A Review of Preschool Children’s Physical Activity and Sedentary Time Using Objective Measures Jill A. Hnatiuk, MSc, Jo Salmon, PhD, Trina Hinkley, PhD, Anthony D. Okely, PhD, Stewart Trost, PhD Context: Identifying current physical activity levels and sedentary time of preschool children is important for informing government policy and community initiatives. This paper reviewed studies reporting on physical activity and time spent sedentary among preschool-aged children (2–5 years) using objective measures. Evidence acquisition: Databases were searched for studies published up to and including April 2013 that reported on, or enabled the calculation of, the proportion of time preschool children spent sedentary and in light- and moderate to vigorous–intensity physical activity. A total of 40 publications met the inclusion criteria for physical activity and 31 met the inclusion criteria for sedentary time. Objective measures included ActiGraph, Actiwatch, Actical, Actiheart, and RT3 accelerometers, direct observation, and Quantum XL telemetry heart rate monitoring. Data were analyzed in May 2013. Evidence synthesis: Considerable variation in prevalence estimates existed. The proportion of time children spent sedentary ranged from 34% to 94%. The time spent in light-intensity physical activity and moderate to vigorous–intensity physical activity ranged from 4% to 33% and 2% to 41%, respectively. Conclusions: The considerable variation of prevalence estimates makes it difficult to determine the “true” prevalence of physical activity and sedentary time in preschool children. Future research should aim to reduce inconsistencies in the employed methodologies to better understand preschoolers’ physical activity levels and sedentary behavior. (Am J Prev Med 2014;47(4):487–497) & 2014 American Journal of Preventive Medicine

Introduction

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ptimizing time spent being physically active and minimizing sedentary time are important foci for children’s health from a very young age.1,2 Recommended amounts of physical activity for preschool children (i.e., those aged 2–5 years who have not yet started school) have been operationalized into public health guidelines. The National Association for Sport and Physical Education (NASPE) in the U.S. recommends that preschool-aged children participate in at least 60 minutes of structured and 60 minutes of unstructured physical From the School of Exercise and Nutrition Sciences (Hnatiuk, Salmon, Hinkley), Deakin University, Melbourne, Victoria; Interdisciplinary Education Research Centre (Okely), University of Wollongong, Wollongong, New South Wales; and the School of Human Movement Studies (Trost), University of Queensland, Brisbane, Queensland, Australia Address correspondence to: Jill A. Hnatiuk, MSc, School of Exercise and Nutrition Sciences Deakin University, 221 Burwood Hwy, Burwood, Victoria 3125, Australia. E-mail: [email protected]. 0749-3797/$36.00 http://dx.doi.org/10.1016/j.amepre.2014.05.042

& 2014 American Journal of Preventive Medicine

activity (intensity of physical activity not specified) each day and should not be sedentary for more than 60 minutes at a time except when sleeping.3 In Australia, the United Kingdom, and Canada, recently endorsed guidelines suggest preschool children should (1) be physically active for at least 3 hours every day (accumulated throughout the day including light-, moderate-, and vigorous-intensity physical activity); (2) spend less than 1 hour per day using electronic entertainment media; and (3) not be sedentary, restrained, or kept inactive for more than 1 hour at a time (with the exception of sleeping).4–7 Accurately quantifying preschool children’s participation in these health behaviors is important for informing government policy and community initiatives: first, for determining the necessity of a focus on this age group (i.e., if most young children are highly active then they would be less of a priority compared with other groups), and second, for monitoring the effectiveness of any existing strategic government initiatives targeting these behaviors. However, little is known about the proportion of preschool children meeting physical


