The main findings of this study are that who children spend time with after school is an important influence on physical activity, and that in particular, time spent outdoors with other children is a key context for participation in MVPA. Previous studies have investigated children’s independent mobility and independent physical activity, demonstrating that greater license to leave the home unaccompanied is positively associated with time outdoors  and physical activity . This work builds upon those findings by quantifying the time children spend alone, with adults, or with other children, and matching this with objective measures of physical activity and indoor/outdoor location. Participants reported spending most time alone or with their parents, especially during indoor time which was very rarely spent with other children. Although children spent few minutes outdoors after school, when they were outdoors they were most likely to be with friends. The accumulation of long periods spent indoors alone or supervised by adults and comparatively little time spent outdoors with other children supports the view that there are limited opportunities for primary school children to go outdoors without an adult [24, 44]. This is concerning given that independent mobility has an established association with children’s physical activity [25–27], and that time outdoors is approximately three times more likely to spent engaging in MVPA .
It has been reported in other UK-based studies that approximately one third of children are only allowed outdoors without an adult when in the company of other children . Previous work also suggests that neighbourhood relations and friends are linked to perceptions of safety for both parents and children [45, 46]. Neighbourhood relations and having someone to play with may also positively influence parental decisions about independent mobility . However, from the present cross sectional data it is not possible to distinguish whether time outdoors facilitates being with friends, or whether the companionship of other children is a pre-requisite of parents’ willingness to grant independent mobility. Parents may be vulnerable to a cycle of increased safety concerns linked to limited independent mobility and the subsequent social norm of children not being allowed out to play in the local environment [47, 48]. Valentine & McKendrick  suggest that a move from public play to organised forms of physical activity has prompted suspicion of those children in public space without adults. Similarly, Ergler, Kearns & Witten  report that the normalisation of indoor play is especially pronounced in urban areas because children are unable to use informal areas such as sidewalks. The regression analyses report that alongside outdoor time with friends, indoor time with friends was also positively associated with MVPA. Time spent with other children therefore appears crucial for physical activity, and this is augmented by being outdoors. Recently published longitudinal data report that an increase in the number of friends between primary and secondary school is associated with an increase in girls MVPA . Further longitudinal work is necessary to understand whether the formation of friendship groups is a product of, or fundamental determinant for independent mobility and outdoor physical activity. Based on such work it may be possible to promote physical activity by developing neighbourhood community links amongst children and parents, and seeking to restore the social norm of children using the outdoors as a setting for physical activity.
Parents are reported to be more protective of girls due to greater perceived risk and to subsequently limit their independence [21, 22, 50]. This paper supports this position indicating that indoor contexts are more important for girls’ physical activity than for boys’. Time spent indoors with friends was important for both genders, however periods indoors with siblings or parents were only associated with MVPA amongst girls. These findings echo qualitative work by Brockman, Fox & Jago  which reported that girls were more likely to report active play centred on the home and with family members. Previous research has reported that similar numbers of boys and girls are allowed outdoors without an adult, but that for girls this was more likely to be conditional on other children being present . The strength of association between time spent outdoors with friends or siblings and MVPA in this study supports the hypothesis that girls who do have other children to accompany them outdoors are likely to be more active. Thus while safety in numbers and fostering friendship groups may be important to facilitate after school MVPA , it is encouraging that despite their limited independence girls appear to find ways to be active indoors. These findings tie with those of Atkin, Gorely, Biddle et al.  who found that technology based sedentary behaviour during the ‘critical hours’ was higher amongst boys than girls. Future research and interventions may benefit from not only increasing the time children spend outdoors with others, but also seeking to maximise the potential of indoor environments for physical activity and limiting the time children spend alone.
It is not clear why time outdoors with friends is a particularly valuable source of MVPA. It may be due to the freedom from adult rules and structure [19, 52]. Alternatively, it is possible that children’s movement patterns and behaviours vary depending on whether they are with adults or other children. It has been reported that children’s movement is more meandering when away from adults , and some children like to do activities (such as non-permitted behaviours) outside the view of adults . This paper emphasises the importance of time spent with other children, however it should also be highlighted that many children rely on adults to supervise their activity. Strategies and policy that enable adults to supervise physical activity and encourage families to be active together may be beneficial for individuals across the lifespan. This study also suggests that time spent indoors with adults other than mum/dad is positively associated with MVPA for both boys and girls, and this may be indicative of after school supervision. It is necessary to understand more about what behaviours indoors contribute to MVPA and how these may be manipulated to increase opportunities for physical activity. For example, after school clubs offer a safe indoor environment for physical activity but opportunity for this may be limited due to the inclusion of academic and snack times .
Strengths and limitations
A key strength of this study is the combination of accelerometer, GPS and diary data to describe the context of children’s physical activity. This allowed exploration of not only who children spend their time with, but whether this related to objectively measured location (indoors vs. outdoors) and physical activity. Whilst the sample size was large and was drawn from a number of different primary schools representing a large English city, the results may not be generalisable to other locations or age groups. Furthermore, given that only children who provided matched accelerometer, GPS and diary data were included, the sample may not be wholly representative of the wider population. Some included participants only provided one day of combined data which may limit the reliability of the findings, however the methodology developed and the rich context-specific nature of the physical activity data provide valuable insight into children’s leisure time behaviour and is informative for further research and interventions.
Consistent with previous studies that have combined diary and objective data, there may be errors in the children’s report of their activities and consequent MVPA classification . For example, children may have recorded time spent with friends when in fact they were also under the supervision of an adult. In addition a significant proportion of time between 15:00 and 22.00 was unaccounted for due to missing diary entries reporting who the children were with. Diary records were not available for 40.5% of girls and 38.3% of boys after school time, and the proportion of missing diary data increased by hour up until bedtime. The participants were asked to record what they did after school, and as such periods where their behaviour was unstructured or intermittent may be more difficult to report . This may especially be the case for children who lack the cognitive and linguistic ability to describe their behaviour . Unstructured activity may be more likely to reflect low intensity physical activity so a greater proportion of this might be missing data. This is supported by the fact that missing diary data contributed disproportionally fewer minutes of MVPA. However, approximately one third of MVPA was not recalled and described by children in their diary. Examining the source of this unreported physical activity should be the subject of further research.
It is also likely that there were errors in the differentiation of physical activity location by the GPS receiver. The GPS signal can be lost when outdoors and although children were trained to turn the GPS on when leaving school it is possible that some delay may have occurred leading to loss of outdoor data. Some outdoor data may therefore erroneously be classified as indoors. The present study was cross sectional, only recorded after school weekday activities and did not adjust for clustering within schools. Longitudinal work is required to fully understand the impact of the variables explored here, particularly the influence of the companionship of other children on independent mobility and unstructured outdoor physical activity. Whilst at present it is clear that children’s outdoor time with friends represents a very small proportion of leisure time, this represents an important intervention target. This is because of the potential for change during the after school period, the greater accumulation of MVPA during time spent in this context, the additional social benefits of this type of activity , and the harmful effects of sedentary behaviours occurring indoors .