title: Magnitude and determinants of change in objectively-measured physical activity, sedentary time and sleep duration from ages 15 to 17.5y in UK adolescents: the ROOTS study.
abstract: BACKGROUND
  Self-reported physical activity (PA) and sleep duration (SLP) change markedly throughout adolescence. We sought to quantify changes in objectively-measured PA, sedentary time (ST) and SLP through adolescence, and to investigate baseline body composition and baseline activity levels as determinants of change.
  
  
  METHODS
  Individually calibrated combined heart rate and movement sensing was used to estimate PA energy expenditure (PAEE), SLP, daily ST and time in light (LPA), moderate (MPA), vigorous (VPA), and moderate-to-vigorous physical activity (MVPA) in 144 adolescents (50% boys) of mean age 15.1(±0.3)y at baseline and 17.5(±0.3)y at follow-up. Changes in PA (ΔPA), ST (ΔST) and SLP (ΔSLP) were calculated as follow-up minus baseline values. Waist circumference (WC) was measured at baseline and follow-up, as was fat mass index (FMI) and fat-free mass index (FFMI) by a pooled estimation method including bio-impedance. Comparison of baseline and follow-up activity was made by mixed-model ANOVA. Linear regression adjusted for baseline demographics, total and weekend hours of monitor wear time and the seasons of activity measurements, was used to investigate baseline body composition as determinants of ΔPA, ΔST and ΔSLP. A further model adjusted for baseline of the outcome assessed baseline activity as a predictor of behaviour change, and investigated associations for baseline body composition independent of the baseline level of the outcome.
  
  
  RESULTS
  From baseline to follow-up levels of MPA and VPA declined (p ≤ 0.039). The annual decline in MVPA was equivalent to -4.5 and -3.0 min/d in boys and girls, respectively. Baseline FMI, FFMI and WC were positively associated with ΔLPA and negatively associated with ΔST in boys when adjusted for baseline of the outcome (p ≤ 0.037 for all). SLP increased from baseline to follow-up (p = 0.004) but ΔSLP was not associated with baseline body composition (p ≥ 0.13). For all variables, higher baseline levels were associated with greater declines over time (p ≤ 0.003).
  
  
  CONCLUSIONS
  Levels of higher-intensity PA decline from mid-to-late adolescence, whereas the duration of sleep increases. Changes in LPA and ST may be associated with baseline body composition, but the baseline level of the outcome is consistently the strongest predictor of changes in adolescent activity.
authors.affiliation: Institute of Metabolic Science, MRC Epidemiology Unit, University of Cambridge, Box 285, Addenbrookes Hospital, Cambridge, CB2 0QQ, UK. paul.collings@mrc-epid.cam.ac.uk.
authors.forename: Paul J
authors.initials: PJ
authors.lastname: Collings
authors.affiliation: Institute of Metabolic Science, MRC Epidemiology Unit, University of Cambridge, Box 285, Addenbrookes Hospital, Cambridge, CB2 0QQ, UK. katrien.wijndaele@mrc-epid.cam.ac.uk.
authors.forename: Katrien
authors.initials: K
authors.lastname: Wijndaele
authors.affiliation: Institute of Metabolic Science, MRC Epidemiology Unit, University of Cambridge, Box 285, Addenbrookes Hospital, Cambridge, CB2 0QQ, UK. kirsten.corder@mrc-epid.cam.ac.uk.
authors.forename: Kirsten
authors.initials: K
authors.lastname: Corder
authors.affiliation: Institute of Metabolic Science, MRC Epidemiology Unit, University of Cambridge, Box 285, Addenbrookes Hospital, Cambridge, CB2 0QQ, UK. kate.westgate@mrc-epid.cam.ac.uk.
authors.forename: Kate
authors.initials: K
authors.lastname: Westgate
authors.affiliation: Institute of Metabolic Science, MRC Epidemiology Unit, University of Cambridge, Box 285, Addenbrookes Hospital, Cambridge, CB2 0QQ, UK. charlotte.ridgway@mrc-epid.cam.ac.uk.
authors.forename: Charlotte L
authors.initials: CL
authors.lastname: Ridgway
authors.affiliation: Institute of Metabolic Science, MRC Epidemiology Unit, University of Cambridge, Box 285, Addenbrookes Hospital, Cambridge, CB2 0QQ, UK. stephen.sharp@mrc-epid.cam.ac.uk.
authors.forename: Stephen J
authors.initials: SJ
authors.lastname: Sharp
authors.affiliation: Developmental Lifecourse Research Group, Department of Psychiatry, University of Cambridge, Cambridge, UK. vjd20@cam.ac.uk.
authors.forename: Valerie
authors.initials: V
authors.lastname: Dunn
authors.affiliation: Developmental Lifecourse Research Group, Department of Psychiatry, University of Cambridge, Cambridge, UK. ig104@cam.ac.uk.
authors.forename: Ian
authors.initials: I
authors.lastname: Goodyer
authors.affiliation: Institute of Metabolic Science, MRC Epidemiology Unit, University of Cambridge, Box 285, Addenbrookes Hospital, Cambridge, CB2 0QQ, UK. Ulf.Ekelund@nih.no.|Department of Sport Medicine, Norwegian School of Sports Science, Oslo, Norway. Ulf.Ekelund@nih.no.
authors.forename: Ulf
authors.initials: U
authors.lastname: Ekelund
authors.affiliation: Institute of Metabolic Science, MRC Epidemiology Unit, University of Cambridge, Box 285, Addenbrookes Hospital, Cambridge, CB2 0QQ, UK. soren.brage@mrc-epid.cam.ac.uk.
authors.forename: Soren
authors.initials: S
authors.lastname: Brage
mesh_terms: D050154:Adiposity / Q000502:physiology; D000293:Adolescent; D000294:Adolescent Behavior / Q000523:psychology*; D001823:Body Composition / Q000502:physiology; D015992:Body Mass Index; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D008297:Male; D009043:Motor Activity / Q000502:physiology*; D057185:Sedentary Behavior; D012890:Sleep / Q000502:physiology*; D013997:Time Factors; D006113:United Kingdom; D055105:Waist Circumference / Q000502:physiology