Email updates

Keep up to date with the latest news and content from IJBNPA and BioMed Central.

Open Access Highly Accessed Review

Seasonal variation in accelerometer-determined sedentary behaviour and physical activity in children: a review

Carly Rich*, Lucy J Griffiths and Carol Dezateux

Author Affiliations

MRC Centre of Epidemiology for Child Health, UCL Institute of Child Health, 30 Guilford Street, London, UK, WC1N 1EH

For all author emails, please log on.

International Journal of Behavioral Nutrition and Physical Activity 2012, 9:49  doi:10.1186/1479-5868-9-49


The electronic version of this article is the complete one and can be found online at: http://www.ijbnpa.org/content/9/1/49


Received:28 October 2011
Accepted:18 April 2012
Published:30 April 2012

© 2012 Rich et al.; licensee BioMed Central Ltd.

This is an Open Access article distributed under the terms of the Creative Commons Attribution License ( http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Aim

To undertake a review of the methods and findings of published research evaluating the influence of season on accelerometer-determined sedentary behaviour (SB) and physical activity (PA) in children.

Methods

A literature search was carried out using PubMed, Embase, Medline and Web of Science up to, and including, June 2011. The search strategy focused on four key elements: children, SB or PA, season and accelerometer. Articles were eligible for inclusion if they were published in English, included healthy study participants aged ≤ 18 years, reported at least one outcome variable derived from accelerometer-determined measurements, and compared SB or PA between two or more seasons, or controlled for season of measurement. Eligible papers were reviewed and evidence tables compiled reporting on publication year, country studied, study recruitment, consent rate, sample descriptives, study design, accelerometer protocol, valid accelerometer data receipt, season definition, statistical methods and key findings.

Results

Sixteen of 819 articles were eligible for inclusion: children aged two to five years, six to twelve, or six to 18 years were included in five, six and five articles respectively. Six articles were from the UK, six from other European countries, three from the USA and one from New Zealand. Study sample sizes ranged from 64 to 5595. PA was reported in all articles but SB in only three. Only four studies were longitudinal and none of these reported SB. Seasonal variation in PA was reported in all UK studies, being highest in summer and lowest in winter. In four non-UK studies seasonal variation in PA was not found. Findings were inconclusive for SB.

Conclusion

There is sufficient evidence to support public health interventions aimed at increasing PA during winter in UK children. No conclusions can be drawn regarding the effect of season on children’s SB reflecting few studies of small sample size, lack of repeat measures, incomparable definitions of season and inconsistent accelerometer protocols. Future research should determine factors that drive seasonal patterns in PA and SB in children such as age, sex, and geographic and climatic setting to inform interventions and target populations.

Keywords:
Accelerometer; Season; Child; Physical activity; Sedentary Behaviour; Review

Introduction

Accelerometer-based measures of physical activity (PA) and sedentary behaviour (SB) are being increasingly obtained in large-scale studies to determine the level and pattern of children’s PA and SB, their determinants and relation to health outcomes. Such studies can help to identify optimal patterns of activity associated with future health and well-being and inform interventions to help populations meet these optimal patterns. This is essential as there is increasing evidence that PA patterns are established in childhood [1], that maintenance of an active lifestyle in adult life is protective against many chronic diseases [2-4], and that - in children as well as in adults - levels of PA have declined with successive generations in developed countries and those in economic transition [5].

A season is a division of the year marked by changes in weather, ecology and hours of daylight which have the potential to influence PA and SB. Periods of low temperatures, high rain fall, strong winds and snow may reduce the likelihood of children being physically active. Although the meteorological factors associated with seasons cannot be changed, the ability to identify specific seasons that are characterised by low PA levels and/or high periods of SB is important for the design of future public health interventions aimed at promoting PA and reducing SB.

It is also important to account for seasonal influence on PA and SB as large-scale cross-sectional studies rely on measurements made in different individuals and in different seasons, potentially introducing bias in between-subject differences in the assessment of habitual activity levels.

While the influence of season on PA levels has been reviewed by others these have, for the most part, focussed on adults [6-8] rather than children, and the extent to which findings can be reliably extrapolated to children is uncertain. Furthermore, the only review to concentrate on children [9] did not focus on accelerometer-determined activity.

In recent years accelerometers have been regarded as the ‘gold standard’ method to examine PA in childhood populations [10,11], as reliance on self or parent proxy reports may overestimate PA levels [12]. Carson et al.[9], summarising data from 35 studies in children, reported that in the majority (83%) a seasonal variation in children’s PA was found. However, studies based on accelerometer-determined activity were not reported separately, a number of major studies [13-19] published on this topic were omitted and the influence of seasonality on SB was not examined. As SB is not simply the absence of PA, but involves purposeful engagement in activities that involve minimal movement and low energy expenditure [20], seasonality may exert a different influence on SB than on PA in children.

We aimed to update and extend the review published by Carson et al.[9] by reviewing the influence of season on accelerometer-determined measures of PA and SB in children including studies published up to and including June 2011. The main purpose is to describe the current studies and their population coverage, critically appraise the study design and analytic methods used, and identify any significant gaps in evidence to inform public health guidelines and policy on optimal PA and SB levels in children.

