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VAN MECHELEN, W., J. W. R. TWISK, G. B. POST, J. SNEL, and H. C. G. KEMPER. Physical activity of young people: The Amsterdam longitudinal growth and health study. Med. Sci. Sports Exerc., Vol. 32, No. 9, pp. 1610–1616, 2000.To describe the natural development of habitual physical activity behavior (HPA) of young Dutch male and female subjects between the ages of 13 and 27, using data from the Amsterdam Longitudinal Growth and Health Study.HPA was measured by means of a structured interview at ages 13, 14, 15, 16, 21, and 27 and concerned all activities (at work, school, during leisure, organized and nonorganized sports, and active transportation) exceeding a level of intensity of 4 METs. Complete longitudinal data concern 98 female and 83 male subjects. MANOVA for repeated measurements were done for total HPA (expressed in min·wk−1 and in METs·wk−1). Similar analyses were done for organized sports activities, leisure time activities, and all “other” activities separately (also min·wk−1 and METs·wk−1), as well as for weekly time at three different levels of intensity of HPA, i.e. 4–7 MET (moderate), 7–10 MET (vigorous), and >10 MET (very vigorous).Our data showed regarding total HPA (min·wk−1) in male, but not in female, subjects a significant decrease in weekly time spent on HPA between the ages of 13 and 27. Regarding the three different levels of intensity in male and female subjects, a significant increase was found in time spent on moderate activities, where female subjects were spending significantly more time on moderate activities than male subjects; both in male and female subjects, a significant decrease was found in time spent on vigorous activities: in male subjects a significant decrease was found in time spent on very vigorous activities, whereas in female subjects time spent at this level of activity remained more or less stable. For the total weighted activity score (MET·wk−1), a significant decrease was found for both male (42%) and female subjects (17%). This decrease was significantly greater for male than for female subjects. Regarding more specific activities, it was found that in the course of time organized sports activities became relatively more important contributors to both weekly HPA time and energy expenditure, both in male and female subjects.Our data show a considerable decrease in HPA over a 15-yr period of time, both in male and female subjects. Differences between male and female subjects are predominantly caused by differences in time spent in moderate and very vigorous activities. In the course of time, organized sports activities became a relatively more important contributor of weekly HPA.