Background/Purpose Tracking of physical fitness at a specific school level will help evaluate the effectiveness of nutrition and physical activity policies. This study was intended to examine (a) stability of middle school students' BMI for a three-year period and (b) differences in stability of BMI between genders.
Method Participants were 204 children (96 boys and 108 girls) from two middle schools in the US. Body weight and height were assessed at the beginning of 2006-07 and the end of 2008-09 school years. The three-year stability of BMI was examined with Spearman rank correlation and Kappa statistics, and tracking the extremes (lower and upper quartiles) was measured by calculating percentages of participants remaining in the lower and upper quartiles.
Analysis/Results Spearman rank correlation coefficient r was .873, and .833, and 8.95 for the entire cohort, boys, and girls respectively, all significant at p < .001 level. Kappa statistics was .539, .569, and .583 for the entire cohort, boys, and girls respectively, all significant at p < .001 level. Percentages of participants remaining in the lower and upper quartiles across the three years were 73.58% and 79.25% for the entire cohort, 70.80% and 76.00% for boys, and 71.43% and 93.10% for girls.
Conclusions Middle school children tend to stay in their original BMI rankings. Girls originally in the upper quartile are much more likely to stay in the same quartile than boys across the three years. The results suggest that middle schools need to initiate more interventions to decrease childhood overweight prevalence, especially for girls.