Methods

Search strategy

We searched Embase, Medline, PubMed, and the Web of Science electronic databases (see search strategy illustrated in Additional file 1). This search strategy focused on four key elements: children, PA or SB, season, and accelerometer. Further studies were identified by hand-searching the bibliographies of published reviews and all included studies.

Additional file 1. Literature search strategy. Literature search strategy used in electronic databases.

Format: DOC Size: 37KB Download file

This file can be viewed with: Microsoft Word ViewerOpen Data

Inclusion criteria

Studies were included if a full article was available, published in English, and included in the database from the year of inception up to and including June 2011 when the final searches were run. Any study or methodological design was included provided at least one outcome variable of PA or SB derived from accelerometer-determined measurements was reported, PA or SB were compared between at least two seasons, or adjustment was made for season of measurement. Articles were included if study participants, or a clearly defined subgroup, comprised children aged two to 18 years that were not selected on the basis of having a specific disease or health problem. Articles were not restricted according to study sample size or country of origin.

Data extraction and analysis

Eligible papers were reviewed and evidence tables compiled reporting on year of publication, country studied, recruitment procedure, consent rate, sample size, age range, sex distribution, study design (cross sectional vs. longitudinal), valid accelerometer data receipt (children providing accelerometer data meeting individual study definition of ‘valid data’), definition of season employed, accelerometer protocol and outcome variables reported, statistical methods and key findings. The NHS Centre for Reviews and Dissemination [21] proposes a simple assessment to guarantee a minimum level of quality based on study design. However, a preliminary review of abstracts suggested that nearly all of the studies included in this review were observational studies and therefore poorly differentiated by this study hierarchy. Nevertheless, information relevant to the methodological quality of each article including the recruitment and sampling procedures, bias in consent and valid accelerometer data receipt are reported. The results of the review are presented as a qualitative review; no attempt has been made to synthesise statistical outcomes as the lack of uniformity across research methodology was too great to permit this.

Searches and abstract review

A total of 819 abstracts were identified by the final electronic searches (219 in PubMed, 157 in Embase, 134 in Medline and 309 in Web of Science), of which 585 were duplicates, leaving 234 unique articles (Figure 1). Of these, 158 were not relevant to the research objective and 46 did not meet inclusion criteria, leaving 14 review and 16 original research articles. No further articles were identified by hand-searching of bibliographies, leaving 16 articles to be included in the review. Of these 11 were found in three databases, two in two and three in one only. No articles were identified through Embase (Figure 2).

thumbnailFigure 1. Flowchart of articles (n= 16) included in review.

thumbnailFigure 2. Articles (n= 16) included in the review identified by each electronic database.

Results

Overview of studies

Sixteen articles (Additional file 2: Table S1) were identified that reported 13 different studies assessing seasonal variation in accelerometer-determined PA in children, and in three this included SB. The dates of publication ranged from 2002 [22] to 2010 [14] and date of data collection from 1997 [16,23] to 2007 [14,18,24,25]. Twelve authors reported studies from Europe [13-18,23,24,26-29] (including six from the UK), three from the USA [19,22,30], and one from New Zealand [25]. In 12 studies between-subject seasonal differences in PA were assessed [14-16,18,19,22,23,25,26,28-30], and in the remaining four within-subject differences were evaluated through repeated accelerometer measurements in the same individuals (referred to here as longitudinal studies) [13,17,24,27].

Additional file 2. Table S1. Articles (n = 16) evaluating the influence of season on accelerometer-determined PA and SB in children.

Format: DOC Size: 92KB Download file

This file can be viewed with: Microsoft Word ViewerOpen Data

Sample characteristics

The sample sizes for studies that used a repeated measures design ranged between 64 [24] and 315 [27], and between 209 [26] and 5595 [28] for studies examining seasonal differences between subjects. Children aged two to five years, six to 12 years, or six to 18 years were included in five [19,22,25,26,30], six [14,17,18,24,27,28] and five [13,15,16,23,29] studies respectively. In 13 articles a measure of weight status was reported [14-17,19,22-29] while in only four was ethnicity [18,19,22,30] or socioeconomic status [13,14,22,23] of the sample reported.

Accelerometer measurement protocol

The Actigraph was the most commonly used accelerometer by studies included within this review and there is extensive literature on its use to measure PA in children supporting its reproducibility, validity and feasibility in large samples of children [31]. In nine studies [13-16,23,26-29] the Actigraph 7164 were used, in two the Actigraph GT1M [18,24], in three the Actiwatch [17,19,22], with one study each for the Tritac R3D accelerometer [30] and the mini Mitter Actical accelerometer [25].

