view of fat reduction. Also, all exercises including strength training are very similar concerning energy output (study).


But let's take a look at it in detail.
Studies that talk about energy deficit: here and here

If you look at the amount of fat burnt during cardio, the number is far lower that most people think. It's usually less than 100g of fat (study)

Let's start with oh so popular HIIT training (which includes almost all interval trainings, tabat etc.)

One of the main reasons why HIIT is so perfect is EPOC. The immense burning even after the training finnished is overhyped. In fact, EPOC takes up only 6-15% of the overall oxygen consupmtion during the training (study) In other words, EPOC will be around 50kcal.

Then we often see sprinters and American footbool players in comparison to HIIT. Make no mistake, the sprinters' training and their interval running has nothing to do with HIIT. Standard resting time between sprints is 5+ minutes for a 5-15s dash. Not a 20s exercise + 10s rest as in tabata. There are even studies that compare the two (here) and they found out that with HIIT you train more like a football player than a sprinter. 

Difference in body composition – weight loss/muscle gain – is almost zero with both HIIT and LISS (study for fat and muscle)

Even worse, HIIT has (unlike LISS) the worst impact on strength training. (studies here and here) Let's take the second study as an example:

There were 3 test groups:

  • Strength training (ST) – 3x fullbody
  • ST + LISS on a bike
  • ST + HITT on a bike

Those who did only strength training gained 4,1% pure muscle mass, ST + LISS gained 3,6% and with HIIT only 1,8%.

This next study here shows 17% gain over 0% with those who had added HIIT. They were people with no training which iseven a bigger problem, because those who are only starting to exercise usually react to any stimulus, so even HIIT should be enough, but is isn't.

We can even go as far as to claim that LISS is far better than HIIT because of one single argument: you can do it longer, thus burn more (back to thermodynamics) (study here)

The only aspect in which HIIT undoubtedly wins, is saving time. It's not supposed to sound sarcastic, because even this aspect can be crucial for some people. If you can only find 10minutes 3 times a week, some form of sprints or intervals would still be the better solution than doing nothing.


Burning fat and cardio on an empty stomach

As mentioned above, it's all about energy deficit. So it doesn't matter if we're burning sugar or fat. The biggest problem about exercising on an empty stomach is the risk of burning muscle. Which is the last thing you want when trying to lose weight (study)

Strength training:

is energetically speaking very demanding, only people do not see it as such. Calorie burn works the same. The heavier you lift, the harder it is for your body metabolically. One of the reasons why is, that the body uses anaerobic system a lot. Which is why people who are starting with light weight have no problem to lift many times as opposed to people with 100-200kg. The higher number of reps, the more you use your aerobic system as well. Of course resting time betwee series is also on aerobic system. The main benefit of strength training for burning of fat is muscle gain. We have to strive for elevating out REE, moving a body with bigger muscle mass is energetically more demanding than moving with a body of the same weight but no muscle.

With cardio there is one problem – it disrupts building of muscles and strength. With HIIT we have seen it's the worst ever. It's mainly about AMPK vs mTOR, but it's probably being made into a much bigger monster than it actually is (here).

Moreover, the aerobic system is best at preventing all sorts of cardiovascular diseases and stress, which is also a huge problem nowadays. HIIT however works as total opposite. Also, training in itself is stress for the body, more so if you go to one of the supr circuit trainings where you lay on the ground by the end in a pool of your own vomit... (here is a study showing this kind of exercise is nonsense) Our heart also reacts differently to LISS and HIIT since it's a muscle. If your only concern isn't loosing weight or building muscle but you are also active in some other sports, having a good aerobic system is good for you in order to avoid injury because your game won't deteriorate as it progresses and you start to get more and more tired. This article however is focused on cardio and not the fuctioning of energetic systems. In other words LISS is not the only way of improving your aerobic system. Especially athletes use many different ways (study here)

For most people a well built strength training is sufficient to build an adequate aerobic system as well without any additional cardio trainings.

Let's see once again that there is almost no difference between energy needed for cardio and strength training (study here)

the comparison was between a 30 minute strength training (3×10 fullbody training), HIIT/circuit training and steady cardio on 70% intesity and there was not much of a difference. 

The biggest problem with HIIT is, that people think they are burning way more than during e.g., strength training. Which, as we have shown, is simply not true. 


So the main things you want to focus on if you want to loose weight:

  1. Have a well built strength training as many times per week as possible long-term (no, that does not mean a few weeks)
  2. Modify your diet to be in a slight deficit (don't try to reduce it as much as possible)
  3. Have enough protein and fat in your diet, women more so than men
  4. If you don't want to ro cannot add more trainings or reduce your diet more, add cardio
  5. As for me, combination of everything is the best. Since you need maximum energy output, you need to do things your body is not yet efficient in. The body is constantly trying to adapt. That's why you can run longer, lift more and more etc. Choosing one type of exercise only is thus nonsense. 
  6. Right after the combination of everything I'd pick LISS. It's benefit is work with stress and not disrupting strength training. It's sustainable for a long time for most people because it doesn't hurt, but it takes a long time. 
  7. HIIT should be used as a diversification of your training. If you really don't have time, you can do some interval exercises, the best being sprints, but be careful about sprints after heavy strength training which could disrupt regeneration. As for me, I'd do 100% sprint up to 15s and 3-5 minute rest or until your heart rate returns under 130 bpm.

Please don't realy on your fitness bands as the numbers they show, especially of calories burnt, are usually off.


I hope this article ends nonsensical wars about which cardio is better.


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Strength and fitness trainer
Owner and founder of Barbar gym
Holder of Czech and international nutrition and fitness certificates