I’ve been talking lately about how the way we eat activates different “metabolic machines” in our bodiesEither to burn sugar (glucose) or to burn fat (and ketones). But what is really healthy is metabolic flexibility, that is, to be able to efficiently use the different mechanisms that our body offers, instead of being “chronic sugar burners”, which happens to most of the population (with the serious associated problems).

Today I want to talk about a closely related topic, which is how the way we move (or train) also impacts the type of energy we burn, and how our body uses different “metabolic pathways” or energy systems to carry out the activities we demand of it.

And the conclusion will be similar: the really important thing is to accustom our body to use all these systems efficiently, and not turn to one only to the detriment of the others, as is also the case with the majority of the population.

Aerobic and anaerobic exercise

You’ve surely heard of aerobic and anaerobic exercise, right? Aerobic is one that requires oxygen to perform and anaerobic is that which can be performed without oxygen. You will have heard that aerobic is for example running and anaerobic lifting weights. That aerobic is good for cardiovascular health and to burn fat and anaerobic to build muscle.

As almost always when things are oversimplified, these ideas are half-truths. They have made those who understand at least the importance of working both think that the best way to do it is the typical 30 minute weight routine (hopefully without machines!) Followed by 30 minutes on the treadmill (hamster style). Or the typical kind of «aerobics»With colored weights of 500 grams, to match the clothes.


Both options are better than running on the treadmill for 1 hour, but there are better ways to stay fit. To really understand what is behind the concepts of aerobic and anaerobic, and therefore know how we should train, you have to dig a little deeper.

I introduce you to your energy systems

Simplifying a thousand pages of biochemistry in a few lines of text, we could say that the only thing our muscles can consume to function is ATP (Adenosine Triphosphate). It is like the basic unit of energy.

exist three ways our body achieves ATP what do you need:

  • Phosphagen system (or alactic anaerobic): Our muscles store small amounts of ATP and phosphocreatine (convertible to ATP almost immediately). This energy is present for explosive movements, where there is no time to convert other fuels into ATP. It allowed our ancestors to escape a predator, or us to avoid being run over by a car. With this energy we can carry out intense activity, but for a very short time, no more than 8-10 seconds.
  • Glycolytic system (or lactic anaerobic): Our muscles (and liver) also store glycogen. Through this system, that glycogen is converted into glucose and then into ATP, allowing intense activity (although less than in the previous case) for a few minutes.
  • Oxidative (or aerobic) system: When glycogen stores decrease, we must make use of our oxidative system, which requires oxygen to function. It’s the slowest way to get ATP, but it can generate energy for hours.

In short, we have two energy systems that work without oxygen (anaerobic), and one system that requires a constant input of oxygen (aerobic), with very different levels of energy release.

All three may be contributing energy simultaneously, but there will always be one that predominates over the others at any given time.


Just like most people put excessive emphasis on carbohydrate intake (all year round) and thus become glucose (sugar) burners, most people focus on aerobic training.

I have nothing against aerobics of ‘reasonable’ duration, and everyone should be able to run 10km in decent time or swim 200m without drowning. The problem comes when people become “addicted” to cardio, when they put on their nike plus shoes connected to the iPod (not knowing that their feet prefer to run barefoot), their GPS heart rate monitor, and compete with their friends to see who does more Kilometers per month. This is a poor and limited indicator of your actual physical condition.

In a previous article, I discussed the dangers of an excessive focus on aerobic activities, and the impact this has on other qualities of what it means to be really Fit.

In that same article, I recommended long-distance lovers to alternate their long runs with training based on sprint intervals (meaning sprinting to run as fast as you can, as if your life really depended on it).

Sprinting is, for me, the anaerobic exercise par excellence; work your two anaerobic systems to the maximum. In fact, most runners don’t even breathe during the queen of athletics.

The 100 meter dash

Tyson Gay and Charles Silmon compete in the men's 100 meter semi-final at the US Olympic athletics trials in Eugene

In a typical corridor, the energy to travel the first 60-80 meters comes almost exclusively from the phosphagen system. The explosiveness of this system makes it in this section of the race where maximum speed is reached. In the last 20-40 meters, as the energy input of the phosphagen system decreases and that of the glycolytic system increases, the speed of the runners decreases. Not even Usain Bolt can maintain top speed (maximum intensity) during the last few meters, and we are talking about less than 10 seconds of effort.

The great runners are those who can to stretch their phosphagen system a few hundredths more, and that make the entry of the glycolytic system faster (they slow down as little as possible).

Aerobic vs anaerobic: the reality and the relationship

As the graph above shows, there is no absolute separation between one and the other. Actually it’s a spectrum, Where we have on one side almost 100% aerobic activities (run a marathon or do an Iron man) and at the other extreme almost 100% anaerobic activities (sprint 100 meters or do a squat of 200 kilos).

While neither extreme is good, I think it is indicative from an evolutionary perspective that we have two systems for doing short intense activities, and only one for long duration activities.

In fact, if we compare these extremes, we could affirm the following:

  • The training aerobic destroys muscle, he anaerobic builds muscle.
  • The training aerobic generates catabolic hormones (such as cortisol), anaerobic generates anabolic hormones (like testosterone and growth hormone).
  • The training aerobic is good for the heart, he anaerobic too.
  • The training aerobic fat burning, he anaerobic too.
  • The training aerobic worsens anaerobic, he anaerobic improves aerobic.
  • The training aerobic does not improve strength, power, or speed, he anaerobic yes.
  • The training aerobic is inflammatory, he anaerobic is anti-inflammatory.
  • The training aerobic is boring, he anaerobic is fun. Well, maybe this is a personal opinion, but that’s what my blog is for :).

In all fairness, it is true that the aerobic is more effective in improving cardiovascular endurance than anaerobicAnd when I talk about muscle-destroying and inflammatory, I mean chronic cardio, frequent long-distance running, not 40-minute run twice a week. If you run long hours, better polarize your workouts.

The question then is, how can I get the best of both while avoiding the negative effects of ‘chronic cardio’?

Effective mixes

There’s really no single answer, but there are far better alternatives than long aerobic sessions or the typical 30-minute weight and 30-minute treadmill routine at a conventional gym.

Alternative 1

Do HIIT workouts (high intensity interval training) frequently. You can use body exercises, kettlebells with ballistic exercises etc … This type of training triggers anabolic hormones, enhancing muscle building and fat burning (study). Besides being more efficient than aerobic exercise at burning fat, it is also better for your heart health. Remember that your heart is a muscle, and you must also train it in different ranges of effort.

Alternative 2

Perhaps one of the main drawbacks of HIIT is that it is not the ideal option for building a lot of muscle and strength (although it helps). If this is your goal, it is more advisable to follow some type of programming that works all the qualities of fitness, but enhancing those aspects that you want to develop more. In “How to Design a Training Program” I detailed an example of how to do this. The idea is to establish “blocks”, giving your body enough time to adapt to new types of stress, without becoming stagnant or generating excessive volume wastes.

And of course you can mix both alternatives, as I propose in my Program.

In short, there are many combinations that work, and in their balance is the key to gaining muscle and losing fat. So it is sad to see how people have become used to always doing the same activities and the same canned routines; not very efficient, not very sustainable and not very useful to achieve a functional body and good health.