CSCS Chapter 3 Energy Systems and Bioenergetics Involved in Exercise

bioenergetics Aug 31, 2022

Edited by: Danielle Abel

Ever felt confused by the differences in energy systems? 

You're not alone; energy systems can definitely be hard to understand because they are not "on/off" systems. 

If you like learning from videos, you can check out the video that correlates with this article below. 

Keep in mind, depending upon the type of exercise, the exercise duration, and the level of intensity of the exercise, specific energy systems will be more or less activated. 

  • For example, an endurance athlete will be fueled by more aerobic energy systems like slow glycolysis and oxidative processes (Krebs cycle/ oxidative phosphorylation), whereas a strength athlete will be filed by more anaerobic energy systems like phosphagen and fast glycolysis. 

Before we dive into the energy systems, it's also important for you to recognize the differences between the rate of ATP production and the capacity of ATP production. 

  • Rate of ATP production refers to how quick (or slow the system is)
  • Capacity of ATP refers to the duration of the energy system (how long it lasts)

The 5 Energy Systems

  • Phosphagen

  • Fast glycolysis

  • Slow glycolysis

  • Oxidation of carbohydrates
  • Oxidation of fats and proteins


The phosphagen system produces ATP at the fastest rate, but it does not last very long and does not produce much ATP. 1 ATP is produced when a phosphocreatine molecule splits in half, and the phosphate is given to an ADP molecule to reform into ATP, which can be used for energy. 

Fast glycolysis

Fast glycolysis is also referred to as anaerobic glycolysis and has a net output of 2 ATP. Fast glycolysis takes longer to form ATP than the phosphagen system, but it lasts longer as well. Fast glycolysis includes breaking a blood glucose (sugar) molecule into pyruvate and then breaking the pyruvate down into lactate. 

An example, a shotputter would primarily be a phosphagen athlete with some contributions from fast glycolysis. 

It's important to note that there are some contributions from other systems as well, but for the most part, there will be primary and secondary systems. 

Slow glycolysis

This energy system is also known as aerobic glycolysis, and it's right in the middle of the different systems. It's moderate from a rate of ATP production and capacity for ATP production perspective. With slow glycolysis, glucose is broken down into pyruvate, but because oxygen is present (aerobic glycolysis), it's then sent through the Kreb's cycle.

Oxidation of carbohydrates

The system is the full oxidation of carbohydrates, so taking a single glucose molecule is broken down into pyruvate then goes through the Kreb's cycle. Then it goes through the process of oxidative phosphorylation. This system is slow to produce ATP, but it has the second-highest level of ATP capacity.

This is the primary energy system for distance events like 800 meters and running a number of miles, for example, a 5k.

Oxidation of fats

This system is the full oxidation of fats through the process of lipolysis (breaking down triglycerides) and beta-oxidation (breaking down fatty acids). Through either of these processes, the fatty acids become available to be fully oxidized through the Kreb's Cycle as discussed above with substrate level phosphorylation, and oxidative phosphorylation. 

Oxidation of fats is the slowest, but it produces the greatest amount of ATP and therefore lasts the longest. 

Oxidation of fats would be the primary energy system of marathon runners. 

Support & Courses Available

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