Although there is a well-known association between cardiorespiratory endurance and health outcomes in adults, the measurement of cardiorespiratory endurance in youth and of its relationship to health outcomes is relatively new to the literature. The committee's review revealed clear relationships between cardiorespiratory endurance and several health risk factors, including adiposity and cardiometabolic risk factors. Other studies point to a potential relationship between cardiorespiratory endurance and other, less studied risk factors, such as those related to pulmonary function, depression and positive self-concept, and bone health. Limitations of the studies reviewed by the committee relate mainly to the design of the studies, specifically the lack of analysis of the independent effect of cardiorespiratory endurance on health. A paucity of studies explore the effects of several potential modifiers, such as age, gender, body composition, maturation status, and ethnicity, on performance on the various tests of cardiorespiratory endurance. While such effects have been suggested in the past, the committee could draw no conclusions based on the evidence reviewed. The cardiorespiratory endurance tests most commonly associated with a positive change in a health marker are the shuttle run and tests conducted with the treadmill and cycle ergometer. Available evidence indicates that these three types of tests demonstrate acceptable validity and reliability. The health markers most frequently assessed are related to body weight or adiposity and cardiometabolic risk factors. Based on its relationship to health, as well as its reliability, validity, and feasibility, a timed or progressive shuttle run, such as the 20-meter shuttle run, is appropriate for measuring cardiorespiratory endurance in youth. If the test is to be administered in a setting with space limitations (e.g., a mobile test center for a national survey), a submaximal treadmill or cycle ergometer test should be used. The shuttle run is advantageous when there are time constraints and when cost may be a problem, such as in schools and other educational settings. Although the evidence for a relationship between distance/timed runs and health is insufficient at this time, this type of test is valid and reliable and could be an alternative in schools and other educational settings. Until more data are collected with which to establish criterion-referenced cut-points (cutoff scores), interim cut-points corresponding to the lowest 20th percentile of the distribution of cardiorespiratory endurance should be used to interpret results of all cardiorespiratory endurance tests and to determine whether individuals are at risk of negative health outcomes. Cardiorespiratory endurance has been recognized as a key component of physical fitness throughout the history of the field. This chapter presents the committee's review of the scientific literature that explores the relationship between specific field tests of cardiorespiratory endurance and health outcomes in youth. The committee's recommendations for the selection of fitness tests are based primarily on an extensive review of the literature provided by the Centers for Disease Control and Prevention (CDC) described in Chapter 3. In making its recommendations, the committee considered not only the evidence for a relationship to health, but also the scientific integrity (reliability and validity) of putative health-related tests, as well as the administrative feasibility of implementing these tests. After presenting these results, the chapter offers guidance for setting interim cut-points (cutoff scores) for the selected tests. The final section presents conclusions. Recommendations regarding specific tests for measuring cardiorespiratory endurance for national surveys and in schools and other educational settings are found in Chapters 8 and 9, respectively. Future research needs are addressed in Chapter 10. Cardiorespiratory endurance is the ability to perform large-muscle, whole-body exercise at moderate to high intensities for extended periods of time (Saltin, 1973). Numerous terms have been used to denote this component of physical fitness, including aerobic fitness and aerobic capacity. These terms are essentially synonymous with cardiorespiratory endurance, which is the term used in this report. Forms of exercise that depend on cardiorespiratory endurance include vigorous distance running, swimming, and cycling. This fitness component also affects a person's ability to perform, without undue fatigue, less intense, sustained whole-body activities, such as brisk walking, stair climbing, and home chores. People with good levels of cardiorespiratory endurance can perform large-muscle, whole-body exercise at high intensity for at least moderate durations before experiencing fatigue, and they can comfortably perform light- to moderate-intensity exercise for extended periods. Cardiorespiratory endurance depends on the body's ability to support skeletal muscle activity through high rates of aerobic metabolism. The ability to produce energy at high rates through aerobic metabolism during exercise depends on three physiologic functions: (1) transport of oxygen from the atmosphere to the active muscles through the actions of the cardiorespiratory system, (2) consumption of oxygen in the aerobic metabolic process in the cells of the active muscles, and (3) removal of waste products. People with high levels of cardiorespiratory endurance typically have highly functional cardiorespiratory systems (i.e., heart, lungs, blood, blood vessels), and their skeletal muscles are well adapted to the use of oxygen in aerobic metabolism. Higher levels of cardiorespiratory endurance have been associated with a wide range of health benefits in adults, including a lower risk of cardiovascular disease (Arraiz et al., 1992; Blair et al., 1989; Sandvik et al., 1993), type 2 diabetes (Colberg et al., 2010), hypertension (Blair et al., 1984), certain cancers (Oliveria et al., 1996), and premature mortality from all causes (Blair et al., 1989, 1993, 1995). The linkage between cardiorespiratory endurance and health in youth is discussed later in the