The influence of age, sex, maturity, body mass and body fatness on the physical activity (PA) of 11-13-y-olds was examined longitudinally. Body mass, triceps and subscapular skinfold thickness and pubic hair were recorded and 3-d continuous heart rate (HR) monitoring was used to estimate PA on each annual measurement occasion. At the onset, subjects were 11.0 (0.4)-y-old and data were available on 202, 143 and 160 subjects in years 1 to 3, respectively with an almost equal sex distribution. Multilevel regression modelling examined age-, sex- and maturity-related changes in time spent with HR above 139 (moderate activity) and 159 (vigorous activity) bpm. Sustained (10 or 20 min) periods of moderate or vigorous activity were not characteristic of PA patterns. Both PA measures declined with age, with a consistent sex difference reflecting the lower PA levels of girls. Body mass and fatness were not significant explanatory variables, but an additional decrement in activity was evident in late maturity.
Conclusion: Few children experience extended bouts of PA, and from 11-13 y, PA decreases, with more girls than boys becoming inactive. The data emphasize the importance of promoting active lifestyles during youth.