Fitness Measures and Health Outcomes in Youth (2012)

8


Fitness Measures for a National Youth Survey

One of the uses of fitness measures is in the assessment of youth populations through statewide or national surveys. Chapter 2 provides a brief history of fitness testing and an overview of national fitness surveys (Tables 2-1 through 2-5) and a list of surveys currently in use in the United States and other countries (Table 2-6).

As its statement of task requested, the committee reviewed the evidence for tests for four components of fitness that historically have been recognized as health related using the methodology described in Chapter 3. The scientific rationale for the committee’s selection of tests based on their relationship to health and their validity, reliability, and feasibility is given in Chapters 4 (body composition), 5 (cardiorespiratory endurance), 6 (musculoskeletal fitness), and 7 (flexibility). This chapter provides the committee’s conclusions and recommendations regarding fitness test items to be included in a battery for use in a national survey. In presenting these conclusions and recommendations, the committee emphasizes that a national youth fitness survey should be implemented in schools by skilled national survey administrators (i.e., those familiar with the procedures for conducting large surveys and the protocols for fitness testing). Although this report does not include recommendations for specific fitness test protocols, the committee recognizes the need to develop standardized protocols for field-based tests for youth to enable meaningful comparisons of results from different studies and surveys.

CURRENT STATUS OF NATIONAL YOUTH FITNESS TESTING

As discussed in Chapter 2, no national fitness survey has been conducted since the 1980s. However, the currently active National Health and Nutrition Examination Survey (NHANES) includes components pertinent to physical fitness, such as body composition, cardiovascular fitness, and physical activity (Morrow et al., 2009). This set of fitness tests was recently extended for youth, and a 2012 NHANES Youth Fitness Survey is currently under way. The survey includes the following test items: body mass index (BMI), arm length and circumference, waist circumference, skinfolds, and whole-body dual-energy X-ray absorptiometry (DXA) scans

(body composition); the handgrip, plank, and knee extension (muscle strength); the treadmill (cardiovascular endurance); and a test of gross motor development. Selection of these items was based on expert opinion and feasibility.¹

CONCLUSIONS AND RECOMMENDATIONS

Based on its review of the scientific literature, as presented in Chapters 4 through 7, the committee concluded that there is enough scientific evidence to support recommending specific health-related items for youth fitness tests for the components of body composition, cardiorespiratory endurance, and musculoskeletal fitness. The committee concluded enough evidence has not been accumulated to support recommending a measure of flexibility for a national survey at this time. While the literature reviewed also did not provide enough information to support specific conclusions on the relationship between age and performance on fitness test items, the committee based its recommendations in part on feasibility of administration, selecting test items that are the most practical and relatively independent of age. The committee generally recognized that age can affect the validity, reliability, and safety of test items, so considering the age-appropriateness of test items is important when developing a national survey. The committee also concluded that there are many modifiers, such as demographic factors, that must be considered to avoid introducing errors in the interpretation of performance on fitness tests. In addition to age and gender, which are often recorded and used to interpret results, it is essential for survey developers to include a race/ethnicity questionnaire as part of a national survey. Although the evidence is not as substantial for maturation status and motor skill, measurements of these factors should also be considered in the survey design to enhance understanding of their mediating effects on performance on fitness tests. Variables that are more challenging to account for are those related to differences in the training of administrators and in the previous level of practice or physical activity of participants. To minimize those differences, care should be taken in training administrators and in providing opportunities for participants to become familiar with the tests when feasible. It is also important for the trained administrators to record adverse events associated with test taking so that safety issues can be better understood and addressed appropriately.

The committee’s statement of task included determining how scores should be interpreted for test items selected for inclusion in a national

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¹Personal communication, L. G. Borrud and V. L. Burt, National Center for Health Statistics, January 5, 2012. Additional information about the NHANES National Youth Fitness Survey is available online at http://www.cdc.gov/nchs/nyfs.htm (accessed August 16, 2012).

youth fitness survey and in youth fitness tests conducted in other settings. Cut-points (cutoff scores) are critical to interpretation of the results of health-related fitness tests since they serve as a way to distinguish individuals and populations that may be at risk of poor health outcomes from those that are not. As explained in Chapter 3, the committee concluded that a criterion-referenced method using cut-points associated with health outcomes or health markers is the ideal approach. Except for BMI, however, insufficient evidence exists with which to develop age- and gender-specific criterion-referenced cut-points related to health outcomes for any of the recommended tests; instead, age- and gender-specific interim cut-points corresponding to percentiles for youth or for adults should be used until enough data have been collected to enable establishing age- and gender-specific criterion-referenced cut-points. The committee provides general guidance for developing cut-points and interpreting performance results in Chapter 3; specific recommendations for developing cut-points depend on data available for each of the components of fitness. The following is a summary of the basis for the committee’s recommendations for deriving interim cut-points² (additional explanation can be found in the chapters on the fitness components [Chapters 4-7]):

• Body composition—BMI cut-points were set based on the already established 2000 Centers for Disease Control and Prevention (CDC) growth charts and percentiles. Until population-based evidence in youth is available for skinfolds and waist circumference, the 85th percentile (borrowed from the BMI percentiles) should be used to derive interim cut-points for these measures.
• Cardiorespiratory endurance—Until population-based evidence in youth is available, the recommended interim cut-points should be based on data from both youth and adult populations on the relationship between treadmill test performance and health outcomes. For adults, the lowest quintile has been determined as appropriate (Blair et al., 1989). For youth, the 30th percentile has been established as identifying those at risk of poor health outcomes (Lobelo et al., 2009; Welk et al., 2011). Based on those two determinations, the committee recommends that interim cut-points be derived from

²Depending on the nature of the test, the risk of a poor health outcome is defined by an individual’s being either below or above a specific percentile of a population. For BMI, for example, individuals above the 85th percentile of the 2000 Centers for Disease Control and Prevention (CDC) growth charts are considered to be overweight or obese and therefore at risk of a poor health outcome. In contrast, for cardiorespiratory endurance tests, those below a certain percentile (e.g., 20th percentile) of the population are the ones who are less able to perform exercise without fatigue and therefore at risk of a poor health outcome.

