Background
Outdoor play, sedentary behavior, and moderate-to-vigorous physical activity are related to youth’s health [
1‐
5]. Sedentary behavior describes any behavior in a sitting or declined position requiring less than 1.5 metabolic equivalents [
6], and physical activity describes any skeletal muscle movement leading to energy expenditure [
7]. While increased sedentary behavior is related to physical, behavioral, and psychosocial health problems [
1,
5], increased physical activity is associated with numerous physical and mental health benefits in youth [
2]. Thus, the World Health Organization recommends limiting sedentary behavior, specifically screen-time sedentary behavior, in children and adolescents, and recommends that children and adolescents engage on average in at least 60 min moderate-to-vigorous physical activity a day across the week [
8]. However, prevalence data show that youth across the US and Europe spend between 4 and 12 h a day sedentary [
9] and that globally only about 20% of youth conduct at least 60 min moderate-to-vigorous physical activity daily [
10], which is similar to physical activity levels in Germany when assessed via self-report [
11].
An activity especially relevant for children is outdoor play, defined as any unstructured physical activity taking place outdoors during children’s leisure time [
12]. In children between two and five years, outdoor play was positively related to prosocial outcomes, such as openness and cooperation [
3,
4]. In three- to four-year-old preschoolers, more outdoor play was related to a body-mass-index (BMI) decrease and a reduced risk for obesity at the end of the preschool year [
13]. In another study with about 20,000 Canadian youth in grades six to 10, Janssen [
14] showed that replacing one hour active outdoor play with one hour active video gaming per day was associated with a 6% reduced probability of prosocial behaviour, a 3% reduced probability of high life satisfaction, and a 7% increased probability in high emotional problems. In the Canadian Health Behaviour of School Children study with youth aged 11–15 years, more than 30 min outdoor play were related to a 24% decrease in the prevalence of psychosomatic symptoms, including feeling low and nervous, sleep problems, and bad temper in females, while no significant associations were observed for males [
15]. Although outdoor play is related to several health benefits, across the last two decades, older children and adolescents in Germany decreased outdoor play: Between 2003 and 2006, 62% of children (11–13 years) and 37.2% of adolescents (14–17 years) engaged in more than three days outdoor play per week, however, between 2014 and 2017, this decreased to 50.1% and 14.6%, respectively, with similar trends for boys and girls [
16], whereas outdoor play remained stable in children between four and ten years with 79.8%–91.2% engaging in outdoor play on more than three days per week [
16]. Thus, although the highest outdoor play levels are observed in children up to 10 years, the data shows that outdoor play is still salient in older children and adolescents [
16].
As outdoor play is understood as a behavior that includes activity [
12], several studies investigated relationships between acute sedentary behavior, physical activity, and outdoor play. A meta-analysis of mostly cross-sectional studies revealed that children (two to five years) in childcare centers spent 44% of outdoor play time in total physical activity, but only 14% in moderate-to-vigorous physical activity, whereas 53% of outdoor play time was sedentary based on accelerometer measurement [
17]. Other studies investigated associations between habitual physical activity, sedentary behavior, and outdoor play. A recent systematic review about correlates of outdoor play in children between three and 12 years supported positive associations between outdoor play and physical activity, while there was largely no relationship observed between screen time or screen exposure as a proxy for sedentary behavior and outdoor play [
18]. Again, most of the studies were cross-sectional and most of the studies looked at children in preschool age [
18]. Fewer studies investigated associations between habitual outdoor play, moderate-to-vigorous physical activity, and sedentary behavior in older children: In 11-year-old children, high levels of outdoor play were associated with more moderate-to-vigorous physical activity and less sedentary behavior on weekend days for boys and girls, while on weekdays, these associations were only observed for boys [
19]. In another cross-sectional study with six-to-13 year-olds, engaging in outdoor play on one or more days per week was associated with a 23–62 min increase in moderate-to-vigorous physical activity in boys, while those associations were not observed for girls [
20]. A long-term follow-up study investigated participation in outdoor play in ten-year-old children and sports and physical activity participation at the age of 42 years, showing that outdoor play was not predictive of physical activity in adulthood [
21]. However, this study did not investigate longitudinal associations across the developmentally important period of childhood and adolescence.
In summary, several studies have shown a positive relationship between acute and habitual outdoor play and physical activity cross-sectionally, with most investigations targeting children in the elementary school age, while there is little known about the relationship between outdoor play and sedentary behavior. Thus, there are research gaps regarding 1) associations between outdoor play, sedentary behavior, and physical activity, and in a broad age range of children and adolescents, and 2) longitudinal associations between outdoor play, sedentary behavior, and physical activity across childhood and adolescence.
