title: Sedentary behaviours and its association with bone mass in adolescents: the HELENA Cross-Sectional Study.
abstract: BACKGROUND
  We aimed to examine whether time spent on different sedentary behaviours is associated with bone mineral content (BMC) in adolescents, after controlling for relevant confounders such as lean mass and objectively measured physical activity (PA), and if so, whether extra-curricular participation in osteogenic sports could have a role in this association.
  
  
  METHODS
  Participants were 359 Spanish adolescents (12.5-17.5 yr, 178 boys,) from the HELENA-CSS (2006-07). Relationships of sedentary behaviours with bone variables were analysed by linear regression. The prevalence of low BMC (at least 1SD below the mean) and time spent on sedentary behaviours according to extracurricular sport participation was analysed by Chi-square tests.
  
  
  RESULTS
  In boys, the use of internet for non-study was negatively associated with whole body BMC after adjustment for lean mass and moderate to vigorous PA (MVPA). In girls, the time spent studying was negatively associated with femoral neck BMC. Additional adjustment for lean mass slightly reduced the negative association between time spent studying and femoral neck BMC. The additional adjustment for MVPA did not change the results at this site. The percentage of girls having low femoral neck BMC was significantly smaller in those participating in osteogenic sports (≥ 3 h/week) than in the rest, independently of the cut-off selected for the time spent studying.
  
  
  CONCLUSIONS
  The use of internet for non-study (in boys) and the time spent studying (in girls) are negatively associated with whole body and femoral neck BMC, respectively. In addition, at least 3 h/week of extra-curricular osteogenic sports may help to counteract the negative association of time spent studying on bone health in girls.
authors.affiliation: School of Sport and Health Sciences, University of Exeter, Heavitree Road, Exeter, EX1 2LU, UK. L.A.Gracia-Marco@exeter.ac.uk
authors.forename: Luis
authors.initials: L
authors.lastname: Gracia-Marco
authors.forename: Juan P
authors.initials: JP
authors.lastname: Rey-López
authors.forename: Alba M
authors.initials: AM
authors.lastname: Santaliestra-Pasías
authors.forename: David
authors.initials: D
authors.lastname: Jiménez-Pavón
authors.forename: Ligia E
authors.initials: LE
authors.lastname: Díaz
authors.forename: Luis A
authors.initials: LA
authors.lastname: Moreno
authors.forename: German
authors.initials: G
authors.lastname: Vicente-Rodríguez
mesh_terms: D000293:Adolescent; D000294:Adolescent Behavior / Q000502:physiology*; D015519:Bone Density; D003430:Cross-Sectional Studies; D005260:Female; D006801:Humans; D016014:Linear Models; D008297:Male; D057185:Sedentary Behavior; D013030:Spain; D013177:Sports / Q000706:statistics & numerical data; D013997:Time Factors