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KIMM, S. Y. S., N. W. GLYNN, A. M. KRISKA, S. L. FITZGERALD, D. J. AARON, S. L. SIMILO, R. P. McMAHON, and B. A. BARTON. Longitudinal changes in physical activity in a biracial cohort during adolescence. Med. Sci. Sports Exerc., Vol. 32, No. 8, pp. 1445–1454, 2000.This report describes the development and use of two self-report methods and an objective measure to assess longitudinal changes in physical activity in a large biethnic cohort of young girls from childhood through adolescence.The NHLBI Growth and Health Study (NGHS) is a multicenter study of obesity development in 2379 black and white girls followed from ages 9–10 yr to 18–19 yr (NGHS years 1–10). A Caltrac activity monitor was used to objectively quantify activity levels in years 3–5. A 3-d diary (AD) and a habitual patterns questionnaire (HAQ) were administered annually and biannually, respectively, to subjectively quantify physical activity levels. The changing pattern of activities as the girls matured during the 10-yr study period necessitated periodic form changes. Empirical analytic approaches were developed to help distinguish between true longitudinal changes in activity levels from potential numerical artifacts resulting from modifications in forms.The longitudinal activity data indicate a steep decline in the level of reported activity from baseline to year 10 as indicated by AD scores (446.8 to 292.1 MET-min·d−1, 35%) as well as by HAQ scores (29.3 to 4.9 MET-times·wk−1, 83%). This parallel trend in the pattern of the decline in activity among the two self-report methods was mirrored by a similar decline using the Caltrac method of physical activity assessment. From years 3 to 5, the AD decreased by 22%, whereas both the HAQ and Caltrac declined by 21%.The longitudinal data on physical activity collected in the NGHS cohort further confirm a dramatic decrease in the overall level of physical activity during the transition from childhood to adolescence. The consistency among the three methods indicate that both the AD and HAQ are useful tools for the assessment of activity levels in adolescent girls.