Longitudinal changes in 11-13-year-olds' physical activity

The influence of age, sex, maturity, body mass and body fatness on the phys ical 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 an d data were available on 202, 143 and 160 subjects in years 1 to 3, respect ively with an almost equal sex distribution. Multilevel regression modellin g examined age-, sex- and maturity-related changes in time spent with WR ab ove 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 w ere not significant explanatory variables, but an additional decrement in a ctivity 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 emphas ize the importance of promoting active lifestyles during youth.