The aim is to address those research gaps with two studies. To address the first research gap, Study 1 aims to investigate relationships between outdoor play and accelerometer-assessed moderate-to-vigorous physical activity and sedentary behavior in a cross-sectional sample of children and adolescents between six and 17 years. Study 2 aims to address the second research gap by investigating prospective associations with outdoor play and self-reported moderate-to-vigorous physical activity and screen-time sedentary behavior in a sample of children with three measurement timepoints across 11 years. The different measurement instruments are due to the fact that accelerometer data were not available at the first two measurement time points.
The STROBE statement [
22] guided the reporting of this study.
Discussion
The aim of this study was to investigate cross-sectional and prospective associations between outdoor play, sedentary behavior (screen-time), and moderate-to-vigorous physical activity.
Study 1 showed that engagement in outdoor play was associated with lower accelerometer-measured sedentary behavior, which is supported by previous findings that used device-based assessment of sedentary behavior [
19]. Regarding socio-demographic predictors, older age was the strongest predictor of sedentary behavior among youth. In our sample, each one-year increase in age was associated with about half an hour-increase in sedentary behavior per day. A similar strong result has been observed in a previous study with nine- to 11-year-old children, with a one-year increase being associated with a 26-min increase in sedentary behavior per day [
43]. Also, other studies have consistently reported positive associations between sedentary behavior and children’s age [
9,
44]. The strong increase in sedentary behavior may be explained via more study time in higher school grades [
45], more sedentary behavior in school [
46], as well as more leisure-time engagement in screen-time behaviors such as gaming and recreational internet use [
11]. Also, our results indicate that girls spend more time in sedentary behavior than boys, which is in line with previous findings [
47]. A reason for this could be that girls engage in various types of sedentary behavior that go beyond screen-time sedentary behavior, such as homework and talking on the phone whilst being similar to boys in screen-time sedentary behavior [
48], resulting in more total sedentary behavior. Interestingly, in contrast to physical activity, there was no interaction observed with age, which means that at any age between six and 17 years, more outdoor play is associated with lower sedentary behavior. A reason for this could be that older children and adolescents still engage in active types of outdoor play, which may not be intense enough to translate into moderate-to-vigorous physical activity, but still be more active than sedentary behavior (e.g., playing at a swimming pool, meeting for a round of badminton).
Regarding moderate-to-vigorous physical activity, having a normal weight and a high socio-economic status were positively related to moderate-to-vigorous physical activity, whereas being a girl was negatively related to moderate-to-vigorous physical activity, which is in line with previous findings [
49,
50]. The gender gap observed in physical activity in our data is comparable to other high-income Western countries in Europe [
51]. The reasons why girls have lower physical activity levels in general are complex, including less enjoyment and confidence in their sporting abilities, gender norms [
52], stronger perceived physical activity barriers in girls, such as lack of time, social influence, and willpower [
53], as well as limited independent mobility of girls compared to boys [
54]. Engagement in ≥2 h of outdoor play was associated with increased moderate-to-vigorous physical activity, with this relationship being moderated by sex, showing that more outdoor play is only related to more moderate-to-vigorous physical activity in boys. This is similar to findings in previous studies, showing that outdoor play is mostly unrelated to moderate-to-vigorous physical activity in girls [
19,
20]. One explanation could be that girls in Germany engage in less active types of outdoor play than boys [
55], which might result in lower activity intensities that do not add to moderate-to-vigorous physical activity. Another reason could be that boys across countries are allowed to play outdoors with less supervision and with a larger spatial range from the residential home [
56], resulting in more outdoor play opportunities.
The association between outdoor play and moderate-to-vigorous physical activity was also moderated by age, indicating that outdoor play was only related to increased moderate-to-vigorous physical activity until around 15 years. One reason could be that children engage in more intense types of outdoor play, such as rope skipping, running around in the garden, or playing games such as tag with other children. In contrast, adolescents may engage in less intense types of outdoor play, such as passing a ball to each other. However, to investigate the underlying mechanisms for this interaction, a more specific analysis of outdoor play would be necessary to investigate in which types of outdoor play children and adolescents engage at which age and how the different types relate to moderate-to-vigorous physical activity [
18].
