***TASK*** the task is to summarize an input biomedical literature in six sentences 
    ***INPUT*** the input is a biomedical literature 
    ***OUTPUT*** the output is the summary of an input biomedical literature in six sentences 
    ***DOCUMENTATION*** 
    ***EXAMPLES*** 
    Input: During childhood , physical activity is likely the most important modifiable factor for the development of lean mass . However , the effects of normal growth and maturation must be controlled . To distinguish effects of physical activity from normal growth , longitudinal data are required . One hundred nine boys and one hundred thirteen girls , participating in the Saskatchewan Pediatric Bone Mineral Accrual Study , were repeatedly assessed for 6 yr . Age at entry was 8 - 15 yr .  Stature , body mass , and physical activity were assessed biannually . Body composition was assessed annually by dual-energy X-ray absorptiometry . Physical activity  was determined using the physical activity question naires for children and adolescence . Biological age was defined as years from age of peak height velocity . Data were analyzed using multilevel r and om-effects models . In boys , it was found that physical activity had a significant time-dependent effect on lean mass accrual of the total body ( 484.7+/-157.1 g ) , arms ( 69.6+/-27.2 g ) , legs ( 197.7+/-60.5 g ) , and trunk ( 249.1+/-91.4 g ) ( P<0.05 ) . Although the physical activity effects were similar in the girls ( total body : 306.9+/-96.6 g , arms : 31.4+/-15.5 g , legs : 162.9+/-40.0 g , and trunk : 119.6+/-58.2 g ; P<0.05 ) , boys for the same level of activity accrued , depending on the site , between 21 and 120 % more absolute lean mass ( g ) . In conclusion , habitual physical activity had a significant independent influence on the growth of lean body mass during adolescence , once biological maturity and stature were controlled CONCEPT Postmenarchal girls and premenopausal women have 3 - 4 % higher cortical bone density ( CoD , milligrams per cubic centimeter ) , compared with postpubertal boys and men , respectively . Females ' denser cortical bone is thought to serve as a calcium reservoir for reproductive needs . However , prospect i ve data are lacking that describe CoD development and bone mineral density distribution during puberty in both sexes . OBJECTIVE Thus , our objectives were to assess maturity and sex differences in the 20-month change of CoD and radial distribution of bone mineral density ( RDBMD , milligrams per cubic centimeter ) in early- , peri- , and postpubertal girls and boys . Maturity groups were based on change in menarcheal status ( girls , n = 68 ) and pubic hair stage ( Tanner ) ( boys , n = 59 ) .  Peripheral quantitative computed tomography was used to measure  CoD and RDBMD at the tibial middiaphysis . RESULTS The increase in average CoD was 1.9 % [ 22.8 mg/cm(3 ) ; 95 % confidence interval ( CI ) , 10 - 36 ] , 2.8 % ( 33.8 mg/cm(3 ) ; 95 % CI , 21 - 47 ) , and 1.5 % ( 55.0 mg/cm(3 ) ; 95 % CI , 17 - 93 ) greater in early , peri- , and postpubertal girls , compared with boys , respectively . Analysis of RDBMD revealed that the change in density distribution varied across pubertal groups in girls . Across puberty , all girls showed an increase in the high density midcortical region , whereas only peripubertal girls showed an increase in the lower density subcortical region . A sex-difference in RDBMD change was noted within early and peripubertal groups . CONCLUSIONS Our findings of sexual dimorphism in CoD development give support to the hypothesis that female bone deposits calcium for reproductive needs by consolidation of cortical bone during puberty The objective of this work was to examine the prediction of incident hip fracture risk by composite indices of femoral neck strength , constructed from dual X-ray absorptiometry ( DXA ) scans of the hip . These indices integrate femoral neck size and body size with bone density , and reflect the structure ’s ability to withst and axial compressive forces and bending forces , and the ability to absorb energy in an impact . We studied a prospect i ve cohort of community-dwelling women , 55 years or older at baseline , average age 72 years , that was established in 1988 , with follow-up until 1998 . Higher baseline values of each composite strength index were associated with lower risk of incident hip fracture in this cohort . After adjusting for age and body mass index , relative risks associated with each st and