chapter. The gold standard measure of cardiorespiratory endurance is maximal aerobic power (VO2max)—the greatest rate at which a person is able to consume oxygen during sustained, exhaustive exercise. In the laboratory, VO2max is typically measured while a person performs maximal, graded exercise on a treadmill or cycle ergometer. VO2max can be expressed in terms of liters of oxygen consumed per minute (l/min), or the values can be normalized for differences in body size and expressed as milliliters of oxygen consumed per kilogram of body weight per minute (ml/kg/min). VO2max is known to be a key physiologic determinant of cardiorespiratory endurance and has typically been used as the criterion measure in the validation of field measures of cardiorespiratory endurance. Many field measures of this fitness component have been studied and used in various fitness test batteries around the world (see Table 2-6 in Chapter 2). The most commonly used field tests involve distance/timed runs of varying length and graded-pace shuttle runs. Various types of distance/timed runs have been used to measure cardiorespiratory endurance in fitness test batteries since the advent of large-scale fitness testing in the post–World War II era. The tests vary in structure, some being based on a distance limitation in which performance is measured as time required to complete the specified distance (often 1 or 1.5 miles), and others on a time limitation in which performance is measured as the distance covered in the specified time (often 9 or 12 minutes). While runs as short as 600 yards were used in early versions of fitness test batteries, distance runs using the 1 mile or 9-minute format have been most common since the 1970s. Shuttle runs measure cardiorespiratory endurance when an individual runs to and from two different points, usually around 20 meters apart, at a set pace. The progressive aerobic cardiovascular endurance run (PACER), a variation on the shuttle run, is a maximal cardiorespiratory endurance test in which lines are placed 15 or 20 meters apart, and the participant runs repeatedly between the two lines within prescribed times. The time decreases periodically while the distance remains the same until the participant cannot run fast enough to reach the finish line in the prescribed time. Alternatively, some fitness surveys use quasi-laboratory tests (i.e., those that measure VO2max but can be conducted in the field). These tests involve the performance of graded, submaximal exercise on a treadmill or cycle ergometer. As noted, the evidence for the committee's recommendations for fitness tests for cardiorespiratory endurance was derived mainly from an extensive review of the literature provided by the CDC, which selected studies measuring the associations between various components of fitness and health. The CDC search strategy and data extraction procedures are described in detail in Chapter 3. For cardiorespiratory endurance, the CDC screened 4,795 studies; of these, only 260 longitudinal, experimental, and quasi-experimental studies satisfied the CDC's search criteria for further consideration. Of this subset, the committee reviewed 47 experimental studies, 24 longitudinal prospective studies, and 29 quasi-experimental studies. In addition to this review, the committee considered the integrity and the feasibility of the tests in a stepwise process, also described in Chapter 3. This section describes the committee's evaluation of the relationship between specific tests of cardiorespiratory endurance and health; the subsequent sections address the integrity and feasibility of the tests. The committee selected only studies of high quality for review (see Chapter 3 for a list of general selection criteria). Studies for in-depth review were limited to those with designs appropriate to the committee's purpose, that is, only experimental, longitudinal, and quasi-experimental studies. (Cross-sectional studies or experimental studies with no control were excluded.) An additional literature search (utilizing search terms similar to those of the CDC review) was undertaken to cover studies published in 2011. The set of studies was further narrowed on the basis of the following criteria. First, the study provided important evidence linking a particular candidate measure of cardiorespiratory endurance—distance/timed run, shuttle run, treadmill, cycle ergometer—to a positive health outcome, marker, or risk factor in four categories (adiposity, metabolic risk, cognitive, and other). Studies also were categorized as presenting direct or associational evidence. A study was defined as presenting direct evidence when a change in a fitness measure resulted in a positive change in a health risk factor or outcome, and when the study used appropriate controls and statistical methods to analyze the independent effect of the intervention and potential confounders. When making its recommendations, the committee also considered associational evidence (i.e., from studies that did not consider all possible confounders) as it may constitute supporting evidence. In general, studies were excluded based on the following criteria: poor study design (e.g., no control population), inappropriate population (e.g., obese children with complex health issues), lack of power to detect changes (e.g., small sample size), inability to assess the independent effect of a dietary intervention or other important known confounder, or insufficient change in the fitness measure of interest. The following sections review the strength of the evidence for a relationship between health outcomes and the four categories of fitness tests for cardiorespiratory endurance (distance/timed run, shuttle run, treadmill, and cycle ergometer). The discussion is organized by test because, in contrast with measures for other fitness components (i.e., musculoskeletal fitness and flexibility), the committee found sufficient evidence linking specific cardiorespiratory endurance tests to health markers, particularly cardiometabolic risk factors and body composition. The strength of the evidence is categorized as sufficient or insufficient based on the number of studies linking a measure to a particular