Prior research suggests a cross sectional association between childhood television exposure and future body mass index and health-related behavior
[23, 24]. Our findings suggest that early childhood television viewing may also undermine future explosive leg strength and contribute to the accumulation of abdominal fat. In the present study, increases in television exposure between the ages of 29 and 53 months predicted shorter long jump performance in grade 2 and larger waistline measurements in grade 4. In our population-based sample of typically developing children, these findings were observed above and beyond a number of potentially confounding child and family characteristics. These findings also support research showing that time spent in sedentary pursuits, regardless of time spent in physical activity, can predict health outcomes in pediatric populations
We previously observed an association between preschool television exposure and fourth grade BMI
. Because studies have linked visceral adiposity to the development of a number of chronic health conditions, it is informative to examine if preschool televiewing is also associated with later child waist circumference. Indeed, every hour of television watched accounted for a 0.042 cm increase in waist circumference. Given that average television exposure in our sample was 8.82 hours per week, these levels correspond to an expected .41 cm increase in waistline measurement by age 10. In addition, close to 15% of our sample consumed over 18 hours of television per week, corresponding to a 0.76 cm increase in waist circumference by the age of 10. Because risk for obesity can be tracked as early as middle childhood, a surplus of weight by grade 4 represents an important health concern
. Finally, our observed relations persisted over 7 years later, and remained above and beyond the statistical control of a number of potentially confounding variables.
Sedentary habits from early childhood are likely to encourage the accumulation of visceral fat by interfering or competing with more vigorous physical activities, which partially explains the relationship between television viewing and childhood obesity
. Risks for weight gain may also operate through increased caloric intake during screen-time snacking
[25, 26]. The diet of children under the age of 5 largely reflects parental choices. Nevertheless, by preschool, children become increasingly autonomous. The quantity of early childhood television viewing has been shown to influence preferences as children grow older and gain more control over their diets
. Specifically, television advertisements for fast food, which target young audiences and seldom include healthful choices such as fruits and vegetables, aim to sway child preferences towards foods that contain high amounts of sugar and saturated fats
Watching more television in early childhood forecasted lesser performance on a test of explosive muscular strength in later childhood and was further associated with an increased probability of scoring in the bottom 5%. This suggests that for some children, excessive television exposure was associated with the experience of as a substantial level of impairment. This finding is of concern given that explosive leg strength is a robust indicator of individual general muscular strength. Eventually, reduced muscular strength that persists into adulthood can predict a number of negative health outcomes
Explosive leg strength is likely to contribute to the performance of movements that require speed and power
. A number of high intensity activities such as skating and sprinting or sports such as soccer, basketball, and football involve these kinds of physical skills. In addition to providing an edge in most sports, explosive leg strength may also help children complete basic exercises during physical education classes and facilitate participation in vigorous activities during leisure time. Children’s pursuit of sports depends in part on their perceived athletic competence. As such, the ability to perform well during athletic tasks may promote later involvement in physical activities. In a context of increasingly sedentary lifestyles, children’s lack of physical prowess may therefore represent a barrier to habitual sports participation
A statistically significant association between television exposure and long jump performance suggests an association between early screen-time and later muscular fitness. Sufficient physical activity during childhood is crucial for tissue anabolism, growth, and development
[29, 30]. Habitual sedentariness, which limits time windows of opportunity for physically effortful activity, may predict reduced muscle mass and bone density,
 which then may undermine subsequent explosive leg strength.
Strengths of the present study include the use of rich longitudinal birth-cohort data which facilitates the establishment of the temporal precedence of the independent variable in time. The present design also made it possible to control for a number of potential confounders. The observed effects remained significant above and beyond the effect of child weight for gestational age, eating habits, and early levels of physical activity. Furthermore, the observed relationships remained significant after controlling for maternal BMI, education, and immigration status. The use of directly measured outcomes also increases confidence in the observed results.
Despite these strengths, several limits merit discussion. First, the observational nature of our study limits our ability to infer causality. Second, social desirability motives could have influenced parental under-reporting of child televiewing. Nevertheless, an unreliable measurement is more likely to lead in the underestimation of effect sizes, rather than result in a spurious association. Sample attrition also represents a limit, which could have compromised the representativeness of our analytic sample. In order to minimize the effects of attrition, we employed multiple imputation techniques, which have been shown to provide more robust estimates of true effect sizes than simple imputation and pairwise and listwise deletion strategies. Finally, the observed effect sizes per hour of television exposure are small. However, our results gain importance when we consider the cumulative effects of excessive television exposure.
The present study warrants a better understanding of whether a causal link exists between quantities of television exposure and subsequent muscular fitness and waist circumference. One way to examine this possibility is to randomly assign children to intervention programs designed to reduce sedentary behaviors such as television exposure, and to examine the long-term effects on later fitness outcomes. Future research should also examine whether television exposure is related to additional child health indicators. Specifically, it would be informative to examine whether reducing television exposure during early childhood predicts cardio-vascular health by middle childhood.
In summary, our findings suggest that as a preferred pass time, television also represents a modifiable lifestyle factor that is associated with later physical prowess and health. The preschool years represents a period of remarkable sensitivity to environments and experiences and thus account for the origins of many lifestyle behaviors and preferences
. As such, early interventions aimed at modifying toddler viewing habits may contribute to subsequent physical health and athletic performance. The economics of early intervention suggest that strategies which target viewing habits in infancy, when brain plasticity is high and behaviors and preferences are not yet crystallized, offer fiscal benefits for population health