The cardiorespiratory fitness level of an individual can be defined as the highest rate at which oxygen is taken up and consumed by the body during incremental but intense exercise, such as when on a motorized treadmill or a cycle ergometer. The gold standard measurement for cardiorespiratory fitness is the maximal oxygen uptake (VO2max), where V is the abbreviation for volume, O2 for oxygen, and max for maximum. This measurement can be directly obtained from gas exchange measurement during maximal exercise testing or estimated from the results of submaximal or maximal exercise tests.
The Fick equation can be used to properly define VO2max: VO2max 1/4 Q (CaO2 — CvO2max); where Q refers to cardiac output, CaO2 to the content (C) of oxygen in arterial (a) blood, and CvO2max to the maximum content of oxygen in venous (v) blood. The terms within parentheses are called, as a group, the arterovenous oxygen difference.
VO2max is also sometimes called maximal oxygen consumption, peak oxygen uptake, and maximal aerobic capacity. Terms frequently used when referring to cardiorespiratory fitness include cardiovascular fitness, fitness, aerobic power, aerobic fitness, and peak metabolic equivalents (METs). The units for reporting cardiorespiratory fitness levels are commonly either in an absolute rate (liters [of oxygen] per minute; abbreviated L/min) or a relative rate (milliliters of oxygen per kilogram of body mass per minute; mL/kg/min).
Traditionally, cardiorespiratory fitness has been viewed as a measure of overall health, with studies consistently revealing an inverse relationship between VO2max values and risk of death from all causes. Cardiorespiratory fitness can be used to accurately prescribe exercise intensity. Typically, intensity of exercise is prescribed as a range with a desirable lower and upper limit target workload established. These lower and upper limits can be defined in terms of a percentage of VO2max or peak METs.
Compendiums of physical activity or metabolic calculations are then used to identify specific activities/exercises that fall within the target workload. Additionally, cardiorespiratory fitness has long been considered necessary for success in endurance-related events. A classic study conducted at Ball State University (Muncie, Indiana) in the 1960s confirmed the importance of VO2max for endurance performance. The results of the study demonstrated a strong correlation between VO2max values and 10-mile (16-kilometer) run times. In a group of individuals with comparable levels of VO2max, however, various other factors may be better predictors of performance or be used in conjunction with VO2max to produce a more accurate measurement.
Maximal oxygen uptake may be determined using numerous exercise modes that activate large groups of muscle mass, provided the intensity of effort and protocol duration are sufficient to maximize aerobic energy transfer. Treadmill exercise and cycle ergometry are the most common modes utilized for VO2max testing. Other types of exercise modes, have been employed to achieve VO2max, including bench stepping; free, tethered, and flume swimming; swim-bench ergometry; inline skating; cross-country skiing; rollerskating; simulated arm-leg climbing; arm crank and wheelchair exercises; and rowing ergometry.
Regardless of mode, variations in VO2max typically reflect the quantity of muscle mass activated during exercise. Treadmill exercise generally elicits the highest VO2max values for the same untrained and/or recreationally trained individual performing different exercise mode VO2max tests, although subject training specificity influences the magnitude of VO2max values attained among different exercise modes. Elite-trained cyclists have similar treadmill and cycle ergometry VO2max values. Likewise, untrained and trained collegiate swimmers achieve VO2max values during swimming versus treadmill tests of around 80% and 90%, respectively. Elite swimmers attain similar or greater VO2max values.
Although genetics (the ability to pass down traits from generation to generation) explains a considerable proportion of the variation in cardiorespiratory fitness, it is well known that VO2max is higher in the majority of individuals who participate in regular and properly structured exercise programs. Physiological processes involving the cardiorespiratory system (heart, lungs, and blood vessels) and peripheral physiological functions (i.e., oxygen extraction at the skeletal muscle level) contribute to the overall magnitude of VO2max. The ability of the cardiovascular system to transport oxygen to exercising skeletal muscles is referred to as the central component of VO2max. The role of this component is to transport oxygen from the atmosphere and deliver it to the muscles where it is used in mitochondrial respiration to produce adenosine triphosphate (ATP), which transports energy within cells for metabolism.
Oxygen delivery may be limited by maximal cardiac output, pulmonary diffusion, and blood volume and flow. The peripheral aspect of VO2max involves the capacity for the exercising skeletal muscle to extract and use the oxygen delivered by the cardiovascular system. Factors that may hinder VO2max are skeletal muscle diffusion capacity, capillary density, and mitochondrial enzyme levels.
The risk of acute myocardial infarction or sudden death during vigorous exercise is higher in older adults compared to their younger counterparts. The higher prevalence of cardiovascular disease in older adults is responsible for this elevated risk. Similarly, the risk of cardiac events associated with maximal exercise testing, during which cardiorespiratory fitness is frequently determined, is related to the incidence of cardiovascular disease. The overall risk remains relatively low; it has been reported that 6 cardiac events per 10,000 exercise tests can occur.
