V̇O_2max

Essentials of the Most Widely Used Test in Exercise Physiology

Abstract

Learning Objective 

To gain a better understanding of maximal aerobic capacity and physiologic factors that influence it.

In Brief

Maximal oxygen consumption (V̇O_2max) is the most widely obtained measurement in exercise physiology. This article gives a brief historical review of V̇O_2max and discusses the factors that influence it and the current concepts when measuring it.

The single most widely obtained measurement in exercise physiology is maximal oxygen consumption (V̇O_2max). The actual or estimated V̇O_2max is one of the most helpful, meaningful, and motivational physiologic measurements that fitness professionals and personal trainers can track with their clients. The most important factors that influence maximal oxygen consumption are age, gender, heredity, body composition, state of training, and mode of exercise. This article begins with a brief historical review of V̇O_2max, followed by a discussion of factors that influence it, and concludes with current concepts when measuring it.

V̇O_2max, Its Historical Roots, and Why We Measure It

According to ACSM (¹), maximal oxygen consumption (also called maximal oxygen uptake, maximal aerobic power, aerobic capacity, functional aerobic capacity, or simply V̇O_2max) is accepted as the criterion measure of cardiorespiratory fitness. It is the greatest rate at which oxygen can be consumed during exercise or the maximal rate at which oxygen can be taken in, distributed, and used by the body during physical activity. V̇O_2max is the volume used per minute (in scientific notation, a dot sometimes appears over the V to indicate "per unit of time"), O₂ is oxygen, and max represents maximal oxygen uptake. V̇O_2max is usually expressed in relative (uptake relative to body weight) terms as milliliters of oxygen consumed per kilogram of body weight per minute (ml·kg⁻¹·min⁻¹).

The early scientists who measured oxygen consumption during exercise in athletes were probably J. Linhard at the University of Copenhagen (in 1915) and G. Liljestrand and N. Stenstrom (in 1920) at the Karolinska Institute in Stockholm (²). These early exercise physiology researchers established that oxygen consumption increased with running and swimming speed. The first English-speaking scientist to study oxygen consumption during exercise was Nobel laureate Archibald Vivian Hill, professor of physiology at University College, London (²). Hill and colleagues (in 1925) concluded in their pioneering investigations that the oxygen requirement increases continuously as running speed increases; however, it reaches a maximum beyond which no effort can drive it, owing to the limitations of the circulatory and respiratory system (²).

The realization that physical work capacity, V̇O_2max, and cardiovascular fitness are interrelated has resulted in a convergence of athletic performance and medical (clinical) definitions of fitness. From the athletic perspective, cardiorespiratory functions determine V̇O_2max, which in turn determine physical work capacity or fitness. From the medical perspective, increased fitness is associated with decreased risk of disease. Because cardiovascular disease is the greatest threat to the health of individuals in contemporary Western society, medical aspects of fitness is largely concerned with cardiovascular fitness. One of most popular ways of determining cardiovascular fitness is measuring V̇O_2max. Therefore, oxygen consumption is not only a distinctive parameter of metabolism but also a good measure of cardiovascular fitness. In fact, V̇O_2max is so important from both the athletic and medical perspectives, it has emerged as one of the most significant criterion of physical fitness (³).

The Influence of Age Upon V̇O_2max

Aging is an ongoing transformation of body appearance and function, and after maturation it is associated with a gradual reduction in the capacity of all bodily systems. Physiologic and performance measures improve rapidly during childhood and reach a maximum between late adolescence and approximately 30 years of age (⁴).

There are noteworthy differences between children and adults in terms of physiologic responses to exercise, such as with V̇O_2max. For example, both maximal stroke volume (blood pumped by the heart per beat) and cardiac output (blood pumped by the heart per minute) are lower in children compared with adults, because of the child's immature heart. In addition, children have a smaller blood volume and lower hemoglobin (oxygen carrying molecule on red blood cell) concentration than adults (⁵). These differences combine to reduce the maximal oxygen transport capacity of children. Although total maximal oxygen consumption of children (expressed as liters per minute) is much lower than that of adults, when corrected for body weight the V̇O_2max of boys is similar to that found in young men. In addition, young teenage girls often have higher V̇O_2max values (expressed per kilogram of body weight) than young women (⁶). Values of V̇O_2max increase at approximately the same rate in both genders until age 12. After that age, boys continue to increase their relative maximal oxygen uptake until the age of 18, whereas girls show minimal improvement after age 14 (⁷).