category of health markers, the study designs (evidence from experimental and longitudinal studies having more weight than that from quasi-experimental studies), and the statistical significance of the association. The selected longitudinal, experimental, and quasi-experimental studies are summarized in Tables 5-1, 5-2, and 5-3, respectively. For each study, the tables include (1) the fitness test(s) used, (2) the health outcomes/markers examined, (3) the size and characteristics of the sample, and (4) a summary of the results and the quality and level of the evidence. The committee identified two experimental, one longitudinal, and one quasi-experimental quality studies that utilized various distance or timed runs to measure cardiorespiratory endurance and its association with health risk factors. There were fewer studies in this category of fitness tests than in the other categories. The distances and times varied, and the tests were primarily school based (Chang et al., 2008; Mota et al., 2009; Sidiropoulou et al., 2007). The studies reviewed also varied in their purposes, and none were designed specifically to answer the questions the committee was tasked to address. One high-quality experimental study examined the associations between an intervention and a change in fitness (as measured by the specified distance run), adiposity, or cardiometabolic risk factors (Chang et al., 2008). The authors implemented school-based interventions and found increases in physical fitness, along with decreases in adiposity and improvements in cardiometabolic risk factors. This study did not examine in more depth whether the improvements in cardiometabolic risk factors were independently associated with improvements in fitness and/or adiposity. One longitudinal study found associations between baseline fitness and gain in body mass index (BMI) over time, but did not find an association between changes in fitness and changes in BMI over time (Mota et al., 2009). Other studies included only a special population (Sidiropopulou et al., 2007). Distance run protocols were used in children aged 7 and older, although the majority of studies were of youth aged 10 and older. The influence of gender on the relationship between fitness and health risk factors was not determined. Only one study specifically examined obese children (Chang et al., 2008). Although running is a weight-bearing activity, these studies did not adjust for BMI, which often was the major health outcome of interest (Chang et al., 2008; Mota et al., 2009). As with the other categories of tests discussed below, these studies overall found that results of distance/timed run tests used to measure fitness in youth corresponded to health risk factors, especially body fatness and cardiometabolic risk, in youth. The committee identified three experimental, six longitudinal, and one quasi-experimental studies that used various shuttle run testing protocols to measure cardiorespiratory endurance and health outcomes. The protocols included the 6-minute, 20-meter shuttle run and various 20-meter multistage incremental tests. The majority of associations were found with adiposity (as measured by BMI), perhaps because many of the studies took place in a school setting, where it may be difficult to take more invasive measures of health risk factors, such as blood lipids (Aires et al., 2010; Chen et al., 2007; Kim et al., 2005; Martins et al., 2009). One high-quality experimental study showed improvements in cardiorespiratory endurance in schools randomized to a comprehensive physical activity intervention (Reed et al., 2008). These students also showed improvements in blood pressure; however, there were no detectable changes in BMI. Blood cardiometabolic risk factors also were measured in a smaller subset of children, but changes in these risk factors were not significant. Similarly, Murphy and colleagues (2006) found that improvements in cardiorespiratory endurance corresponded to improvements in bone ultrasound results independent of any changes in BMI after a 6-month physical activity intervention. Longitudinal studies by Martins and colleagues (2009) and Kim and colleagues (2005) demonstrated an inverse relationship between baseline cardiorespiratory endurance and increasing BMI and incidence of overweight over 1 year and 5 years, respectively. A few studies also found that improved performance on the shuttle run resulted in improvements in cardiometabolic risk factors (total cholesterol, low-density lipoprotein [LDL], high-density lipoprotein [HDL], and triglycerides) and blood pressure (Puder et al., 2011; Reed et al., 2008). While Puder and colleagues (2011) found that low baseline fitness was associated with an increase in homeostasic model assessment-insulin resistance (HOMA-IR) level over 1 year, changes in fitness over this time and the corresponding relationship to HOMA-IR were not examined and likely would have been difficult to detect with the study's small sample size (N = 83). In general, there were relatively few studies measuring cardiometabolic risk factors, and the relationships to health markers were not as strongly supported by the study design (e.g., the nonoptimal statistical approach of Martins et al. [2009]). The shuttle run often is used as a measure of cardiorespiratory endurance in the school setting because it requires no special equipment, and training the administrators of the test is relatively easy. As the shuttle run is typically school based, studies of this type of test examined children aged 4-17. Many studies examined results by gender, but only Martins and colleagues (2009) specifically state that gender did not impact the relationship between fitness and BMI. Puberty was self-assessed in two studies (Martins et al., 2009; Puder et al., 2011), only one of which controlled for this factor (Puder et al., 2011) (an ommision that is understandable given that collecting data on pubertal status may be difficult in a school-based environment, where the majority of shuttle run testing takes place). Most of these studies also were inclusive of all weight categories and races/ethnicities and did not specifically examine the potential impact of these