title: Sitting time and mortality from all causes, cardiovascular disease, and cancer. Extracurricular physical activity participation modifies the association between high TV watching and low bone mass. Size-corrected BMD decreases during peak linear growth: implications for fracture incidence during adolescence. Lower body negative pressure treadmill exercise as a countermeasure for bed rest-induced bone loss in female identical twins. Television, computer, and video viewing; physical activity; and upper limb fracture risk in children: a population-based case control study. Adiposity and TV viewing are related to less bone accrual in young children. Low back joint loading and kinematics during standing and unsupported sitting Association between bone stiffness and nutritional biomarkers combined with weight-bearing exercise, physical activity, and sedentary time in preadolescent children. A case-control study.
abstract: PURPOSE Although moderate-to-vigorous physical activity is related to premature mortality , the relationship between sedentary behaviors and mortality has not been fully explored and may represent a different paradigm than that associated with lack of exercise . We prospect ively examined  sitting time  and mortality in a representative sample of 17,013 Canadians 18 - 90 yr of age . METHODS Evaluation of  daily sitting time  ( almost none of the time , one fourth of the time , half of the time , three fourths of the time , almost all of the time ) ,  leisure time physical activity , smoking status , and alcohol consumption  was conducted at baseline . Participants were followed prospect ively for an average of 12.0 yr for the ascertainment of mortality status . RESULTS There were 1832 deaths  ( 759 of cardiovascular disease ( CVD ) and 547 of cancer ) during 204,732 person-yr of follow-up . After adjustment for potential confounders , there was a progressively higher risk of mortality across higher levels of sitting time from all causes ( hazard ratios ( HR ) : 1.00 , 1.00 , 1.11 , 1.36 , 1.54 ; P for trend < 0.0001 ) and CVD ( HR:1.00 , 1.01 , 1.22 , 1.47 , 1.54 ; P for trend < 0.0001 ) but not cancer . Similar results were obtained when stratified by sex , age , smoking status , and body mass index . Age-adjusted all-cause mortality rates per 10,000 person-yr of follow-up were 87 , 86 , 105 , 130 , and 161 ( P for trend < 0.0001 ) in physically inactive participants and 75 , 69 , 76 , 98 , 105 ( P for trend = 0.008 ) in active participants across sitting time categories . CONCLUSIONS These data demonstrate a dose-response association between  sitting time  and mortality from all causes and CVD , independent of leisure time physical activity . In addition to the promotion of moderate-to-vigorous physical activity and a healthy weight , physicians should discourage sitting for extended periods 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 UNLABELLED Peak adolescent fracture incidence at the distal end of the radius coincides with a decline in size-corrected BMD  in both boys and girls . Peak gains in bone area preceded peak gains in BMC in a longitudinal sample of boys and girls , supporting the theory that the dissociation between skeletal expansion and skeletal mineralization results in a period of relative bone weakness . INTRODUCTION The high incidence of fracture in adolescence may be related to a period of relative skeletal fragility result ing from dissociation between bone expansion and bone mineralization during the growing years . The aim of this study was to examine the relationship between changes in size-corrected BMD ( BMDsc ) and peak distal radius fracture incidence in boys and girls . MATERIAL S AND METHODS Subjects were 41 boys and 46 girls measured annually ( DXA ; Hologic 2000 )  over the adolescent growth period and again in young adulthood . Ages of peak height velocity ( PHV ) , peak BMC velocity ( P BMC V ) , and peak bone area ( BA ) velocity ( PBAV ) were determined for each child . To control for maturational differences , subjects were aligned on PHV . BMDsc was calculated by first regressing the natural logarithms of BMC and BA . The power coefficient ( pc ) values from this analysis were used as follows : BMDsc = BMC /BA(pc ) . RESULTS BMDsc decreased significantly before the age of PHV and then increased until 4 years after PHV . The peak rates in radial fractures ( reported from previous work ) in both boys and girls coincided with the age of negative velocity in BMDsc ; the age of peak BA velocity ( PBAV ) preceded the age of peak BMC velocity ( P BMC V ) by 0.5 years in both boys and girls . CONCLUSIONS There is a clear dissociation between P BMC V and PBAV in boys and girls . BMDsc declines before age of PHV before rebounding after PHV . The timing of these events coincides directly with reported fracture rates of the distal end of the radius . Thus , the results support