title: Development of sedentary behavior across childhood and adolescence: longitudinal analysis of the Gateshead Millennium Study.
abstract: BACKGROUND
  In many parts of the world policy and research interventions to modify sedentary behavior of children and adolescents are now being developed. However, the evidence to inform these interventions (e.g. how sedentary behavior changes across childhood and adolescence) is limited. This study aimed to assess longitudinal changes in sedentary behavior, and examine the degree of tracking of sedentary behavior from age 7y to 15y.
  
  
  METHODS
  Participants were part of the Gateshead Millennium Study cohort. Measures were made at age 7y (n = 507), 9y (n = 510), 12y (n = 425) and 15y (n = 310). Participants were asked to wear an ActiGraph GT1M and accelerometer epochs were defined as sedentary when recorded counts were ≤25 counts/15 s. Differences in sedentary time and sedentary fragmentation were examined using the Friedman test. Tracking was examined using Spearman's correlation coefficients and trajectories over time were assessed using multilevel linear spline modelling.
  
  
  RESULTS
  Median daily sedentary time increased from 51.3% of waking hours at 7y to 74.2% at 15y. Sedentary fragmentation decreased from 7y to 15y. The median number of breaks/hour decreased from 8.6 to 4.1 breaks/hour and the median bout duration at 50% of the cumulative sedentary time increased from 2.4 min to 6.4 min from 7y to 15y. Tracking of sedentary time and sedentary fragmentation was moderate from 7y to 15y however, the rate of change differed with the steepest increases/decreases seen between 9y and 12y.
  
  
  CONCLUSIONS
  In this study, sedentary time was high and increased to almost 75% of waking hours at 15y. Sedentary behavior became substantially less fragmented as children grew older. The largest changes in sedentary time and sedentary fragmentation occurred between 9y to 12y, a period which spans the transition to secondary school. These results can be used to inform future interventions aiming to change sedentary behavior.
authors.affiliation: University of Strathclyde, School of Psychological Science and Health, Glasgow, Scotland, UK. xanne.janssen@strath.ac.uk.
authors.forename: Xanne
authors.identifier: http://orcid.org/0000-0003-1355-0792
authors.initials: X
authors.lastname: Janssen
authors.affiliation: Institute of Health & Society, Newcastle University, Newcastle upon Tyne, England, UK.|Human Nutrition Research Centre, Newcastle University, Newcastle upon Tyne, England, UK.
authors.forename: Kay D
authors.initials: KD
authors.lastname: Mann
authors.affiliation: Institute of Health & Society, Newcastle University, Newcastle upon Tyne, England, UK.|Human Nutrition Research Centre, Newcastle University, Newcastle upon Tyne, England, UK.
authors.forename: Laura
authors.initials: L
authors.lastname: Basterfield
authors.affiliation: Human Nutrition Research Centre, Newcastle University, Newcastle upon Tyne, England, UK.
authors.forename: Kathryn N
authors.initials: KN
authors.lastname: Parkinson
authors.affiliation: Institute of Health & Society, Newcastle University, Newcastle upon Tyne, England, UK.
authors.forename: Mark S
authors.initials: MS
authors.lastname: Pearce
authors.affiliation: Human Nutrition Research Centre, Newcastle University, Newcastle upon Tyne, England, UK.
authors.forename: Jessica K
authors.initials: JK
authors.lastname: Reilly
authors.affiliation: Institute of Health & Society, Newcastle University, Newcastle upon Tyne, England, UK.|Human Nutrition Research Centre, Newcastle University, Newcastle upon Tyne, England, UK.
authors.forename: Ashley J
authors.initials: AJ
authors.lastname: Adamson
authors.affiliation: University of Strathclyde, School of Psychological Science and Health, Glasgow, Scotland, UK.
authors.forename: John J
authors.initials: JJ
authors.lastname: Reilly
mesh_terms: D000293:Adolescent; D000294:Adolescent Behavior; D000367:Age Factors; D002648:Child; D002652:Child Behavior; D015331:Cohort Studies; D015444:Exercise; D005260:Female; D015438:Health Behavior; D006801:Humans; D016014:Linear Models; D008137:Longitudinal Studies; D008297:Male; D012574:Schools; D057185:Sedentary Behavior
keywords: Accelerometry; Adolescents; Children; Cohort; Longitudinal; Sitting