Body Energy Systems. Do we work out within the correct one?

When we talk about energy systems, we reffers to the fuel’s source when we are in the middle of our workout. Our muscles fibers use a fuel that comes from a molecule called Adenosin Triphosphate (ATP), which releases the energy needed thanks to a few internal reactions.

ATP + H2O → ADP + Pi + 7,3 Kcals

ATP energy lasts between 2 and 4 seconds due to the small amount available in our bodies. When it happens, we need to keep obtaining ATP from other sources in order to maintain the current muscles activity. There are 3 ways to obtain ATP:

Creatine Phosphate Anaerobic Alactic System

Phosphocreatine (PCr) is a phosphorylated creatine molecule that serves as a rapidly mobilizable reserve as a high energy phosphates in skeletal muscles and brain. It is around 5 or 6 times bigger present than ATP. Thanks to this molecule, we can keep obtaining quick energy for a few more seconds (5–10 secs) without using oxygen.

ADP + PCr → ATP + Cr

The problem here (again) is the small amount of phosphocreatine stored in our bodies. Once it is depleted, in order to continue exercising, we need to leave our body to rest in order to restore it.

The set Creatine Phosphate — ATP is called Anaerobic alactic system since energy is obtained without the implication of oxygen and there is not trash product such as lactic acids (that we will see later in this post).

This system is used in exercises lasting between 0 and 20 seconds, where the intensity reaches the 100%.

How do we workout this system? A good way is doing multi — jumps, or small sprints uphills and resting for more than 120 seconds, so our ATP gets restored for the next effort. To completely recover of this type of workout takes around 72 hours.

Anaerobic lactic system

This system provides energy quickly, although it is slower than the anaerobic alactic system commented above.

It uses the muscles glycogen as fuel that, through glycogenolysis, is transformed to glucose. After that, it’s metabolized and converted to lactic acid. This process allows to get ATP:

Glucose (C6H12O6) → 2 lactic acids (c3H6O3) + 2 ATP

It’s a quick way to obtain energy but it has a few important inconveniences:

  1. Low energy available.
  2. Accumulation of lactic acids in muscles and in bodily fluids (intra and extra cell liquids).

The reduction of muscular Ph affects negatively to the muscular contraction and the capability of transforming glycogen into glucose.

This system uses exclusively carbs as energy source, whilst the aerobic system uses also proteins and fats.

This system is used in exercises lasting between 45 and 120 seconds where the intensity is something between 85% and 95%.

We could workout this system doing 4–8 series of 200m — 400m long with a rest period around 5–7 minutes. We have to make sure that the effective time is not longer than 10/12 minutes since we would be targeting the aerobic system described below.

Oxidative aerobic system

The aerobic system is a very slow provider of energy. It depends on the oxygen and is an almost unlimited source of energy (thanks to the amount of fat that our body stores).

Through this system our body is able to burn out carbs, fats, proteins and even alcohol when it is present, using oxygen as the reactive source. The remaining product is CO2 y H2O (and urea when proteins are metabolized).


To use glucose through the oxidative system is different than doing it through the anaerobic path. Now, the glucose is completely burnt out (in your interested it’s done thanks to intermediate metabolites within the Krebs cycle), which means that there is not lactic acids anymore but CO2 and H2O, products that don’t provoke fatigue (as we saw with the lactic acids before). Thus, the amount of energy obtained is much bigger (around 19 times bigger) than the obtained through the anaerobic way (see the difference between the ATP produced here and in the formula in the lactic anaerobic path above. Awesome, isn’t it!?):

Glucose + 6 O2 → 6 CO2 + 6 H2O + 38 ATP


Fatty acids, either stored as intramuscular tryglicerids or coming from the blood flow, go within the metabolic path, which provokes the creation of acetilCO-A and the enter into the Krebs cycle. It provokes the creation up to 9 moles of ATP for each carbon’s atom. The glucose added only 6 moles for each carbon’s atom.

For instance, taking 3 kg of body fat, the caloric value would be of 27.000 KCals. This is the energy needed for running 8 marathons!!!

That’s is the reason of why I do encourage people, which take part in long endurance events such as marathons, triathlons, cycling, etc… to teach their bodies to burn body fat as main energy source, instead of being glucose dependents.


And last but not less, we have proteins. Proteins are also a big source of energy, but smaller than fats and less desirable to use. Theoretically, the amount of useful protein is around 12.000 kcals. However, the capability of use protein is small (not more than 5%). Also, if the amount of carbs and fat stores are big enough, fortunately, the proteins utilization decrease.

Our bodies does not have a protein warehouse as they do for fats and carbs. Proteins in our bodies are part of body tissues. To use those proteins mean that our body need to transform tissue into amino acids (which are the protein’s base) and then, through glyconeogenesis, they are transformed into glucose.

How do we work out our aerobic system? As I said in one of my previous posts (5 tips to lose body fat!), the aerobic capability must be the base of our training (as well as the strength). imagine that you are building a house without a solid base. The house might stand up for some time but, later or sooner, it would end dropping… The same happens with our body. When we don’t have a strong aerobic system, we might build big muscles, be explosive and strong, but the aerobic capacity allows us to recover after each workout. If we don’t have a strong one, we will end up injuring ourselves, with having problems to get over our body workouts.

There are many ways of building the aerobic system:

  1. Continuous training: is a long distance training with the same intensity (between 60% and 80% of our VO2Max). It is believed that this type of workout produces the best adaptations for the peripheral and central systems.
  2. HIT (high interval training): such as Tabata. This training not only taxes the aerobic system but also the anaerobic one.
  3. Fartlek: it consists of a few exercises, mainly race one, performed in an interval way. There is no rest between exercises.
Aerobic vs Anaerobic


McArdle et. al., 1991

Chicharro, J. L., & Campos, V. C Vaquero. (2013). Fisiología del entrenamiento aeróbico. Ed. Médica Panamericana.

Almendros, M. M. (2013). Nutrición para la salud y actividad física. Ediciones Díaz de Santos

Maughan, R. J., & Shirreffs, S. M. (2012). Nutrition for sports performance: issues and opportunities. Proceedings of the Nutrition Society, 71(01), 112–119

Baar K. (2014). Nutrition and the adaptation to endurance training. Sports Med. (44 Suppl 1:S5–12).

Coletta et al. (2013). The influence of commercially-available carbohydrate and carbohydrate-protein supplements on endurance running performance in recreational athletes during a field trial. Journal of the International Society of Sports Nutrition

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