VO2 max (also maximal oxygen consumption, maximal oxygen uptake, peak oxygen uptake or maximal aerobic capacity) is the maximum rate of oxygen consumption measured during incremental exercise; that is, exercise of increasing intensity. The name is derived from three abbreviations: "V" for volume, "O2" for oxygen, and "max" for maximum. Maximal oxygen consumption reflects cardiorespiratory fitness and endurance capacity in exercise performance.
Relationship to cardiovascular disease and life expectancy
VO2 max is widely used as an indicator of cardiorespiratory fitness. In 2016, the American Heart Association published a scientific statement recommending that cardiorespiratory fitness (CRF), quantifiable as VO2 max, be regularly assessed and used as a clinical vital sign. This statement was based on mounting evidence that lower fitness levels are associated with high risk of cardiovascular disease, all-cause mortality, and mortality rates stemming from various types of cancers. In addition to risk assessment, the AHA recommendation cited the value measuring fitness for validating exercise prescription, physical activity counseling, and improving both patient management and patient health.
VO2 max is expressed either as an absolute rate in (for example) litres of oxygen per minute (L/min) or as a relative rate in (for example) millilitres of oxygen per kilogram of body mass per minute (e.g., mL/(kg·min)). The latter expression is often used to compare the performance of endurance sports athletes. However, VO2 max generally does not vary linearly with body mass, either among individuals within a species or among species, so comparisons of the performance capacities of individuals or species that differ in body size must be done with appropriate statistical procedures, such as analysis of covariance.
Measurement and calculation
Accurately measuring VO2 max involves a physical effort sufficient in duration and intensity to fully tax the aerobic energy system. In general clinical and athletic testing, this usually involves a graded exercise test (either on a treadmill or on a cycle ergometer) in which exercise intensity is progressively increased while measuring:
- ventilation and
- oxygen and carbon dioxide concentration of the inhaled and exhaled air.
VO2 max is reached when oxygen consumption remains at a steady state despite an increase in workload.
Calculation: the Fick equation
VO2 max is properly defined by the Fick equation:
- , when these values are obtained during an exertion at a maximal effort.
- where Q is the cardiac output of the heart, CaO2 is the arterial oxygen content, and CvO2 is the venous oxygen content.
Estimation using submaximal exercise testing
The necessity for a subject to exert maximum effort in order to accurately measure VO2 max can be dangerous in those with compromised respiratory or cardiovascular systems; thus, sub-maximal tests for estimating VO2 max have been developed.
The heart rate ratio method
An estimate of VO2 max is based on maximum and resting heart rates. It is given by:
This equation uses the ratio of maximum heart rate (HRmax) to resting heart rate (HRrest) to predict VO2 max. The researchers cautioned that the conversion rule was based on measurements on well-trained men aged 21 to 51 only, and may not be reliable when applied to other sub-groups. They also advised that the formula is most reliable when based on actual measurement of maximum heart rate, rather than an age-related estimate.
Kenneth H. Cooper conducted a study for the United States Air Force in the late 1960s. One of the results of this was the Cooper test in which the distance covered running in 12 minutes is measured. Based on the measured distance, an estimate of VO2 max [in mL/(kg·min)] is:
where d12 is distance (in metres) covered in 12 minutes.
An alternative equation is:
where d'12 is distance (in miles) covered in 12 minutes.
Multi-stage fitness test
Rockport fitness walking test
Estimation of VO2 max from a timed one-mile track walk incorporating duration in minutes and seconds (, e.g.: 20:35 would be specified as 20.58), gender, age, body weight in pounds (), and heart rate () at the end of the mile. The constant x is 6.3150 for males, 0 for females. BW is in lbs, time is in minutes.
Effect of training
The average untrained healthy male has a VO2 max of approximately 35–40 mL/(kg·min). The average untrained healthy female has a VO2 max of approximately 27–31 mL/(kg·min). These scores can improve with training and decrease with age, though the degree of trainability also varies very widely: conditioning may double VO2 max in some individuals, and will never improve it in others.
