Determinants of Physical Activity during Early Childhood: a Systematic Review

Physical activity plays an important role for long-term health, and the preschool period is a vital developmental stage to establish healthy habits and behaviors. The purpose of this study was to conduct a systematic review of longitudinal studies examining the factors related to physical activity in young children. A literature search identifying physical activity studies in 2-to 6-year-old children was conducted in 2014. A total of 102 studies were initially identified, of which only nine met the inclusion criteria of prospective or longitudinal designs using objective measures of physical activity. Gender, aging, and seasonality (e.g. temperature, weather) were the three most common factors examined; followed by parental behaviors and weight status. Preschool-aged children are more physically active in better weather, and if parents, particularly fathers, spend more time playing with their children. Overall, few longitudinal studies have examined predictors of physical activity among young children. Of the limited evidence, it appears that the home environment may be a point for intervention, specifically encouraging parents to engage more with their young children in active pursuits. More longitudinal research is required to confirm and better understand salient factors related to physical activity during this critical early childhood period.


Introduction
There are many established health benefits associated with regular participation in physical activity for children * Corresponding author.
the following keywords: physical activity, determinant, correlate, and preschool(er).A study was included if it met the following inclusion criteria: 1) used a quantitative research design published in an English peer-review journal; 2) included a sample of young children between the ages of 2 and 6 years; 3) included any form of physical activity as the dependent variable; and 4) investigated the association between any biopsychosocial factor and physical activity.Two authors (YCL, SKD) independently conducted the initial search, examining titles and abstracts of identified studies.The reviewers subsequently compared studies identified to be potentially relevant for inclusion.A third author (JC) was consulted if consensus between the reviewers was not achieved.The primary search resulted in 94 articles that met the inclusion criteria.Reference checks, including targeted searches of all first authors, resulted in another eight studies identified for a total of 102 papers.

Study Quality
In order to assess bias and to evaluate the quality of the studies included in this review, we used a 10-item checklist, which is specifically modified for prospective studies, spanning four domains: study attrition and follow-up duration, assessment of determinants, assessment of outcome measures, and data analyses (Uijtdewilligen et al., 2011).Two authors (YCL and SKD) independently reviewed all included studies.A third author (JC) was consulted if both raters failed to reach a consensus.A score of "1" was given if the study met the specificcriterion in each domain.If multiple determinants or correlates were measured, the score was calculated by dividing the number of reliable/valid tools by the total number of tools used.A score of 0 indicated that the study failed to meet the criterion, and a question mark (?) indicated that the criterion was either unknown or not mentioned in the manuscript.The quality score is presented as the percentage of the sum of all ten item scores assessed, with higher scores indicating higher study quality.Using the cutoff point suggested, a score of <70% was considered low quality (Uijtdewilligen et al., 2011).

Physical Activity over Time
Given that we restricted the search to only prospective or longitudinal studies, most studies examined the changes in physical activity over time.Specifically, eight studies examined the changes in physical activity behaviors during early childhood (i.e., aging effect).Four of the studies (two high quality and two low quality) found a positive relationship (Bürgi et al., 2011;Jackson et al., 2003;McKee et al., 2012;Saudino, 2012), suggesting that children became more active as they get older.Two other high quality studies found a negative association between aging and physical activity in young children (Jago et al., 2005; R. W. Taylor et al., 2013), showing physical activity decreasing over time.There were two studies (one high quality and one low quality) that did not find a significant relationship (Gabel et al., 2011;Shen et al., 2013).The study by Buss and colleagues (1980) monitored physical activity over one year using actometers.However, the levels of physical activity (e.g., time or counts) were not reported, whereby only the correlations of physical activity between two time points were included, which hindered our ability to appropriately assess the changes in physical activity over time.Overall, the evidence appears to be mixed in terms of how physical activity changes over time, as these young children became older.
Comparatively, our findings are different than previous reviews of preschool children which have identified no clear association between age and physical activity (De Craemer et al., 2012;Hinkley et al., 2008), although we believe the evidence does not appear to be strong enough to draw clear conclusions.Children are generally considered to be the most active segment of the population, whereby physical activity tends to decrease as children enter adolescences and into early adulthood (Ross & Pate, 1987;Sallis, 1993).Our findings suggest that more work is needed to better understand the patterns of physical activity behaviors across the early childhood period, including assessment tools capturing various forms of physical activity (e.g., organized sports, free-play activities) on multiple occasions during this formative period.This is of particular importance, given the increased emphasis on active play rather than structured or organized activity among children, as a way to increase overall physical activity among youth (Brockman, Jago, & Fox, 2010; Canadian Society for Exercise Physiology, 2013).