Study 2 revealed that outdoor play predicts PC/Gaming as one type of screen-time sedentary behavior from early to later childhood, but not TV watching or moderate-to-vigorous physical activity. No other longitudinal studies were found that investigated outdoor play, and screen-time sedentary behavior across childhood and adolescence, while cross-sectional findings to date showed largely no relationship between screen time or exposure and outdoor play [
18]. A reason why outdoor play negatively predicts PC/Gaming but not TV watching could be that outdoor play is an active behavior [
12], while PC/Gaming has also been categorized as an active (sedentary) behavior as it requires mentally active engagement [
57]. Contrary, TV watching is categorized as a passive sedentary behavior [
57]. Thus, if children develop outdoor play as an active behavior in young childhood, they might become less involved in screen-based active behaviors in later childhood, while TV watching as a passive behavior seems to be independent of outdoor play. Hence, outdoor play may be a leisure pursuit that can replace computer and gaming time. When considering outdoor play as a replacement for sedentary behavior, it would be valuable to investigate the type of outdoor play that can replace sedentary behavior in the future. This is especially relevant as replacing sedentary time with some physical activity, including light-intensity physical activity as we may see in older children’s and adolescent’s outdoor play, show favorable associations with cardiometabolic risk factors, such as waist circumference [
58], as well as with mental health outcomes [
59]. No prospective relationship was found between outdoor play and PC/Gaming from late childhood to adolescence. Although the stability coefficient indicated that children’s outdoor play behavior tracks to adolescence, outdoor play decreases in general with age. Thus, in adolescence, outdoor play might be independent of sedentary behavior and moderate-to-vigorous physical activity.
Regarding outdoor play and moderate-to-vigorous physical activity, we are not aware of another study that investigated prospective relationships between the two behaviors during childhood and adolescence. A long-term follow-up study that investigated relationships between childhood outdoor play and sports or physical activity participation in adulthood supports the results of this study [
21]. Outdoor play consists of various activities, however not every activity is conducted with a health-enhancing intensity. In fact, one analysis showed that preschool children spent about half of their outdoor play in physical activity, but only 14% in moderate-to-vigorous physical activity [
17], indicating that outdoor play rather relates to light intensity physical activity. Another reason could be that outdoor play and moderate-to-vigorous physical activity should be assessed more specifically as they both comprise various activities.
As stated, all cross-lags not related to outdoor play have already been investigated in this sample, thus, we refer to Nigg et al. [
5] for a discussion of those results.
Considering results of Study 1 and Study 2, both studies support a relationship between outdoor play and sedentary behavior (screen-time) and that this relationship is present for boys and girls. Hence, outdoor play may be a behavior across the developmental stage of childhood and adolescence that has the potential to replace sedentary behavior and thus promote both physical and mental health in children and adolescents [
58,
59]. Regarding moderate-to-vigorous physical activity, Study 1 and Study 2 had inconsistent results: While Study 1 supported a relationship between outdoor play and moderate-to-vigorous physical activity, no longitudinal associations were observed in Study 2. A reason for this could be the focus on sports-related moderate-to-vigorous physical activity in the self-reported data, which neglects moderate-to-vigorous physical activity in daily life (Study 2), while daily life moderate-to-vigorous physical activity is captured through accelerometer-measurement, which might be more relevant for outdoor play (Study 1). Another reason could be that the assessment of outdoor play in days per week (Study 2) is not sensitive enough to detect associations compared to hours per day (Study 1). Thus, future studies should investigate longitudinal associations between the three behaviors using more fine-grained outdoor play operationalization and device-based assessment of moderate-to-vigorous physical activity and sedentary behavior.
Limitations and strengths
There are some limitations to be considered. In Study 2, data is based on self-report. Temporal order of variables could be established but not causality [
34,
40] and the duration between assessments was long. Only PC/Gaming time along with TV watching were assessed for sedentary behavior, not accounting for other non-screen-based sedentary behavior, e.g., homework or reading. For both Study 1 and Study 2, assessment for one measurement timepoint across the whole year, which may lead to seasonality effects; however, the MoMo physical activity questionnaire specifically asks for activity types across all months to counteract those effects. Finally, differences between study completers and non-completers may have influenced the associations investigated.
Nonetheless, to our best knowledge, this is one of the first studies that investigated prospective associations between children’s and adolescent’s outdoor play, sedentary behavior and moderate-to-vigorous physical across more than a decade. This allowed to explore potential relationships regarding the temporal order of the variables, as to date, mostly cross-sectional studies are available [
60]. Also, to our best knowledge, no studies explored the relationship between outdoor play and accelerometer-measured moderate-to-vigorous physical activity and sedentary behavior in a large sample of children and adolescents in Germany across a broad age range, with this studies’ results mirroring results from previous studies in other countries [
19,
21,
60].
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