Relationship between physical activity and body mass index in adolescents

To examine the associations between physical activity and body mass index (BMI) among females aged 14 to 17 yr.

A convenience sample of 65 Mexican American, 58 African American, and 49 non-Hispanic white girls in an urban high school in Texas participated in this study. Physical activity was assessed by ankle actigraphy. Average activity per period (before, during and after school) of the day and total activity were derived by a software program as movements per minute. The Center for Disease Control and Prevention 2000 BMI charts were used to calculate overweight status. Pearson correlation coefficient and analysis of variance were used to determine the strength of association and to compare activity levels by BMI status.

There was a statistically significant inverse association between total diurnal physical activity level and BMI (r=-0.37; P<0.05); and a statistically significant association between after-school activity and BMI (r= -0.28; P<0.05). After adjusting for the effects of age, race, and sexual maturity, total diurnal activity level was 10% less (P=0.03) for overweight females; for females at risk of overweight, diurnal activity level was 6% less (P=0.04) than for normal-weight females. Ethnic differences showed an 8% lower activity level among African American than for non-Hispanic white and 6% lower than for Mexican American females.

The study indicated that adolescent females in schools are at risk for inactivity at certain times of the day. Schools in partnership with their communities should assume a major role to promote participation in physical activity among adolescents through innovative activity programs in schools.