In thirteen (81%) articles authors reported the epoch selected; this was usually 60 seconds [13,16,17,19,22,27,28,30] with individual studies using shorter epochs, namely two [24], five [18], 10 [15], and 15 [14] seconds. Subjects were asked to wear the accelerometer for between two [22] to seven days [13,14,17,18,27,28], with most requesting seven days wear. The criteria used to determine number of days of accelerometer data per child required to characterise habitual activity were specified in 13 articles [13-18,23-25,27-29]: these ranged from one [13,18,29] to four days [17]. In five at least one weekend day was stipulated [14-16,24,30]. In eleven articles duration of wear in any single day was specified [13,15-18,23,24,26-29]: these ranged from at least 360 minutes (six hours) [26] to at least 600 minutes (ten hours) [16-18,23,27-29]. Moderate to vigorous PA (MVPA) was defined using a range of count values varying from 2000 [16] to 3600 [28] cpm. Vigorous PA was defined using Trost et al.[32] and a threshold of 1000 [22] cpm.

Main findings

A seasonal variation in PA levels was reported in all of the UK studies [14,18,24,26-28]. Mattocks et al.[27] evaluated seasonal and intra-individual variation of PA in 315 11 year old UK children by obtaining four repeat accelerometer measurements over a full calendar year and calculated intraclass correlation coefficient (ICC) values to determine the extent to which activities in different seasons vary. ICC values for variation in activity (cpm) over the course of the year increased from 0.49 to 0.53 after adjusting for month of measurement, indicating an effect of month. PA levels were higher in summer than winter [27]. Using a cross-sectional study design, Owen et al.[18] examined ethnic differences in mean daily PA (cpm) in a cross-sectional study of 144 11 year old UK children, adjusting for month of measurement: PA was higher in summer than winter and did not vary by sex [18]. Riddoch et al.[28] compared PA across all four seasons in 5595 11 year old UK children: PA levels (cpm and MVPA) were lowest in winter compared to summer (n = 5595).

Wennlöf et al.[29] compared the PA levels (cpm) of 969 nine and 15 year old Swedish children who were measured once only in spring, autumn, or winter and found that children were most active during April and May with a significant effect of month of measurement in total PA being observed. Rundle et al.[19] compared the activity levels of 437 four year old US children measured either during summer or winter and found season of measurement to be the strongest predictor of mean activity counts with children more active during summer than winter.

By contrast, no variation according to the season of measurement was found in four studies, none of these were carried out in the UK [13,16,22,30], and only one of which compared PA levels (cpm) across all four seasons [30] in 219 US children measured once throughout the year. Bringolf-Isler et al.[13] examined the association between PA (cpm) and socio-demographic and environmental characteristics, including season in a longitudinal study of 189 six to 14 year old Swiss children who were measured once in either winter or summer. Nilsson et al.[16] determined between- and within- day differences in total PA (cpm), and the daily time spent in MVPA in 1954 nine and 15 year old children from four European countries. They found that adjustment for season of PA measurement (autumn, winter and spring) did not alter their main findings. Finn et al.[22] also found no differences between summer and autumn total PA (cpm) and vigorous activity in 214 three to five year old US children.

In only three articles [14,16,26] was seasonal variation in children’s SB evaluated: two from the UK [14,26], and all cross-sectional. Fisher et al.[26] compared total PA (cpm) and percentage time spent in SB, light PA, and MVPA across all four seasons in 209 Scottish children (mean age 4.8 years): children were more sedentary in spring than summer. King et al.[14] explored 22 potential correlates including season of total PA (cpm), MVPA and SB in 480 seven year old UK children and found that SB was higher in spring, autumn and winter compared to summer. In contrast, Nilsson et al.[16] reported that levels of SB in nine and 15 year old European children were not influenced by season of measurement (including only spring, autumn, and winter).

In three articles seasonal PA data were analysed separately for different age groups [15,23,25]. Kristensen et al.[23] compared PA levels (cpm) across all four seasons and reported an effect of age: season had less influence on 14 to 16 year olds than in eight to 10 year olds. Overall, European children were more physically active in the spring than in winter or autumn, with the exception of adolescents. Kolle et al.[15] compared PA levels (cpm) across spring, autumn, and winter, and found that season had less influence on PA in Norwegian 15 year olds than in Norwegian nine year olds: younger children were more active during spring than during winter and autumn, but the PA levels of 15 year olds did not vary by season. Taylor et al.[25] compared total PA (cpm) across all four seasons in New Zealand children at age three, four and five years. Seasonal variation in PA differed by age group: children were less active during spring than during summer or winter at three years, but these differences were not observed at four and five years.

Rowlands et al.[24] are the only authors to have considered the influence of gender on seasonal variation of PA. In a longitudinal study of only 64 nine to 11 year old UK children measured in only one of two seasons (summer and winter). Total PA (cpm) and moderate activity were higher during summer than winter for boys on weekend and weekdays, and higher for girls on weekend days. For boys, vigorous activity was higher during summer than winter on weekdays, while weekday activity was higher in summer than winter; for girls, weekend activity was higher in summer than winter. Rowlands et al.[24] are also the only authors to have assessed seasonal variation in PA patterns according to the frequency, intensity and duration of PA bouts (lasting greater than four seconds) of light, moderate, and vigorous intensity activity. They found that the mean duration of all intensity PA bouts was greater during summer than winter in boys, and the frequency and intensity of vigorous intensity PA bouts was greater during summer than winter in girls [24]. There are no reports of variation in PA or SB by ethnicity, socioeconomic status, weight status or region of residence.

