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5 Health-Related Fitness Measures for Youth: Cardiorespiratory Endurance KEY MESSAGES Although there is a well-known association between cardiorespiratory endurance and health outcomes in adults, the measurement of cardiore- spiratory 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 endur- ance and other, less studied risk factors, such as those related to pulmo- nary 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 indepen- dent 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 111

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112 FITNESS MEASURES AND HEALTH OUTCOMES IN YOUTH 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. C ardiorespiratory endurance has been recognized as a key component of physical fitness throughout the history of the field. This chap- ter 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 recommenda- tions 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 recommenda- tions, 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 implement- ing 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. DEFINITIONS 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 com-

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CARDIORESPIRATORY ENDURANCE 113 ponent 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 cardio- vascular 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 cardiorespira- tory endurance and health in youth is discussed later in the chapter. CARDIORESPIRATORY ENDURANCE TESTS 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 cardiorespira- tory 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).

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114 FITNESS MEASURES AND HEALTH OUTCOMES IN YOUTH 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 postWorld 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 partici- pant 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. CARDIORESPIRATORY ENDURANCE AND HEALTH IN YOUTH Literature Review Process 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 mea- suring 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 consid- eration. 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 fea- sibility 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.

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CARDIORESPIRATORY ENDURANCE 115 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 pub- lished 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 out- come, 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 interven- tion 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 insuf- ficient change in the fitness measure of interest. The following sections review the strength of the evidence for a rela- tionship 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 cardiore- spiratory 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.

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TABLE 5-1 Longitudinal Studies 116 Health Outcome(s)/Marker(s) Mental and Sample Body Metabolic Cognitive Size and Study Summary, Quality and Reference Fitness Test(s) Composition Health Function Other Characteristics Level of Evidence Eisenmann Treadmill Body mass No relationship N = 48 The Aerobics Center Longitudinal et al., 2005 (Balke, index (BMI)/% with blood Ages 15 and Study (ACLS) tested subjects during maximal)-- fat/waist pressure 26, male (M) adolescence and young adulthood. maximal circumference (BP), total and female (F) Statistics equal only partial correl oxygen (WC) cholesterol (TC), ations; did not test for interactions. consumption high-density Adolescent treadmill time (TT) (VO2max) lipoprotein associated with lower adiposity as (HDL), adults. No association with risk for tryglycerides cardiovascular disease (CVD). (TG), glucose Level of evidence (LE): Associational Johnson Treadmill Fat mass (FM) N = 115 Progressive walking treadmill et al., 2000 (walking)-- by dual- Ages 4.5-11, protocol. Assessed annually for 3-5 VO2max energy X-ray M and F, white years post. Did not examine change absorptiometry and African in VO2max relative to change in (DXA) American adiposity. Adjustments for Tanner, ethnicity, and gender. Relationship between baseline VO2max and rate of increasing adiposity. Rate of increase of adiposity was lower for those who were fit at baseline. No relationship between ethnicity and VO2max. LE: Associational

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Byrd- Treadmill DXA N = 160 VO2max at baseline and DXA and Williams (maximal)-- Ages 8-13, Tanner examined annually for up to 4 et al., 2008 VO2max M and F, years. Linear mixed models measured Hispanic, gender-specific relationships between overweight/ VO2max and increases in adiposity. obese Adjustment for Tanner. Higher baseline fitness in overweight Hispanic boys was protective against increased adiposity. LE: Associational Twisk et Treadmill Skinfolds TC/HDL, TC; N = 181 Six repeated measurements from al., 2000 (maximal)-- no relationship Ages 13-27, ages 13 to 27. Longitudinal linear VO2 max with BP M and F, regression for relationship between Amsterdam fitness and CVD risk factors, adjusting for time, gender, age, diet, and other lifestyle factors. Fitness inversely associated with TC/HDL, skinfolds, TC. Skinfolds mediated the relationship between fitness and CVD risk; no effect on BP. LE: Direct Janz et al., Cycle Skinfolds/WC TC/HDL, N = 125 Examined whether fitness during 2002 ergometer low-density Ages 6-15, first 4 years predicts CVD risk at (maximal)-- lipoprotein M and F 5 years, adjusted for age, gender, VO2max (LDL) and maturation. Multiple linear 117 continued

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TABLE 5-1 Continued 118 Health Outcome(s)/Marker(s) Mental and Sample Body Metabolic Cognitive Size and Study Summary, Quality and Reference Fitness Test(s) Composition Health Function Other Characteristics Level of Evidence regression to examine change in fitness and CVD risk, adjusted for changes in fat-free mass (FFM) and maturation. Change in fitness associated with TC/HDL, LDL, central and total adiposity 4 years later. LE: Direct McMurray Cycle Skinfolds/body Metabolic N = 389 Assessed baseline fitness and 7-year et al., 2008 ergometer mass index syndrome Ages 7-10 follow-up incidence of metabolic (submaximal) (BMI) to 14-17, syndrome/logistic regression. M and F Baseline fitness associated with incidence of metabolic syndrome 7 years later. LE: Direct Ortega Cycle BMI N = 598 Normal-weight subjects at 9 years et al., 2011 ergometer Ages 9.5 to followed for 6 years. Examined (maximal) 15, M and F, change in fitness and incident Estonian and overweight. Binary logistic Swedish regression, adjusted for sex, age, country, maturation.

