Exercising has so many benefits, but each one of us has their own personal goal when it comes to it. It can be losing fat, looking better, or just being healthy!

To exercise wisely and to reach your goals, it’s important to understand how our muscles work and where they take energy from, to maximize your efforts and pursue your goals!

Muscular contraction depends on ATP production. To keep a certain intensity, the ATP needs to be produced at the same rate that it is being consumed.

Muscle can increase its energetic needs by 100 times in a few seconds going from a resting state to a 100% effort!

Muscle use three main systems to produce energy, they are all integrated and used at the same time but we would split them up to analyze them singularly:

1. Phosphagen system.
2. Anaerobic Glycolytic system.
3. Aerobic system.


• Source: PhosphoCreatine.
• It doesn’t require oxygen and it doesn’t produce lactic acid.
• It produces ATP really fast but only for a short amount of time (6-10 seconds if it was the only system working) because the source is very limited.


• Source: Muscular glycogen.
• This source is preferred over blood glucose because it’s already inside the muscle and it produces an extra ATP.
• It doesn’t require oxygen but it produces lactic acid.
• It produces ATP through glycolysis.
• It produces ATP pretty fast but it still doesn’t last long because the source is limited and lactic acid is cleared slowly from the body not allowing the system to reach full potential (lasts for about 40 seconds if it was the only system working)


• Sources: Muscular glycogen, blood glucose, muscle fatty acids, blood fatty acids.
• It produces ATP through oxidative phosphorylation.
• It requires oxygen.
• It can differ depending on the source but overall it produces ATP slower but has a much higher capacity!
• Muscular glycogen through the oxidative system can last for about 80 minutes and blood fatty acids can potentially last until you deplete all your fat in your body.

What system we use the most at a certain moment depends on 4 factors

1. INTENSITY of the exercise.
2. LENGTH of the exercise.
3. GENETIC factors.
4. TRAINING level.

If it’s a high intensity-low length exercise (sprinting for example) we’ll use mainly anaerobic systems because they produce ATP at a faster rate.

If it’s a lower intensity exercise (jogging for example) we’ll mainly use the aerobic system.

Genetic factors involve the type of muscle fibers:
Slow-twitch fibers prefer the oxidative system for example while fast-twitch ones use more of the non-oxidative pathway.


Training can modify what system our body tends to use the most, aerobic training will cause the source to switch to the lipids more rather than glucose.

And we can also modify our fibers, there are intermediate ones that can turn into slow or fast-twitch ones depending on the type of training!