Definition of Aerobic Fitness

Definition of Aerobic Fitness
Definition of Aerobic Fitness (Image: Cineberg/iStock/GettyImages)

Aerobic fitness is a measure of your body's ability to take oxygen from the atmosphere and use it to produce energy for your muscle cells. Many factors influence aerobic fitness, including your lung efficiency, cardiac function, gender, age and genetic makeup. Understanding the various components of aerobic fitness will help you train smarter to achieve optimal performance.

It's All About Oxygen

Aerobic fitness is your body's ability to transport and utilize oxygen. It's measured by VO2 max, the maximal amount of oxygen you can use during intense exercise. VO2 max is influenced by primary and secondary components. The central component involves the ability of your lungs, heart and vascular system to deliver oxygen to your muscles via your blood stream. The secondary component involves the ability of your muscle cells to extract oxygen from your blood and use it to make adenosine triphosphate, or ATP, the fundamental carrier of energy in cells.

Priming the Pump

Your heart and lungs play a central role in aerobic fitness, with your heart being the prime limiting factor. While your lungs must function efficiently in order to transfer oxygen from the atmosphere to your bloodstream, they take a backseat to your heart, which must contract forcefully to inject oxygenated blood into your system to reach your cells.

Aerobic exercise training increases your total blood volume, heart muscle size and contractility, resulting in a greater volume of blood being injected per heart beat. Increased stroke volume means your heart does not have to beat as frequently at rest, resulting in a lower resting heart rate.

African woman running on racetrack
High-intensity exercise increases VO2 max. (Image: jacoblund/iStock/GettyImages)

Max Extraction

Regardless of how efficiently your heart pumps blood, aerobic fitness is also dependent on the ability of your muscle cells to extract oxygen from your blood and use it to make energy. Aerobic energy is produced in the mitochondria of your muscle cells, using carbohydrates and fats for fuel. Mitochondria are microscopic organelles that function as energy factories for aerobic metabolism.

In response to repeated bouts of aerobic exercise, the density and number of mitochondria increase. High-intensity exercise that challenges your aerobic limit has a more profound effect on mitochondria adaptations and oxygen extraction than low- to moderate-intensity activity.

During high-intensity exercise, you exceed your body's ability to produce aerobic energy, forcing it to tap into anaerobic pathways. However, anaerobic energy can only be sustained for a very limited time, usually less than two minutes. Perpetually exceeding your anaerobic threshold causes your muscles to adapt to perform aerobically at higher intensities.

Measuring Up

While VO2 max is the ultimate measurement of aerobic fitness, it relies on a complex evaluation in a laboratory setting, conducted by trained technicians. Other methods may be used to measure your aerobic fitness. One way is to monitor your heart rate during maximal-intensity exercise and translate it as a percent of your maximal heart rate, which you can calculate using the equation: 206.9 – (age in years × 0.67) = estimated maximal heart rate. The fitter you are, the higher your percent of max heart rate during exercise. Aerobic fitness classifications ranging from low to excellent are reflected by values of 60 to 90 percent of max heart rate, respectively.

Read more: VO2 Max Training

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