Context: Being physically active during the early years (age 0-6 years) is vital for healthy development. Identifying correlates and determinants of physical activity (PA) is crucial to guide effective interventions. This systematic review synthesized studies investigating potential correlates and determinants of PA during the early years, accounting for different types of PA assessment.Evidence acquisition: Nine electronic databases were searched from inception year (1900) until September 2014; data were analyzed/interpreted in April 2015. Inclusion criteria were: written in English, published in peer-reviewed journals, participants not in statutory/school education, and an observational design investigating associations between an exposure/variable and a quantitative measure of PA. Correlates/determinants of total, moderate to vigorous, and light PA were reported using an ecological model. Evidence synthesis: Of 22,045 identified studies, 130 were included. All took place in highincome countries and few (6%) were of high quality. Correlates of total PA were sex (male, ++), parental PA (+), parental support (+), and time outdoors (+). Determinants of total PA were sex (+) and time spent playing with parents (+). The only correlate of moderate to vigorous PA was sex (male, ++). No determinants of moderate to vigorous or light PA were found. PA correlates/determinants were relatively consistent between objective and subjective PA measures. Conclusions:Numerous studies investigated potential correlates and determinants of PA, but overall quality was low. A small number of demographic/biological and social/cultural factors were associated with PA. There is a need for high-quality studies exploring correlates/determinants across all domains of the ecological model. ContextPhysical activity (PA) is a key influence upon health across the life course. [1][2][3][4] The "early years" is an umbrella term for an age range that encompasses infants (0-2 years), toddlers (2-4 years), and preschoolers (4-6 years). 5 During this period, PA is reported to be associated with multiple health outcomes. 5 Evidence suggests PA levels track from early to later childhood, 6 and into adulthood, 7 so establishing optimal levels of this health-related behavior early in life is crucial. 8,9 Whether children during the early years are sufficiently active is unclear. Some studies have reported that children largely fail to meet current PA guidelines, 10 and spend most of their time inactive, 11,12 whereas others have reported sufficient activity levels in this age group. 13,14 Given the link between PA and health, it is important to understand correlates and determinants of PA to enable the development and implementation of effective interventions, 15 particularly as previous interventions have had limited efficacy. 16 In this review, the term "correlate" is used when an independent variable is found to be associated with PA in cross-sectional studies and thus causality cannot be determined. The term "determinant" is used when...

Purpose-To investigate associations of objectively-measured physical activity (PA) and sedentary time (ST) with adiposity in a predominantly bi-ethnic (South Asian and White British) sample of young children.Methods-The sample included 333 children aged 11 months to 5 years who provided 526 crosssectional observations for PA and body composition. Total PA volume (vector magnitude counts per minute (cpm)), daily time at multiple intensity levels (the cumulative time in activity >500 cpm, >1000 cpm, >1500 cpm and so on up to >6000 cpm), and time spent sedentary (<820 cpm), in light PA (820-3907 cpm) and moderate-to-vigorous PA (MVPA: ≥3908 cpm) were estimated with tri-axial accelerometry. Indicators of adiposity included BMI, waist circumference, and the sum of subscapular and triceps skinfold thicknesses. Statistical analyses were performed using multilevel regression and isotemporal substitution models adjusted for confounders. Effect modification by ethnicity was examined. Conflicts of Interest:None of the authors reported a conflict of interest related to the study. Authors' ContributionsThe authors' responsibilities were as follows -PJC: designed and conducted the research, analyzed data, wrote the paper, and had primary responsibility for the final content of the manuscript; SB assisted design of the statistical analysis and critiqued the manuscript; DDB assisted processing of accelerometer data; DDB, SC, JWe, RRCM, JWr, and SEB: designed individual studies, organized and managed data collections, and critiqued the manuscript; all authors approved the final manuscript as submitted. Results-There was no evidence for effect modification by ethnicity (p-interaction≥0.13). In the whole sample, the accumulated time spent above 3500 cpm (i.e. high light-intensity PA) was inversely associated with the sum of skinfolds (β (95% CI) = -0.60 (-1.19 to -0.021) mm per 20 min/d) and the magnitude of association increased dose-dependently with PA intensity (peaking for time spent >6000 cpm: -1.57 (-3.01 to -0.12) mm per 20 min/d). Substitution of 20 min/d of ST with MVPA was associated with a lower sum of skinfolds (-0.77 (-1.46 to -0.084) mm). Europe PMC Funders GroupConclusions-High light-intensity PA appears to be beneficial for body composition in young South Asian and White British children, but higher-intensity PA is more advantageous.