title: Developmental trajectories of physical activity and television viewing during adolescence among girls: National Growth and Health Cohort Study.
abstract: BACKGROUND
  Analytic methodology for investigating physical activity patterns over time has been limited. The aim of this study was to demonstrate the group-based trajectory analysis process for identifying developmental physical activity (PA) and television (TV) viewing trajectories and the risk factor of PA trajectories, and for examining a relationship between PA and TV viewing trajectories among adolescent girls.
  
  
  METHODS
  Secondary analysis was conducted using the National Growth and Health Study (NGHS) dataset. The NGHS administered the Habitual Activity Questionnaire and TV viewing questionnaire to White and Black girls at age 10, 12, 14, 16, 17, 18, and 19 years. Group-based trajectory analyses were conducted to identify distinct PA trajectories. Race was chosen to present an example of the risk factor analysis and was added as a predictor in the trajectory model. Dual-trajectory analysis was conducted to estimate probabilities of TV viewing trajectory groups conditional on the PA trajectory groups.
  
  
  RESULTS
  A total of 2,155 girls (52% Black) were included in the data analysis. We identified four PA trajectories: substantially decreasing from high PA (PA group 1, 9.4%), maintaining moderate PA (PA group 2, 31.6%), maintaining high PA (PA group 3, 5.8%), and decreasing from moderate PA (PA group 4, 53.2%). A significantly lower proportion of Black girls had high PA levels at baseline and maintained their baseline PA than White girls. Most girls who were classified as maintaining high PA (88%) were also classified as decreasing TV viewing.
  
  
  CONCLUSIONS
  A group-based trajectory approach provides new insights about the patterns of maintaining moderate or high levels of PA that exist among adolescent girls. However, a lower proportion of Black girls followed the maintenance patterns than White girls. The behavioral development of PA and TV viewing may be intertwined among adolescent girls.
authors.affiliation: Department of Pediatrics, Stanley Manne Children's Research Institute, Ann & Robert H. Lurie Children's Hospital of Chicago, Northwestern University, 225 E Chicago Ave. Box 157, Chicago, IL, 60611, USA. skwon@luriechildrens.org.
authors.forename: Soyang
authors.initials: S
authors.lastname: Kwon
authors.affiliation: Department of Preventive Medicine, Northwestern University, 680 N Lakeshore Dr. Suite 1400, Chicago, IL, 60611, USA. jungwha-lee@northwestern.edu.
authors.forename: Jungwha
authors.initials: J
authors.lastname: Lee
authors.affiliation: Department of Preventive Medicine, Northwestern University, 680 N Lakeshore Dr. Suite 1400, Chicago, IL, 60611, USA. carnethon@northwestern.edu.
authors.forename: Mercedes R
authors.initials: MR
authors.lastname: Carnethon
mesh_terms: D000293:Adolescent; D001741:Black or African American; D002648:Child; D015331:Cohort Studies; D015444:Exercise; D005260:Female; D006306:Health Surveys; D006801:Humans; D008137:Longitudinal Studies; D011998:Recreation; D012307:Risk Factors; D057185:Sedentary Behavior; D013690:Television / Q000706:statistics & numerical data*; D044465:White People; D055815:Young Adult