Physical activity (PA) has been identified as an important agent in the prevention of chronic diseases such as obesity, cardiovascular diseases, and metabolic syndrome. In order to know more precisely the levels of PA during childhood and adolescence, it is necessary to develop and validate instruments able to adequately and widely assess PA to identify the impact on health in primary and secondary school populations. Quantifying PA will be helpful in order to focus school and community interventions on those groups with unhealthy lifestyles.
The Physical Activity Questionnaire (PAQ) is a self-report instrument developed specifically for Children (PAQ-C) and later adapted for Adolescents (PAQ-A), which has been widely used to assess PA in healthy school populations. However, there is a lack of uniformity and information about the meaning of final score, which would differentiate sedentary behaviors in youth.
The overall purpose of this dissertation research was, on the one hand, to validate PAQ psychometric characteristics in Spanish children and to determine cut-off points that would improve the utility of the PAQ-C and PAQ-A for future research applications; and, on the other hand, to study longitudinal changes of body composition and PA behaviors in adolescence using this tool.
The main variables of different studies included in this dissertation were evaluated as follows: PA level was evaluated using the Physical Activity Questionnaire (PAQ-C or PAQ-A) and/or accelerometers (Actigraph GT3X); body composition was assessed by anthropometric measurements; sexual maturity was estimated by predicted percentage of adult stature; and dietary intake was assessed by a self-administered food-frequency questionnaire.
The most important results show:
a) In the first study, test-retest reliability showed an Intraclass Correlation Coefficient of 0.96 for the final score of PAQ-C, which obtained a consistency of Cronbach's α of 0.76. Moreover, few and low correlations (rho=0.228-0.278, all P<0.05) were observed between PAQ-C and triaxial accelerometry and the accuracy analysis performed with the concordance coefficient correlation reported a low accuracy of PAQ-C compared with accelerometry (r=0.192, P=0.092). To sum up, the results suggest that PAQ-C had a high reliability but a questionable validity for assessing PA in our sample of Spanish children.
b) The main finding of the second study was to determine a PAQ-C and PAQ-A score cut-off point of 2.75 to discriminate 60 minutes of MVPA, which is associated within a total volume of 10664 steps/day for children and 9701 steps/day for adolescents. However, area under the curve (AUC) values of PAQ-C score were no significant (P>0.05) and only weak (AUC<0.7) discriminators between “active” and “non-active” children. So, according with the ROC analysis, only the PAQ-A can be a useful tool to classify adolescents as active or inactive following international recommendations as criteria.
c) In the longitudinal study, significant differences for FMP were found among S1, S2 and S3 (23.41±8.24 vs. 21.89±7.82 vs. 22.05±8.06, P<0.05; respectively); a significant interaction with sex was observed (P<0.05), but not for maturation. Regarding PA, S2 was significantly higher than S3 (2.58±0.72 vs. 2.29±0.73, P<0.001). An interaction between PA and maturation was statically significant (P<0.05). Our results suggest that body composition changes observed during adolescence are not driven by changes in PA. Moreover, the interaction analysis suggests that sex affects PA behavior, but not maturation or nutritional variables.
These overall results suggest that the PAQ appears to be a more appropriate tool to measure PA in adolescents than in children, and it may discriminate active and inactive students in adolescence according to international guidelines.
In addition, assessments conducted longitudinally in this dissertation show the problems of progressive decline of PA among adolescents, which seems to be influenced by the gender, regardless of maturation.
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