Discussion

Key findings

In summary, seasonal variation in PA was reported in the majority of studies, particularly in children living in the UK, and in younger rather than older children. These seasonal differences were reported in all UK studies (n = 6), being highest during summer and lowest during winter. Findings from non-UK studies were inconsistent: seasonal variation in PA was reported in seven studies, but not in four. Findings from non-UK studies that were based in the same country were also inconsistent. For example, one US study reported higher levels of PA in summer than winter [19]; however, a different US study reported higher levels of PA in autumn than summer [22], and one found no association between season and PA levels [30]. Findings were inconclusive for SB and vigorous PA.

Strengths and limitations of review

This review identified a large number of potential studies obtained from a systematic literature search conducted in a range of databases. The broad definition of search terms and systematic search strategy should have enabled this review to detect as many potential studies as possible. Only studies with accelerometer-determined measurements of activity were included as they are currently regarded as the optimal method for measurement of PA and SB in population studies [33], providing greater accuracy and reliability than self-report methods [34]. This review provides the best summary to date of the evidence evaluating seasonal variation in accelerometer-determined PA and SB in children.

The heterogeneity in study samples, design, accelerometer protocols, statistical methods and outcome measures included in this review limited interpretation and meant that it was not possible to meta-analyse the results. The semi-quantitative reporting in this review provides only a somewhat arbitrary classification of seasonal variation with the focus on specifying seasons characterised by low or high activity rather than the strength of variation. Only one reviewer undertook the reviewing process and no robust indicator of methodological quality was provided due to the lack of uniformity across research methodology. A number of the articles have drawn data from the same cohort studies, for example the Avon Longitudinal Study of Parents and Children [27,28] and the European Youth Heart Study [16,23,29], which may introduce bias into the analysis sample. As this review was restricted to published studies only, publication bias may be present.

Strengths and limitations of studies

Limitations across the studies included small sample sizes, inconsistent study designs and accelerometer protocols, and the use of varied seasonal definitions and statistical methods. There were also few studies that employed a repeat measures study design, the most appropriate design allowing differentiation between seasons and between subjects. The studies included in the current review comprised geographically clustered samples, limiting the representativeness of their findings. Few studies evaluated seasonal variation in SB or in vigorous PA independently of total PA. There were also limited and conflicting evidence regarding sex and age effects on seasonal variation in PA therefore no conclusions can be reached on this.

However, there were a sufficient amount of UK studies conducted in a range of settings that reported consistent findings. In addition, only studies using accelerometers were included in this review because they provide more accurate estimates of volumes and intensities of PA than self-report measures [35]. However, there are limitations associated with the use of accelerometers including underreporting levels of cycling, and their inability to accurately measure swimming or load bearing activity. Despite recent advances in accelerometer design allowing researchers to select shorter epochs whilst still being able to measure for a sufficient number of days, a 60 second epoch was used in half the studies included in this review. Due to the sporadic nature of children’s PA, longer epochs may be inappropriate when measuring PA and may underestimate levels of MVPA [36]. It has also been widely documented that the use of different thresholds to define activity intensities limits the ability for researchers to make reliable comparisons of MVPA levels between studies, and at present there is still no consensus on the best threshold to use [37].

Further research needed

Future studies of seasonal activity in children need to use large samples, employ a repeat measures study design, use comparable definitions of season, and use a consistent standardised approach to accelerometer measurement. This should include the use of a short epoch, presenting count data as well as minutes of MVPA, and also take into account activities that cannot be measured accurately by accelerometers.

Further research is also needed to understand seasonal variation in children’s SB and vigorous PA independently of total PA so that public health interventions aimed at reducing SB and increasing vigorous PA can be targeted at specific times of the year. Recent evidence suggests that SB in children is associated with an elevated metabolic risk profile [38,39], elevated blood pressure [40], and increased body weight [41-46], independent of PA. Furthermore, participation in vigorous intensity PA appears more consistently associated with lower adiposity [47-49] than total PA [47,50], and this may even be independent of time spent in SB [43].

Seasonal variation in PA appears to be location specific; therefore additional research is needed in different countries and within different regions of larger countries. Characteristics that define seasons including the weather, ecology, and hours of daylight vary according to country, and even within different regions of large individual countries. For example, winter in the UK is different to winter in Norway in terms of weather and also available sunlight, and the weather in the US state of New York (Rundle et al.[19]) is characterised by cold, snowy winters and usually warm summers, whereas South Dakota (Finn et al.[22]) usually experiences cold winters and hot summers that can bring thunderstorms with high winds, thunder and hail.

Regional differences in seasonal variation may reflect variations in climate. As the meteorological factors associated with seasons in a specific region cannot be altered, there may be a role for future research to study other factors associated with variation of PA and SB throughout the seasons. This will enable us to understand what encourages a child to be more or less physically active in specific seasons. For example, previous research [51] evaluating regional differences in pedometer-determined PA between urban and rural primary children living in Cyprus found an interaction between season and rural/urban regions: rural children were more active in summer, and urban children were more active in winter. To our knowledge, the only study evaluating seasonal differences in accelerometer-determined PA in rural and urban children is reported in a published abstract by Tremblay et al.[52]. The authors found that rural children were more active during summer and urban children more active in the winter.

