Several studies that used DXA or quantitative ultrasound suggested that objective ly measured SB was negatively associated with lower extremity bone outcomes , such as femoral neck bone mineral density .
The magnitude of this negative association was small and independent of moderate-to-vigorous physical activity .
In contrast to the lower extremities , there was insufficient evidence for an association of lumbar spine bone outcomes with objective ly measured SB .
In high- quality studies that used DXA , no association was observed between objective ly measured SB and total body bone outcomes .
In studies using question naires , none of these relationships were observed .
Sedentary  behaviour ( SB ) is increasing in Western societies and some studies suggest a deleterious effect of SB on bone .
The aim of this systematic review was to examine the association between  SB and bone health in children , adolescents and young adults .
Extracurricular physical activity participation modifies the association between high TV watching and low bone mass.
OBJECTIVES To examine whether different sedentary behaviours are associated with the risk of  low bone mineral content in adolescents , and if so , whether extracurricular physical-sporting activity influences this association . MATERIAL S AND METHODS A total of 277 adolescents from Zaragoza ( 168 females and 109 males ) aged 13.0 - 18.5 yr within frame work of the multicentre AVENA study participated in this study . Bone mineral content ( BMC ) , lean mass , and fat mass were measured with DXA . Physical activity and sedentary independent variables : participation in extracurricular physical-sporting activity ( PA ) , h/d of television watching , playing video/computer games during school days and on weekend days and doing homework/ study ing . They all were assessed by question naire . The main outcome was low BMC , as defined by BMC Z-score for age and sex < percentile 10 . Logistic regression was used to test the interaction and association of PA and sedentary variables with low BMC , after controlling for confounders like height , maturational status or lean mass . RESULTS Among the sedentary variables studied , only television watching > or =3 h/d was associated with an increased risk for low BMC in males ( OR , 95 % CI : 7.01 , 1.73 to 28.40 ) , after controlling for sexual maturation . When PA was in the models , television watching was not any longer associated with low BMC , while PA was so ( OR , 95 % CI : 0.23 , 0.09 to 0.55 ) . Involvement in such activity reduced the risk of low bone mass by 76 % ( P<0.01 ) independently of body mass , height and fat mass , but not of the lean mass . CONCLUSION Watching television for 3 or more h/d seems to be associated with an increased risk for  low BMC in male adolescents . However , this association is mediated by participation in PA , suggesting that negative consequences of excessive television watching on adolescent bone health could be counteracted by sport participation . Longitudinal data and r and omized controlled trials will confirm or contrast our findings
Several studies that used DXA or quantitative ultrasound suggested that objective ly measured SB was negatively associated with lower extremity bone outcomes , such as femoral neck bone mineral density .
The magnitude of this negative association was small and independent of moderate-to-vigorous physical activity .
In contrast to the lower extremities , there was insufficient evidence for an association of lumbar spine bone outcomes with objective ly measured SB .
In high- quality studies that used DXA , no association was observed between objective ly measured SB and total body bone outcomes .
In studies using question naires , none of these relationships were observed .
Sedentary  behaviour ( SB ) is increasing in Western societies and some studies suggest a deleterious effect of SB on bone .
The aim of this systematic review was to examine the association between  SB and bone health in children , adolescents and young adults .