title: Moderate-to-vigorous physical activity from ages 9 to 15 years.
abstract: BACKGROUND
  Decreased physical activity plays a critical role in the increase in childhood obesity. Although at least 60 minutes per day of moderate-to-vigorous physical activity (MVPA) is recommended, few longitudinal studies have determined the recent patterns of physical activity of youth.
  
  
  OBJECTIVE
  To determine the patterns and determinants of MVPA of youth followed from ages 9 to 15 years.
  
  
  METHODS
  Longitudinal descriptive analyses of the 1032 participants in the 1991-2007 National Institute of Child Health and Human Development Study of Early Child Care and Youth Development birth cohort from 10 study sites who had accelerometer-determined minutes of MVPA at ages 9 (year 2000), 11 (2002), 12 (2003), and 15 (2006) years. Participants included boys (517 [50.1%]) and girls (515 [49.9%]); 76.6% white (n = 791); and 24.5% (n = 231) lived in low-income families.
  
  
  METHODS
  Mean MVPA minutes per day, determined by 4 to 7 days of monitored activity.
  
  
  RESULTS
  At age 9 years, children engaged in MVPA approximately 3 hours per day on both weekends and weekdays. Weekday MVPA decreased by 37 minutes per year [corrected], while weekend MVPA decreased by 39 minutes per year [corrected]. By age 15 years, adolescents were only engaging in MVPA for 50 minutes per weekday [corrected] and 36 minutes per weekend day [corrected]. Boys were more active than girls, spending 18 and 14 more minutes per day [corrected] in MVPA on the weekdays and weekends, respectively. The rate of decrease in MVPA was the same for boys and girls. The estimated age at which girls crossed below the recommended 60 minutes of MVPA per day was approximately 13.2 years for weekday [corrected] activity compared with boys at 14.9 years [corrected], and for weekend activity, girls crossed below the recommended 60 minutes of MVPA at 12.7 years [corrected] compared with boys at 13.6 years [corrected].
  
  
  CONCLUSIONS
  In this study cohort, measured physical activity decreased significantly between ages 9 and 15 years.
authors.affiliation: Department of Pediatrics, University of California San Diego, 9500 Gilman Dr, La Jolla, CA 92093-0927, USA. pnader@ucsd.edu
authors.forename: Philip R
authors.initials: PR
authors.lastname: Nader
authors.forename: Robert H
authors.initials: RH
authors.lastname: Bradley
authors.forename: Renate M
authors.initials: RM
authors.lastname: Houts
authors.forename: Susan L
authors.initials: SL
authors.lastname: McRitchie
authors.forename: Marion
authors.initials: M
authors.lastname: O'Brien
mesh_terms: D000293:Adolescent; D000367:Age Factors; D015992:Body Mass Index; D002648:Child; D015444:Exercise / Q000502:physiology; D005260:Female; D006306:Health Surveys; D006801:Humans; D007182:Income; D008137:Longitudinal Studies; D008297:Male; D008962:Models, Theoretical; D012737:Sex Factors; D013997:Time Factors; D014481:United States