Additional research is needed that examines possible interactions of season with other factors known to influence PA and SB such as sex, age, ethnic group, weight status and geographic location [53-56]. In contrast to Carson et al.[9], we found some evidence to suggest that season influenced children but not adolescents. This may be because adolescents have less free time to play outdoors than children; therefore, their PA is less influenced by fluctuations in daylight and weather. Adolescents are also more likely to participate in organised PA that takes place all year around [57]. Sex differences in PA across season may relate to the type of playground activities typically undertaken by boys and girls at school [24]. Boys undertake more vigorous activity during play time at school than girls and therefore warm summer weather may have more impact in the amount of PA that they undertake [57].

In the majority of studies identified for this review, authors reported a measurement of weight and several reported the ethnic composition of their sample; however, none determined whether seasonal variation in activity were associated with weight status’, or ethnic group. Overweight children are less likely to be active than underweight children [58,59], and British South Asian children are less likely to be active than European whites and black African-Caribbeans [18,60-63]. It is possible that the influence of season on PA and SB may be enhanced in specific ethnic groups, or in overweight children compared to underweight children. It is important to identify factors that may cause or interact with seasonal effects in PA and SB, so that interventions can target certain groups of children.

Conclusion

From our review we suggest there is sufficient evidence to develop public health interventions to increase PA during winter in UK children. There is insufficient high quality evidence to suggest this for children living in other countries and this may reflect regional variations in climate as well as varying definitions of season. No conclusions can be drawn regarding the effect of season on children’s SB or vigorous PA reflecting few studies of small sample size, lack of repeat measures, incomparable definitions of season and inconsistent accelerometer protocols. More research is needed to determine whether levels and patterns of vigorous PA and SB vary according to season utilizing a consistent standardised approach to accelerometer measurement, a repeated measures study design and in large geographically dispersed samples. Future research should be aimed at addressing the gaps in evidence identified in this review namely seasonal effects on SB and vigorous PA, and the determinants of seasonal patterns in PA such as age, sex, and geographic and climatic setting so that interventions can be season-specific and target inactive children.

Competing interests

The authors declared that they have no competing interest.

Authors’ contributions

CR was responsible for the conception and design of the review, the literature search, critical appraisal of the included papers, and also drafted the manuscript. LJG and CD made substantial contributions towards the conception and design of the review and revising the manuscript. All authors read and approved the final manuscript.

Acknowledgements

This study received external funding from the Wellcome Trust (grant 084686/Z/08/A).CR was funded by this grant. The Centre for Paediatric Epidemiology and Biostatistics is supported in part by the Medical Research Council in its capacity as the MRC Centre of Epidemiology for Child Health. Research at the UCL, Institute of Child Health and Great Ormond Street Hospital for Children receives a proportion of the funding from the Department of Health’s National Institute for Health Research Biomedical Research Centres funding scheme. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

References

  1. Jose KA, Blizzard L, Dwyer T, McKercher C, Venn AJ: Childhood and adolescent predictors of leisure time physical activity during the transition from adolescence to adulthood: a population based cohort study.

    Int J Behav Nutr Phys Act 2011, 8:54. PubMed Abstract | BioMed Central Full Text | PubMed Central Full Text OpenURL

  2. Gotay CC: Behavior and cancer prevention.

    J Clin Oncol 2005, 23:301-310. PubMed Abstract | Publisher Full Text OpenURL

  3. Kriska AM, Saremi A, Hanson RL, Bennett PH, Kobes S, Williams DE, Knowler WC: Physical activity, obesity, and the incidence of type 2 diabetes in a high-risk population.

    Am J Epidemiol 2003, 158:669-675. PubMed Abstract | Publisher Full Text OpenURL

  4. Fang J, Wylie-Rosett J, Cohen HW, Kaplan RC, Alderman MH: Exercise, body mass index, caloric intake, and cardiovascular mortality.

    Am J Prev Med 2003, 25:283-289. PubMed Abstract | Publisher Full Text OpenURL

  5. Pratt M, Macera CA, Blanton C: Levels of physical activity and inactivity in children and adults in the United States: current evidence and research issues.

    Med Sci Sports Exerc 1999, 31:S526-S533. PubMed Abstract | Publisher Full Text OpenURL

  6. Tucker P, Gilliland J: The effect of season and weather on physical activity: a systematic review.

    Public Health 2007, 121:909-922. PubMed Abstract | Publisher Full Text OpenURL

  7. Shephard RJ, Aoyagi Y: Seasonal variations in physical activity and implications for human health.

    Eur J Appl Physiol 2009, 107:251-271. PubMed Abstract | Publisher Full Text OpenURL

  8. Chan CB, Ryan DA: Assessing the effects of weather conditions on physical activity participation using objective measures.

