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Objective: To investigate associations between engagement in knee bending (stair climbing, kneeling, squatting, heavy lifting, getting in/out of a squatting position) and synovitis prevalence on noncontrast magnetic resonance imaging (MRI) in individuals at risk of and with knee osteoarthritis., Methods: We included baseline data from 594 participants (mean ± SD age 61.5 ± 8.9 years, 61% had Kellgren/Lawrence grade ≥2; 59% were female; mean ± SD body mass index was 30.7 ± 4.8 kg/m 2 ) of the Osteoarthritis Biomarker Consortium Foundation for the National Institutes of Health project. Knee bending activities were queried by a standard questionnaire, and the severity of Hoffa synovitis and effusion synovitis (surrogate outcomes of synovitis) were graded using the MRI OsteoArthritis Knee Scoring system. Logistic regression was used, unadjusted and adjusted, for metabolic syndrome, physical activity level, and sex. A grade ≥1 defined synovitis prevalence, with a grade ≥2 cutoff implemented in sensitivity analyses., Results: The prevalence of grade ≥1 Hoffa synovitis and effusion synovitis equaled 59% (n = 353) and 62% (n = 366), respectively. Adjusted for confounders, kneeling for ≥30 minutes during a single day was associated with grade ≥1 Hoffa synovitis prevalence (odds ratio [OR] 1.65 [95% confidence interval (95% CI) 1.11-2.47]). Participants engaging in this activity ≤1 day per week had greater odds for prevalent Hoffa synovitis than those who did not perform the activity (OR 1.88 [95% CI 1.11-3.18]). No other significant associations were found. Sensitivity analyses yielded similar findings., Conclusion: In this selected sample with a preponderance of grade ≥1 Hoffa and/or effusion synovitis on noncontrast MRI, only prolonged kneeling was associated with Hoffa synovitis prevalence. Replication in other samples is warranted., (© 2018, American College of Rheumatology.)