Time for our Workout Of The Month. Each month we take a look at a different exercise and break it down by movement, how to do it, and discuss some of the positive versus negative impact that it can have to you and your level of fitness. Some workout’s of the month breakdown how to do a common exercise, others feature a specific exercise. This month we’re going to take a look at a more simple concept: aerobic exercise. We’ll break it down and show you just what happens inside your body during aerobic exercise and show you how you can truly benefit from this type of working out.

Aerobic Exercise

This will actually be a two month long series. We’re going to be breaking down aerobic exercise on exactly what it is and how you can use it for benefiting your body. Next, we’ll be comparing anaerobic exercise and breaking that down to show you how you can use both types of exercise to help you get closer to your fitness and health goals.

What Is Aerobic Exercise

You probably heard the term aerobic exercise in the news or at the gym before. Every now and again there might be sayings like, “burn more fat with aerobic exercise,” or, “to be healthier just include more aerobic exercise.” Is it really that easy to burn fat or lose weight or get in better shape just by doing aerobic exercise? No. Not really. If it was, we would all be doing it. Let’s dive further into what’s happening inside your body during aerobic activity.

Aerobic Exercise: Aerobic activity is any type of exercise where activity is at a sustained intensity where it needs to use oxygen in order to produce energy for the body to use. Just what kind of energy does our body use? Well it comes in the form of something called ATP.

Just what is ATP? Adenosine tri-phosphate. It’s a type of cellular structure needed for muscular contraction. Under aerobic exercise, the body uses oxyten to break down carbohydrates and fats in order to make ATP – it’s known as an oxidative pathway. In comparison to anerobic exercise (which we’ll cover next month), in the presence of oxygen, one molecule of sugar will generate about thirty-six to thirty-eight molecules of ATP versus two from anerobic exercise. This makes aerobic exercise a much more sustainable type of activity, and is one reason why people can’t sprint forever. (51-53 182-185 NASM).

How Aerobic Exercise Works

The production of ATP through the breakdown of carbohydrates and fats requires the use of oxygen in order to match the musular requirement of the exercise. If the intensity of the exercise is too much, oxygen demand can’t keep up and it then becomes anerobic exercise – which we will cover in another post.

Energy Sources Of Muscles: While ATP is a major mechanism behind muscle contraction, the energy has to come from somewhere.

1. Carbohydrates: The main source of fuel. The body uses free sugar in the blood (which comes from the food we eat). Once that supply of free sugar runs out our body turns to stored up supplies of sugar called glycogen that comes from our liver or our muscles.
2. Fats: Once the stores of sugars have been used up, fats are the next to break down and be used for energy.

Energy Sources Of Aerobic Exercise

So you’re saying sugars get used before fats?

Kind of, but wouldn’t it be nice if it were really that simple?

In order to get that oxygen you need for metabolism, you obviously have to breathe. This is where something called the respiratory exchange ratio comes into play.

The Respiratory Exchange Ratio (RER):

The respiratory exchange ratio is a concept of the ratio of carbon dioxide (what we exhale) to the oxygen we take in when we breathe. When our bodies are at rest, we generally take in more oxygen than we make in carbon dioxide. As we exercise we start generating more carbon dioxide from our muscle tissue and the ratio increases. This ratio of carbon dioxide production to oxygen consumption dictates what type of fuel our bodies use for energy. The body uses the majority of its fuel consumption from fats when our bodies are at rest. Again, as we exercise, the carbon dioxide produced in our bodies increases, also changing what type of fuel we burn. See the table below on how our bodies switch from burning fats to primarily burning carbohydrates as activity changes.

RER Percent From Carbohydrates Percent From Fat
0.71 0.0 100
0.75 15.6 84.4
0.80 33.4 66.6
0.85 50.7 49.3
0.90 67.5 32.5
0.95 84.0 16.0
1 100 0

For a general rule of thumb, when your heart rate is around sixty to sixty-five to seventy-five percent of your max heart rate, you’re burning an approximate equal proportion of carbohydrates to fats. At seventy-five to eighty-five percent of your max heart rate, the body is primarily burning carbohydrates. When our bodies are at rest, we primarily burn fat for energy. As you can see from the table above, as the carbon dioxide production to oxygen consumed increases, so does our reliance on carbohydrates for energy.

Determining your max heart rate: 220-your age is your max heart rate. Multiply that by whatever percentage you want and that’s what you get for the percentage of your heart rate.

The Body Is Constantly Adapting

Will your body constantly burn the same amount of sources of fuel at the same rate? No. Your body likes to do things in the most efficient way possible. You will get used to whatever level of fitness stress you put on your body – which is the reason why workouts need to evolve, adapt, or change. It’s also another good reason why keeping track of your progress by writing things down is such a good idea.

Duration of exercise and type of fuel available in the body also plays a role. The longer the duration of activity the more energy used. Eventually, any free sugar in your blood will get used up. Next up is sugar supplies in the liver and muscle. When that’s exhausted, your body then turns to fat. Naturally, diets higher in carbohydrates increase the use of glycogen (sugar) as fuel whereas a diet higher in fat increases the use of fat as fuel. Some studies show an increase in performance associated with consumption of a high fat diet but only with low intensity levels of exercise.

What Kind Of Exercise Is Considered Aerobic Activity

Lower intensity, longer duration activities that generally last over at least two minutes. It can really be any type of sustained activity that keeps your heart rate up between the 65%-85% of your max heart rate range. A few examples would be, running, weight circuits, Crossfit (depending on the WOD), walking up hills, etc. It’s really that simple.

Have any questions on what it means to have an aerobic workout? Let us know. Hopefully this can help you plan your workouts accordingly and help you with your fitness goal.


1. Clark, M., Corn, R., Lucett, S. NASM Essentials Of Personal Training. National Academy Of Sports Medicine. Baltimore: Lippincott Williams & Wilkins. 2008. 60-61. Print. 50-54. 182-185