    Int J Environ Res Public Health 2009, 6:2639-2654. PubMed Abstract | Publisher Full Text | PubMed Central Full Text OpenURL

  9. Carson V, Spence JC: Seasonal variation in physical activity among children and adolescents: a review.

    Pediatr Exerc Sci 2010, 22:81-92. PubMed Abstract OpenURL

  10. Cliff DP, Reilly JJ, Okely AD: Methodological considerations in using accelerometers to assess habitual physical activity in children aged 0–5 years.

    J Sci Med Sport 2009, 12:557-567. PubMed Abstract | Publisher Full Text OpenURL

  11. Rowlands AV: Acceleromter assessment of physical activity in children: an update.

    Pediatr Exerc Sci 2009, 19:252-266. OpenURL

  12. Pate RR, Freedson PS, Sallis JF, Taylor WC, Sirard J, Trost SG, Dowda M: Compliance with physical activity guidelines: prevalence in a population of children and youth.

    Ann Epidemiol 2002, 12:303-308. PubMed Abstract | Publisher Full Text OpenURL

  13. Bringolf-Isler B, Grize L, Mader U, Ruch N, Sennhauser FH, Braun-Fahrlander C: Assessment of intensity, prevalence and duration of everyday activities in Swiss school children: a cross-sectional analysis of accelerometer and diary data.

    Int J Behav Nutr Phys Act 2009, 6:50. PubMed Abstract | BioMed Central Full Text | PubMed Central Full Text OpenURL

  14. King AC, Parkinson KN, Adamson AJ, Murray L, Besson H, Reilly JJ, Basterfield L: Correlates of objectively measured physical activity and sedentary behaviour in English children.

    Eur J Public Health 2011, 21(4):424-431. PubMed Abstract | Publisher Full Text OpenURL

  15. Kolle E, Steene-Johannessen J, Andersen LB, Anderssen SA: Seasonal variation in objectively assessed physical activity among children and adolescents in Norway: a cross-sectional study.

    Int J Behav Nutr Phys Act 2009, 6:36. PubMed Abstract | BioMed Central Full Text | PubMed Central Full Text OpenURL

  16. Nilsson A, Anderssen SA, Andersen LB, Froberg K, Riddoch C, Sardinha LB, Ekelund U: Between- and within-day variability in physical activity and inactivity in 9- and 15-year-old European children.

    Scand J Med Sci Sports 2009, 19:10-18. PubMed Abstract | Publisher Full Text OpenURL

  17. Nyberg G, Ekelund U, Marcus C: Physical activity in children measured by accelerometry: stability over time.

    Scand J Med Sci Sports 2009, 19(1):30-35. PubMed Abstract | Publisher Full Text OpenURL

  18. Owen CG, Nightingale CM, Rudnicka AR, Cook DG, Ekelund U, Whincup PH: Ethnic and gender differences in physical activity levels among 9-10-year-old children of white European, South Asian and African-Caribbean origin: the Child Heart Health Study in England (CHASE Study).

    Int J Epidemiol 2009, 38:1082-1093. PubMed Abstract | Publisher Full Text | PubMed Central Full Text OpenURL

  19. Rundle A, Goldstein IF, Mellins RB, shby-Thompson M, Hoepner L, Jacobson JS: Physical Activity and Asthma Symptoms among New York City Head Start Children.

    J Asthma 2009, 46:803-809. PubMed Abstract | PubMed Central Full Text OpenURL

  20. Reilly JJ, Coyle J, Kelly L, Burke G, Grant S, Paton JY: An objective method for measurement of sedentary behavior in 3- to 4-year olds.

    Obes Res 2003, 11:1155-1158. PubMed Abstract | Publisher Full Text OpenURL

  21. Centre for Reviews Dissemination: Undertaking systematic reviews of research on effectiveness: CRD's guidance for those carrying out or commissioning reviews. University of York, York; 2001. PubMed Abstract | Publisher Full Text OpenURL

  22. Finn K, Johannsen N, Specker B: Factors associated with physical activity in preschool children.

    J Pediatr 2002, 140:81-85. PubMed Abstract | Publisher Full Text OpenURL

  23. Kristensen PL, Korsholm L, Moller NC, Wedderkopp N, Andersen LB, Froberg K: Sources of variation in habitual physical activity of children and adolescents: the European youth heart study.

    Scand J Med Sci Sports 2008, 18:298-308. PubMed Abstract | Publisher Full Text OpenURL

  24. Rowlands AV, Pilgrim EL, Eston RG: Seasonal changes in children's physical activity: an examination of group changes, intra-individual variability and consistency in activity pattern across season.

    Ann Hum Biol 2009, 36:363-378. PubMed Abstract | Publisher Full Text OpenURL

  25. Taylor RW, Murdoch L, Carter P, Gerrard DF, Williams SM, Taylor BJ: Longitudinal study of physical activity and inactivity in preschoolers: the FLAME study.

    Med Sci Sports Exerc 2009, 41:96-102. PubMed Abstract | Publisher Full Text OpenURL

  26. Fisher A, Reilly J, Montgomery C, Kelly L, Williamson A, Jackson DM, Paton JY, Grant S: Seasonality in physical activity and sedentary behaviour in young children.

    Pediatr Exerc Sci 2005, 17:31-40. OpenURL

  27. Mattocks C, Leary S, Ness A, Deere K, Saunders J, Kirkby J, Blair SN, Tilling K, Riddoch C: Intraindividual variation of objectively measured physical activity in children.