In sports where endurance is an important component in performance, such as cycling, rowing, cross-country skiing, swimming and running, world-class athletes typically have high VO2 maxima. Elite male runners can consume up to 85 mL/(kg·min), and female elite runners can consume about 77 mL/(kg·min). Five time Tour de France winner Miguel Indurain is reported to have had a VO2 max of 88.0 at his peak, while cross-country skier Bjørn Dæhlie measured at 96 mL/(kg·min). Dæhlie's result was achieved out of season, and physiologist Erlend Hem who was responsible for the testing stated that he would not discount the possibility of the skier passing 100 mL/(kg·min) at his absolute peak. Norwegian cyclist Oskar Svendsen is thought to have recorded the highest VO2 max of 97.5 mL/(kg·min), when aged 18.
The highest values in absolute terms for humans are often found in rowers, as their much greater bulk makes up for a slightly lower VO2 max per kg. Elite oarsmen measured in 1984 had VO2 max values of 6.1±0.6 L/min and oarswomen 4.1±0.4 L/min. Rowers are interested in both absolute values of VO2 max and in lung capacity, and the fact that they are measured in similar units means that the two are often confused. British rower Sir Matthew Pinsent is reported to have had a VO2 of 7.5 L/min . He also had a lung capacity of 8.5 litres. New Zealand sculler Rob Waddell has one of the highest absolute VO2 max levels ever tested. These VO2max figures are absolute figures (ie the total amount of oxygen metabolised in a minute). Rowers tend to be more interested in absolute VO2max whereas runners are generally more interested in relative VO2 max (ie the total amount of oxygen metabolised per minute per kilogram of body weight).
VO2 max has been measured in other animal species. During loaded swimming, mice had a VO2 max of around 140 mL/(kg·min), Thoroughbred horses had a VO2 max of around 193 mL/(kg·min) after 18 weeks of high-intensity training. Alaskan huskies running in the Iditarod Trail Sled Dog Race had VO2 max values as high as 240 mL/(kg·min). Estimated VO2 max for pronghorn antelope was as high as 300 mL/(kg·min).
The factors affecting VO2 are often divided into supply and demand. Supply is the transport of oxygen from the lungs to the mitochondria (including lung diffusion, stroke volume, blood volume, and capillary density of the skeletal muscle) while demand is the rate at which the mitochondria can reduce oxygen in the process of oxidative phosphorylation. Of these, the supply factor is often considered to be the limiting one. However, it has also been argued that while trained subjects probably are supply limited, untrained subjects can indeed have a demand limitation.
Factors that affect VO2 max are: age, gender, fitness and training, altitude, among others. VO2 max can be a poor predictor of performance in runners due to variations in running economy and fatigue resistance during prolonged exercise. Cardiac output, pulmonary diffusion capacity, oxygen carrying capacity, and the peripheral limitations of muscle diffusion capacity, mitochondrial enzymes, and capillary density are all examples of VO2 max determinants. The body works as a system. If one of these factor is sub-par, then the whole system loses its normal capacity to function properly.
The drug erythropoietin (EPO) can boost VO2 max by a significant amount in both humans and other mammals. This makes EPO attractive to athletes in endurance sports, such as professional cycling. EPO has been banned since the 1990s as an illicit performance-enhancing substance. But by 1998 it had become widespread in cycling and led to the Festina affair as well as being mentioned ubiquitously in the USADA 2012 report on the U.S. Postal Service Pro Cycling Team. Greg LeMond has suggested establishing a baseline for riders' VO2 max (and other attributes) to detect abnormal performance increases.
British physiologist Archibald Hill introduced the concepts of maximal oxygen uptake and oxygen debt in 1922. Hill and German physician Otto Meyerhof shared the 1922 Nobel Prize in Physiology or Medicine for their independent work related to muscle energy metabolism. Building on this work, scientists began measuring oxygen consumption during exercise. Notable contributions were made by Henry Taylor at the University of Minnesota, Scandinavian scientists Per-Olof Åstrand and Bengt Saltin in the 1950s and 60s, the Harvard Fatigue Laboratory, German universities, and the Copenhagen Muscle Research Centre among others.
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- Cardiorespiratory fitness
- Comparative physiology
- Oxygen pulse
- Training effect
- Running economy
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