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The importance of regular physical activity and of physical fitness for health has been widely accepted and extensively documented. However, before developing and implementing strategies to encourage physical activity and improve physical fitness, it is important to 1) be able to accurately assess physical activity and physical fitness, 2) be well informed about the status and (generation- or age-related) change of physical activity and physical fitness and 3) have clear insight into the causes underlying the phenotypic variation in physical activity and physical fitness. The purpose of this thesis was to document each of these aspects in the Flemish population using data from the Leuven Longitudinal Study on Lifestyle, Fitness and Health (LLSLFH) 1969-2004. Therefore this thesis was divided into a methodological, a longitudinal epidemiological and a genetic epidemiological part each consisting of two chapters addressing one specific research question. In the first chapter of the methodological part, three difficulties associated with longitudinal research, i.e. the “imperfect” design, the evolution of data collection methods and the representativeness of the ongoing study sample, were documented by means of findings from the LLSLFH. The LLSLFH provides unique research opportunities, especially in studying longitudinal and genetic epidemiological aspects of physical activity and physical fitness, but is also faced with several limitations. However, when aware of these possible “pitfalls” several measures (e.g., thorough planning, appropriate sampling procedures, reimbursement of travel expenses, personal results, newsletters, telephone interviews with dropouts) should be taken to prevent or limit them as much as possible and to interpret the results in the right perspective. In the second chapter two-week test-retest reliability and concurrent validity of the Flemish Physical Activity Computerized Questionnaire (FPACQ) developed for employed/unemployed and retired people was investigated. The FPACQ was used in the most recent phase of the LLSLFH. The RT3 Tri-axial Research Tracker, in combination with a written seven-day activity record was used as objective criterion measure. Particularly in employed/unemployed people and slightly less in retired people, the FPACQ was a reliable and reasonably valid questionnaire for the assessment of different dimensions of physical activity and sedentary behaviour. In the first chapter of the longitudinal epidemiological part the secular trend in physical activity and physical fitness was investigated in adolescents from 1969 to 2005. Several anthropometric characteristics in both sexes and skeletal age in boys demonstrated a positive secular trend, while a negative secular trend was observed for most physical fitness tests. No secular trend was apparent for sports participation. In the second chapter tracking of physical activity and physical fitness was studied from adolescence to middle adulthood in females. Most anthropometric and physical fitness characteristics were relatively stable from adolescence to adulthood. On the other hand, sports participation was not a stable characteristic. In the first chapter of the genetic epidemiological part the familial aggregation in physical activity and physical fitness was studied and the proportion of the variability attributable to genetic, common and unique environmental factors was quantified. The observed familial aggregation was explained by genetic factors for body mass index, sum of five skinfolds, trunk-extremity skinfold index, flexibility and muscular endurance and by common environmental factors for balance and sports participation, while both factors were important for waist circumference and explosive strength. However, for all variables at least about half of the variance was explained by unique environmental factors. In the second chapter parent-offspring resemblance in physical activity and physical fitness was compared between adolescent chronological age-matched parent-offspring pairs and date-of-exam-matched (adult parent-adolescent offspring) parent-offspring pairs. Only for height, weight, trunk-extremity skinfold index and vertical jump in males, the stated hypothesis of higher adolescent chronological age-matched parent-offspring correlations could be confirmed. The findings of this thesis have important implications with regard to the necessity and the efficacy of strategies to improve physical activity and physical fitness. From the longitudinal epidemiological studies it can be concluded that 1) during adolescence measures are necessary to encourage physical activity and improve physical fitness and 2) during adulthood constant efforts should be made to keep people active and to reactivate those who have shifted from an active to a more sedentary lifestyle. Moreover, the identification of environmental factors, viewed against the background of genetic susceptibility, as major causes of the phenotypic variance in physical activity and physical fitness in the genetic epidemiological studies emphasizes the potential efficacy of such strategies. Table of contents iii Acknowledgements/Dankwoord v Professional career ix Publications xi Abbreviations xiii English summary xv Dutch Summary/Samenvatting xvii Section 1. General introduction and outline 19 Section 2. Research articles 45 Part 1. Methodological issues 47 Chapter 1. Methodological issues associated with longitudinal research: Findings from the Leuven Longitudinal Study on Lifestyle, Fitness and Health (1969-2004) Journal of Sports Sciences, accepted for publication 49 Chapter 2. Reliability and validity of the Flemish Physical Activity Computerized Questionnaire (FPACQ) in adults Research Quarterly for Exercise and Sport, accepted for publication 75 Part 2. Longitudinal epidemiological studies 103 Chapter 1. Secular trends in anthropometric characteristics, physical fitness, physical activity, and biological maturation in Flemish adolescents between 1969 and 2005 American Journal of Human Biology, accepted for publication 105 Chapter 2. Tracking of physical fitness and physical activity from youth to adulthood in females Medicine and Science in Sports and Exercise 38 (6): 1114-1120, 2006 129 Part 3. Genetic epidemiological studies 145 Chapter 1. Aggregation of anthropometric characteristics, physical fitness and physical activity in Flemish nuclear families Twin Research and Human Genetics, in revision 147 Chapter 2. Parent-offspring resemblance in physical fitness and physical activity: date-of-exam-matched versus adolescent age-matched analysis American Journal of Epidemiology, submitted 169 Section 3. Summary and general discussion 189 nrpages: 215 status: published |