ard deviation increment in the composite indices were 0.39 for compression strength ( 95 % confidence interval 0.25–0.60 ) , 0.43 for bending strength ( 95 % confidence interval 0.27–0.70 ) , and 0.34 for impact strength ( 95 % confidence interval 0.20–0.58 ) . Larger bone size relative to body size was associated with lower risk of incident hip fracture . After adjusting for age , body mass index , and bone density , each st and ard deviation increment in femoral neck width normalized by body weight was associated with incident hip fracture relative risk of 0.38 ( 95 % confidence interval 0.20–0.70 ) . Our results suggest that composite indices of femoral neck strength , constructed from DXA measurements , have the potential to improve hip fracture risk assessment . This needs further exploration in larger cohorts and in different population sub-groups Exercise during growth has a positive influence on bone mineral accrual , yet little is known about how bone geometry and strength adapt to loading during growth . Our primary objective was to compare changes in proximal femur bone geometry and strength between 31 prepubertal ( Tanner Stage 1 ) boys who participated in a school-based , high-impact circuit intervention ( 12 min , three times a week ) for 20 months and 33 maturity-matched controls . Our secondary objective was to compare changes in total body ( TB ) , proximal femur ( PF ) , and lumbar spine ( LS ) bone mineral content ( BMC ) and bone area ( BA ) in these groups . We assessed geometric variables and bone strength at the narrow neck ( NN ) , intertrochanteric ( TR ) region , and femoral shaft regions by applying the Hip Structure Analysis program to proximal femur dual energy X-ray absorptiometry scans ( DXA , Hologic QDR 4500 ) . Further , we assessed total body , lumbar spine , and proximal femur BMC and BA by DXA and derived total body lean mass and fat mass from total body scans . Intervention ( 10.2 + /- 0.5 years ) and control boys ( 10.1 + /- 0.5 years ) had similar baseline height ( 140.8 vs. 141.3 cm ) and weight ( 36.9 vs. 35.4 kg ) , and average 20-month physical activity scores ( Physical Activity Question naire for Children , PAQ-C ) and calcium intakes ( 861 vs. 852 mg/day , food frequency question naire ) . Twenty-month height and weight changes were not significantly different between groups ; lean mass changed more ( P < 0.05 ) in intervention boys ( 22.8 % ) than control boys ( 18.6 % ) . At the NN region , intervention boys had greater bone expansion on both the periosteal ( + 2.6 % , P = 0.1 ) and endosteal ( + 2.7 % , P = 0.2 ) surfaces , result ing in significantly greater changes in section modulus ( bone bending strength ) ( + 7.5 % , P = 0.02 , ANCOVA , adjusting for height change , final Tanner Stage , and baseline bone values ) . Changes at the intertrochanteric and femoral shaft regions were not significantly different between groups . Femoral neck ( FN ) BMC changes were significantly greater in intervention boys ( + 4.3 % , P < 0.01 ) ; changes in BA and BMC for other regions were not significantly different between groups . In summary , a school-based , high-impact exercise intervention implemented three times a week for 12 min is an effective strategy for site-specific gains in bone strength at the narrow neck region of the proximal femur Background We have previously reported that a one-year school-based exercise intervention program influences the accrual of bone mineral in pre-pubertal girls . This report aims to evaluate if also hip structure is affected , as geometry independent of bone mineral influences fracture risk . Methods Fifty-three girls aged 7 – 9 years were included in a curriculum-based exercise intervention program comprising 40 minutes of general physical activity per school day ( 200 minutes/week ) . Fifty healthy age-matched girls who participated in the general Swedish physical education curriculum ( 60 minutes/week ) served as controls . The hip was scanned by dual X-ray absorptiometry ( DXA ) and the hip structural analysis ( HSA ) software was applied to evaluate bone mineral content ( BMC ) , areal bone mineral density ( aBMD ) , periosteal and endosteal diameter , cortical thickness , cross-sectional moment of inertia ( CSMI ) , section modulus ( Z ) and cross-sectional area ( CSA ) of the femoral neck ( FN ) . Annual changes were compared . Group comparisons were done by independent student 's t-test between means and analyses of covariance ( ANCOVA ) . Pearson 