Measures taken to lower this risk include sufficient pretesting screening to identify individuals with health conditions that could be affected by exercise testing and determining an individual's risk stratification as either low, moderate, or high. The degree of monitoring and supervision required before, during, and after the maximal exercise test can be adjusted to suit the individual's risk stratification.
The most accurate method for determining individual cardiorespiratory fitness is through the measurement of VO2max. It is also considered the oldest measure to determine human performance. The measurement of VO2max is generally accomplished during an incremental exercise test by using indirect calorimetry. Various protocols may be used for maximal exercise testing; however, the specific increases in workload at each stage vary considerably, depending on the individual. In fitter individuals and athletes, it is common for the workload to be increased by two to three METs each stage. In contrast, for lower fit individuals or individuals with health complications, such as cardiac disease, the workload increment for each stage is 0.5 to 1.0 METs per stage.
Either continuous or discontinuous protocols may be employed during maximal exercise testing. Continuous protocols call for the workload to be progressively increased over the course of the test until the individual being tested fatigues. Discontinuous protocols are intermittent in nature with workload stages separated by rest periods; each successive stage in a discontinuous protocol has a higher workload. The duration of each stage in either a continuous or discontinuous protocol stage can last from a few seconds to several minutes.
Oxygen consumption is determined by collecting and measuring expired gases (indirect calorimetry) throughout maximal exercise testing. The relationship between oxygen consumption and workload is linear across the early part of an exercise test, but the relationship becomes more curvilinear as an individual approaches fatigue. Near and at fatigue, many individuals exhibit a plateau in oxygen consumption; the plateau in VO2max is one of the criteria frequently used in research to confirm VO2max has been achieved. This phenomenon, however, is not a consistent finding across all populations.
Other criteria used to confirm the attainment of VO2max include a heart rate within 10 to 15 beats per minute of age-predicted maximum, a respiratory exchange ratio value exceeding 1.10, and rating of perceived exertion (RPE) values of 18 or 19 on the 6–20 Borg Scale Rating of Perceived Exertion, one of the common tools used to assess fatigue during physical exercise.
Although it is the most precise method, direct measurement of VO2max via indirect calorimetry may be unsuitable and unfavorable because the calorimetry equipment necessary for the measurement of VO2max is expensive, requires specially trained laboratory personnel, and calls for maximal subject effort to ensure accurate testing results. Furthermore, the VO2max testing procedures are time consuming, making the assessment less than ideal when testing a large number of individuals.
Generally, exercise professionals select either treadmill exercise or cycle ergometry for submaximal exercise-testing assessments. Other exercise modes and field tests have also been used in estimating VO2max, including bench stepping, rowing ergometry, stair climbing, track walking, track running, and swimming. Among the most common tests for predicting VO2max are the Balke 15-minute run, the Bruce treadmill test, the Cooper 12-minute run, and the Rockport 1-mi. (1.6-km) walk test.
Low cardiorespiratory fitness is exceptionally modifiable. Improvements in cardiorespiratory fitness are generally more favorable when compared to other risk factors. It has been has reported that, following three months of aerobic training, the typical improvement in cardiorespiratory fitness is between 10% and 30%. These findings are comparable for both previously sedentary adults and older adults.
These improvements in cardiorespiratory fitness pay big dividends in terms of long-term health. Research suggests a 15% reduction in mortality for a 10% improvement in cardiorespiratory fitness.
Cardiorespiratory fitness has been coined the ultimate health outcome. Research has reported that low cardiorespiratory fitness is associated with premature mortality. Low levels of cardiorespiratory fitness are also linked to an increased risk of developing cardiovascular disease, which also increases the risk of mortality. Low cardiorespiratory fitness accounts for more deaths in both men and women than any other cardiovascular disease risk factor, including smoking, obesity, hypertension, and high cholesterol.
In addition, individuals with poor fitness are less likely to be able to perform activities of daily living. Cardiorespiratory fitness tends to decline by approximately 10% per decade after the age of 25 years. Undeterred, these reductions in physiological functional capacity can eventually result in loss of independence.
An individual's cardiorespiratory fitness level depends on variables including age, sex, race, training status, and genetics. Typical values for VO2max with specific reference to age and sex are available at the Sports Fitness Advisor website ( http://www.sportfitness-advisor.com/VO2max.html ). The proportion of low, moderate, and high cardiorespiratory fitness differs among race and race-sex groups. Low cardiorespiratory fitness is most prevalent in non-Hispanic black women. Nearly one of every three women is estimated to have low levels of fitness, whereas slightly more than one of every ten non-Hispanic white women is estimated to have low fitness levels. Comparatively, high cardiorespiratory fitness is greatest among non-Hispanic men and lowest amongst nonHispanic black women.
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Lance C. Dalleck, BA, MS, PhD
Revised by William A. Atkins, BB, BS, MBA