The muscle endurance search screened 6,563 reports, 38 of which were retained for further consideration and were abstracted. Of this subset, 12 studies were experimental, 15 experimental with no control, 6 quasi-experimental, and 5 longitudinal. The committee chose to review only the experimental (including those with no control and quasi-experimental) and longitudinal prospective studies in making its recommendations. In addition to the CDC search strategy, the committee reviewed the reference lists in the selected articles and relevant studies published before 2000 or after 2010.

The committee developed a set of criteria with which to assess the scientific quality of the studies (see Chapter 3). Each study was evaluated against those criteria and categorized as of low, moderate, or high quality. Only those of high quality were reviewed further; they are described in Table 6-2. The evidence for a link between a test item and a health marker in the top high-quality studies was categorized as direct or associational based on the strength of the study design and the rigor of the statistical analysis. The strength of the evidence was categorized as sufficient or insufficient based on the number of studies with direct or indirect evidence, the study designs, and the statistical significance of the association.

Review of the Scientific Literature

Chronic, hypokinetic-related diseases are manifestations of latent progressive poor health over a protracted period of time. Because these diseases are relatively less prevalent in youth, there is substantially less scientific evidence supporting the association of musculoskeletal fitness with health outcomes in youth than in adults.

The relationship between health and musculoskeletal fitness in youth has been reviewed recently in relation to the development of the Fitnessgram®/Activitygram® (Welk and Blair, 2008) and a new health-related physical fitness test battery for European youth—the Assessing Levels of Physical Activity (ALPHA) study (Castro-Piñero et al., 2010; Ortega et al., 2008b; Ruiz et al., 2009). In a recent review, Ortega and colleagues (2008b) report significant inverse associations of lower-limb explosive strength (i.e., power) and abdominal endurance with lower-abdominal obesity in youth (e.g., p < 0.001 between performance on the standing long jump and waist circumference in 8-year-old males) (Brunet et al., 2007). The same review also highlights inverse associations (p = 0.048) between a composite muscle fitness index score and a standardized composite measure of cardiovascular risk among adolescent girls (Garcia-Artero et al., 2007), and between putative cardiovascular inflammatory markers and muscle strength in normal-weight and overweight adolescents (for C-reactive protein, p = 0.02 and p = 0.09, respectively) (Ruiz et al., 2008). Additionally, positive associations were found between muscle

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