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activity and sedentary recommendations, as few countries have conducted national prevalence surveys in this age group. Lack of prevalence information may be attributed to the challenges associated with assessing these behaviors in young children.8 Population prevalence surveys typically employ self- or proxy-report questionnaires,9 and capturing these behaviors in young children using these subjective techniques may result in problems with the data such as recall bias, lack of comprehension, and socially desirable responses.10 In addition, use of different survey measures also makes it difficult to compare across samples. Objective measures such as heart rate telemetry and direct observation techniques have traditionally provided more valid and reliable measures of young children’s physical activity and sedentary time than proxy reports.11 However, the development of motion sensors has provided the field with a viable alternative. Accelerometers enable objective estimates of young children’s physical activity12 and more recently have also been used to assess time spent sedentary.13 Accelerometers, heart rate telemetry, and direct observation capture the intensity of movement and can therefore provide estimates of young children’s time spent sedentary and in light-, moderate-, and vigorous-intensity physical activity.13 There is considerable conjecture over which accelerometer data cut-points accurately reflect various intensities of activity among preschool children, which has previously been described thoroughly in papers.8,13 Several validated preschool-age cut-points exist in the literature, and these cut-points are considerably different from one another. The variability in applied cut-points has been shown to have a strong influence on the interpretation of moderate to vigorous–intensity physical activity prevalence estimates.14 Four review papers8,14–16 have been published that synthesize aspects of preschool children’s objectively assessed physical activity. However, none of the reviews reported the percentage of time children spent in lightintensity physical activity or in moderate- or vigorousintensity physical activity, none reported the percentage of time children spent sedentary, and none synthesized evidence from studies using only objective measures of physical activity, including heart rate telemetry and direct observation. Reporting prevalence estimates using the percentage of time spent in these various intensities of activity is important for standardizing different amounts of wear time between participants and studies, and making estimates between different measures more comparable. Consideration of young children’s participation in light-intensity physical activity and time spent

sedentary, particularly prolonged sitting, are important given that these behaviors are a part of the public health recommendations.4–6 It is also important to distinguish between time spent in light-intensity physical activity and moderate to vigorous–intensity physical activity for two reasons: (1) emerging evidence suggests that engagement in light-intensity physical activity may elicit health benefits in older children17–19; and (2) it may be easier to reduce sedentary time by incorporating light-intensity physical activity rather than moderate to vigorous–intensity physical activity in preschool children. Additionally, reporting young children’s participation in physical activity and sedentary behavior using objective techniques other than accelerometry is vital given that considerable variability exists in accelerometer cut-points validated in preschool children.13 The inclusion of other objective measures enables the comparison of these estimates to accelerometer studies using different cut-points. Therefore, the aim of this paper was to review studies reporting on, or allowing the calculation of, the percentage of time spent in light- and moderate to vigorous–intensity physical activity and being sedentary among preschool children (aged 2–5 years) using a range of objective measures.

Methods The following electronic databases were searched: Medline, PubMed, Education Resources Information Center (ERIC), Australian Education Index, PsycINFO, Current Contents, Social Science Index, SPORTDiscus, Child Development Abstracts, Google Scholar, and Health Reference Center–Academic. Manual searches of the reference lists of recovered articles and the authors’ extensive personal files were also conducted. The search targeted primary research articles and systematic reviews published up to and including April 2013. The key words used in the computer searches included physical activity, physical inactivity, sedentary behavior, guidelines, recommendations, compliance, prevalence, measurement, assessment, toddler, early childhood, preschool, accelerometer, heart rate, observation, and objective. The annual subject and author indexes of relevant journals in the field of physical activity and health, which are not included in the databases listed above, were also examined. In this paper, studies that met the following inclusion criteria were reviewed: a minimum of 30 children20; incorporated children with a mean age of 5.9 years or younger; utilized an objective measure to assess light-intensity physical activity (typically defined as activity o3 or 5 METs), moderate to vigorous–intensity physical activity (typically defined as activity 43 or 4 METs), or sedentary time; and collected data across the whole day and provided estimates of physical activity or sedentary behavior as the percentage of time spent in these behaviors or included sufficient information within the paper to be able to calculate such percentages (i.e., the average www.ajpmonline.org