A study conducted by Walker et al. (⁸) indicated that during weight-bearing walking and running, V̇O₂ use (ml·kg⁻¹·min⁻¹) of children averaged 10-30% higher than that of adults at a designated submaximal pace. This may be attributed to a lower exercise economy (physical factors affecting energy usage), which is affected by shorter stride lengths and greater stride frequency. In addition, children have lower ventilatory (or breathing) efficiency. These factors make a standard walking or running pace physiologically more challenging for children.

After the age of 30 years, functional capacity declines. This occurs as a natural consequence of aging but is enhanced by an inactive lifestyle (⁴). With sedentary individuals, the decline in functional capacity is characterized by a decrease in maximal oxygen consumption, maximal cardiac output, muscle strength and power, neural function, flexibility, and increased body fat (³). The decline in physical functional capacity attributable to a decline in V̇O_2max results in older individuals working closer to maximal effort when performing some submaximal tasks. Eventually, reductions in physical functional capacity with age lead to increased disability, loss of independence, and reduced quality of life.

It has been shown that with normal aging, V̇O_2max declines approximately 8-10% per decade after age 30 (⁹). These decreases in V̇O_2max with age are extremely variable but may be attributed to a decline in maximal heart rate, stroke volume, fat-free mass, and arteriovenous oxygen difference (³). Remaining physically active as one ages may minimize these physiologic losses.

The Influence of Gender Upon V̇O_2max

The gender differences in V̇O_2max have generally been explained by differences in body composition and hemoglobin concentration (⁴). At puberty, testosterone production in boys increases, leading to larger bones and increased muscle mass. In girls, estrogen secretion increases, broadening the pelvis, stimulating breast development, and increasing the amount of fat in the thighs and hips. These unique sex differences continue into adulthood and contribute to why, in general, men and women differ in size, strength, and athletic performance.

Untrained young adult women generally average about 25% body fat, whereas men average 15% (¹⁰). Therefore, when V̇O_2max is expressed relative to body mass (ml·kg⁻¹·min⁻¹), women generally have approximately 20% lower aerobic capacities than those of their male counterparts (¹⁰). However, expressing aerobic capacity in terms of fat-free mass balances out the gender difference. Men have a 10-14% greater hemoglobin (which carries oxygen in the blood) concentration than women because of higher testosterone levels (¹¹). Women also tend to have smaller lungs and hearts. These differences in the blood's oxygen carrying and transporting capacity enables men to transport more oxygen during exercise, thus potentially increasing their aerobic capacity above that of women (¹¹).

Figure

The Influence of Heredity Upon V̇O_2max

Research to explain the genetic contribution (effect) to individual differences in physiologic and metabolic capacity shows much variability. Bouchard and colleagues (¹²) studied 42 brothers and 66 dizygotic twins of both sexes and reported that the magnitude of the genetic effect upon V̇O_2max is 40% and 50% for maximal heart rate. Currently, most researchers estimate that 25-40% of a person's V̇O_2max is directly attributed to heredity. Thus inherited factors contribute a fairly significant component to maximal aerobic capacity.

The Influence of Body Composition Upon V̇O_2max

The profound effect of body composition upon aerobic capacity has led to the common practice of expressing oxygen consumption in relation to body mass or fat-free mass. Variations in body mass explain nearly 70% of the difference in V̇O_2max scores among individuals (⁴). As discussed earlier in the gender section, the relationship of fat to fat-free mass also is quite influential upon V̇O_2max. Denoting oxygen consumption per unit of lean body mass often negates the difference in V̇O_2max between men and women of similar training status.

The Influence of State of Training Upon V̇O_2max

Improvement in maximal oxygen consumption with training depends upon initial status of fitness and the type of training used. Progressive overload in endurance training results in a maximum increase in V̇O_2max of approximately 20%-30% (³). Studies show that the relative increases in V̇O_2max over an 8- to 26-week period are similar between youth and adults. Of course, greater increases in V̇O_2max are possible if the initial physical fitness of the individual is low.