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Improvements in fitness from childhood to adolescence associated with lower risk of overweight/obesity in adolescence. LE: Direct Aires et al., Shuttle run BMI N = 345 Fitnessgram shuttle run grouped 2010 (progressive Ages 11-16 into healthy zone or under aerobic and 14-19, healthy zone. Linear mixed effects cardiovascular M and F modeling to examine fitness and endurance run change in BMI. Unclear whether [PACER]) baseline fitness or change in fitness was used. No adjustment made for BMI, maturation. Under healthy zone predicted increase in BMI over 3 years. LE: Associational Chen et al., Shuttle run BMI N = 307 Examined which factors were 2007 (PACER) Ages 7-8, associated with changes in BMI M and F, over 1 year. Stepwise regression Chinese to examine predictors (fitness, mother's weight status, screen time, BMI at baseline) of BMI at 1-year follow-up. Cardiorespiratory endurance was independently associated with BMI after 1 year. LE: Direct 119 continued

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TABLE 5-1 Continued 120 Health Outcome(s)/Marker(s) Mental and Sample Body Metabolic Cognitive Size and Study Summary, Quality and Reference Fitness Test(s) Composition Health Function Other Characteristics Level of Evidence Kim et al., Shuttle run BMI N = 2,927 Examined relationship between 2005 (20 m/6 min) Ages 5-14, cardiorespiratory endurance and M and F incident overweight over 1 year in schoolchildren. Multivariate logistic regression models. Adjusted for sociodemographics. Not passing cardiorespiratory endurance test associated with incident overweight. LE: Direct Martins Shuttle run BMI BP, TC N = 153 Study examined the association et al., 2009 (PACER) Ages 8-9, M between VO2 max at baseline and F, Portugal and changes in CVD risk over 5 years. Multilevel modeling to examine effect of fitness over time. Maturation measured; only age, gender adjusted for. No association between cardiorespiratory endurance and BP or TC. Lower level of cardiorespiratory endurance associated with higher BMI over 5 years. LE: Associational

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Puder et Shuttle run BMI/WC, Homeostatic N = 83 Association of adiposity and fitness al., 2011 (PACER) skinfolds model First and fifth with changes in inflammation and assessment- graders, insulin resistance. Multiple linear insulin resistance M and F regression, adjusted for age, gender, (HOMA-IR), and puberty. No association with Creactive CRP. protein (CRP) Low baseline fitness associated with increases in HOMA-IR. LE: Associational Mota et al., 9-min run BMI N = 135 Baseline BMI and fitness with 2009 Ages 6-12, changes in BMI over 2 years; M and F, logistic regression; did not specify Portugal controlling for covariates. Baseline fitness associated with change in BMI over 2 years. Unfit children at baseline were 3.9 times more likely to be BMI gainers; however, a change in CRP was not associated with a change in BMI over time. LE: Associational 121

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142 FITNESS MEASURES AND HEALTH OUTCOMES IN YOUTH 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 Cen- ter 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 perfor- mance 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). Distance/Timed Run Tests 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 dem- onstrate 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). ADMINISTRATIVE FEASIBILITY Several factors should be considered with respect to administrative fea- sibility 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

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CARDIORESPIRATORY ENDURANCE 143 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 cur- riculum). 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 fac- tors are related to the test subject, the facility and equipment, the adminis- trator 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 rel- evant 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 commit- tee that used the treadmill and cycle ergometer tests, respectively, utilized maximal protocols. Maximal tests on either the treadmill or cycle ergom- eter 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 tread- mill and cycle ergometer tests require substantially less space. On the other hand, the treadmill and cycle ergometer tests require complex and expen- sive 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 proto- cols, test administration, and factors to consider is key to successful admin- istration 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 interpreta-

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144 FITNESS MEASURES AND HEALTH OUTCOMES IN YOUTH tion 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 psy- chological 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, find- ing 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. GUIDANCE FOR INTERPRETATION OF TEST RESULTS Chapter 3 presents a detailed discussion of the interpretation of fitness tests. Discussion of mathematical models for estimating cut-points, percen- tiles, 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, dys- lipidemia, and cardiometabolic risk. There is also some evidence that cardi orespiratory 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 cardiometa- bolic 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 tread- mill 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 conserva- tive 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 com- mittee'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

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CARDIORESPIRATORY ENDURANCE 145 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 appropri- ate 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 back- ground 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 administra- tors, teachers, physicians, and others and on future health behaviors. CONCLUSIONS There is a well-known association between the fitness component car- diorespiratory 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 cardiorespira- tory 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 arti- cles indicating a positive relationship between results of cardiorespira- tory 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 relation-

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146 FITNESS MEASURES AND HEALTH OUTCOMES IN YOUTH ships 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 cardiore- spiratory 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 accept- able 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. Submaxi- mal 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 relation- ship 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 pre- sented in Chapters 8 and 9, respectively.

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