Energy Pathways and How They Fuel Your Activities

The human body uses three different energy pathways to fuel its activities (1):

  • The Phosphagen System
  • The Glycolytic System, and
  • The Aerobic System

The phosphagen and glycolytic systems don’t use oxygen to produce energy.  Together, they’re also known as anaerobic systems.

The phosphagen system breaks down the adenosine triphosphate (ATP) and phosphocreatine (CP) molecules stored in your muscles for quick energy.  This system powers activities that last a very short time, e.g. less than 10 seconds long.  Think max power intensity, such as a 1-rep max lift or a 100m sprint.

The phosphogen system, unfortunately, depletes very quickly and takes considerable time for the body to recover.

Meanwhile, for further high intensity work, the body moves onto its next energy pathway: the glycolytic system.

The glycolytic system is also anaerobic.  Here, the body breaks down glycogen (e.g. chains of glucose molecules) (2) stored in the liver and muscles to metabolize glucose.  When this happens, the body produces ATP for more energy.  The same process, however, creates lactic acid, which causes the “burn” felt in tired muscles.  This system fuels intense activities that last between 1-3 minutes.

Once the glycolytic system depletes, the body now requires oxygen for energy.

The aerobic (or oxidative) system uses oxygen to metabolize substrates from carbohydrates, fats, and proteins to produce energy (3).  The body can rely on the aerobic pathway for a long time, e.g. running a marathon, but it’s limited to low-to-moderate power activities.  Typically, under 50% of max power.

Energy Expenditure During a High Intensity Workout

During a high intensity workout, when these energy systems are graphed together, it looks like this (4):

Energy Pathways Graph

When we exercise for 1-2 minutes at high intensity, we rely primarily on the the glycolytic system, and very briefly on the the phosphagen system.

Then, as we inevitably slow to a less intense pace (e.g. 50% of max effort) while breathing very hard, we rely on the the oxidative system for energy.  During this same time, however, we’re recruiting more glycogen to kickstart the more powerful glycolytic system again.

It turns out that developing the glycolytic system also develops the oxidative system.  People who do mostly short, high intensity workouts find themselves (surprisingly) being able to work for longer periods of time, even though they haven’t done endurance training.

It’s Important To Utilize All Energy Systems

We want you to be good at utilizing all three energy systems.

If you only do workouts less than 10 minutes long, you’ll only occasionally use the oxidative system and miss out on training your body for longer workouts.

If you only do long aerobic workouts, you’ll miss out on training and improving your anaerobic systems.

Thus, every week, the Keelo app recommends both short and long workouts in your program.


  4., page 17.

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