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Hnatiuk et al / Am J Prev Med 2014;47(4):487–497 accelerometer wear time or observation period was reported). A minimum wear time criterion for studies using accelerometry was not applied. Additionally, studies that included data in a particular setting (e.g., preschool or child care setting) were included, provided that data were collected over most of the day and included indoor and outdoor settings. Only baseline data from intervention studies were included. If data were reported separately for groups relevant to the study (e.g., by seasons, intervention/control), the percentage of time spent physically active and sedentary was averaged between groups. Similarly, if no significant difference in physical activity or sedentary behavior was reported between boys and girls, only the group mean was reported. If a significant difference was observed, boys’ and girls’ data were presented separately in the summary table with an overall average calculated and used in the figures. Studies were excluded if they used accelerometry but only reported counts or counts per minute, or if they used pedometers only. Where two or more publications came from the same sample, the decision was made to only include data from the study with the larger sample size. Data were analyzed in May 2013.

Results Description of Physical Activity Prevalence Studies A total of 40 publications from 37 samples met the inclusion criteria (Appendix Table 1, available online). Two sets of publications came from the same sample,28,39,58–60; thus, only the studies with the largest sample sizes were included.28,39 The results presented in the publications from the same sample group reported comparable prevalence estimates. The highest proportion of samples (18) came from the U.S.,21–23,29,30,32,33,36,41,42,46,49,50,52,54,55–57 six were from Scotland, 27,28,34,38,44,45 five were from Australia,25,26,31,37,47 with one each from Belgium,24 Switzerland,39 Portugal,48 Ireland,35 England,51 Mexico,53 and Canada.40 Additionally, one study reported results on children from Sweden and the U.S.43 Twenty-nine studies used ActiGraph accelerometers (ActiGraph, Pensacola, FL); one study each used Actiwatch accelerometers (MiniMitter Co., Sun River, OR), Actical accelerometers (MiniMitter Respironics, Bend, OR), Actiheart accelerometers (CamNtech Ltd., Cambridge, UK), and RT3 accelerometers (StayHealthy Inc., Monrovia, CA); three used direct observation; and one used Quantum XL telemetry heart rate monitoring (AMF Co., Jefferson, IA). Table 1 summarizes the different cut-point thresholds in the studies that used accelerometers. Of these studies, 1121–26,31,37,41,43,47 used Sirard’s64 cut-points; six29,39,40,42,48,49 used Pate’s62 cut-points; eight27,28,34,35,38,44–46 used Puyau’s66 cut-points (children and youth); three30,32,36 used Freedson’s67 cut-points for 6-year-old children; and one33 used Freedson’s cut-points extrapolated for 5-year-old children. October 2014

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The study that used the Actiheart accelerometer used Actiheart cut-points equivalent to the Pate cut-points, and the studies that used the Actical52 and RT353 accelerometers used the cut-points of Pfeiffer63 and Rowlands,68 respectively. The study that used the Actiwatch accelerometer50 did not specify the counts per minute (CPM) threshold used to identify moderate to vigorous–intensity physical activity. The three studies that used direct observation to assess children’s physical activity used the Observation System for Recording Activity in Preschools (OSRAP). One of the studies54 used eight 32-minute observation periods over two consecutive days (direct observation periods excluded nap and meal times) during child care whereas the other observed children for 5–6 hours over 1 day at preschool.55 The third study observed children for ten to 12 observation periods of 30 minutes over 10 days at preschool.56 The study using heart rate telemetry considered a heart rate 4140 beats/minute as moderate to vigorous–intensity physical activity.57 Sample sizes of the studies ranged from 30 to 703. Most studies reported physical activity levels among 3–5-year-old children, few included children aged younger than 3 years, and some also incorporated children aged 6–7 years. Most studies (22)21–25,27,28,31,34,35,37,38,40,44–46,48–50,54–56 directly reported the percentage of time spent in physical activity, but a number (15) 26,29,30,32,33,36,39,41–43,47,51–53,57 were calculated indirectly by the authors.

