title: Effect of physical activity on functional performance and knee pain in patients with osteoarthritis :  analysis with marginal structural models.
abstract: BACKGROUND
  A previous analysis of the Osteoarthritis Initiative study reported a dose-response relationship between physical activity and improved physical function in adults with knee osteoarthritis, using conventional statistical methods. These methods are subject to bias when confounders are affected by prior exposure.
  
  
  METHODS
  We used baseline and 1-, 2-, and 3-year follow-up data from the Osteoarthritis Initiative study of 2545 US adults with knee osteoarthritis recruited between 2004 and 2006 from 4 clinical sites. Physical activity was measured using the Physical Activity Scale for the Elderly, and outcomes were functional performance measured by the timed 20-meter walk test and self-reported knee pain measured by the Western Ontario and McMaster Universities Osteoarthritis Index. We estimated the effect of physical activity on each outcome using inverse probability-weighted (IPW) estimators of marginal structural models. For each outcome, we fitted 2 separate IPW models adjusting for concurrent or lagged confounders.
  
  
  RESULTS
  The mean differences in walking speed for the second, third, and fourth quartiles of physical activity relative to the first were 0.48 (95% confidence interval = -0.12 to 1.08), 0.45 (-0.23 to 1.13), and 0.46 (-0.29 to 1.22) meters/min based on the IPW model adjusting for concurrent confounders. When adjusting for lagged confounders, the results were 1.35 (0.64 to 2.07), 1.33 (0.54 to 2.14), and 1.26 (0.40 to 2.12). Both IPW models indicated that physical activity did not affect knee pain.
  
  
  CONCLUSIONS
  Physical activity has no effect on knee pain and may have either a very small effect or no effect on functional performance in adults with knee osteoarthritis.
authors.affiliation: Sports Medicine Research Center, Tehran University of Medical Sciences, Tehran, Iran. mansournia_ma@yahoo.com
authors.forename: Mohammad Ali
authors.initials: MA
authors.lastname: Mansournia
authors.forename: Goodarz
authors.initials: G
authors.lastname: Danaei
authors.forename: Mohammad Hossein
authors.initials: MH
authors.lastname: Forouzanfar
authors.forename: Mahmood
authors.initials: M
authors.lastname: Mahmoodi
authors.forename: Mohsen
authors.initials: M
authors.lastname: Jamali
authors.forename: Nasrin
authors.initials: N
authors.lastname: Mansournia
authors.forename: Kazem
authors.initials: K
authors.lastname: Mohammad
mesh_terms: D000368:Aged; D015984:Causality; D015986:Confounding Factors, Epidemiologic; D015444:Exercise; D005080:Exercise Test; D005260:Female; D005500:Follow-Up Studies; D006796:Household Work; D006801:Humans; D007899:Leisure Activities; D008297:Male; D008875:Middle Aged; D015233:Models, Statistical; D020370:Osteoarthritis, Knee / Q000503:physiopathology*; D010147:Pain Measurement; D011446:Prospective Studies; D057566:Self Report; D016896:Treatment Outcome; D016138:Walking; D014937:Work