Objectively measured physical activity in four-year-old British children: a cross-sectional analysis of activity patterns segmented across the day

Physical activity plays an important role in children’s physical and mental wellbeing [1, 2], with particular benefits for preschool children’s (3-5 years) social, gross motor and development skills [3, 4]. Although the long-term benefits of being physically active from a very young age have yet to be confirmed, emerging evidence suggests that active children under 5 have better health and cognitive outcomes compared to less active peers, with positive habit formation early in life likely to be beneficial [5]. Establishing a high baseline level of physical activity when young could be especially important given that activity levels are known to decrease from late childhood, through adolescence into adulthood [6, 7].

Whilst there is a perception that young children are constantly active [8, 9], prevalence estimates suggest that this may not be the case. A review in 2010 found that preschool children accrue only limited amounts of physical activity during the preschool day, are sedentary for large amounts of time, and fail to meet the then recommended activity guidelines (>60 minutes MVPA) [10]. These guidelines, from National Association for Sport and Physical Education (NASPE), advocated 60 minutes of structured physical activity, with at least 60 minutes and up to several hours of further unstructured play each day for the under-5 s [11]. Yet when adherence to these recommendations was assessed across 39 studies, only 54% of children met these guidelines [12].

More recently, new physical activity guidelines for preschool-aged children have been published in Canada [13], Australia [14], the UK [15], and the USA [16]. Broadly similar, they recommend preschool-aged children should be active for 180 minutes daily. Importantly, these guidelines do not specify activity intensity, instead focusing on maximising activity throughout the day [13–15]. Recommendations for sedentary behaviour in this age group also vary internationally, with UK recommendations stating that the time children spend sedentary or restrained when not asleep should be limited [15]. Other guidelines advocate limiting screen time to 1 hour or less per day (Australia [14]) or restricting sedentary time to less than 1 hour at a time (US [16]). Using these new guidelines, published evidence from Australia [17] suggests young children are active for around 130 minutes on average each day, equating to roughly 16% of measured time. The amount of moderate-to-vigorous physical activity (MVPA) children engaged was approximately 4.8% [17].

Most studies investigating activity levels in preschool children have focused on average activity levels. However, there is some suggestion that activity patterns differ over the course of the day and week [18]. Understanding differences in activity levels over the course of the day allows focussed intervention efforts to be developed, taking into account when children may be less active and therefore predisposed to efforts to increase activity. This paper aims to describe levels and patterns of activity in a population-based sample of 4-year-old British children. In addition to assessing children’s average activity levels, we were also able to explore how activity patterns change across the day, segmenting activity using hourly time-stamped data. These novel analyses therefore allow us to determine how children’s differing intensities of activity change throughout the day and how temporal and demographic factors influence this activity.

All children met the current UK recommended guidelines of 180 minutes of activity per day (at any intensity above sedentary) on 100% of days measured. On just over half of the days measured, children engaged in >60 minutes of MVPA (former activity guideline), with boys meeting this guideline on significantly more measurement days than girls (55 vs. 49% of measurement days, p < 0.001).

Children’s average hourly activity levels across the day are shown in Figure 1. Although average daily time spent in sedentary activity, LPA and MVPA showed few differences by subgroups, multiple differential associations for all outcomes were seen when activity was segmented across the day (Tables 3, 4 and 5). Sensitivity analyses showed that results did not differ when children ≤2 days of valid activity data were included or excluded from analyses, nor when children with and without one valid weekday and weekend day were included or excluded. All children with ≥1 day of valid activity data were therefore included in analyses.

Girls were more sedentary (β = 6.3, (95% C.I. 1.8, 10.8)) and less active (LPA: -8.5 (-14.6, -2.4)) in the mornings, engaging in 18% less MVPA (GMR:0.82 (0.73, 0.91)) compared to boys. Girls also engaged in less MVPA in the evenings (0.83 (0.74, 0.94)). Underweight children were more sedentary in the afternoons compared to their normal weight peers. Compared with children whose mothers left school at or before 16, children whose mothers left school after 18 were more sedentary and less active (engaging in less LPA and MVPA) in the evenings.

Children who attended preschool full-time were more sedentary and less active in the mornings compared to children attending part-time. Moreover, children were also more sedentary and less active on weekend mornings compared to weekdays. The reverse was true on weekend afternoons and evenings: children were less sedentary and engaged in more MVPA on weekend afternoons, and were more active in the evenings compared to weekdays. Compared to winter months, children engaged in less LPA in the morning during summer, but were also less sedentary and engaged in more MVPA on summer evenings.

This study showed that British 4-year old children spend on average 67% of their day being active, engaging predominately in LPA (~88% of active time). Although few differences in average activity levels were observed, the current study provides novel information about how temporal and demographic factors differentially influence children’s activity when segmented across the day. Child’s sex and weight status, age their mother left full-time education, attending childcare full-time, time of the week and season were all independently associated with children’s activity levels at different times of the day. These time-specific observations are likely to be important for intervention development, indicating that focussing on (specific subgroups in) periods when children are less active, for example girls in the mornings, or during winter months, may result in larger increases in children’s activity.

