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The Aerobic Energy System


The basics

If you’ve read our overview of how the body uses energy, you’ll know that there are three different energy systems that work together to fuel and sustain you as you exercise.

As mentioned in our articles on the two other energy systems articles, our body’s usable source of energy is called Adenosine Triphosphate, or ATP for short, and without it, we would not be able to survive or function.

For intense, short bursts of exercise, our bodies call upon the two anaerobic energy systems, (the ATP-PC system, and the glycolytic system) to provide the bulk of the energy and power the body.

However, both systems can only last for a relatively short duration before oxygen is required and the body must call on the aerobic, or oxidative, system to provide a larger portion of energy production in order to continue with the physical activity.

The Aerobic Energy System in action

The aerobic system (also known as the “oxidative system”) is the primary source of ATP and energy for the body at rest and during low-intensity activities.

The body uses carbohydrates and fats while producing energy using this method. However, when the intensity increases and the demands on the body change, the body must use other methods for ATP production in addition to what is produced by the aerobic system.

When the body begins to exercise, the demands change, and you will begin using mostly anaerobic systems to power the body.

Typically, the aerobic system works minimally at the start of exercise but will start to provide a significant amount of energy once a physical activity lasts for longer than one to three minutes, or when a steady state is achieved.

It also becomes the primary energy system that powers you once the other energy systems have fatigued.

Types of exercise which predominantly use the aerobic energy system include lower-intensity cardiovascular activities such as walking, or long runs, and bike rides.

What you need to know

The oxygen needed by this system to function is provided by the cardiovascular and respiratory systems via blood flow to the tissues.

During a period in which the aerobic energy system is dominant, two processes are used for energy production: The Krebs Cycle and the Electron Transport Chain.

In the Krebs cycle, and further down the metabolic pathway, you begin to produce much more ATP than was possible in the first two anaerobic energy systems.

This production of ATP is completed in the Electron Transport Chain where oxygen is present. Because oxygen must be present, this means that the system runs when you are working at a low to moderate level of activity, or even when you are sitting still.

The aerobic system is active all of the time, and although it mainly burns carbohydrates and fat, it can also metabolize some protein for energy production.

This does not typically happen, unless you have been exercising for a long time, i.e., longer than two to three hours, or if you have not eaten in a long time.

Putting this knowledge to use

The more you effectively train your aerobic system, the fitter you are, the better your body will become at using fat as a fuel and at saving your stored glucose until you really need it, like when you increase the intensity of the exercise you are doing.

The best ways to train your aerobic system include:

  1. Interval training – i.e., alternating periods of intense work with rest or periods which are still active but at a lower intensity that can still be sustained.
  2. Continuous training – i.e., when the exercise maintains a constant intensity and lasts for a longer time, for example, longer than 15 minutes.
  3. “Fartlek” training – “Fartlek” is a running-specific term which refers to workouts in which the runner’s speed and effort are raised and lowered throughout the session (without stopping for rest). Although Fartlek training usually means running workouts in that mode, you can apply the same principle to any steady workouts -- mix things up with your pace and effort level to improve endurance and stamina.

By working out regularly, you increase your body’s ability to use its fuel effectively, decreasing your reliance on carbohydrates for energy production and using more fat.

Our bodies have a seemingly unlimited capacity for storing fat, and chemically fat provides over twice as much energy per gram than protein or carbohydrates, making it a very attractive choice for sustained energy production.

This means that at rest and during exercise, you are burning a greater amount of fat, and over time this will no doubt impact your body composition.

However, although you may be burning mostly fat, a steady supply of carbohydrates is still necessary for the breakdown of fat into an energy source. You can develop your aerobic system so that it is able to store more ATP molecules at a time, meaning that you can exercise for longer, which is an attractive prospect for many.

In short, aerobic training is good for building endurance and improving your cardiovascular and respiratory function. In simpler terms, this means that your heart and lungs become stronger and more efficient, enabling you to train harder and longer as your fitness levels improve.

Training starts here