factors on the relationship between fitness and health risk. Two quality experimental trials demonstrated improvements in fitness and corresponding improvements in bone health (Murphy et al., 2006) and blood pressure (Reed et al., 2008). Several quality studies also examined the longitudinal relationship between fitness as measured by the shuttle run and changes in adiposity in schoolchildren over time. Overall, the strongest established relationships between cardiorespiratory endurance as assessed by the shuttle run and health markers were with adiposity as measured by BMI. The committee identified six experimental, four longitudinal, and one quasi-experimental quality studies that used various treadmill protocols (both maximal and submaximal) to measure VO2max and associated health risk factors. Longitudinal studies demonstrated a strong link between changes in cardiorespiratory endurance as measured by treadmill testing and changes in adiposity measures such as BMI, waist circumference, and adiposity (as measured by skinfold and dual-energy X-ray absorptiometry [DXA]) (Byrd-Williams et al., 2008; Eisenmann et al., 2005; Johnson et al., 2000; Twisk et al., 2000). A strength of the longitudinal studies examined is that fitness levels in adolescence are related to body composition and total cholesterol levels in young adulthood (Eisenmann et al., 2005; Twisk et al., 2000). Twisk and colleagues (2000) showed that an increase in VO2max over this period was positively associated with a healthy cardiovascular risk profile in adulthood. Controlling for body fatness, the authors demonstrated that fitness had an independent effect on the ratio of total cholesterol to HDL. Another longitudinal study spanning adolescence to adulthood (mean age 26) found that time to reach exhaustion on the treadmill in adolescents and the change in this time experienced from adolescence into adulthood were correlated with both adult body fatness and changes in body fatness from adolescence to adulthood, respectively (Eisenmann et al., 2005). However, this study did not demonstrate this positive relationship with adult risk factors for cardiovascular disease and may have been limited by its relatively small sample size (N = 48) (Eisenmann et al., 2005). Other longitudinal studies used simply baseline VO2max in relationship to changes in adiposity (Byrd-Williams et al., 2008; Johnson et al., 2000) without examining the change in VO2max over this period. Still, these studies highlight the important relationship between cardiorespiratory endurance and body fatness at a later time. Experimental studies conducted in schools and laboratories also demonstrated positive relationships between changes in cardiorespiratory endurance and adiposity measures, as well as cardiometabolic risk, blood pressure, and executive functioning (Barbeau et al., 2002; Carrel et al., 2005; Davis et al., 1985; Farpour-Lambert et al., 2009; Gately et al., 2005). Several studies used well-controlled maximal treadmill test protocols (Farpour-Lambert et al., 2009; Walther et al., 2009), while others utilized submaximal protocols that involved use of a metabolic cart for measurement of gas exchange (Barbeau et al., 2002; Carrel et al., 2005; Davis et al., 1985). Cardiorespiratory endurance and body fatness are highly interrelated, and both are factors in the risk for cardiovascular disease. Therefore, when examining associations of changes in cardiorespiratory endurance with body fatness and cardiometabolic risk factors, it is difficult to determine the independent causes and effects. Few of the well-controlled studies using the treadmill examined these effects independently (Twisk et al., 2000). After a 1-year school-based randomized physical activity intervention, Walther and colleagues (2009) demonstrated an increase in VO2max and endothelial progenitor cells in youth without a corresponding decrease in adiposity. Even though fat mass has an inverse relationship to weight-bearing cardiorespiratory endurance measures, several studies did specifically examine overweight and obese youth using a treadmill protocol and observed measurable changes in cardiorespiratory endurance (Barbeau et al., 2002; Byrd-Williams et al., 2008; Carrel et al., 2005; Davis et al., 1985; Farpour-Lambert et al., 2009). Some studies also adjusted for maturation stage (Byrd-Williams et al., 2008; Johnson et al., 2000), and some examined the interaction between specific ethnicities and the relationship of fitness to health markers (Byrd-Williams et al., 2008; Johnson et al., 2000). Although Johnson and colleagues did not find an effect of ethnicity, an interaction between gender and the relationship of baseline fitness to changes in adiposity was observed in Hispanic boys, but not Hispanic girls (Byrd-Williams et al., 2008). Overall, the studies reviewed indicate that, among the health markers measured, adiposity and cardiometabolic risk factors show the strongest evidence for an association with treadmill-measured cardiorespiratory endurance. The committee identified four experimental, three longitudinal, and three quasi-experimental quality studies that utilized various cycle ergometry tests to measure cardiorespiratory endurance and health outcomes. The protocols in these studies varied widely, with the majority utilizing maximal exercise tests. Among those studies, several, both longitudinal and experimental, were of high quality (Ben Ounis et al., 2010; Janz et al., 2002; Kelly et al., 2004; Ortega et al., 2011). The weight of the evidence for an association between cardiorespiratory endurance as measured by cycle ergometry and health risk factors was particularly strong for measures of adiposity (BMI, waist circumference, percent body fat, fat mass), cardiometabolic risk factors (including total cholesterol, HDL, LDL, triglycerides, insulin resistance, glucose, and vascular stiffness), and blood pressure (Ben Ounis et al., 2010; Dunton et al., 2007; Janz et al., 2002; Kelly et al., 2004; Lee et al., 2010; McMurray et al., 2008; Ortega, et al., 2011; Stella et al., 2005). The strongest evidence for a relationship between cycle ergometry test results and health risk factors is found with measures