's correlation test was used to evaluate associations between activity level and annual changes in FN . All children remained at Tanner stage 1 throughout the study . Results No between-group differences were found during the 12 months study period for changes in the FN variables . The total duration of exercise during the year was not correlated with the changes in the FN traits . Conclusion Evaluated by the DXA technique and the HSA software , a general one-year school-based exercise program for 7–9-year-old pre-pubertal girls seems not to influence the structure of the hip Cross-sectional studies of elite athletes suggest that growth is an opportune time for exercise to increase areal bone mineral density ( BMD ) . However , as the exercise undertaken by athletes is beyond the reach of most individuals , these studies provide little basis for making recommendations regarding the role of exercise in musculoskeletal health in the community . To determine whether moderate exercise increases bone mass , size , areal , and volumetric BMD , two socioeconomically equivalent schools were r and omly allocated to be the source of an exercise group or controls . Twenty boys ( mean age 10.4 years , range 8.4 - 11.8 ) allocated to 8 months of 30-minute sessions of weight-bearing physical education lessons three times weekly were compared with 20 controls matched for age , st and ing and sitting height , weight , and baseline areal BMD . Areal BMD , measured using dual-energy X-ray absorptiometry , increased in both groups at all sites , except at the head and arms . The increase in areal BMD in the exercise group was twice that in controls ; lumbar spine ( 0.61 + /- 0.11 vs. 0.26 + /- 0.09%/month ) , legs ( 0.76 + /- 0.07 vs. 0.34 + /- 0.08%/month ) , and total body ( 0.32 + /- 0.04 vs. 0.17 + /- 0.06%/month ) ( all p < 0.05 ) . In the exercise group , femoral midshaft cortical thickness increased by 0.97 + /- 0 . 32%/month due to a 0.93 + /- 0.33%/month decrease in endocortical ( medullary ) diameter ( both p < 0.05 ) . There was no periosteal expansion so that volumetric BMD increased by 1.14 + /- 0.33%/month , ( p < 0.05 ) . Cortical thickness and volumetric BMD did not change in controls . Femoral midshaft section modulus increased by 2.34 + /- 2 . 35 cm3 in the exercise group , and 3.04 + /- 1.14 cm3 in controls ( p < 0.05 ) . The growing skeleton is sensitive to exercise . Moderate and readily accessible weight-bearing exercise undertaken before puberty may increase femoral volumetric BMD by increasing cortical thickness . Although endocortical apposition may be a less effective means of increasing bone strength than periosteal apposition , both mechanisms will result in higher cortical thickness that is likely to offset bone fragility conferred by menopause-related and age-related endocortical bone resorption The POWER PE study was an 8-mo , r and omized , controlled , school-based exercise intervention design ed to apply known principles of effective bone loading to practical opportunities to improve life-long musculoskeletal outcomes . A total of 99 adolescents ( 46 boys and 53 girls ) with a mean age of 13.8 + /- 0.4 yr ( peri- to postpubertal ) volunteered to participate . Intervention subjects performed 10 min of jumping activity in place of regular physical education ( PE ) warm up . Control subjects performed usual PE warm-up activities .  Bone mass ( DXA and QUS ) was assessed at baseline and follow-up along with anthropometry , maturity , muscle power , and estimates of physical activity and dietary calcium . Geometric properties ( such as femoral neck [ FN ] moment of inertia ) were calculated from DXA measures . Boys in the intervention group experienced improvements in calcaneal broadb and ultrasound attenuation ( BUA ) ( + 5.0 % ) and fat mass ( -10.5 % ) , whereas controls did not ( + 1.4 % and -0.8 % , respectively ) . Girls in the inter 
    Output:
    
Changes in bone structure ( eg , bone cross-sectional area , cortical thickness , alone or in combination ) rather than bone mass most often accompanied significant bone strength outcomes .
Prepuberty and peripuberty may be the most opportune time for boys and girls to enhance bone strength through PA , although this finding is tempered by the few available studies in more mature groups .
Despite the central role that muscle plays in bones ' response to loading , few studies discerned the specific contribution of muscle function ( or surrogates ) to bone strength .