Physical Activity Prevalence Estimates Figure 1 illustrates the proportion of time spent in moderate to vigorous–intensity physical activity as assessed by accelerometers (ActiGraph, Actical, Actiwatch, RT3); heart rate monitoring; and direct observation. The accelerometer estimates ranged from 1.7% to 41.2% of the day reported as being spent in moderate to vigorous–intensity physical activity. Standardized to a 13-hour day (assuming an average of an 11-hour sleep per night for a 4-year-old child),69 this corresponded to between approximately 13 minutes and 5.4 hours spent in moderate to vigorous–intensity physical activity. The only heart rate monitoring study in this review estimated that young children spent 7% (approximately 54.6 minutes) of their day in moderate to vigorous– intensity physical activity. The three direct observational studies54–56 estimated that children spent between 3% and 12% (approximately 23 minutes to 1.6 hours) of their day in moderate to vigorous–intensity physical activity. The median prevalence estimate reported by all studies in this review was 5.9% (approximately 47 minutes) of the day spent in moderate to vigorous–intensity physical activity.

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Table 1. Accelerometer cut-points for physical activity or sedentary time used in included studies Study population in which cut-points validated (years)

Study

Accelerometer

Cut-points

Reilly61

ActiGraph

3–4

Sedentary: o1,100 CPM

Pate62

ActiGraph

3–5

Sedentary: 0–37.5 counts/15 seconds

Cut-points (standardized to counts/15 seconds if applicable) Sedentary: o275 counts/15 seconds N/A

Light: 437.5 counts/15 seconds Moderate: 4420 counts/15 seconds Vigorous: 4842 counts/15 seconds Pfeiffer

63

Actical

3–5

Moderate: Z715 counts/15 seconds

N/A

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Vigorous: Z1,411 counts/15 seconds Sirard

64

ActiGraph

3

Sedentary: 0–301 counts/15 seconds

N/A

Light: Z302 counts/15 seconds Moderate: 4615 counts/15 seconds Vigorous: Z1,231 counts/15 seconds 4

Sedentary: 0–363 counts/15 seconds

N/A

Light: Z364 counts/15 seconds Moderate: 4812 counts/15 seconds

N/A

Vigorous: Z1,235 counts/15 seconds 5

Sedentary: 0–398 counts/15 seconds

N/A

Light: Z399 counts/15 seconds Moderate: 4891 counts/15 seconds Vigorous: Z1,255 counts/15 seconds Evenson65

ActiGraph

5–8

Sedentary: 0–25 counts/15 seconds

N/A

Light: 26–573 counts/15 seconds Moderate: 574–1,002 counts/15 seconds

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Vigorous: Z1,003 counts/15 seconds Actical

5–8

Sedentary: 0–11 counts/15 seconds

N/A

Light: 12–507 counts/15 seconds (continued on next page)

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Figure 2 shows the percentage of time spent in lightintensity physical activity per day for each of the identified studies. All but two used accelerometers to assess physical activity, and the results of time spent in light-intensity physical activity ranged from 3.9% to 32.6%. The studies that used direct observation reported that approximately 8%–11% of the day was spent in lightintensity physical activity. Over a 13-hour waking day, the estimates from accelerometry and direct observation corresponded to between approximately 4.2 hours and 30 minutes in light-intensity physical activity. The median prevalence estimate reported by all studies in this review was 16.9% (approximately 2.2 hours) of the day spent in light-intensity physical activity.

Vigorous: Z583 counts/15 seconds

Moderate: Z243 counts/15 seconds

Vigorous: Z743 counts/15 secondsa

Moderate: Z154 counts/15 secondsa

Vigorous: Z2,050 counts/15 seconds

Moderate: o2,050 counts/15 seconds

Light: o800 counts/15 seconds

Sedentary: o200 counts/15 seconds

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Vigorous: Z2,330 CPM

Moderate: Z970 CPM

a

Cut-points derived from equation for a 6-year-old child. CPM, counts per minute.

