ACSM Fitness Focus: What is Oxygen Consumption?

Oxygen consumption is frequently used when discussing exercise, but what it is, what it means, and why it is important is often misunderstood.

Background
Our bodies need a constant supply of energy to sustain life. Energy is used to contract skeletal muscles, pump blood, sustain tissues, and many other functions. The form of energy the body needs for these processes is adenosine triphosphate (ATP). This high-energy compound is split apart, releasing energy to perform cellular work. Think of it as the gasoline for your cells. Because the body does not store large quantities of ATP, our bodies must continually make more. Although there are processes that can produce ATP without oxygen (O2), these anaerobic systems are limited in their capacity to produce ATP. The greatest capacity to produce ATP is through the breakdown of carbohydrates, fats, and to some extent proteins. To make ATP from these nutrients, O2 is required. Ultimately, oxygen-requiring - also known as aerobic - processes are necessary to sustain the body's energy needs.

What is Oxygen Consumption?
Oxygen consumption (V˙O2) is the amount of oxygen taken in and used by the body per minute; thus, it is the rate of oxygen use. V˙O2 is sometimes expressed in liters of O2 per minute (liters per minute). Alternately, it can be expressed as a function of body weight, frequently as milliliters of O2 per kilogram of body weight per minute (milliliters per kilogram per minute). Expressing V˙O2 as a function of body weight is important when comparing people of different sizes during exercises such as running.

How is Oxygen Consumption Measured?
When we inhale, 20.93% of the air that we bring into our lungs is O2 and 0.03% of the air is carbon dioxide (CO2). When we exhale, the percentage of O2 is lower (we consume O2), and the percentage of CO2 is higher (we produce CO2). In exercise physiology laboratories, machines that measure the amount of air inhaled or exhaled and devices that measure the percentage of O2 and CO2 in expired air are used to calculate V˙O2. There also are portable devices that can be used outside laboratory settings to measure V˙O2 during various types of activities (e.g., playing golf, performing yard work, etc).

Why is it Important?
V˙O2 reflects energy expenditure, meaning that measuring V˙O2 provides an estimate of calories burned. Approximately 5 kcal of energy are expended for every liter of O2 consumed. Thus, a person with a V˙O2 of 1 L/min is burning 5 kcal each minute. As one exercises harder, V˙O2 rises, meaning that energy expenditure is higher. The interrelationship of the type and intensity of activity, O2 consumed, and energy used allows researchers to estimate the caloric expenditure for different tasks. This information can be used to shape exercise plans to meet weight control goals.

Information about V˙O2 also can be useful for athletes. Economy tests examine the relationship between work performed and energy expenditure during exercise. Athletes who are able to move smoothly, without wasting energy, perform better. For example, a person who runs with his arms out to the side of his body uses extra energy but that doesn't help him run faster. Coaches closely examine the movement patterns of athletes to improve the economy of effort. Maximal V˙O2 (V˙O2max) is another measurement often used by coaches and athletes (see Fitness Focus from issues 9:3, 9:4, and 9:5 published in 2005). V˙O2max provides a measure of the maximal ability to perform high-intensity aerobic work. This value is strongly associated with performance and health.

© 2010 American College of Sports Medicine Author: Thompson, Dixie L. Ph.D., FACSM, ACSM'S Health & Fitness Journal: January/February 2010 - Volume 14 - Issue 1 - p 4