Based on the data presented here, all children met the current UK activity guidelines. Although these guidelines are comparable to Australian recommendations, previous estimates indicate that only 5.1% of Australian children met these guidelines [17]. The proportion of time children spent active in this study (~67%) was also higher than reported previously in studies in Europe (~16%) [37, 38] and Australia(~16%) [17]. Several factors, including the population studied, the measure of physical activity and wear and processing protocols, are likely to contribute to these differences across studies. These data were collected in a population-based sample, as opposed to a pre-school or care-setting based sample as is more commonly used [17, 38]. Moreover, previous studies have predominantly used Actigraph accelerometers worn on the hip during waking hours and taken off during water-based activities [17, 37, 38]. The waterproof Actiheart monitor, worn continuously for 24 hours each day, is therefore likely to have captured more of children’s daily activity. Further, the cut points used here, equivalent to those previous applied to Actigraph data [30, 31, 39], were derived experimentally and validated against doubly labelled water [29]. As other studies have used higher cut points to classify active time [17, 37, 38], they are likely to report lower levels of activity. Taken together, varying study methods and lack of raw count per minute data to compare between studies [40], make comparison of prevalence estimates of young children’s activity, and their compliance with guidelines, problematic [27].

We observed that children spend the majority of their time in LPA. Provided that ≥100 cpm is a valid threshold for LPA [30, 31], and even accounting for differences in classification according to accelerometer cutpoints [41], children’s light activity levels are ‘sufficient’ to comfortably satisfy current activity guidelines. It may however be important to consider whether specifying activity intensity along with a total activity guideline is necessary for this age-group. LPA may confer limited health benefits for young children, if, as is the case in older children and adults, benefits to health are associated with more vigorous intensity activity [42, 43]. Indeed, in this sample of children, MVPA (and not LPA) has been found to be positively associated with bone density [44] and vigorous activity with lower fat mass [45]. The relevance of this debate is further highlighted by the recent observation that more than 1 in 5 UK children are either overweight or obese on entering school at age 5 [46], which seems at odds with observations that all children are sufficiently active. The importance of activity intensity in younger children therefore requires further investigation to inform future activity guidelines for the under-5 s.

Increasingly, evidence suggests that even at a young age boys are more active than girls [17, 37, 47], which was also observed here. This study adds to the current literature by highlighting that these differences are not consistent throughout the day. Sex differences were most apparent in the mornings, with girls being relatively less active and more sedentary than boys. In contrast, children’s weight status did not appear to have an influence on children’s activity across the day. As similar finding have been seen previously in this age group [18], these patterns may be indicative of preschool-aged children’s activity across the day, and future interventions may need to consider a temporal focus to differentially target boys and girls.

Interestingly, children who attended nursery full-time compared to part-time were more sedentary and less active in the mornings. It has been suggested previously that childcare may influence children’s physical activity levels [10], but the measure used here provides only a basic idea of whether children were usually in childcare full- or part-time. As it was not possible to determine where children were during the measurement week, further work is required to determine whether activity levels differ by time spent in childcare and what factors in the childcare environment influence children’s activity.

The differences in daily patterns observed between week and weekend days are likely to reflect families’ normal working weeks, with children typically rising and going to bed earlier on weekdays. What is not clear is why children whose mothers left school at a later age were also more likely to be sedentary and less active in the evenings. As maternal employment status was not collected at the age four visit, it is not possible to determine whether this finding relates to more of these mothers being in full-time work. However, it has been shown previously that three-year-old children with more highly educated mothers are more likely to have a set bedtime and bedtime routine [48], which may plausibly account for the children of higher educated mothers being less active in the evenings here.

Although not studied directly, it is likely that the observed seasonal variation, with greater activity and less sedentary time in the evenings during the summer compared to winter, reflects the longer daylight hours of summer months. Contrary to previous observations in preschool children [49], children’s average daily MVPA was higher in spring compared to winter, which appears to be the result of a consistent accumulation of more activity throughout the day. Through this novel and detailed investigation of the temporal differences in behaviour, we are able to highlight opportunities to encourage activity in children, such as specifically targetting activity during the mornings for girls, or facilitating day-time activity opportunities for preschoolers during the autumn and winter months.

Four-year old British children accumulate sufficient activity over the day to meet current guidelines, although this activity is predominantly of light intensity for which the health benefits remain to be determined. In addition, patterning of activity varies over the day by demographic and temporal factors. These should be taken into account when developing new interventions to increase preschoolers’ physical activity, for example focusing on encouraging girls’ morning activity and providing opportunities for daytime activity in winter months.