In this prospective study of 10- to 15-year-old US adolescents, we found that changes in television viewing, or sedentary behaviors in general, were not associated with changes in leisure-time moderate/vigorous physical activity. This finding applied equally to girls and boys and to younger (10–12-year-old) and older (13–15-year-old) adolescents.
Our findings of essentially no relationship between television viewing and leisure-time moderate/vigorous physical activity are consistent with those of cross-sectional studies in children and adolescents. In a national cross-sectional sample of high school youths, Heath et al3
found no relationship between television hours and physical activity. In analyses from Planet Health,15
a school-based obesity prevention intervention for children in sixth and seventh grades, Gortmaker et al found the coefficient for the correlation between television viewing and total physical activity to be −0.04 (S. L. Gortmaker, PhD, written communication, 2006). Almost all studies of television viewing and physical activity among adults20,21
also showed weak relationships between television viewing and moderate/vigorous physical activity.
To our knowledge, only 2 longitudinal studies of the association between television viewing and physical activity among children and adolescents have been published. In a study of sixth- and seventh-grade girls attending 4 northern California middle schools, Robinson et al22
found that baseline hours of after-school television viewing were associated only marginally with changes in levels of physical activity over time (Spearman r
= 0.04; P
= 0.48). However, in a crossover trial of three 3-week phases with 58 adolescents, Epstein et al24
observed that targeting decreasing sedentary behaviors was an effective strategy to increase physical activity. Several factors might explain the opposite results found in these 2 studies. The study by Epstein et al24
was a randomized, controlled trial of adolescents in an experimental setting, whereas the study by Robinson et al22
was an observational study within a general population. Similar to the study by Robinson et al,22
ours was an observational study; our sample included >10 000 girls and boys. The overall effect estimates for changes in television viewing and total inactivity that we observed in our study confirm the findings of Robinson et al22
and extend the observation of a null relationship between sedentary behavior and leisure-time physical activity to adolescent boys.
In comparison with the cross-sectional studies published to date, our study had several strengths, including its prospective design, the ability to adjust for several child characteristics that have been found to be associated with television viewing and physical activity (such as gender, age, and race/ethnicity), and a relatively large sample size. One limitation is that we relied on self-reports of television viewing and physical activity; the attendant random error could have resulted in a bias toward the null results that we observed. In previous analyses of this cohort, however, both television viewing and physical activity were associated with changes in BMI, lending confidence to the validity of their measurement.18
Also, although the subjects in this study were from all 50 US states, generalizability might be limited because the subjects were sons and daughters of registered nurses, the cohort was >90% white, and the children lived in households with higher household incomes than the general population. However, the narrow socioeconomic range might act to remove confounding by socioeconomic status. Finally, we observed relatively small mean changes in sedentary behaviors. Our results might not be able to guide fully the design of intervention studies that will likely be designed to produce much larger changes in sedentary behaviors.