Cross-sectional and longitudinal associations between 24-hour movement behaviors and growth, motor, and social-emotional development in early childhood.

Background: To enhance evidence on optimal 24-hour movement behaviors (physical activity, sedentary behavior, and sleep) in early childhood, this study investigated cross-sectional and longitudinal associations of the composition of these behaviors with social-emotional development, gross motor development and growth in 0-4-year-olds.

Methods: Data were collected at two timepoints (baseline and 9 months later) in two sub-cohorts from the My Little Moves study: one examining social-emotional development (sub-cohort-SE) and one gross motor development and growth (sub-cohort-GM). Children's time spent in 24-hour movement behaviors was assessed via parent-report using the My Little Moves app. Isometric log-ratios were calculated to represent 24-hour movement behavior composition. Social-emotional and gross motor development were assessed using the Bayley Scales of Infant and Toddler Development-III, with both total raw and norm-referenced scaled scores. Children's weight and height were measured to calculate BMI z-scores. Linear regression and mixed-model analyses examined cross-sectional and longitudinal associations, with significant results further explored using compositional isotemporal reallocation analysis.

Results: Sub-cohort-SE provided data from 101 children at timepoint 1 (age 20.6 ± 12.5 months) and 62 children at timepoint 2 (age 25.7 ± 9.8 months). Sub-cohort-GM provided data from 60 children at timepoint 1 (age 20.4 ± 10.8 months) and 46 children at timepoint 2 (age 27.6 ± 9.6 months). The composition of 24-hour movement behaviors was significantly associated with raw gross motor development scores in both cross-sectional (p < .001, R²Δ = 0.042) and longitudinal (p < .001, R²Δ = 0.033) analyses. The association with BMI z-scores was significant only in the cross-sectional analysis (p = .015, R²Δ = 0.130). Reallocating 10 min from sedentary behavior to physical activity or sleep increased raw gross motor development scores by 0.22 (95% CI [0.11, 0.33]), and 0.27 (95% CI [0.08, 0.45]). Reallocating 10 min from sedentary behavior to sleep increased BMI z-scores by 0.04 (95% CI [0.01, 0.06]).

Conclusions: The composition of 24-hour movement behaviors was significantly associated with BMI z-scores and gross motor development, but not social-emotional development in children aged 0-4 years. Evidence on the optimal distribution of movement behaviors remains unclear and needs further examination in larger longitudinal studies.