8–10 (boys) RT3 Rowlands68

Vigorous: Z2,972 CPMa

Vigorous: Z8,200 CPM

Moderate: o8,200 CPM

Moderate: Z615 CPMa 6–18 ActiGraph Freedson67

Puyau

Light: o3,200 CPM

Sedentary: o800 CPM 6–16 ActiGraph

Vigorous: Z719 counts/15 seconds

Description of Sedentary Time Prevalence Studies

66

Moderate: 508–718 counts/15 seconds

Cut-points Study population in which cut-points validated (years) Accelerometer Study

Table 1. Accelerometer cut-points for physical activity or sedentary time used in included studies (continued)

Cut-points (standardized to counts/15 seconds if applicable)

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Thirty-one publications from 30 different samples met the inclusion criteria (Appendix Table 2, available online). Two publications came from the same sample28,58; thus, the results from only one of the studies were included.28 Thirteen studies were from the U.S.,21–23,33,41,42,46,49,54–57,70 six were from Scotland,27,28,34,38,44,45 three were from Australia,25,26,37 and the remainder were from Belgium,24 Ireland,35 Portugal,48 Switzerland,60 England,51 New Zealand,71 and the U.S. and Sweden.43 Sample sizes ranged from 32 to 545. Children’s ages ranged from 3 to 7 years, with most studies reporting mean ages of 3–5 years. Most studies21–25,27,28,34,35,37,38,44–46,48,49,54–56,71 directly reported the percentage of time spent sedentary, but some26,33, 41–43,51,57,60,70 were calculated by the authors. Twenty-four studies used ActiGraph accelerometry, one study used Actiheart accelerometry, and one study used both Actical accelerometers and ActivPAL inclinometers. Further, four studies used direct observation to assess sedentary behavior. The accelerometry studies used a low threshold range of CPM to estimate time, or proportion of wearing time, that the child was sedentary. This lower threshold varied between studies; for example, the Reilly group in Scotland45 applied a cut-point of o1,100 CPM, whereas Pate et al.41 and Janz and colleagues33 from the U.S. applied substantially lower cut-points of 104 CPM and 152 CPM, respectively. The study71 that examined physical activity using ActivPAL inclinometers considered a recording of 10 seconds of sitting/lying data per 15-second epoch to be sedentary behavior. Three54–56 of the four studies that used direct observation to assess children’s sedentary behavior used the OSRAP. The fourth direct observation study57 used the Children’s Activity Rating Scale (CARS) to determine time spent sedentary. That study observed children for 6–12 hours per day over 4 days in their own home.

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Figure 1. Percentage of time spent in moderate to vigorous–intensity physical activity from ActiGraph, Actiwatcha, Acticalb, Actiheartc, and RT3d accelerometer, heart rate monitoring\widehat, and direct observation # studies. Note: “Unknown” indicates cut-points not reported. MVPA, moderate to vigorous–intensity physical activity, N/A, not applicable.

Sedentary Time Prevalence Estimates Figure 3 illustrates the proportion of time children spent sedentary according to accelerometry and direct observation. The proportion of time spent sedentary based on accelerometry ranged from 23% to 95% of the time. Over a 13-hour waking day, this would equate to between approximately 2.9 and 12.4 hours being sedentary. The observational studies estimated that between 55% and 89% (approximately

7.2–11.5 hours) of a child’s day is spent sedentary. Across all studies in this review, the median amount of time spent sedentary was 77% (approximately 10 hours).

Discussion This paper is the first to review the prevalence (percentage of time) of light- and moderate to vigorous–intensity

Figure 2. Percentage of time spent in light-intensity physical activity derived from ActiGraph accelerometers, Actiheartc accelerometers, and direct observation# studies. Note: “Unknown” indicates cut-points not reported. LPA, light-intensity physical activity, N/A, not applicable.