of adiposity and cardiometabolic risk. The strength of the evidence in cycle ergometry studies appears to be similar to that in the treadmill studies discussed above. Cycle ergometry studies include several well-executed experimental studies (Ben Ounis et al., 2010; Kelly et al., 2004; Wong et al., 2008) and two longitudinal studies that occurred over a period of 4-6 years (Janz et al., 2002; Ortega et al., 2011). Kelly and colleagues (2004) found improvements in cardiorespiratory endurance, HDL cholesterol, and endothelial function following exercise training in overweight youth without changes in adiposity, demonstrating the independent effects of changes in fitness on these cardiovascular disease markers (Kelly et al., 2004). Others found positive effects of exercise and improvements in cardiorespiratory endurance on cardiovascular risk markers and adiposity, but were unable to determine whether these were independent effects (Ben Ounis et al., 2010; Wong et al., 2008). In longitudinal studies, cardiorespiratory endurance improvements over 6 years from childhood to adolescence were associated with a lower risk of becoming overweight/obese during adolescence (Ortega et al., 2011). Adjustment was made for confounding by baseline BMI, and no interactions by gender were identified. Similarly, Janz and colleagues (2002) demonstrated the relationship between changes in cardiorespiratory endurance over 5 years and health markers at the 5-year point. The authors demonstrated a relationship between changes in cardiorespiratory endurance and ratio of total cholesterol to HDL, LDL, and adiposity measures. Jekal and colleagues (2009) designed their quasi-experimental study to evaluate the effect of a 12-week exercise program (Jekal et al., 2009). Although the study did not include a control group, measurements before and after the intervention in this small study demonstrated a significant association of cardiorespiratory endurance with fatness and risk factors for cardiovascular disease. Another study examined the effect of 12 weeks of aerobic training on plasma visfatin and insulin resistance in normal-weight and obese female adolescents; unfortunately, analyses were not conducted to elucidate whether the improvement in these risk factors was due to weight loss or improvements in fitness or both (Lee et al., 2010). The results suggest that the interaction between weight and cardiorespiratory endurance is important, even though the authors did not analyze the independent contributions of each of these variables to insulin resistance. Two studies identified other health risk factors—depression and positive self-concept—that were mitigated by increased fitness (Dunton et al., 2007; Stella et al., 2005). Of interest is the fact that the population of Dunton and colleagues (2007) included various ethnic groups, even though there was no analysis of ethnic origin as a potential modifier of the relationship between performance on the test and self-concept. As with the treadmill tests, few cycle ergometry studies evaluated interactions with modifiers such as age or gender. Studies utilizing cycle ergometry have focused mainly on children aged 10 and older, with one longitudinal study examining those aged 7-10 (McMurray et al., 2008). Given the non-weight-bearing nature of cycle ergometry tests, body weight is not a modifying factor for these tests; a number of cycle ergometry tests were conducted with overweight/obese children (Ben Ounis et al., 2010; Kelly et al., 2004; Lee et al., 2010; Stella et al., 2005). In sum, the strength of the evidence from the use of cycle ergometry to measure cardiorespiratory endurance and associated health risk factors lies with adiposity, cardiometabolic risk factors, and blood pressure. Among the four types of cardiorespiratory endurance tests evaluated in these studies, all but the distance/timed run tests showed significant relationships to health risk factors, specifically adiposity measures and cardiometabolic risk factors. The studies considered of highest quality for each of the tests were as follows:
The most important limitation of other studies reviewed was the lack of in-depth examination of confounders, specifically whether improvements in cardiometabolic risk were independently associated with improvements in cardiorespiratory endurance or were also due to a decrease in adiposity that is often experienced when individuals participate in physical activity. In several of the experimental studies, concurrent changes were seen in adiposity measures, cardiometabolic risk factors, and fitness (Barbeau et al., 2002; Ben Ounis et al., 2010; Carrel et al., 2005; Farpour-Lambert et al., 2009; Wong et al., 2009). In such cases, it is impossible to determine the independent effects of improvements in cardiorespiratory endurance on cardiometabolic risk factors. The committee also considered whether studies examined the effects of various modifiers (e.g., age, gender) on the relationship between cardiorespiratory endurance and health. Many of the studies reviewed were of good quality but were not designed with these questions in mind. Only a subset of the studies specifically examined potential modifiers of the relationship between cardiorespiratory endurance and health risk factors. For example, studies included potential differences by gender (Barbeau et al., 2002; Johnson et al., 2000; Martins et al., 2009; Puder et al., 2011; Twisk et al., 2000), race/ethnicity (Barbeau et al., 2002; Byrd-Williams et al., 2008; Johnson et al., 2000; Tremblay and Lloyd, 2010), age/maturation stage (Byrd-Williams et al., 2008; Johnson et al., 2000; Puder et al., 2011), weight status (Twisk et al., 2000), and training status (Sidiropoulou et al., 2007). In most cases, however, studies included no analysis of these factors as modifiers of performance or as modifiers of the effect of performance on health. In general, the committee considered experimental studies to be of higher quality because, by their design, such studies can demonstrate causes and effects in a more direct manner than is possible with other designs. Yet, it should be noted that many of the experimental studies measured health risk factors using