the theory that there is a period of relative skeletal weakness during the adolescent growth period caused , in part , by a draw on cortical bone to meet the mineral dem and s of the exp and ing skeleton result ing in a temporary increased fracture risk Supine weight-bearing exercise within lower body negative pressure ( LBNP ) alleviates some of the skeletal deconditioning induced by simulated weightlessness in men . We examined this potential beneficial effect in women . Because dietary acid load affected the degree of bone resorption in men during bed rest , we also investigated this variable in women . Subjects were 7 pairs of female identical twins assigned at r and om to 2 groups , sedentary bed rest ( control ) or bed rest with supine treadmill exercise within LBNP .  Dietary intake was controlled and monitored . Urinary calcium and markers of bone resorption were measured before bed rest and on bed rest days 5/6 , 12/13 , 19/20 , and 26/27 . Bone mineral content was assessed by dual-energy X-ray absorptiometry before and after bed rest . Data were analyzed by repeated- measures two-way analysis of variance . Pearson correlation coefficients were used to define the relationships between diet and markers of bone metabolism and to estimate heritability of markers . During bed rest , all markers of bone resorption and urinary calcium and phosphorus increased ( P<0.001 ) ; parathyroid hormone ( P=0.06 ) , bone-specific alkaline phosphatase ( P=0.06 ) , and 1,25-dihydroxyvitamin D ( P=0.09 ) tended to decrease . LBNP exercise tended to mitigate bone density loss . The ratio of dietary animal protein to potassium was positively correlated with urinary calcium excretion for all weeks of bed rest in the control group , but only during weeks 1 and 3 in the exercise group . Pre-bed rest data suggested that many markers of bone metabolism have strong genetic determinants . Treadmill exercise within LBNP had less of a protective effect on bone resorption during bed rest in women than previously published results had shown for its effect in men , but the same trends were observed for both sexes . Dietary acid load of these female subjects was significantly correlated with calcium excretion but not with other bone resorption markers UNLABELLED The effect of physical activity on upper limb fractures was examined in this population -based case control study with 321 age- and gender-matched pairs .  Sports participation increased fracture risk in boys and decreased risk in girls . Television viewing had a deleterious dose response association with wrist and forearm fractures while light physical activity was protective . INTRODUCTION The aim of this population -based case control study was to examine the association between television , computer , and video viewing ; types and levels of physical activity ; and upper limb fractures in children 9 - 16 years of age . MATERIAL S AND METHODS A total of 321 fracture cases and 321 r and omly selected individually matched controls were studied .  Television , computer , and video viewing and types and levels of physical activity were determined by interview-administered question naire . Bone strength was assessed by DXA and metacarpal morphometry . RESULTS In general , sports participation increased total upper limb fracture risk in boys and decreased risk in girls . Gender-specific risk estimates were significantly different for total , contact , noncontact , and high-risk sports participation as well as four individual sports ( soccer , cricket , surfing , and swimming ) . In multivariate analysis , time spent television , computer , and video viewing in both sexes was positively associated with wrist and forearm fracture risk ( OR 1.6/category , 95 % CI : 1.1 - 2.2 ) , whereas days involved in light physical activity participation decreased fracture risk ( OR 0.8/category , 95 % CI : 0.7 - 1.0 ) . Sports participation increased h and ( OR 1.5/sport , 95 % CI : 1.1 - 2.0 ) and upper arm ( OR 29.8/sport , 95 % CI : 1.7 - 535 ) fracture risk in boys only and decreased wrist and forearm fracture risk in girls only ( OR 0.5/sport , 95 % CI : 0.3 - 0.9 ) . Adjustment for bone density and metacarpal morphometry did not alter these associations . CONCLUSION There is gender discordance with regard to sports participation and fracture risk in children , which may reflect different approaches to sport . Importantly , television , computer , and video viewing has a dose-dependent association with wrist and forearm fractures , whereas light physical activity is protective . The mechanism is unclear but may involve bone-independent factors , or less likely , changes in bone quality not detected by DXA or metacarpal morphometry OBJECTIVE To examine the relation between baseline fat mass and gain in bone area and bone mass in preschoolers studied prospect ively for 4 years , with a focus on the role of physical