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physical activity and sedentary time in preschool children. The results are highly variable, reporting that children spend 2%–41% of their day in moderate to vigorous–intensity physical activity, 4%–33% in lightintensity physical activity, and 34%–94% sedentary. There are a number of reasons that such variability may exist, namely, the use of different cut-points for the accelerometer data, variations in the applied inclusion criteria, and sample differences. These issues have been discussed in detail in previous methodologic papers8,13; however, their relevance to the present findings are discussed below. Among the accelerometer studies, four examining time spent in moderate to vigorous–intensity physical activity30,32,33,36 used Freedson cut-points, which have been validated in an older population group (6–18 years) rather than preschool children, resulting in a low threshold count for moderate to vigorous–intensity physical activity. This low threshold count may have overestimated the time spent in moderate to vigorous–intensity physical activity in these studies.72 Although the Puyau cut-points (also not validated in the preschool population) were also applied in some studies,27,28,34,35,38,44–46 the count threshold for moderate to vigorous–intensity physical activity was much closer in magnitude to that of the other preschool-age cut-points and did not result in such high prevalence estimates. When the four studies using the Freedson cut-points for moderate to vigorous–intensity physical activity were not

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included in the prevalence estimates, the range of the proportion of time spent in this intensity dropped to between 2% and 18%. Although most of the studies reporting the percentage of time spent sedentary used cutpoints validated in the preschool-age group, the estimates still varied when Sirard64 or Reilly61 cut-points were used compared with the Pate62 cut-points. Because these studies used different classification systems as their criterion measure, it is possible that these different criterion measures resulted in different cut-point values for sedentary, light-intensity physical activity and moderate to vigorous–intensity physical activity. Given the discrepancies in cut-points for various intensities, there is much interest in identifying the most appropriate accelerometer cut-points for preschool children. Recent work comparing accelerometer cut-points for physical activity and sedentary behavior in 4–6-year-old children using whole-room calorimetry and direct observation (i.e., CARS) has found that the best classification accuracy is observed when using the Evenson cut-point for sedentary behavior (100 CPM) and the Pate cut-points for moderate to vigorous–intensity physical activity.73 However, another study using only direct observation (i.e., OSRAC-P) as the criterion identified the Sirard cut-points as having the greatest classification accuracy.74 Similar to the development of cut-points, identifying the best classification accuracy may be influenced by the criterion measure used. Additionally, it has been

Figure 3. Percentage of time spent sedentary from ActiGraph, Acticalb, and Actiheartc accelerometers, ActivPalþ inclinometers, and direct observation# studies. October 2014

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suggested that accelerometers may have lower accuracy of detecting young children’s sedentary behavior,75,76 resulting in an overestimation of sedentary time.76 Future research should aim to determine which age-appropriate, validated cut-points should be used in studies to assist in identifying more accurate estimates of the prevalence of physical activity and sedentary behavior in this age group. It is also important to recognize that the volume of “valid” monitoring time varied between studies, likely because there were limited established criteria on which to base these decisions when most of the studies were conducted. Studies included in this review applied a validity criteria from 1 hour41 to 10 hours21,48 of monitoring time over 1 day,51,55,71 2 days,29,54 3 days, 25–28,32–34,37–39,41,44,47,52,60 or 4 days,21,22,24,30,31,36,40,43,53,57 and some required the inclusion of a certain number of weekdays versus weekend days, whereas others did not. Although this review only included studies that reported the percentage of time spent in the various activity intensities or sedentary time or studies that reported sufficient data to calculate that percentage, differences in inclusion criteria may still affect the results if acceptable reliability31,77,78 is not achieved with the monitoring time chosen. It is difficult to determine how much of the variability in findings is a result of these methodologic decisions; how much is due to the sampling population (particularly given that over half of studies came from either U.S. or Scotland); and how much is true variability. Additionally, given that preschool boys tend to be more active than preschool girls,79 samples that comprise a higher or lower proportion of boys to girls may overestimate or underestimate prevalence estimates. With such diverse methods employed, it becomes difficult to come to a consensus on “true” physical activity levels and sedentary time in preschool-aged children. Given the inconsistencies in accelerometer-derived measurements of physical activity and sedentary time, a strength of this review was that it sought to include prevalence estimates from studies that used other objective measures of physical activity. Some discrepancy was seen between the direct observation studies, though the variability was smaller than with the studies using accelerometry. Brown et al.55 and Pate and colleagues41 reported quite high observed sedentary behavior (83%–89%) and low moderate to vigorous–intensity physical activity (3%), whereas Bower et al.54 reported a lower prevalence of sedentary behavior (55%) and a higher prevalence of moderate to vigorous–intensity physical activity (12%). These three studies used the same direct observation system with the same classification codes for sedentary behavior and light- and moderate to vigorous–intensity

