VO2 max

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.

The measurement of VO₂ max in the laboratory provides a quantitative value of endurance fitness for comparison of individual training effects and between people in endurance training. Maximal oxygen consumption reflects cardiorespiratory fitness and endurance capacity in exercise performance. Elite athletes, such as competitive distance runners, racing cyclists or Olympic cross-country skiers, have a VO₂ max of around 90 mL/(kg·min), while endurance animals, such as race horses or pronghorn antelopes, have VO₂ max values above 200 mL/(kg·min).

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.

Expression

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

Measurement

VO₂ max measurement using instruments on a modern metabolic cart during a graded treadmill exercise test

Gas exchange of VO₂ and VCO₂ during max test. Begin for 3 mins at 60 watts and add 35 watts every 3 minutes until exhaustion

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

Main article: Fick principle

VO₂ max is properly defined by the Fick equation:

$\mathrm {VO_{2}\;max} =Q\times \ (\mathrm {C_{a}O_{2}} -\mathrm {C_{v}O_{2}} )$, when these values are obtained during an exertion at a maximal effort.

where Q is the cardiac output of the heart, C_aO₂ is the arterial oxygen content, and C_vO₂ is the venous oxygen content.

(C_aO₂ – C_vO₂) is also known as the arteriovenous oxygen difference.

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:

${\text{V}}{\ce {O2}}{\text{ max}}\approx {\frac {{\text{HR}}_{\text{max}}}{{\text{HR}}_{\text{rest}}}}\times 15.3{\text{ mL}}/({\text{kg·minute}}).$

This equation uses the ratio of maximum heart rate (HR_max) to resting heart rate (HR_rest) 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.

In around 40-year-old normal weight never-smoking men with no cardiovascular diseases, bronchial asthma, or cancer, the HR_max to HR_rest ratio should be multiplied by approximately 14 to estimate VO₂ max. Every 10 years of age reduces the coefficient by one, as well as does the change in body weight from normal weight to obese or the change from never-smoker to current smoker. Consequently, VO₂ max of 60-year-old obese current smoker men should be estimated by multiplying the HR_max to HR_rest ratio by 10.

Cooper test

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:

$\mathrm {VO_{2}\;max} \approx {d_{12}-504.9 \over 44.73}$

where d₁₂ is distance (in metres) covered in 12 minutes.

An alternative equation is:

$\mathrm {VO_{2}\;max} \approx {(35.97*d'_{12})-11.29}$

where d'₁₂ is distance (in miles) covered in 12 minutes.

Multi-stage fitness test

There are several other reliable tests and VO₂ max calculators to estimate VO₂ max, most notably the multi-stage fitness test (or beep test).

Rockport fitness walking test

Estimation of VO₂ max from a timed one-mile track walk incorporating duration in minutes and seconds ($t$, e.g.: 20:35 would be specified as 20.58), gender, age, body weight in pounds ($BW$), and heart rate in 10 sec ($HR$) 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.

$\mathrm {VO_{2}\;max} \approx 132.853-0.0769BW-0.3877{\text{age}}-3.2649t-0.1565HR+x$

Effect of training

Non-athletes

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 widely.

Athletes

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 max values. Elite male runners can consume up to 85 mL/(kg·min), and female elite runners can consume about 77 mL/(kg·min).

High values in absolute terms for humans may be found in rowers, as their greater bulk makes up for a slightly lower VO₂ max per body weight. Elite oarsmen measured in 1984 had VO₂ max values of 6.1±0.6 L/min and oarswomen 4.1±0.4 L/min. New Zealand sculler Rob Waddell has one of the highest absolute VO₂ max levels ever tested.

Animals

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).

Limiting factors

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.

History

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.

See also

• Anaerobic exercise
• Arteriovenous oxygen difference
• Cardiorespiratory fitness
• Comparative physiology
• VDOT
• Oxygen pulse
• Respirometry
• Training effect
• vVO₂max
• Running economy