• the lowest performers (e.g., 20th percentile) to ensure a high probability that an individual identified as being low fit is really low fit and thus at risk for poor health outcomes.

• Musculoskeletal fitness—Until population-based evidence in youth is available, interim cut-points borrowed from the interim cardiorespiratory endurance percentile (e.g., the 20th percentile) are recommended. There are no data sets from youth or adult populations for any musculoskeletal fitness test. As stated in Chapter 3, in such cases, cut-points should be derived by borrowing the percentile from a test that is as comparable as possible in nature (e.g., requires movement of the body) and in the dimension it measures (e.g., upper-body strength) to the test of interest.

Further research is needed to better understand the relationship between fitness and health and to support selection and validation of the ideal criterion-referenced cut-points for fitness measures. This research should involve the implementation of national surveys that include measures of health outcomes. This and other research recommendations are presented in Chapter 10.

While the use of percentiles in establishing interim cut-points is appropriate until more evidence is collected, the committee recommends that once age- and gender-specific interim cut-points have been established, those cut-points rather than percentiles be used in communicating test results to those being tested, health and school officials, and parents. Doing so will minimize the confusion that might arise in communicating in terms of the percentiles used to derive the cut-points. For example, the CDC has used the 95th percentile from the 1960s to the 1990s as a cut-point for obesity in children, yet more than 15 percent of youth currently exceed the 95th percentile. In instances where percentiles may allow a clearer presentation of the results, as for BMI, the year of data collection should be reported with the percentile. The committee also notes that test administrators, those interpreting and communicating the results, and researchers should be fully familiar with the derivation of cut-points so they can interpret distribution changes in the population. To this end, survey administrators and those communicating the results should receive the appropriate training in interpreting tests to minimize classification errors. In addition, researchers developing percentile data for use in deriving interim cut-points should report the year of data collection. Fitness data from large populations are needed to derive the appropriate cut-points from percentiles. If such data are not available, developers of interim cut-points should consult with statisticians to design a small study with a representative sample of U.S. youth to provide such data.

RECOMMENDATION 8-1.³ A national survey of health-related physical fitness in youth should include measures of cardiorespiratory endurance, body composition, and musculoskeletal fitness. The survey should include the following fitness test items: (1) measures of BMI, waist circumference, and skinfold thickness (triceps and subscapular sites) to assess body composition; (2) a progressive shuttle run, such as the 20-meter shuttle run (or a submaximal treadmill or cycle ergometer test if there are space limitations) to measure cardiorespiratory endurance; and (3) handgrip strength and standing long jump tests to measure musculoskeletal fitness.

RECOMMENDATION 8-2. Standard protocols for the administration of measures of youth fitness in national surveys should be developed and implemented. The focus should be on maximizing the measures’ reliability, validity, and safety. Trained personnel should be used for test administration and data collection.

RECOMMENDATION 8-3. Developers of fitness test batteries should use age- and sex-specific cut-points to determine which individuals are at risk of poor fitness-related health outcomes. Optimum cut-points should be based on criterion values when population-based evidence is available on the relationship between the level of performance on a fitness test and a health outcome or marker. In the absence of criterion values, interim population-based percentile values should be applied. These values might be derived from adults on tests for the same component or from youth on tests for a different or the same component. Specifically, the guidance of the committee should be applied as follows:

• Body composition: For BMI, the CDC-established cut-points for underweight, overweight, and obesity evaluations should be used. Interim cut-points for skinfold and waist circumference measures could be derived from the CDC-established percentiles for BMI.
• Cardiorespiratory endurance: For measures of cardiorespiratory endurance, interim cut-points could be derived from the lowest performers (e.g., 20th percentile) on the cardiorespiratory endurance distribution curve.
• Musculoskeletal fitness: For musculoskeletal fitness tests, interim cut-points could be derived by borrowing the percentile (e.g., 20th percentile) from the cardiorespiratory endurance tests.

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³The committee’s recommendations are numbered according to the chapter of the main text in which they appear.

REFERENCES

Blair, S. N., H. W. Kohl, III, R. S. Paffenbarger, Jr., D. G. Clark, K. H. Cooper, and L. W. Gibbons. 1989. Physical fitness and all-cause mortality. A prospective study of healthy men and women. Journal of the American Medical Association 262(17):2395-2401.

Lobelo, F., R. R. Pate, M. Dowda, A. D. Liese, and J. R. Ruiz. 2009. Validity of cardiorespiratory fitness criterion-referenced standards for adolescents. Medicine and Science in Sports and Exercise 41(6):1222-1229.

Morrow, J. R., Jr., W. Zhu, B. D. Franks, M. D. Meredith, and C. Spain. 2009. 1958-2008: 50 years of youth fitness tests in the United States. Research Quarterly for Exercise and Sport 80(1):1-11.

Welk, G. J., K. R. Laurson, J. C. Eisenmann, and K. J. Cureton. 2011. Development of youth aerobic-capacity standards using receiver operating characteristic curves. American Journal of Preventive Medicine 41(4, Suppl. 2):S111-S116.

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