Declining physical activity appears to be a major factor, along with the loss in fat-free mass and increase in fat mass, leading to the downswing in aerobic power and V̇O_2max in adults and elderly (¹³). Research has shown that the rise in aerobic power with training is just as rapid as its fall without it. Notable improvements in V̇O_2max occur within 3 weeks of beginning vigorous cardiorespiratory training (3 to 4 times per week, moderate to high intensity). In addition, once the desired V̇O_2max is achieved, it is possible to maintain it by reducing the frequency and maintaining the intensity of training.

The Influence of Mode of Exercise Upon V̇O_2max

V̇O_2max may be determined using a variety of exercises that activate large muscle groups, provided the intensity and duration of effort are sufficient to maximize aerobic energy transfer (⁴). The usual exercise modes include treadmill running or walking and stationary cycling. Other forms of testing use bench stepping, tethered or flume swimming (specially designed for swim tank testing), swim-bench ergometry, in-line skating, roller-skating, simulated arm-leg climbing, arm ergometry, and wheelchair exercise.

Lewis et al. (¹⁴) showed that variations in V̇O_2max with different forms of exercise generally reflect the quantity of muscle mass activated. Studies that have determined V̇O_2max for the same subjects during different exercise modes indicate that treadmill exercise usually produces the highest values (¹⁴). For instance, most studies show that V̇O_2max measured on a cycle ergometer (with non-cyclists) is 10-15% less than that measured on a treadmill (³).

Figure

Options for Measuring V̇O_2max

Tests that measure or estimate cardiorespiratory endurance are used to evaluate an individual's capacity to acquire and transport oxygen in the blood to fuel muscle mitochondrial (organelle where cell respiration occurs) respiration. One of the most common tests used for measuring cardiorespiratory endurance capacity is the direct measurement of V̇O_2max from expired gas fractions and ventilation during exercise. A variety of gas analyzing systems have been developed over the years. Today, with advances in computer and microprocessor technology, the trained exercise technician can measure metabolic and physiologic responses to exercise accurately and rapidly.

However, the direct measurement of V̇O_2max requires a fairly extensive laboratory, specialized equipment, and considerable subject motivation to perform a V̇O_2max test. Consequently, laboratory tests are usually impractical for the personal trainer and fitness professional working in fitness facilities, in private studios, and out of clients' homes. In addition, strenuous exercise could prove risky to adults who do not receive proper medical clearance and appropriate safeguards or supervision. These considerations increase the importance of submaximal exercise testing to predict V̇O_2max (⁴).

Although direct measurement of V̇O_2max with computerized metabolic carts is the most valid and reliable marker of cardiorespiratory fitness, because of the challenges noted above and when testing larger groups of people, several researchers have developed statistical (known as regression or multiple regression) equations to predict V̇O_2max. Most often these estimation equations include two or more predictor variables such as resting or submaximal heart rate, age, gender, body weight, and body composition. The choice of prediction versus direct measurement of V̇O_2max is highly dependent upon the purpose of the testing. Research and clinical evaluation requires direct measurement of physiologic capacities. Personal trainers and fitness professionals may best serve their clients and students with the use of administratively easy V̇O_2max prediction tests.

Theory to Practice: Resources for Choosing V̇O_2max Protocols

The assessment of V̇O_2max is an invaluable assessment for aerobic capacity that fitness professionals and personal trainers can use to evaluate and track the progress of students and clients. It also is a motivational tool that may prove to be an incentive for students to continue their cardiorespiratory training. See the list of Recommended Readings to learn how to correctly administer all types of V̇O_2max tests.

Condensed Version and Bottom Line

A chief purpose of exercise testing in most non-clinical settings is to assess the aerobic capacity of healthy adults for developing aerobic exercise prescriptions and evaluating progress of exercise programs. Consequential factors that influence maximal oxygen consumption are age, gender, heredity, body composition, state of training, and mode of exercise. Because of the expense of equipment, the time required, and the risks associated with maximal exercise, direct measurement of V̇O_2max is not practical for fitness testing in health and fitness clubs or when testing large groups. Several submaximal exercise protocols have been developed to accurately estimate V̇O_2max from heart rate and other independent variables, such as age, gender, resting or exercise heart rate, and body weight. Some excellent resources are available and listed in the Recommended Readings for fitness professionals and personal trainers to choose aerobic capacity testing protocols for use with their students and clients.