methods requiring invasive (i.e., blood draws) and/or precise (e.g., body composition by DXA) techniques (Barbeau et al., 2002; Ben Ounis et al., 2010; Farpour-Lambert et al., 2009; Slaughter et al., 1988; Walther et al., 2009). Likewise, many studies measured fitness with laboratory protocols using either a treadmill or cycle ergometer and with small sample sizes (Ben Ounis et al., 2010; Farpour-Lambert et al., 2009; Kelly et al., 2004; Nourry et al., 2005). An experimental study using precision measurements would be more likely to detect relationships between changes in fitness and changes in adiposity and health risk factors, even with small sample sizes. As discussed above, a number of tests have been used to measure cardiorespiratory endurance in studies linking this component of physical fitness to indicators of health in youth. The most common tests used in large-scale surveys and youth fitness test batteries are distance/timed runs, shuttle runs to volitional fatigue, and graded-exercise heart rate extrapolation tests (treadmill or cycle ergometer). The validity and reliability of these tests have been studied extensively, and several authors have summarized the literature on their psychometric properties. In reviewing this literature, the committee relied primarily on review articles identified through searches using PubMed and Web of Science with the following keywords: fitness assessment, fitness testing, validity, reliability, children, and adolescents. As necessary, the committee reviewed original research articles identified as above or from citations listed in review articles for the three categories of tests for which the committee found the strongest evidence for a relationship to health—the shuttle run as well as the treadmill and cycle ergometer (i.e., heart rate extrapolation) tests. The distance run also is reviewed here since it could be used as an alternative in schools and other educational settings, even though the literature on this type of test is sparser. Several permutations of graded-intensity shuttle runs to volitional fatigue have been used in youth fitness test batteries. The most common is the 20-meter shuttle run as developed by Léger, and it is this version of the shuttle run that has been examined most frequently in validity/reliability studies (Léger et al., 1988). Performance on the test is scored as 20-meter laps completed before the participant falls behind the pace set by an auditory timer, and validity has been examined as the correlation between laps completed and measured VO2max. In Léger's developmental study, the correlation between laps completed and VO2max was r = 0.71 in a group of 8- to 19-year-olds (Léger et al., 1988). Boreham and colleagues (1990) and Liu and colleagues (1992) completed similar validation studies and reported validity coefficients of r = 0.87 and r = 0.72, respectively. Clearly there is strong and consistent evidence that performance on the shuttle run in young people correlates highly with weight-relative VO2max. This test also has been shown to be a highly reliable measure. In a recent review article, Artero and colleagues (2011) report that test-retest reliability coefficients for this test have ranged from r = 0.78 to r = 0.93. Overall, the available evidence suggests that the 20-meter shuttle run has excellent validity and reliability as a measure of cardiorespiratory endurance. It is well known that heart rate increases linearly with increasing intensity of endurance exercise; maximal heart rate and VO2max tend to occur at the same exercise intensity, and therefore power output (e.g., exercise intensity) at a standard heart rate correlates highly with power output and VO2 at maximal exercise. These relationships are the basis for tests of cardiorespiratory endurance that involve the performance of graded, submaximal exercise with heart rate monitoring. Perhaps the best known and most widely used of such tests is the Physical Working Capacity-170 (PWC-170) test (Wahlund, 1948). This test is performed on a cycle ergometer at three progressively increasing intensities. Performance on the test is quantified as power output at a heart rate of 170 beats per minute as estimated from the linear plot of heart rate versus power output. Similar treadmill tests based on the same principles have been developed (Gutin et al., 1990; National Center for Health Statistics, 2004). Performance on the PWC-170 test has been validated against VO2max as a criterion measure. Rowland and colleagues (1993) found moderate correlations between absolute VO2max and performance on the PWC-170 in boys and girls (r = 0.70 and 0.71, respectively), but relationships were weaker when VO2 was expressed relative to body weight (r = 0.65 and 0.48, respectively). Boreham and colleagues (1990) reported a high correlation (r = 0.84) between performance on the PWC-170 and VO2max in 48 adolescent boys and girls. Of interest, in the same study, Boreham and colleagues (1990) found that performance on the PWC-170 and 20-meter shuttle run was highly correlated (r = 0.89). The PWC-170 is highly reliable, with test-retest correlation coefficients ranging from 0.89 to 0.98 (Watkins and Ewing, 1983; Watson and Odonovan, 1976). The validity of distance/timed runs typically has been established by examining the correlation between a criterion measure—directly measured VO2max (ml/kg/min) as determined during exhaustive treadmill running—and test performance (distance or time). The reviewers of this literature have consistently concluded that distance runs of 1 mile or greater demonstrate acceptable validity versus VO2max. As noted by Safrit (1990) and Freedson and colleagues (2000), correlations between VO2max and performance on distance/timed runs typically have been observed in the good to high range (r = –0.63 to –0.90; a negative correlation has been seen between time to complete and VO2). Also, distance/timed runs have been found to be reliable based on test-retest correlations. In summarizing studies examining the reliability of distance runs, Freedson and colleagues (2000, pp. S80-S81) conclude that the “reliability of distance run tests has been generally high with correlation coefficients