This dissertation contains an introduction chapter (Chapter 1), a review of the literature (Chapter 2), two manuscripts (Chapters 3 and 4) and a summary chapter (Chapter 5). The first manuscript is a mixed longitudinal study that examined change in walking and running economy over four years in children and adolescents. The second manuscript is a mixed longitudinal study that examined change in energy expenditure in four lifestyle physical activities (dance aerobics, laundry, basketball, and sweeping) over four years in children and adolescents. To examine change in economy and energy expenditure, VO2 was expressed in both absolute (L/min) and relative terms (ml/kg/min). Change in VO2 over time in youth is traditionally expressed in relative terms. However, by expressing VO2 in absolute terms while using weight as a covariate in the analyses, the purpose was to obtain results comparable to those used with ratio scaling of VO2. However, this was not the case and the data did not fit the hierarchical linear model for the six activities when VO2 was expressed in L/min. Similar to previous literature, relative VO2 (ml/kg/min) decreased over time for walking and running in children and adolescents. Relative VO2 also decreased over time in the four lifestyle activities, which is a new contribution to the literature. Factors found to influence change in relative VO2 over time included: body surface area (laundry, sweeping, walking), chronological age (aerobics, laundry, sweeping, walking), leg length (running, walking), and ventilation (aerobics, basketball, laundry, sweeping, walking). For laundry and sweeping (VO2 ml/kg/min), the predictor variables (body surface area, chronological age, and ventilation) explained all of the variance in slope; meaning individual variation in change in VO2 over time was explained. For walking, running, basketball, and aerobics, slope was still significant in the final model. In conclusion, relative VO2 for six different physical activities decreases over time in children and adolescents and is explained by a variety of factors depending on the activity. Other factors not included in this study should be examined to further explain the individual variation in slope in these activities.
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