presents the distribution of variables in 1997 and 1999. The average age was 12.9 (range 9.9–16.6) for girls and 12.8 (range 9.9–16.9 years) for boys in 1997, and 15.0 (range 11.8–18.6) and 14.8 (range 11.9–18.8 years) respectively in 1999. In general, girls reported fewer hours of physical activity than boys (12.2 hrs/wk compared to 14.7 hrs/wk in 1997, and 11.9 compared to 14.9 in 1999). The number of hours of physical activity stayed relatively stable (+/−5 hours) from 1997–99 for the majority of girls (56%) and boys (48%). During this time period, 18% of girls and 25% of boys changed their activity levels by 10 or more hrs/wk. (either increased or decreased). shows the trends in physical activity by age among both boys and girls in the cohort. Among both girls and boys, activity appears to increase from ages 10 to 15 and steadily decline thereafter. No evidence of a cohort effect on physical activity was found.
Distribution of variables 1997 and 1999
Age trends of self-perception measures among girls and boys.
For and , age was rounded down and used as a whole number, such that a participant aged 11.0 years and a participant aged 11.9 years would both be considered 11 years old. Ages were further combined such that anyone aged 14 in any of the years 1997, 1998, or 1999 was included in the age 14 category. Participants were therefore included in multiple age categories, based on their ages at the time that each questionnaire was completed. Because of the aging of the cohort, most participants were in the middle age categories (13, 14, 15) and fewer were in the youngest and oldest categories.
Weekly hours of physical activity among boys and girls by age (pooled)
Comparing 1997 and 1999, self-perception scores also remained relatively stable. In each of the 4 areas of self-perception studied, at least 50% of participants changed less than 2 points. shows the age-related trends in social, athletic, global and scholastic self-perception scores among boys and girls from age 10 to 18 years. No evidence of a cohort effect was found.
For both girls and boys, linear regression models showed that increase in physical activity was positively associated with change in social and athletic self-perception scores (p<0.0001) but was not associated with a change in either scholastic or global self-perception. ()
Linear regression of one-hour change in weekly physical activity from 1997–1999 on change in self-perception scores 1997–1999
reports the odds ratios (OR) and 95% confidence intervals (CI) for change in self-perception score by 2 or more points compared to little or no change (0 or 1 point), by varying levels of activity. Those who increased their physical activity levels were more likely to have increased self-perception scores. (.) In girls this link was noted with social and athletic self-perception; those who increased their activity time by 5 or more hrs/wk were at least 33% more likely to have also increased their social self-perception (OR 1.33, 1.61), and at least 44% more likely to have also increased their athletic self-perception (OR 1.44, 2.05). In boys, there was a significant association with increased activity and increased social self-perception; those who increased activity by 10 or more hrs/wk were 45% more likely to have increased social self-perception. Conversely, those who decreased activity by 10 or more hours were significantly less likely to have increased self-perception. With this level of activity decrease, the OR for girls for increased athletic self-perception was 0.59, and the OR for boys for increased self-perception was 0.63 for social, and 0.56 for athletic.
Table III a
Odds ratios for increasing self-perception scores by 2 or more points vs. little or no change (0 or 1 point change) 1997–1999
Compared to those with little or no change, the odds of decreasing self-perception scores were greater among those with decreased physical activity levels. () For girls who decreased activity by 5 or more hrs/wk, the odds of decreased social (OR 1.37, 1.66) and athletic (OR 1.37, 1.89) self-perception were significant. For boys, this relationship with decreased self-perception was seen among those who had an activity decrease of 10 or more hrs/wk (OR 1.44 for social, 1.89 for athletic). In girls and boys, those with increased activity of 5 or more hrs/wk were significantly less likely to have decreased athletic self-esteem (OR 0.70 and 0.57 for girls, 0.61 and 0.63 for boys).
Table III b
Odds ratios for decreasing self-perception scores by 2 or more points vs. little or no change (0 or 1 point change) 1997–1999
No statistically significant associations between physical activity and either scholastic or global self-perception scores were found in girls or boys.