The Three Energy Systems Part 1

After my blogs on ATP, a natural progression would be to discuss our energy systems and the roles they play in energy regeneration. Before I move onto this new topic, here’s a brief summary on ATP (adenosine triphosphate) from my write ups in my two previous blogs on the topic. What happens is, once we expend ATP (energy molecule) and lose one of the three phosphagens in the process, it turns into ADP (adenosine diphosphate). In order to regenerate the ADP molecule back into ATP, we must add back the third phoshpagen via one of our three energy systems. For a more in depth clarification on what I’m referring to, see these blogs; ( ) & ( )

We have trillions of cells within our body and billions of ATP molecules in each cell. The process of ATP expenditure occurs multiple times every second, so we need effective methods of ATP regeneration to simply survive, let alone perform exercise. We store approximately 80 -100g of ATP for immediate usage during bodily functions such as muscular contraction. However, these levels are not enough for significant exercise and as our effort levels increase, these stores become quickly spent. This is when our three energy systems, our phosphagen, anaerobic and aerobic systems, become vital in the regeneration of energy aka ATP.

Each energy system is responsible for regenerating cellular fuel in the form of ATP from it’s “spent” form ADP. All three energy systems are active at once; however, the contribution from each will depend on the intensity and duration of the activity. Below is a brief summary of our three energy systems.

The phosphagen system AKA the creatine phosphate system, is active at the start of all activity, regardless of the primary energy system at work. Once ATP loses its third molecule and turns to ADP, our phosphagen system attempts to replenish the molecule back to ATP, by breaking down another high energy phosphagen known as creatine phosphate (CP) to replace it. In order to breakdown CP to a bioavailable form to use in ATP regeneration, an enzyme known as creatine kinase reacts with creatine phosphate to synthesise/replenish a new ATP molecule, by adding back the third “spent” phosphagen bond.

This reaction occurs fast, but cannot supply energy for long (approximately 10 seconds), as there are only small amounts of CP readily available within our muscles and the body will not completely deplete itself of ATP. People with a higher percentage of type II (fast twitch) muscle fibres have higher percentages of CP available in their muscles than those with type I (slow twitch). This means they are able to replenish ATP faster and maintain higher levels of explosive activity for longer periods of time.

The phosphagen system will continue to supply ATP until the exercise ceases or the CP stores become too low, when this happens the anaerobic or aerobic system takes over as the main generator of cellular energy. Next week, I will continue and look to cover the remaining energy systems the anaerobic and aerobic system.

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