title: Physical activity during life course and bone mass: a systematic review of methods and findings from cohort studies with young adults.
abstract: BACKGROUND
  The purpose of this paper was to review the literature of the cohort studies which evaluated the association between physical activity during the life course and bone mineral content or density in young adults.
  
  
  METHODS
  Prospective cohort studies with bone mineral density or content measured in the whole body, lumbar spine and femoral neck by dual energy x-ray absorptiometry as outcome and physical activity as exposure were searched. Two independent reviewers selected studies retrieved from electronic databases (Medline, Lilacs, Web of Science and Scielo) and reviewed references of all selected full text articles. Downs & Black criterion was used in the quality assessment of these studies.
  
  
  RESULTS
  Nineteen manuscripts met inclusion criteria. Lumbar spine was the skeletal site most studied (n = 15). Different questionnaires were used for physical activity evaluation. Peak strain score was also used to evaluate physical activity in 5 manuscripts. Lack of statistical power calculation was the main problem found in the quality assessment. Positive associations between physical activity and bone mass were found more in males than in females; in weight bearing anatomical sites (lumbar spine and femoral neck) than in total body and when physical activity measurements were done from adolescence to adulthood - than when evaluated in only one period. Physical activity during growth period was associated with greater bone mass in males. It was not possible to conduct pooled analyses due to the heterogeneity of the studies, considering mainly the different instruments used for physical activity measurements.
  
  
  CONCLUSIONS
  Physical activity seems to be important for bone mass in all periods of life, but especially the growth period should be taking into account due to its important direct effect on bone mass and its influence in physical activity practice in later life. Low participation in peak strain activities may also explain the lower number of associations found in females.
authors.affiliation: Post-Graduate Program in Epidemiology, Federal University of Pelotas, Pelotas, Brazil. renatabielemann@hotmail.com
authors.forename: Renata M
authors.initials: RM
authors.lastname: Bielemann
authors.forename: Jeovany
authors.initials: J
authors.lastname: Martinez-Mesa
authors.forename: Denise Petrucci
authors.initials: DP
authors.lastname: Gigante
mesh_terms: D015502:Absorptiometry, Photon; D056044:Actigraphy; D000293:Adolescent; D000328:Adult; D000367:Age Factors; D015519:Bone Density; D002648:Child; D015331:Cohort Studies; D015444:Exercise; D005260:Female; D005272:Femur Neck / Q000000981:diagnostic imaging*; D006801:Humans; D008159:Lumbar Vertebrae / Q000000981:diagnostic imaging*; D008297:Male; D009043:Motor Activity; D012107:Research Design; D057185:Sedentary Behavior; D012737:Sex Factors; D055815:Young Adult