Behavior Tracking and 3-Year Longitudinal Associations Between Physical Activity, Screen Time, and Fitness Among Young Children

Purpose: Understanding the correlates of children's fitness as they develop is needed. The objectives of this study were to 1) examine the longitudinal associations between physical activity (PA), screen time (ST), and fitness; 2) determine if sex moderates associations; and 3) track PA and ST over 3 years.

Methods: Findings are based on 649 children [baseline = 4.5 (0.5) y; follow-up = 7.8 (0.6) y] from Edmonton, Canada. Parental-reported hour per week of PA and ST were measured at baseline and 3 years later. Fitness (vertical jump, sit and reach, waist circumference, grip strength, predicted VO_2max, push-ups, and partial curl-ups) was measured using established protocols at follow-up. Sex-specific z scores or low/high fitness groups were calculated. Linear or logistic multiple regression models and Spearman correlations were conducted.

Results: Baseline ST was negatively associated with follow-up grip strength [β = -0.010; 95% confidence interval (CI), -0.019 to -0.001]. Associations between baseline PA and follow-up overall fitness (β = 0.009; 95% CI, 0.002 to 0.016) were significant, whereas baseline PA and follow-up VO_2max (β = 0.014; 95% CI, 0.000 to 0.027) approached significance (P < .06). No sex interactions were observed. Moderate and large tracking were observed for PA (r_s = .30) and ST (r_s = .53), respectively.

Conclusions: PA and ST may be important modifiable correlates of overall fitness in young children.