Thompson, Dixie L. Ph.D., FACSM
Dixie L. Thompson, Ph.D., FACSM, is the director of the Center for Physical Activity and Health and an associate professor in the Department of Exercise, Sport, and Leisure Studies at the University of Tennessee, Knoxville.
This is the first of a three-part series examining maximal oxygen consumption, abbreviated V̇O2max. This column describes what V̇O2max is and how to measure it. Part two will explore links between V̇O2max, health, and performance. The third column will provide a description of how to improve V̇O2max.
V̇O2max is defined as the highest rate at which O2 can be taken in and used during high-intensity dynamic exercise. Most fitness experts agree that V̇O2max is the best measure of cardiovascular fitness. Other terms used for V̇O2max are maximal aerobic power and maximal O2 uptake.
Symbol Why Is Oxygen Needed During Exercise?
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Muscle contraction requires energy. For submaximal exercise and all-out activities lasting 3 minutes or longer, most of the energy used comes from processes involving O2. Oxygen is needed so that carbohydrates, fats, and proteins can be broken down, resulting in the release of energy. At rest, humans need O2 to support cellular activities in all tissues and to maintain body temperature with the heat generated. The O2 requirement at rest is taken as 3.5 ml of O2 every minute for each kilogram a person weighs (i.e., 3.5 ml/kg/min), or 1 MET. During exercise, the need for energy increases. The higher the exercise intensity, the higher O2 uptake climbs to meet that need, until it can go no higher (V̇O2max). Each person has an upper limit for O2 uptake based upon factors such as age, health, training, and genetics. Good marathon runners may have a V̇O2max in excess of 70 ml/kg/min, whereas in a low-fit person, it may be less than 20 ml/kg/min.
Symbol What Factors Determine V̇O2max?
For muscles to use O2, the lungs must take in air and transfer O2 to the blood. The blood must then be pumped to the muscles. Finally, the muscles must take the O2 from the blood and use it. For healthy individuals, the lungs' ability to get O2 into the blood does not compromise V̇O2max. Likewise, the muscles' ability to extract O2 from the blood is not fully taxed during maximal exercise. Scientists generally agree that, in most healthy people, the primary physiologic factor limiting V̇O2max is the ability of the heart to pump blood to the exercising muscles. This is the reason that V̇O2max is considered a measure of cardiovascular fitness.
Symbol How Is V̇O2max Measured?
To measure V̇O2max, a laboratory must have an exercise device (e.g., treadmill or cycle ergometer), as well as a way to measure the volume of air breathed per minute and the O2 percentage in expired air. Current computer-based systems track breathing volume and O2 percentage continuously and print out the results minute by minute. The highest O2 uptake at the end of the test is considered V̇O2max. Many people do not have access to a laboratory that provides this type of testing. However, there are a number of ways that V̇O2max can be estimated. Equations using physical characteristics such as age, sex, and exercise habits can give a rough estimate of V̇O2max. Estimates of V̇O2max also can be obtained from field tests like the 12-minute run or the timed 1-mile walk. Many gyms offer exercise tests, which estimate V̇O2max based upon a person's heart rate response to a standard exercise routine (e.g., YMCA test). Although these tests provide an estimate, rather than a measure, of V̇O2max, they can be very useful for developing training programs and tracking changes over time.