Gender and Physical Activity
There were a total of eight studies that examined the effect of gender on physical activity, but the evidence regarding the association between gender and physical activity in young children was inconsistent.Overall, there were four studies (three of which were high quality) that found boys to be more active over time than girls (Bürgi et al., 2011;Jackson et al., 2003;Saudino, 2012;R. W. Taylor et al., 2013), and four studies (three that were high quality) that found no differences in physical activity between genders (Buss et al., 1980;Jago et al., 2005;McKee et al., 2012;Shen et al., 2013).Although the remaining one study by Gabel and colleagues (2011)  At 5 years, PA had declined substantially to around half that observed at 3 years.Activity levels were similar at 6-7 years as they were just prior to s tarting school.Boys were more physically active than girls.
Children showed higher counts per minute but a lower LMV: S ratio on weekend compared with weekdays.
Rain and cold weather significantly decreased on activity.
Parental activity correlated weakly with the child's activity at 3 and 4 yr, but only the father's activity remained a significant predictor of the child's activity after adjustment for confounders.also recruited both boys (n = 10) and girls (n = 7) and tracked physical activity over one year, this study did not compare the difference in physical activity between boys and girls, and this makes it impossible to examine the gender effect.
Our findings from the current review are inconsistent with Hinkley and colleagues (2008) and the broader literature, which shows a robust and consistent gender effect: young boys being more active than girls throughout childhood and into adolescence (Bauman et al., 2012;Hinkley et al., 2008;Sallis, Prochaska, & Taylor, 2000).Rather, our findings are consistent with the review by De Craemer and colleagues ( 2008), indicative of no clear gender effect on overall physical activity.It is posited that gender differences during childhood occur because boys begin to confront greater pressure from their peers to join organized sports when they begin school (Cairney, Hay, Faught, Mandigo, & Flouris, 2005), while girls are encouraged to participate in quieter activities (Rose, Larkin, & Berger, 1997).During the preschool years, however, boys and girls may not be influenced to the same degree by these socially constructed roles (Cairney et al., 2005;Ismael & Lawson, 2012;Nicaise, Kahan, & Sallis, 2011;Rose et al., 1997).Consistent with this reasoning, the unequivocal gender differences in the physical activity literature may not be evident until later in childhood, when children begin primary school.Given the limited longitudinal evidence, there is an obvious need for more rigorous studies to concretely show how gender influences physical activity during early childhood, and potentially how this changes over time.

Seasonal Effects and Weather Conditions
Three studies (two high quality and one low quality) investigated the effect of seasonality and/or weather.While previous systematic reviews conclude that no relationship between physical activity and different seasons or weather conditions, results of the current review suggest that preschool children tend to be more physically active during warmer seasons or in better weather (McKee et al., 2012;Shen et al., 2013;R. W. Taylor et al., 2013).For example, the study by Shen and colleagues (2013) found young children spend more after-school time in light physical activity and more time in total physical activity during weekends in the fall season compared to the winter.Similarly, McKee et al. (2012) reported children accruing close to 2000 more steps in the spring than winter.Taylor and colleagues (2013) concluded that an increase in one mm of rainfall and a decrease in temperature by one degree Celsius equated to a 2% decrease in physical activity participation.Overall, these findings are consistent with some recent cross-sectional studies showing shorter hours of daylight and lower daily temperature during cooler seasons negatively impact physical activity in young children (Lovasi et al., 2011;McKee et al., 2012).While promising, it is important to acknowledge that only a limited number of longitudinal studies have investigated seasonal and/or weather effects on physical activity in preschool children, and more research is needed to better understand how temperature, season, and other related factors such as daylight and climate change impact physical activity behaviors.Despite the limitations, findings from the current review suggest that intervention efforts should focus on facilitating greater physical activity behaviors for children during the colder seasons, or when the weather is less than ideal for engagement in outdoor activities.

Parental Behaviors
Previous systematic reviews had inconsistent findings in regard to the relationships between parental behaviors and physical activity in preschool children.While Hinkley and colleagues (2008) suggests that role modeling and direct parental involvement are associated with increased physical activity in young children, De Craemer and colleagues (2012) did not find support for parental modeling and involvement nor other parental behaviors (e.g.parenting style, parental encouragement or enjoyment) in their review.Results from two of three studies in the current review examining this relationship (one high quality and one low quality) found parental physical activity and time spent playing with children to be significant factors related to young children's physical activity (McKee et al., 2012;R. W. Taylor et al., 2009).Specifically, preschool-aged children engage in more physical activity if their parents, in particular fathers, participate in more physical activity and spend more time playing with them (McKee et al., 2012;R. W. Taylor et al., 2009).Beets & Foley (2008) conducted a similar cross-sectional study and found paternal time spent with children being both directly and indirectly related to their child's participation in physical activity.Another study has found the frequency of a father's walking being positively associated with their children's outdoor play (Spurrier, Magarey, Golley, Curnow, & Sawyer, 2008).More research, however, is required to improve our understanding of how paternal and maternal behaviors, and family structures (e.g., dual parent family, single parent family, etc.) impact physical activity in young children.Similarly, it may also be important to understand how parental influences interact with other environmental factors such as weather, climate, and season, as these could have a potential moderating effect.

