Aerobic activity is any physical activity that uses the large muscles of your body continually and rhythmically at a constant level of intensity. Some examples of high-intensity aerobic activity include step aerobics classes, swimming laps in a pool and long-distance running and cycling.
These are all challenging activities, but you can sustain them for more than a few minutes. However, there’s a point at which high-intensity aerobic exercise becomes anaerobic exercise, and that’s when your body switches its primary source of energy.
Aerobic Vs. Anaerobic
The term aerobic means "with oxygen." During aerobic activity your body is able to sustain the activity using oxygen from the environment. The activity may still feel intense -- you may be sweating buckets and out of breath -- but you can keep going for an extended period of time.
However, above a certain intensity, your body can no longer use oxygen to meet the energy demands on your muscles. These high-intensity activities, such as running sprints, can't be sustained for very long. It's at this point that your body switches to the anaerobic energy system.
The Fuel in Your Tank
Your body uses different sources of energy depending on how hard you work. For very intense exercise in the anaerobic zone, your body uses a high-energy compound called creatine phosphate, which it breaks down into adenosine triphosphate, or ATP -- the main unit of energy in all your cells. In the absence of oxygen your body can also use a carbohydrate source called glycogen to quickly produce ATP.
During aerobic exercise, even challenging, vigorous aerobic exercise, the energy demands are not as intense. Your body has more time for the slower processes that break down glucose for energy. These glycogen stores come from leftover glucose in the muscle cells, glycogen in the liver and fat reserves. Aerobic respiration is much slower than anaerobic, but the energy created lasts much longer.
Intensity and Fuel Sources
Energy production is a complicated process and it varies depending on many factors including your fitness level and your genetic makeup. Typically, when you're doing light- to moderate-intensity aerobic activity, such as brisk walking or leisurely riding a bike, your body is primarily using fat for energy, followed by carbohydrate.
Up the intensity a little more to high-intensity endurance exercise -- the highest intensity you can maintain for an extended period of time -- and your body begins to use more carbohydrates than fat for energy.
Finally, when you cross the anaerobic threshold to an intensity you can't maintain for more than a couple minutes, your body is using carbohydrate almost entirely as its energy source.
The Myth of the Fat-burning Zone
You might have heard people talk about the "fat-burning zone," or the alleged aerobic zone in which your body uses primarily fat for energy. The idea is that if you exercise in this zone, you'll burn more fat; but this concept is widely misunderstood.
Fat loss depends on the total calories expended, not on the source of those calories. Exercising at a lower intensity doesn't burn as many calories as exercising at a higher intensity. Although a greater percentage of calories burned during low-intensity exercise come from fat, you burn far more total calories -- and more fat calories overall -- during high-intensity exercise, therefore increasing your net calorie deficit.