Objective: To investigate the relationships between the time spent in specific intensities of activity and inactivity, cardiorespiratory fitness and body composition in children.
Methods: A cross-sectional study was conducted in a random sample of schools. Height, weight and waist circumference were measured in 224 children aged 7-10 years. Cardiorespiratory fitness was estimated by the 20 m multistage running test, and physical activity was measured over 4 days by the RT3 (a triaxial accelerometer). Time each day spent in moderate and vigorous intensities of activity was calculated.
Results: Twelve schools agreed to participate in the study. Body composition and fitness data were obtained for 224 children and activity data for 152 children. Boys were found to take part in about twice as much vigorous and hard activity as girls (mean (95% confidence interval) 64.3 (53.2 to 75.4) min in boys compared with 37 (33.1 to 40.9) min in girls; p<0.001). In boys there was significant difference between those defined as normal, overweight and obese in the time spent in vigorous activity (p<0.05), but no such difference was found in girls. A significant negative correlation between waist circumference and time spent in vigorous activity (r = -0.31, p<0.05) was found in boys but not in girls. Time spent sedentary was positively correlated with waist circumference in boys (r = 0.33, p<0.01) but not in girls. In both boys and girls there were significant negative correlations between fitness and both body mass index (r = -0.274, p<0.001) and waist circumference (boys: r = -0.503, p<0.01; girls: r = -0.286, p<0.01).
Conclusion: In boys, body composition was inversely related to fitness and to vigorous activity and was positively related to inactivity. In girls, body composition was related to fitness but not to specific components of physical activity.