activity and TV viewing . STUDY DESIGN Children were part of a longitudinal study in which measures of fat , lean and bone mass , height , weight , activity , and diet were taken every 4 months from ages 3 to 7 years . Activity was measured by accelerometer and TV viewing by parent checklist . We included 214 children with total body dual energy x-ray absorptiometry ( Hologic 4500A ) scans at ages 3.5 and 7 years . RESULTS Higher baseline fat mass was associated with smaller increases in bone area and bone mass over the next 3.5 years ( P < .001 ) . More TV viewing was related to smaller gains in bone area and bone mass accounting for race , sex , and height . Activity by accelerometer was not associated with bone gains . CONCLUSIONS Adiposity and TV viewing are related to less bone accrual in preschoolers The aim was to examine lumbar spine kinematics , spinal joint loads and trunk muscle activation patterns during a prolonged ( 2 h ) period of sitting .  This information is necessary to assist the ergonomist in design ing work where posture variation is possible — particularly between st and ing and various styles of  sitting  . Joint loads were predicted with a highly detailed anatomical biomechanical model ( that incorporated 104 muscles , passive ligaments and intervertebral discs ) , which utilized biological signals of spine posture and muscle electromyograms ( EMG ) from each trial of each subject . Sitting result ed in significantly higher ( p < 0.001 ) low back compressive loads ( mean±SD 1698±467 N ) than those experienced by the lumbar spine during st and ing ( 1076±243 N ) . Subjects were equally divided into adopting one of two sitting strategies : a single ‘ static ’ or a ‘ dynamic ’ multiple posture approach . Within each individual , st and ing produced a distinctly diVerent spine posture compared with sitting , and st and ing spine postures did not overlap with flexion postures adopted in sitting when spine postures were averaged across all eight subjects . A rest component ( as noted in an amplitude probability distribution function from the EMG ) was present for all muscles monitored in both sitting and st and ing tasks . The upper and lower erector spinae muscle groups exhibited a shifting to higher levels of activation during sitting . There were no clear muscle activation level diVerences in the individuals who adopted diVerent sitting strategies . St and ing appears to be a good rest from sitting given the reduction in passive tissue forces . However , the constant loading with little dynamic movement which characterizes both st and ing and sitting would provide little rest/change for muscular activation levels or low back loading Physical activity ( PA ) and micronutrients such as calcium ( Ca ) , vitamin D ( 25OHD ) , and phosphate ( PO ) are important determinants of skeletal development . This case-control study examined the association of these nutritional biomarkers and different PA behaviours , such as habitual PA , weight-bearing exercise ( WBE ) and sedentary time ( SED ) with bone stiffness ( SI ) in 1819 2 - 9-year-old children from the IDEFICS study ( 2007 - 2008 ) .  SI was measured on the calcaneus using quantitative ultrasound .  Serum and urine Ca and PO and serum 25OHD were determined . Children 's sports activities were reported by parents using a st and ardised question naire . A sub sample of 1089 children had accelerometer-based PA data ( counts per minute , cpm ) . Moderate-to-vigorous PA ( MVPA ) and SED were estimated . Children with poor SI ( below the 15th age-/sex-/height-specific percentile ) were defined as cases ( N=603 ) . R and omly selected controls ( N=1216 ) were matched by age , sex , and country . Odds ratios ( OR ) for poor SI were calculated by conditional logistic regression for all biomarkers and PA behaviour variables separately and combined ( expressed as tertiles and dichotomised variables , respectively ) . ORs were adjusted for fat-free mass , dairy product consumption , and daylight duration . We observed increased ORs for no sports ( OR=1.39 , p<0.05 ) , PA levels below 524 cpm ( OR=1.85 , p<0.05 ) and MVPA below 4.2 % a day ( OR=1.69 , p<0.05 ) compared to WBE , high PA levels ( < 688 cpm ) and high MVPA ( 6.7 % ) , respectively . SED was not associated with SI . ORs were moderately elevated for low serum Ca and 25OHD . However , biomarkers were not statistically significantly associated with SI and did not modify the association between PA behaviours and SI . Although nutritional biomarkers appear to play a minor role compared to the osteogenic effect of PA and WBE , it is noteworthy that the highest risk for poor SI was observed for no sports or low MVPA combined with lower serum Ca ( < 2.5 mmol/l ) or lower 25OHD ( < 43.0 nmol/l )
target: 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 .
background: 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 .