What are Energy Systems?

Energy Systems

What type of exercise do you do?

Do you know what type of energy system your body is using?

Are you fueling your body properly?

To keep things simple, there are three energy systems that your body uses:

  • Phosphagen System (~10-20 seconds)
  • Glycolysis (~20-120 seconds)
  • Aerobic System (~120 seconds or longer)

Phosphagen System

The phosphagen system is the energy pathway that your body uses for all out exercise bouts lasting about 10-20 seconds. Phosphocreatine is the energy source for this system that allows you to execute short intense activities such as sprinting a 40 yard dash. Because there is a high amount of power needed, your body requires a high amount of ATP. No carbohydrates or fats are needed in this process, because the energy needed is coming from stored creatine phosphate. This is a anaerobic process, meaning no oxygen is required.


Glycolysis is the preferred energy system for high intensity exercise lasting anywhere from 20 seconds to 2 minutes. During glycolysis, carbohydrates in the storage form of either muscle glycogen (storage form of glucose) or blood glucose (sugar) is broken down and used as energy. As you get closer to that 2 minute window, muscles begin to lose their ability to contract effectively and in turn muscle force production & exercise intensity decrease.

Aerobic System

The aerobic system, which requires oxygen, is the most complex out of the 3 energy systems. The metabolic reactions that take place in the presence of oxygen during this phase are what provide most of the cellular energy for your body. Aerobic energy exercise can last for 2 minutes and longer.

Energy System Power Capacity Fuel Used

  • Phosphagen – Very High
    • Very Low Creatine Phosphate
    • Stored ATP
  • Glycolysis – High
    • Low Blood Glucose
    • Muscle & Liver Glycogen
  • Aerobic – Low
    • High Blood Glucose
    • Muscle & Liver Glycogen
    • Adipose & Intramuscular Fat