    Med Sci Sports Exerc 2007, 39:622-629. PubMed Abstract | Publisher Full Text OpenURL

  28. Riddoch CJ, Mattocks C, Deere K, Saunders J, Kirkby J, Tilling K, Leary SD, Blair SN, Ness AR: Objective measurement of levels and patterns of physical activity.

    Arch Dis Child 2007, 92:963-969. PubMed Abstract | Publisher Full Text | PubMed Central Full Text OpenURL

  29. Wennlof AH, Yngve A, Nilsson TK, Sjostrom M: Serum lipids, glucose and insulin levels in healthy schoolchildren aged 9 and 15 years from Central Sweden: reference values in relation to biological, social and lifestyle factors.

    Scand J Clin Lab Invest 2005, 65:65-76. PubMed Abstract | Publisher Full Text OpenURL

  30. Burdette HL, Whitaker RC, Daniels SR: Parental report of outdoor playtime as a measure of physical activity in preschool-aged children.

    Arch Pediatr Adolesc Med 2004, 158:353-357. PubMed Abstract | Publisher Full Text OpenURL

  31. de Vries SI, Bakker I, Hopman-Rock M, Hirasing RA: van MW: Clinimetric review of motion sensors in children and adolescents.

    J Clin Epidemiol 2006, 59:670-680. PubMed Abstract | Publisher Full Text OpenURL

  32. Trost SG, Pate RR, Sallis JF, Freedson PS, Taylor WC, Dowda M, Sirard J: Age and gender differences in objectively measured physical activity in youth.

    Med Sci Sports Exerc 2002, 34:350-355. PubMed Abstract | Publisher Full Text OpenURL

  33. Reilly JJ, Penpraze V, Hislop J, Davies G, Grant S, Paton JY: Objective measurement of physical activity and sedentary behaviour: review with new data.

    Arch Dis Child 2008, 93:614-619. PubMed Abstract | Publisher Full Text OpenURL

  34. Welk GJ, Corbin CB, Dale D: Measurement issues in the assessment of physical activity in children.

    Res Q Exerc Sport 2000, 71:S59-73. PubMed Abstract OpenURL

  35. Adamo KB, Prince SA, Tricco AC, Connor-Gorber S, Tremblay M: A comparison of indirect versus direct measures for assessing physical activity in the pediatric population: a systematic review.

    Int J Pediatr Obes 2009, 4:2-27. PubMed Abstract | Publisher Full Text OpenURL

  36. Baquet G, Stratton G, Van PE, Berthoin S: Improving physical activity assessment in prepubertal children with high-frequency accelerometry monitoring: a methodological issue.

    Prev Med 2007, 44:143-147. PubMed Abstract | Publisher Full Text OpenURL

  37. Corder K, Brage S, Ekelund U: Accelerometers and pedometers: methodology and clinical application.

    Curr Opin Clin Nutr Metab Care 2007, 10:597-603. PubMed Abstract | Publisher Full Text OpenURL

  38. Mark AE, Janssen I: Relationship between screen time and metabolic syndrome in adolescents.

    J Public Health (Oxf) 2008, 30:153-160. Publisher Full Text OpenURL

  39. Ekelund U, Brage S, Froberg K, Harro M, Anderssen SA, Sardinha LB, Riddoch C, Andersen LB: TV viewing and physical activity are independently associated with metabolic risk in children: the European Youth Heart Study.

    PLoS Med 2006, 3:e488. PubMed Abstract | Publisher Full Text | PubMed Central Full Text OpenURL

  40. Martinez-Gomez D, Tucker J, Heelan KA, Welk GJ, Eisenmann JC: Associations between sedentary behavior and blood pressure in young children.

    Arch Pediatr Adolesc Med 2009, 163:724-730. PubMed Abstract | Publisher Full Text OpenURL

  41. Marshall SJ, Biddle SJ, Gorely T, Cameron N, Murdey I: Relationships between media use, body fatness and physical activity in children and youth: a meta-analysis.

    Int J Obes Relat Metab Disord 2004, 28:1238-1246. PubMed Abstract | Publisher Full Text OpenURL

  42. Mitchell JA, Mattocks C, Ness AR, Leary SD, Pate RR, Dowda M, Blair SN, Riddoch C: Sedentary behavior and obesity in a large cohort of children.

    Obesity(SilverSpring) 2009, 17:1596-1602. OpenURL

  43. Steele RM, van Sluijs EM, Cassidy A, Griffin SJ, Ekelund U: Targeting sedentary time or moderate- and vigorous-intensity activity: independent relations with adiposity in a population-based sample of 10-y-old British children.

    AmJ ClinNutr 2009, 90:1185-1192. OpenURL

  44. Hancox RJ, Milne BJ, Poulton R: Association between child and adolescent television viewing and adult health: a longitudinal birth cohort study.

    Lancet 2004, 364:257-262. PubMed Abstract | Publisher Full Text OpenURL

  45. Rey-Lopez JP, Vicente-Rodriguez G, Biosca M, Moreno LA: Sedentary behaviour and obesity development in children and adolescents.

