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.
From the aSports Medicine Research Center, Tehran University of Medical Sciences, Tehran, Iran; bDepartment of Epidemiology and Biostatistics, School of Public Health, Tehran University of Medical Sciences, Tehran, Iran; cDepartment of Global Health and Population, Harvard School of Public Health, Boston, MA; dInstitute for Health Metrics and Evaluation, University of Washington, Seattle, WA; eAerospace Medical Research Unit, Department of Physiology, McGill University, Montreal, Quebec, Canada; and fDepartment of Endocrinology, AJA University of Medical Sciences, Tehran, Iran.
Submitted 8 August 2011; accepted 8 December 2011; posted 10 March 2012.
Supported by Tehran University of Medical Sciences. The Osteoarthritis Initiative (OAI) is a public-private partnership comprised of 5 contracts (N01-AR-2-2258; N01-AR-2-2259; N01-AR-2-2260; N01-AR-2-2261; N01-AR-2-2262) funded by the National Institutes of Health, a branch of the Department of Health and Human Services, and conducted by the OAI Study Investigators. Private funding partners include Merck Research Laboratories; Novartis Pharmaceuticals Corporation, GlaxoSmithKline; and Pfizer, Inc. Private sector funding for the OAI is managed by the Foundation for the National Institutes of Health. This manuscript was prepared using an OAI public-use data set and does not necessarily reflect the opinions or views of the OAI investigators, the NIH, or the private funding partners.
The authors reported no financial interests related to this research.
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Correspondence: Mohammad Ali Mansournia, Department of Epidemiology and Biostatistics, School of Public Health, Tehran University of Medical Sciences, PO Box: 14155–6446, Tehran, Iran. E-mail: email@example.com.