The Three Energy Systems Part 2

Following on from last weeks part 1, in which I provided a brief summary on energy production and subsequently explained in further detail our three energy systems. This week I’ll continue and summarise the next steps in energy production.

Last week I covered the phosphagen system and explained that it’s used primarily for short duration high intensity activities, it uses a phosphagen known as creatine phosphate to replenish spent energy from ADP back to ATP, but it only has enough to provide replenishment for short periods of time (approximately 10 seconds worth).

Once creatine phosphate stores run too low, the body will look to alternative methods to replenish stores. This is when either the anaerobic or aerobic system becomes involved. If the activity you’re performing still requires substantial effort, our anaerobic system will take over as the main system in charge, in an attempt to sustain that intensity.

Anaerobic means the process does not require the presence of oxygen. The only macro nutrient that can be metabolised without oxygen is carbohydrates, which signifies this as the main source of fuel for our anaerobic system. We can source carbohydrates via two methods, immediate supply within our blood from glucose or the utilisation of glycogen. Glycogen is stored carbohydrates and we have approximately 400-500 grams stored in our muscles and 100g stored in our liver, this total varies depending on the training age and metabolic make up of the individual. A useful formula to calculate total glycogen storage capacity is 15kg of glycogen per kg of bodyweight.

In order to use glucose or glycogen, we must breakdown the carbohydrate molecule to replenish the spent ATP molecule which is now ADP as it’s been spent and without the third phosphagen bond (refer to previous blog for clarification). The breakdown of carbohydrates is referred to as glycolysis and involves many catabolic enzyme reactions, this makes the energy production process slightly slower than the phosphagen system, but it has a larger capacity to supply for a longer period of time. Depending on the individual and their training history, the anaerobic system can supply energy from anywhere between 20-120 seconds, with variations in speed & power output. The reason for this variation in time is due as our “lactate threshold” and is defined as an individuals ability to clear lactate faster than their ability to produce it.

We produce lactate if we continue to maintain high intensity, in an effort to keep this requirement our body will continue to use carbohydrates as an energy source. However, a byproduct of carbohydrate breakdown (glycolysis) is the formation of an acid called pyruvate, which can do one of two things. A.) be converted to lactate, which is referred to as anaerobic glycolysis or B.) Be sent to the mitochondria for oxidation, which is known as aerobic glycolysis.

If we keep energy demand high and continue with carbohydrate breakdown, the pyruvate produced will be converted to lactate, known by many as lactic acid, which is often blamed for the “burning” sensation experienced in our muscles, this is incorrect. I will elaborate further in my podcast this week. However, if we reduce energy demands and before the pyruvate is converted to lactate, we can continue to move/generate energy if we reduce the intensity of the activity. This will then send the pyruvate to our mitochondria to be oxidised by our aerobic system. I will explain this further next in the final blog in this trilogy.

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