The Aerobic Energy System

The aerobic system, also known as the oxidative system, is the main source of ATP and energy for the body when at rest and during low-intensity activities. At the start of exercise, it takes one to three minutes for the aerobic system to ramp up and become the dominant energy source  once your heart rate, breathing, and oxygen delivery stabilize into a rhythm.

The aerobic energy system takes over more for exercises we typically refer to as "cardio," like walking, easy-paced long runs, and Zone 2 bike rides.

Read on to learn more about the aerobic energy system, the health benefits of aerobic training, and how to apply this knowledge to your workout routine to improve performance.

If you are an athlete, you have probably heard the phrase "food is fuel." This is true, especially when you think about how your body breaks down food for energy. Your body needs nutrients to power different types of workouts using three primary energy systems to sustain physical activity.

The food we eat is broken down into a usable form of energy called Adenosine Triphosphate, or ATP for short. And without ATP, we would not be able to function – or even survive.

For intense, short bursts of exercise, our bodies use two anaerobic (without oxygen) energy systems. These are the ATP-PC system and the glycolytic system. They provide most of the energy and power we need.

Both systems can only work for a short time before they need oxygen. Then, the body relies on the aerobic (with oxygen) system to produce more energy for physical activity.

Aerobic vs Anaerobic Energy Systems

During low-intensity types of exercise and at rest, the aerobic system provides the body with ATP, or energy, by using carbohydrates and fats. If, however, the intensity increases or the body's demands change, it must use other methods for ATP production in addition to what is produced in the aerobic system.

When you start to exercise, the body relies more on the anaerobic systems to meet the rapid increase in energy demand. The aerobic system doesn’t provide much energy at the start, but after about one to three minutes, when you reach a “steady state” and the oxygen delivery improves, it becomes the main source.

Think of a car driving on cruise control on a highway – that’s your body in a steady state after you’ve picked up speed and settled into a consistent pace.

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

The aerobic system requires oxygen, which is delivered to your tissues by the cardiovascular and respiratory systems through oxygen-rich blood. That’s why your heart rate and breathing increase to meet the increased need for energy by your muscles.

This system is always “on” – constantly providing energy to the body. And it provides the majority of the body’s energy needs while working out at a low to moderate level of activity and even sitting still.

While it primarily burns carbohydrates and fat, the aerobic system can also metabolize small amounts of protein for energy. This only happens when exercise lasts for a long time, like two to three hours, or when carbohydrate stores are low, such as during fasting.

The more you effectively train your aerobic system, the fitter you become and the more efficiently your body functions.

Here are some of the ways your body improves its efficiency through improving your aerobic endurance:

• Reduced body fat through improved fat metabolism
• Increased cardiac output - the amount of blood pumped by the heart per minute
• Higher VO₂ max - a key measure of cardiovascular fitness
• Greater respiratory capacity and oxygen delivery
• Improved ability to sustain higher intensities for longer
• Increased mitochondrial and capillary density in muscles
• Enhanced enzyme activity for aerobic metabolism
• Improved insulin sensitivity and blood sugar control
• Lower resting heart rate and blood pressure
• Reduced inflammation and oxidative stress

As you train your aerobic system, your body also becomes better at using fat as fuel. This means you rely less on carbohydrates for energy and save your stored glucose for when you really need it, like during high-intensity interval training exercises.

Our bodies can store a lot of fat, and chemically, it provides more than twice as much energy per gram as protein or carbohydrates. That makes fats a great 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, even when burning mostly fat, your body still needs a steady supply of carbohydrates to help break down fat into an energy source.

Now that you know why you should train your aerobic system, here are some practical ways to add it to your training routine:

1. Continuous training – when the exercise maintains a constant intensity and lasts for a longer period of time, for example, longer than 30 minutes.
2. Interval training – alternating periods of intense work with rest or periods which are still active but at a lower intensity that can still be sustained.
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.

Training that targets your aerobic energy system helps build endurance and boosts the health of your heart and lungs. It also increases your body’s capacity to efficiently produce ATP over longer periods, making you stronger and more efficient.

This allows you to train harder and longer as your fitness levels improve – a key benefit many athletes strive for.