title: Longitudinal changes in sedentary time and physical activity during adolescence.
abstract: BACKGROUND
  Low levels of physical activity and high time spent in sedentary activities have been associated with unfavourable health outcomes in adolescents. During adolescence, physical activity declines and sedentary time increases, however little is known about whether the magnitude of these changes differs within or between school-time, after-school time, or at weekends.
  
  
  METHODS
  Adolescents (n = 363) participating in the PEACH (Personal and Environmental Associations with Children's Health) project provided accelerometer data at 12 and 15 years of age. Data were collected in 2008/2009 and 2012/2013. Time spent sedentary (<100 cpm), in light physical activity (LPA (100-2295 cpm) and in moderate to vigorous physical activity (MVPA: ≥ 2296 cpm) were generated for school-time, after-school time and for weekends using school-specific start and finish times. All data were analysed in 2014.
  
  
  RESULTS
  The proportion of time spent sedentary significantly increased during school (+8.23%, 95% CI = 7.35 to 9.13), after-school (+6.99%, 95% CI = 5.91 to 8.07) and at weekends (+6.86%, 95% CI = 5.10 to 8.62). A parallel decrease was found in the proportion of time spent in LPA during school (-7.62%, 95% CI = -8.26 to -6.98), after-school (-7.01%, 95% CI = -7.74 to -6.28) and at weekends (-6.72%, 95% CI = -7.80 to -5.65). The proportion of time spent in MVPA remained relatively stable during school (-0.64, 95% CI = -1.11 to -0.18), after-school (0.04%, 95% CI = -0.58 to 0.67) and at weekends (-0.14%, 95% CI = -1.18 to 0.90).
  
  
  CONCLUSIONS
  Objectively measured sedentary time increased between 12 and 15 years of age during-school, after-school, and at weekends, suggesting that interventions aiming to reduce the age-associated changes in sedentary time are needed in all three time contexts. Future work should identify which sedentary activities change more than others to inform interventions which aim to minimise the increase in time spent sedentary during adolescence.
authors.affiliation: Centre for Exercise, Nutrition and Health Sciences, University of Bristol, Bristol, UK. sarah.harding@bristol.ac.uk.
authors.forename: Sarah K
authors.initials: SK
authors.lastname: Harding
authors.affiliation: Centre for Exercise, Nutrition and Health Sciences, University of Bristol, Bristol, UK. A.S.Page@bristol.ac.uk.
authors.forename: Angie S
authors.initials: AS
authors.lastname: Page
authors.affiliation: National Institute for Health Research, Bristol Biomedical Research Unit in Nutrition, Diet and Lifestyle, Bristol, UK. catherine.falconer@bristol.ac.uk.
authors.forename: Catherine
authors.initials: C
authors.lastname: Falconer
authors.affiliation: Centre for Exercise, Nutrition and Health Sciences, University of Bristol, Bristol, UK. Ashley.Cooper@bristol.ac.uk.|National Institute for Health Research, Bristol Biomedical Research Unit in Nutrition, Diet and Lifestyle, Bristol, UK. Ashley.Cooper@bristol.ac.uk.
authors.forename: Ashley R
authors.initials: AR
authors.lastname: Cooper
mesh_terms: D000293:Adolescent; D000294:Adolescent Behavior; D000367:Age Factors; D002648:Child; D015444:Exercise; D005260:Female; D015438:Health Behavior; D006801:Humans; D008137:Longitudinal Studies; D008297:Male; D009043:Motor Activity; D012574:Schools; D057185:Sedentary Behavior