title: Exploring the Adolescent Fall in Physical Activity: A 10-yr Cohort Study (EarlyBird 41).
abstract: BACKGROUND
  Contemporary adolescents are deemed inactive, especially girls, but whether for biological reasons associated with their maturation, changes in their behavior or because of environmental constraints, is uncertain. We examined the trends in physical activity (PA) in relation to both biological and environmental factors in an attempt to establish what drives activity patterns from childhood through adolescence.
  
  
  METHODS
  Physical activity (7-d Actigraph accelerometry) was measured annually from 5 to 15 yr in a single cohort of some 300 UK children. Total PA (TPA; in-school and out-of-school separately and combined as whole day) and intensity-specific PA (sedentary, light, and moderate-and-vigorous [MVPA]) were analyzed. Biological age (years before/after measured peak height velocity) and pubertal stage (self-reported pubic hair development-Tanner staging) were also measured as was socioeconomic status (postcode-derived index of multiple deprivation [IMD]).
  
  
  RESULTS
  Total PA was stable from 5 to 8 yr (trend P = 0.10) but fell progressively from 9 to 15 yr (by approximately 30% in girls and approximately 20% in boys, both P < 0.001; sex interaction, P < 0.01). Half of this fall was attributable to light intensity PA and only a quarter to MVPA. The decline in PA was related similarly to chronological and biological age, whereas pubertal stage explained the more rapid PA decline in girls (puberty-adjusted sex interaction, P = 0.51). Total PA fell to the same extent for in-school and out-of-school settings (both P < 0.001), and for lower and higher IMD areas (both P < 0.001). Total PA tracked moderately to strongly from childhood into adolescence (r = 0.58; P < 0.001).
  
  
  CONCLUSIONS
  The adolescent decline in PA is consistent across different environmental settings, attributable to falls in light-intensity/habitual activity and influenced by puberty, suggesting that the inactivity of adolescence may, in part, be under biological control.
authors.affiliation: 1Institute of Health Research, University of Exeter Medical School, Exeter, UNITED KINGDOM; 2Sport and Health Science, College of Life and Environmental Sciences, University of Exeter, Exeter, UNITED KINGDOM; and 3Department of Endocrinology and Metabolism, Peninsula Schools of Medicine and Dentistry, Plymouth University Plymouth, UNITED KINGDOM.
authors.forename: Brad S
authors.initials: BS
authors.lastname: Metcalf
authors.forename: Joanne
authors.initials: J
authors.lastname: Hosking
authors.forename: Alison N
authors.initials: AN
authors.lastname: Jeffery
authors.forename: William E
authors.initials: WE
authors.lastname: Henley
authors.forename: Terence J
authors.initials: TJ
authors.lastname: Wilkin
mesh_terms: D061725:Accelerometry; D000293:Adolescent; D000294:Adolescent Behavior / Q000502:physiology*; D000367:Age Factors; D002648:Child; D002675:Child, Preschool; D015331:Cohort Studies; D004777:Environment; D005260:Female; D006801:Humans; D008297:Male; D009043:Motor Activity / Q000502:physiology*; D011446:Prospective Studies; D011627:Puberty / Q000502:physiology; D012737:Sex Factors; D012923:Social Class