ranging from r = 0.61 to 0.92.” A more recent review of studies examining the 1-mile run/walk test found intraclass correlation coefficients ranging from 0.39 to 0.90 in samples of children and adolescents (Artero et al., 2011). Several factors should be considered with respect to administrative feasibility for tests that are to be used as part of a national survey or in schools and other educational settings. Although many of these factors apply to all settings (e.g., cost of the equipment), others relate more closely to schools specifically (e.g., whether the test is appropriate as part of the school curriculum). The latter considerations are discussed in more depth in Chapter 9. The factors to be considered regarding administrative feasibility are summarized in the checklist in Box 3-2 in Chapter 3. In general, these factors are related to the test subject, the facility and equipment, the administrator of the test, and the parents of the test subject. The reader is referred to other publications that expand on these general factors (Mahar and Rowe, 2008). This section focuses on factors that are particularly relevant to conducting cardiorespiratory endurance tests and that apply to all settings. Of interest is that 7 of 11 and 8 of 11 studies reviewed by the committee that used the treadmill and cycle ergometer tests, respectively, utilized maximal protocols. Maximal tests on either the treadmill or cycle ergometer are likely not to be administratively feasible in larger studies, especially if they are school based. Nonetheless, all three types of tests for which the committee found the strongest evidence for a relationship to health—the shuttle run, the treadmill, and the cycle ergometer—are generally feasible, and the setting will dictate the choice among these types. For example, if space is the major issue in test administration, such as in the case of a national survey, the treadmill and cycle ergometer tests will be preferred. Facility factors are of particular importance as the different tests have different space and equipment requirements. For example, the shuttle run requires the most space—at least 20 meters for the test course; the treadmill and cycle ergometer tests require substantially less space. On the other hand, the treadmill and cycle ergometer tests require complex and expensive equipment. The different space requirements may have an impact on privacy for test subjects, the time required for testing, and the number of subjects who can be tested. Training of the test administrator in test protocols, test administration, and factors to consider is key to successful administration of a test and is another important consideration. For example, training for administration of the shuttle run is likely to be somewhat less complex than that required for the treadmill or cycle ergometer test. The cost of the equipment often is a major consideration in deciding which test should be used. The monetary cost of the equipment and of training the test administrators is relatively easy to assess. However, fitness testing may involve a wide range of additional direct and indirect costs. Ultimately, it is important to know the relative costs versus the relative benefits of using particular tests. No formal cost/benefit analyses have been performed for any of the available tests for cardiorespiratory endurance. Parental factors include concerns about the impact of the test on the child. This may include fears regarding adverse events that could occur during testing, as well as concern about how the results and their interpretation will impact the child. Parents may be especially interested in the health implications of the results. These issues are probably equally important for all recommended cardiorespiratory endurance tests. Adverse events, including injury during testing and the potential psychological effects of testing, should be considered. Adverse events of the various tests for assessment of cardiorespiratory endurance have not been systematically evaluated in the literature. The articles selected for this review do not report any injuries during testing. One recent manuscript (Ruiz et al., 2011) does address the safety of the 20-meter shuttle run, finding that no complications occurred during the testing, with only one report of a lower-body muscle cramp. The authors note that they have experienced no safety issues in more than 10,000 children they have tested. Chapter 3 presents a detailed discussion of the interpretation of fitness tests. Discussion of mathematical models for estimating cut-points, percentiles, or distribution curves is beyond the scope of this report. Low cardiorespiratory endurance clearly is related to a variety of negative health outcomes, including obesity, elevated blood pressure, dyslipidemia, and cardiometabolic risk. There is also some evidence that cardiorespiratory endurance is associated with neurocognitive function. Some studies have suggested that the lowest third of the distribution of cardiorespiratory endurance is the group at highest risk for cardiometabolic risk factors/metabolic syndrome, but the relationship may be more of a continuous one, making specific cut-points more difficult to determine. The committee recommends the use of interim cut-points based on data from both youth and adult populations on the relationship between treadmill performance and health outcomes until population-based evidence in youth is available for cardiorespiratory endurance tests. The bottom quintile of the distribution for cardiorespiratory endurance on a maximal treadmill test is associated with elevated morbidity and mortality (Blair et al., 1989) in adults. When interpreting test results, therefore, interim cut-points could be derived from low performers (e.g., 20th percentile) in the cardiorespiratory endurance distribution curve to identify youth at the highest risk of poor health outcomes and increase the likelihood that an individual identified as low fit is actually low fit. This is a more conservative approach than that taken by Lobelo and colleagues (2009) and Welk and colleagues (2011), who estimate approximately the 30th percentile to derive cut-points for cardiorespiratory