title: Heritability of impaired balance: a nationwide cohort study in twins.
abstract: CONCLUSIONS
  In this large population-based twin study, a self-estimated impaired balance, an important risk factor for osteoporotic fractures, had a modest heritability of 0.27. Individual-specific environmental influences seem to be the dominating cause for impaired balance.
  
  
  BACKGROUND
  The principal causal components of an osteoporotic fracture are falls and weakened bone strength. While bone strength has a strong genetic origin, the heritable influences on impaired balance that contribute to the risk of injurious falls at older age are uncertain.
  
  
  METHODS
  To evaluate the heritability and environmental influence on self-reported impaired balance in older men and women, we used data from a sample of 22,998 Swedish twins, 55 to 99 years of age.
  
  
  RESULTS
  An impaired balance was reported by 2,890 (12.3%) of the twins. The tetrachoric correlation for impaired balance was only slightly lower for like-sex dizygotic twins (0.31) compared to monozygotic twins (0.36). These correlations indicate a modest familial (genetic and shared environmental) influence. Model fitting results indicate that the age- and sex-adjusted heritability for impaired balance was 0.27 (95%CI = 0.01-0.45). Individual-specific environmental influences differed only slightly by sex and age.
  
  
  CONCLUSIONS
  These results imply that a self-reported impaired balance, an independent risk factor for osteoporotic fractures, has a modestly heritable etiology in older subjects. Our observation can partly explain the previously observed modest heritability for osteoporotic fractures even though there is a high heritability for bone mineral density.
authors.affiliation: Department of Surgical Sciences, Section of Orthopaedics, University Hospital, Uppsala, Sweden. Helene.Wagner@akademiska.se
authors.forename: H
authors.initials: H
authors.lastname: Wagner
authors.forename: H
authors.initials: H
authors.lastname: Melhus
authors.forename: N L
authors.initials: NL
authors.lastname: Pedersen
authors.forename: K
authors.initials: K
authors.lastname: Michaëlsson
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D004200:Diseases in Twins / Q000453:epidemiology / Q000209:etiology / Q000235:genetics*; D004777:Environment; D004812:Epidemiologic Methods; D005260:Female; D020022:Genetic Predisposition to Disease; D006801:Humans; D008297:Male; D008875:Middle Aged; D004856:Postural Balance / Q000235:genetics*; D012678:Sensation Disorders / Q000453:epidemiology / Q000209:etiology / Q000235:genetics*; D013548:Sweden / Q000453:epidemiology; D014429:Twins, Dizygotic / Q000235:genetics; D014430:Twins, Monozygotic / Q000235:genetics