title: Body mass index, occupational activity, and leisure-time physical activity: an exploration of risk factors and modifiers for knee osteoarthritis in the 1946 British birth cohort.
abstract: BACKGROUND
  Knee osteoarthritis (kOA) risk is increased by obesity and physical activities (PA) which mechanically stress the joint. We examined the associations of midlife kOA with body mass index (BMI) and activity exposure across adult life and their interaction.
  
  
  METHODS
  Data are from a UK birth cohort of 2597 participants with a clinical assessment for kOA at age 53. At ages 36, 43 and 53 BMI (kg/m2), self-reported leisure-time PA, and occupational activity (kneeling/squatting; lifting; climbing; sitting; assigned using a job-exposure matrix) were ascertained. Associations were explored using the multiplicative logistic model.
  
  
  RESULTS
  BMI was strongly and positively associated with kOA in men and women. Men and women in manual occupations also had greater odds of kOA; there was a weak suggestion that kOA risk was higher among men exposed to lifting or kneeling at work. For men, the only evidence of a multiplicative interaction between BMI and activities was for lifting (p = 0.01) at age 43; BMI conferred higher kOA risk among those most-likely to lift at work (OR per increase in BMI z-score: 3.55, 95% CI: 1.72-7.33). For women, the only evidence of an interaction was between BMI and leisure-time PA (p = 0.005) at age 43; BMI conferred higher kOA risk among those at higher PA levels (OR per increase in BMI z-score: 1.59, 95% CI: 1.26-2.00 in inactive; 1.70, 95% CI: 1.14-2.55 (less-active); and 4.44; 95% CI: 2.26-8.36 (most-active).
  
  
  CONCLUSIONS
  At the very least, our study suggests that more active individuals (at work and in leisure) may see a greater reduction in risk of kOA from avoiding a high BMI than those less active.
authors.affiliation: Laboratory of Epidemiology and Population Sciences, National Institute on Aging, National Institutes of Health, Bethesda, Maryland 20814, USA. kathryn.martin@nih.gov
authors.forename: Kathryn R
authors.initials: KR
authors.lastname: Martin
authors.forename: Diana
authors.initials: D
authors.lastname: Kuh
authors.forename: Tamara B
authors.initials: TB
authors.lastname: Harris
authors.forename: Jack M
authors.initials: JM
authors.lastname: Guralnik
authors.forename: David
authors.initials: D
authors.lastname: Coggon
authors.forename: Andrew K
authors.initials: AK
authors.lastname: Wills
mesh_terms: D000328:Adult; D015992:Body Mass Index; D015897:Comorbidity; D005260:Female; D006801:Humans; D007899:Leisure Activities; D008019:Life Style; D008137:Longitudinal Studies; D008297:Male; D008875:Middle Aged; D009043:Motor Activity; D009765:Obesity / Q000453:epidemiology* / Q000503:physiopathology; D009784:Occupational Diseases / Q000453:epidemiology* / Q000503:physiopathology; D020370:Osteoarthritis, Knee / Q000453:epidemiology* / Q000503:physiopathology; D011788:Quality of Life; D012307:Risk Factors; D006113:United Kingdom / Q000453:epidemiology