endurance tests for youth. The committee's approach is based on its view that identifying a fit individual as low fit (potentially recommending an exercise intervention to a test taker who does not need it) is a more serious error than identifying an individual who is low fit as fit. It should be noted that this approach must take into account covariates such as age and sex, which allow standardization of the interpretation of test results across individuals and, more important, for an individual longitudinally across different ages. To derive the appropriate cut-points from percentiles, fitness data based on large populations for the test of interest are needed. If such data are not available, developers of cut-points should consult with statisticians to design a small study with a representative sample of U.S. youth to collect the necessary data. Accurate interpretation and effective communication of test results are important when improved fitness is a goal of the test. As mentioned in Chapter 3, an individual's results can be presented against the background of a continuous distribution. The continuous background reflects the concept that improved fitness in general, even within a broader range, is associated with a lower risk of negative health outcomes. Ultimately, research should be conducted to evaluate the impact of this approach to classification and interpretation on test subjects, parents, test administrators, teachers, physicians, and others and on future health behaviors. There is a well-known association between the fitness component cardiorespiratory endurance and health outcomes in adults. The measurement of cardiorespiratory endurance and its relationship to health outcomes in youth is relatively new to the literature. The committee's review revealed that sufficient relationships have been established between cardiorespiratory endurance and several health risk factors in youth, including adiposity and cardiometabolic risk factors (blood pressure, blood lipids and glucose, and insulin sensitivity). A few studies have established a relationship with other, less-studied pediatric health risk factors, such as pulmonary function, depression and positive self-concept, and bone health. The literature review provided to the committee included 34 articles indicating a positive relationship between results of cardiorespiratory endurance tests in youth and health risk factors, independent of other interventions. The review included longitudinal, experimental, and quasi-experimental studies. There was substantial variability in the tests used, especially with the protocols for distance/timed runs and cycle ergometry. The characteristics of the subjects (e.g., age, gender, weight) varied as well. The cardiorespiratory endurance tests most often associated with a positive change in a health risk factor were the shuttle run, treadmill, and cycle ergometer tests. The health markers most frequently assessed were related to body weight or adiposity and cardiometabolic risk factors. The shuttle run, treadmill, and cycle ergometer tests all showed strong relationships to health markers. Because of the paucity of studies addressing the influence of several potential modifiers of performance—age, gender, race/ethnicity, body composition, maturation status—on the various cardiorespiratory endurance tests, the committee was unable to examine this issue. Such influences have, however, been suggested in the past (Beets and Pitetti, 2004; Bovet et al., 2007; Chomitz et al., 2010; Cureton et al., 1997; Huang and Malina, 2007, 2010; Mahon and Vaccaro, 1989; Pate et al., 2006; Trowbridge et al., 1997). The treadmill and cycle ergometer tests are quasi-laboratory tests that may be best suited to situations where space is a limitation. Field-based cardiorespiratory endurance tests include both distance/timed runs and the shuttle run. The shuttle run is advantageous when there are time constraints and the purchase of sophisticated equipment and use of expert testers may not be feasible. The available evidence indicates that all of the approaches to measuring cardiorespiratory endurance examined in this chapter demonstrate acceptable validity and reliability. The validity and reliability coefficients for runs of varying distances and time limits are more variable and less consistently high than those reported for the shuttle run and heart rate extrapolation tests (treadmill and cycle ergometer). Based on its relationship to health, as well as its reliability, validity, and feasibility, a timed or progressive shuttle run, such as the 20-meter shuttle run, is appropriate for measuring cardiorespiratory endurance in youth. If the test is to be administered in a setting where there are space limitations, a submaximal treadmill or cycle ergometer test should be used, even though several studies reviewed here were conducted with maximal tests. Submaximal protocols are recommended for feasibility reasons: maximal tests are not suitable for large samples or school settings because they require that participants meet certain criteria, such as reaching a certain number of beats/minute, respiratory quotient, and oxygen consumption. Moreover, there is a proven relationship between performance on a submaximal test and performance on a maximal test. Although the evidence for a relationship to health is not sufficient at this time for distance/timed runs, this test is valid and reliable and could be an alternative in schools and other educational settings. Until population-based evidence in youth is available, the lowest 20th percentile of the distribution of cardiorespiratory endurance should be used to derive interim cut-points for determining whether individuals are at risk of cardiovascular-associated negative health outcomes. The committee's full recommendations on cardiovascular endurance tests for use in national youth fitness surveys and in schools and other educational settings are presented in Chapters 8 and 9, respectively. REFERENCES
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Which type of test is least likely to appear in a test battery for health-related fitness?
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