physical activity, and conducted the direct observation in a preschool or child care setting. However, sample sizes of the studies and the lengths of the observation periods differed. For example, the study by Brown and colleagues55 included observation during meal times (during which children are more likely to be sedentary) and the study of Bower et al.54 did not, perhaps contributing to some of the differences in the observed prevalence estimates. Furthermore, Brown and colleagues55 reported that a greater proportion of the observations were conducted indoors rather than outdoors. Given that time spent outdoors is associated with higher levels of physical activity in young children,79 this may partially explain the low physical activity prevalence estimates and high sedentary behavior prevalence estimates in that study compared with the others. In addition, as the child care setting has been associated with children’s physical activity,79 it is possible that some of these estimates are related to sample differences. It is important to note that although sedentary behavior is typically regarded as “sitting time,” many of the classification systems developed (including those used in the studies in this review) classify sedentary behavior as standing motionless or with little movement, perhaps further limiting comparability between measurement tools. The only heart rate monitoring study in this review found a similar level of sedentary time to Brown et al.55 and reported a prevalence of moderate to vigorous–intensity physical activity that was between the prevalence estimates reported by the research groups using direct observation. Given that physical activity and sedentary time are important foci for children’s health, greater methodologic consistency is required in order to understand the prevalence of these behaviors in preschool children. For future research with accelerometry, a data reduction strategy that is valid across age groups is also needed. Currently, it is challenging to examine longitudinal changes in preschool children’s physical activity or sedentary time as they transition into primary/elementary school owing to different cut-points being applied to younger and older age groups. Without consistent or appropriate methodology, the prevalence of physical activity and sedentary behaviors, their relationship to health outcomes, and the effectiveness of interventions will remain challenging to fully understand. Recent work has shown that machine learning or pattern recognition approaches (e.g., artificial neural networks) to accelerometer data reduction can differentiate between sitting and standing with greater accuracy and provide significantly more precise estimates of physical activity.80 Therefore, the use of these approaches may provide a better understanding of young children’s www.ajpmonline.org

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physical activity and sedentary time and eliminate the challenges associated with age-related cut-points. Future research should evaluate the utility of these approaches in children younger than age 5 years.

Conclusions In summary, this paper reviewed studies reporting on light- and moderate to vigorous–intensity physical activity and time spent sedentary in preschool children. Considerable variability in estimates of these intensities of activity was reported in the studies, making it difficult to determine “true” physical activity levels and time spent sedentary in this population group. Consistent methodologies and the use of age-appropriate cut-points for studies using accelerometers are required in order to gain a better understanding of how preschool children are spending their day. JS, TH, ADO, and ST conceptualized the idea for the study. JH, TH, and JS conducted the literature searches and reviewed manuscripts based on the inclusion criteria. JH and JS drafted the manuscript. TH, ADO, and ST revised the manuscript for important intellectual content. All authors approved the final version to be published. JH is supported by a Deakin University International Postgraduate Research Scholarship. JS is supported by a National Health and Medical Research Council Principal Research Fellowship (no. APP1026216). TH is supported by an Alfred Deakin Postdoctoral Research Fellowship. ADO is supported by a National Heart Foundation Career Development Fellowship (no. CR11S6099).

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Appendix Supplementary data Supplementary data associated with this article can be found at http://dx.doi.org/10.1016/j.amepre.2014.05.042.