Weight Status
Weight status, characterized by either percentage body fat or body mass index (BMI), was examined in two studies (one high quality and one low quality) (Bürgi et al., 2011;Jackson et al., 2003).Consistent with the broader literature, including previous reviews (Hinkley et al., 2008;Sallis et al., 2000;Uijtdewilligen et al., 2011), these studies found weight status to be unrelated or weakly associated to physical activity in preschool children.Overall, weight status appears to be more strongly related to sedentary behaviors (e.g.TV viewing or gaming) rather than physical activity (Cox et al., 2012;Kimbro, Brooks-Gunn, & McLanahan, 2011;Uijtdewilligen et al., 2011).The small sample size, however, severely limits the conclusions that can be drawn-though obesity is important factor that should not be discounted, particularly as the rate of obesity among young children continues to rise (Canning, Courage, & Frizzell, 2004;Colley et al., 2013).

Study Limitations
While this was the first systematic review to examine determinants of physical activity among preschool-aged children over time, there are several limitations that need to be acknowledged.First, the review highlights the very limited number of prospective or longitudinal physical activity studies using objective measures of physical activity among preschool-aged children.The current systematic review added another six studies to the three studies previously included by the Hinkley et al. (2008) review.Despite an overall increase in the number of these studies, the number of longitudinal studies is still very limited.Second, although we reviewed prospective or longitudinal studies, many included only one assessment of the factors related to physical activity over time.This assumes, for example, that these determinants are stable: which may not be the case for factors such as parental physical activity behaviors which may fluctuate, or for the weather corresponding to the physical activity assessment period.In order to better understand the factors related to physical activity during early childhood, future research must repeatedly assess potentially time-varying (e.g., weather; parental behaviors) determinants of physical activity in children.Third, most of the included studies were drawn from a convenience sample, which raises questions concerning generalizability and sample bias.Finally, none of the studies included in the current review explicitly state a theoretical framework that the determinants examined were based upon.More longitudinal research grounded in theory is needed to begin to understand the salient factors predicting physical activity in young children.

Future Directions
Overall, there is a clear need for more longitudinal research using objective assessments of children's physical activity among the preschool-age population.The limited evidence thus far suggests that parental behaviors and season/weather were environmental contextual factors that appear to be important for physical activity in young children.Future research must continue to investigate these environmental contextual factors to confirm the conclusions drawn from the current review.Furthermore, studies need to explore other environmental factors that may be influencing their day-to-day physical activities.For example, some preschool-aged children attend day care centers, while others are primarily exposed to the home environment.It is important to consider the structure of care and how that may influence opportunities for physical activity during this early developmental period.Importantly, these studies may begin to demonstrate how it influences changes in physical activity over time, providing insight towards maintenance of sustained participation in physical activity as young children transition into mid-and later childhood.Similarly, it is important to understand how the interactions between social and environmental factors impact behaviors.For instance, it may be possible that some parents use active transport (e.g., walking, biking) to bring children to care centers, or that greater social interactions with peers in these settings through active play may be positively influencing their physical activity behaviors.No studies to date have examined how these interacting biopsychosocial factors influence young children's physical activity.

Conclusion
This systematic review retrieved longitudinal studies using objective measures of physical activity to examine determinants of physical activity in preschool-aged children.Overall, environmental context such as parental behaviors and weather/season appear to be factors that impact young children's physical activity behaviors.However, as the limited number of studies included in the current review, these findings must be interpreted with caution.Overall, in order to better inform various points for interventions, more theory-or model-based prospective research is required to establish the salient determinants of physical activity behaviors among preschool children.

Figure 1 .
Figure 1.Searching procedure and the result of article retrieved.

Table 1 .
Quality of studies included in the current review.

Taylor et al. (2009, 2013) b Study attrition and follow-up duration
cThe percentage of the positive scores.

Table 2 .
Summary of the studies retrieved and main findings.PA was positively associated with motor skills and aerobic fitness at baseline as well as with the longitudinal changes.Specifically, only VPA was related to changes in aerobic fitness.Higher PA was associated with less %body fat at baseline, but not with its change.Conversely, baseline motor skills, aerobic fitness or %body fat were not related to changes in PA.
Day of the week, season, hours of childcare, or birth order did not affect daily PA at any age.