Where does the energy needed by our muscles come from in order to contract and effect body movement ? Simply from food. Of course, the nutrients cannot be used directly at cell level. They are digested into absorbable substrates producing energy stored in a molecule called adenosine triphosphate (ATP).

Any movement is fuelled by ATP but muscles only store it in a small amount. For example, walking burns it out in 2 minutes and all-out effort in 2 seconds.

Depending on the activity (sprint or long-time run), your metabolism will select almost exclusively or inclusively from a range of three different mechanisms allowing to resynthesize ATP as it is used up.

1- Non-lactate anaerobic process

The energy used to resynthesize ATP comes from a similar compound called phosphocreatine (PC) stored in muscle cells at rest. This compound is mainly used in very intense exercises. Any activity like sprinting or shot put that involves all-out effort in a lapse of time between 3 and 15 seconds is fuelled by the non-lactate anaerobic process (without oxygen and low lactic acid rate).

Substrate in use

Phosphocreatine (PC)

Time to action

It is triggered within seconds of starting exercise.

Power

Very high but enables all-out effort only for 3 seconds.

Endurance

Very low. At full power, it runs out after 7 seconds. Beyond this limit, muscles must use other processes to cover their energy needs.

Restricting factors

This low capacity is due to the small amount of ATP and PC in the muscles all the less as the chemical reactions that resynthesize ATP use a large amount of PC. Therefore, the energy is burnt out rapidly as it cannot be stored effectively.

Effects

ATP consumption in non-lactate anaerobic process has a positive effect: it triggers anaerobic glycolysis and stimulates oxidative processes. Therefore, non-lactate anaerobic training is often used to induce and stimulate the lactate anaerobic or aerobic process.

2- Lactate anaerobic process

Once the ATP and PC have been burnt out completely, the only way to produce more ATP is by breaking down glucose (99% of all sugars found in the bloodstream). This is called glycolysis. As a result of complex chemical reactions, our stored form of glucose (glycogen) produces pyruvic acid, hydrogen and energy (used to resynthesize ATP). The pyruvic acid and hydrogen will combine into lactic acid, hence the lactate anaerobic process which prevails in endurance running (400 and 800 metres).

Substrate in use

Glucose

Time to action

It is also triggered within seconds of starting exercise but less intense than in the non-lactate anaerobic process. It prevails in resynthesizing ATP only after a 10-second time lapse.

Power

Very high, it enables all-out effort for 20 to 45 seconds.

Endurance

If maximum power rate is reached by 45 seconds, the process will keep resynthesizing most of the ATP for 2 to 3 seconds.

Restricting factors

The lack of glucose per se does not prevent muscles from contracting as enough is stored but the accumulation of substances due to breaking down glycogen and more specifically the higher acid rate do block muscle contraction.

Effects

The non-lactate anaerobic process triggers the lactate anaerobic process which in turn will trigger the aerobic process.

3- Aerobic process

The aerobic process (with oxygen) consists in chemical reactions from substrates such as carbohydrates and lipids in order to produce the energy needed to resynthesize ATP.

The aerobic process is vital to human metabolism as well as animals. It produces ATP most efficiently and in high quantity. This is the fuel source for endurance running.

Substrate in use

Mostly carbohydrates and lipids (potentially protides)

Time to action

It is triggered after 3.5 seconds in trained adults at full yield and 2 to 3 seconds for a child. This cardiorespiratory evolution does not depend much on the anaerobic process and it explains the maximal aerobic power in children doing sport. It can go even beyond VMA100 to 130% in short series of 5, 10 to 15 seconds.

Power

It is lower than the anaerobic process and depending on our individual capacity to bring oxygen to our muscle cells. The higher the volume is, the more intense the physical effort will be. When all the oxygen in the muscles is used up, an athlete is said to have reached his maximal aerobic power (PMA) or VO2 Max (the highest rate of oxygen consumption attainable during maximal or exhaustive exercise) or VMA (maximal aerobic speed). However, an athlete that has reached his PMA can intensify his effort further. As he runs out of oxygen, his metabolism will go back to the anaerobic process, which will cause an increase of the lactate rate like for example in the final sprint of a 1500-metre run. Once finished the exercise, the accumulation of “waste” and necessary synthesis of “fresh” substrates such as carbohydrates and lipids will stimulate high volumes of oxygen uptake. We can say the metabolism will repay an oxygen “debt” when recovering from all-out effort. The more intense the effort, the heavier the burden of the debt.

Endurance

Much better than the anaerobic process. The aerobic process can fuel long-lasting muscle contraction thanks to a low consumption of varied substrates such as glycogen, fatty acids and organic proteins and only exhausts CO2, heat and sweat.

Restricting factors

This is not an endless process. The closer we are to the aerobic critical threshold i.e. VMA 100%, the higher the blood lactate rate will be and therefore we will struggle to sustain our effort. A top athlete can hardly keep at VMA 100% more than 7 seconds. If we lower the intensity to VMA 80 or 90%, we can sustain the effort longer. This capability to sustain an effort at high PMA for a long time is key to optimize the aerobic process.

Effects

The aerobic exercises stimulate all breathing functions, especially the circulatory system for the muscles to be fuelled with oxygen. They play a major role in our well-being, help eliminate “waste” due to muscle contraction, supply the necessary substrates and also optimize energy processes. This is undeniably a must to warm up your body.

In summary

Actually, we can compare these three processes with energy “tanks”: great capacity for the aerobic process, medium and low for the lactate and non-lactate anaerobic processes. They supply the muscles with energy flowing at different rates (pipe diameter) and triggered at different times (pipe length). The diameter being the power, the tank size being the endurance to fuel the muscles.

ATP is like a battery. All-out effort burns all its energy within seconds. This is why the metabolism needs to synthesize it continuously. The possibility for each process to synthesize most of the ATP depends on the exercise. On the one hand, we have intense efforts during a short period of time whereby ATP comes from the non-lactate aerobic process. On the other hand, long-lasting efforts of lower intensity mostly covered by the aerobic process. The lactate anaerobic process being in between, most of the ATP is used in a 400 or 800-metre run.