showed low activity levels to be a risk factor for low-back pain. Bloemers and colleagues (2012) also found an increased risk of injury among inactive youth, but no direct link to flexibility or other fitness parameters was established. Finally, Burton and colleagues (1996) tracked 11-year-olds over 4 years (to age 15) and found that flexibility measures were not predictive of back pain. Lower flexibility was reported between ages 11 and 15, and girls were more flexible than boys.

Body composition and cardiometabolic health Two experimental studies (Manios et al., 2002; Serbescu et al., 2006) found that after an exercise training intervention, improvements were seen in body composition or lipids and lipoproteins that were measured as health outcomes, which in theory are not physiologically linked to flexibility. It should be noted, however, that in one of the studies (Manios et al., 2002), the exercise intervention did not change the flexibility of the participants. Five prospective studies provide information relevant to flexibility and health outcomes. Inconsistent results were found with regard to the association between flexibility (as measured by sit-and-reach) and body composition. Two studies showed an association between decreases in flexibility and higher skinfold measurements (Matton et al., 2006) or BMI (Kim et al., 2005). Others (Aires et al., 2010; Chen et al., 2007), however, found no association between performance on the sit-and-reach test and BMI. These inconsistencies could be due to differences in study designs, such as the length of the studies, the ages of the children, or the appropriateness of the health outcome itself (body composition).

Limitations of the Scientific Literature

The committee notes that the quality of the research reviewed was less than optimal for several reasons. In some cases, there were problems with the design of the study (e.g., no controls). There have been no large trials with adequate statistical power to demonstrate a relationship between flexibility and any health outcome or marker. Moreover, studies typically were not designed to test hypotheses central to flexibility. For example, flexibility measures often were included as one of the fitness components measured, but the health outcomes assessed were chosen because of their hypothesized association with fitness variables other than flexibility, such as BMI. Early studies that influenced eventual large-scale fitness testing of youth focused on the importance of flexibility to back health. The six-item Kraus-Weber test, which was clinically derived, was thought to predict future back pain in adults and was subsequently used as a fitness test for youth (Kraus and Hirschland, 1954). Flexibility has not been theoretically linked to metabolic markers as have cardiorespiratory endurance and body composition, nor has it typically been linked to bone density, as has musculoskeletal fitness.

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