    Nutr Metab Cardiovasc Dis 2008, 18:242-251. PubMed Abstract | Publisher Full Text OpenURL

  46. Vicente-Rodriguez G, Rey-Lopez JP, Martin-Matillas M, Moreno LA, Warnberg J, Redondo C, Tercedor P, Delgado M, Marcos A, Castillo M, Bueno M: Television watching, videogames, and excess of body fat in Spanish adolescents: the AVENA study.

    Nutrition 2008, 24:654-662. PubMed Abstract | Publisher Full Text OpenURL

  47. Ortega FB, Ruiz JR, Sjostrom M: Physical activity, overweight and central adiposity in Swedish children and adolescents: the European Youth Heart Study.

    Int J Behav Nutr Phys Act 2007, 4:61. PubMed Abstract | BioMed Central Full Text | PubMed Central Full Text OpenURL

  48. Gutin B, Yin Z, Humphries MC, Barbeau P: Relations of moderate and vigorous physical activity to fitness and fatness in adolescents.

    Am J Clin Nutr 2005, 81:746-750. PubMed Abstract | Publisher Full Text OpenURL

  49. Ruiz JR, Rizzo NS, Hurtig-Wennlof A, Ortega FB, Warnberg J, Sjostrom M: Relations of total physical activity and intensity to fitness and fatness in children: the European Youth Heart Study.

    Am J Clin Nutr 2006, 84:299-303. PubMed Abstract | Publisher Full Text OpenURL

  50. Wittmeier KD, Mollard RC, Kriellaars DJ: Physical activity intensity and risk of overweight and adiposity in children.

    Obesity (Silver Spring) 2008, 16:415-420. Publisher Full Text OpenURL

  51. Loucaides CA, Chedzoy SM, Bennett N: Differences in physical activity levels between urban and rural school children in Cyprus.

    Health Educ Res 2004, 19:138-147. PubMed Abstract | Publisher Full Text OpenURL

  52. Tremblay MS, Barnes JD, Esliger DW, Copeland JL: Seasonal variation in physical activity of Canadian Children assessed by accelerometry [abstract].

    Pediatr Exerc Sci 2005, 17:73. OpenURL

  53. Van Der Horst K, Paw MJ, Twisk JW, van Mechelen W: A brief review on correlates of physical activity and sedentariness in youth.

    Med Sci Sports Exerc 2007, 39:1241-1250. PubMed Abstract | Publisher Full Text OpenURL

  54. Ferreira I, Van Der Horst K, Wendel-Vos W, Kremers S, van Lenthe FJ, Brug J: Environmental correlates of physical activity in youth - a review and update.

    Obes Rev 2007, 8:129-154. PubMed Abstract | Publisher Full Text OpenURL

  55. Dunton GF, Kaplan J, Wolch J, Jerrett M, Reynolds KD: Physical environmental correlates of childhood obesity: a systematic review.

    Obes Rev 2009, 10:393-402. PubMed Abstract | Publisher Full Text OpenURL

  56. Sallis JF, Prochaska JJ, Taylor WC: A review of correlates of physical activity of children and adolescents.

    Med Sci Sports Exerc 2000, 32:963-975. PubMed Abstract | Publisher Full Text OpenURL

  57. Ridgers ND, Stratton G, Fairclough SJ: Assessing physical activity during recess using accelerometry.

    Prev Med 2005, 41:102-107. PubMed Abstract | Publisher Full Text OpenURL

  58. Riddoch CJ, Leary SD, Ness AR, Blair SN, Deere K, Mattocks C, Griffiths A, Davey SG, Tilling K: Prospective associations between objective measures of physical activity and fat mass in 12–14 year old children: the Avon Longitudinal Study of Parents and Children (ALSPAC).

    BMJ 2009, 339:b4544. PubMed Abstract | Publisher Full Text | PubMed Central Full Text OpenURL

  59. Ness AR, Leary SD, Mattocks C, Blair SN, Reilly JJ, Wells J, Ingle S, Tilling K, Smith GD, Riddoch C: Objectively measured physical activity and fat mass in a large cohort of children.

    PLoS Med 2007, 4:e97. PubMed Abstract | Publisher Full Text | PubMed Central Full Text OpenURL

  60. Fischbacher CM, Hunt S, Alexander L: How physically active are South Asians in the United Kingdom? A literature review.

    J Public Health (Oxf) 2004, 26:250-258. Publisher Full Text OpenURL

  61. Rogers A, Adamson JE, McCarthy M: Variations in health behaviours among inner city 12-year-olds from four ethnic groups.

    Ethn Health 1997, 2:309-316. PubMed Abstract | Publisher Full Text OpenURL

  62. Williams R, Shams M: Generational continuity and change in British Asian health and health behaviour.

    J Epidemiol Community Health 1998, 52:558-563. PubMed Abstract | Publisher Full Text | PubMed Central Full Text OpenURL

  63. Sproston K, Mindell J: Health Survey for England 2004: Volume 1 The Health of Minority Ethnic Groups. Sationary Office, London; 2006. OpenURL