Exercises designed to produce changes in flexibility have been shown to be effective in increasing flexibility, and youth who participate in active sports generally have better flexibility than those who do not. There has been a decrease in flexibility among youth in the past 20 years, at a time when body weight has increased dramatically.

Flexibility is specific to joints, and relationships to general systemic health outcomes or health markers are therefore less likely to exist than is the case for other fitness components, such as cardiorespiratory endurance. Clinical theory suggests that complex interaction among multiple musculoskeletal factors (e.g., flexibility, strength, muscular endurance, and neuromuscular factors), rather than any individual variable, is most likely to show a relationship to health. Therefore, establishing an association with health outcomes (e.g., back pain, risk of injury, posture problems) and a single flexibility test item is challenging. Further, possible associations are complicated by the fact that the relationship between flexibility and health outcomes is not linear; that is, risk may be higher for both those with low flexibility and those with exceptionally high flexibility than for those in the middle ranges.

The strength of any association between specific flexibility tests and health outcomes in youth is minimal. There may be various reasons for this. First, in contrast with other fitness variables, there have been no large-scale studies of flexibility and health. Second, flexibility may be associated with health when other musculoskeletal variables are taken into account. Finally, the tests used to measure flexibility, the study designs, and the characteristics of the subjects (e.g., age, gender, weight) have varied substantially, making it difficult to establish any possible link between flexibility and various health outcomes. Data were insufficient to permit assessment of the influence of several potential modifiers, such as age, gender, race/ethnicity, body composition, and maturation status, on performance on the various flexibility tests.

The validity and reliability of some of the flexibility tests used in youth fitness test batteries in the United States and abroad have been confirmed. Among the tests reviewed, the various forms of the sit-and-reach have reasonable validity and reliability when used in both survey and school settings. The degree to which the sit-and-reach test is an indicator of overall systemic flexibility is unclear, however.

Based on the lack of evidence for an association between flexibility tests and health outcomes in youth, the committee does not recommend including such tests in a national survey at this time. At the same time, the committee recognizes that, although the evidence is not yet clear, flexibility in youth may in fact be linked to various health outcomes, such as back

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