Arthritis care & research;
Objective: To investigate thresholds of strength below which people with knee osteoarthritis (OA) may have more difficulty carrying out physical functions of daily life. Persons below such thresholds might benefit more from strengthening interventions than those with greater strength. Methods: We studied persons with symptomatic OA at baseline in the Multicenter Osteoarthritis Study (MOST) who had knee extensor strength measured isokinetically at 60o/sec. Subjects underwent a 20-Meter walk test, a Sit-to-Stand Test and answered the WOMAC. Physical function results were plotted against measures of quadriceps strength (newton-meters, Nm) (and as strength/body weight) for the worse knee. Locally weighted regression scatterplot smoothers (LOESS) were examined for inflection points. Nonlinear relationships were examined in piecewise linear regression models. Differences were tested using linear and logistic regression models. Results: 834 participants (65.8% women) were on average 62.9(±7.9) years old. In women, there were thresholds of strength below which the slope of strength vs. function was steeper: walking speed (<58Nm), chair stand time (<32Nm) and WOMAC functions rising from a chair and getting on/off the toilet (<38Nm). We found no thresholds in men. LOESS analyses using strength/weight showed similar results. Conclusions: In persons with symptomatic knee OA, thresholds in the strength function relationship may help identify individuals, especially women, at the brink of disability insofar as strength and daily tasks. In those with low strength, small increments in strength may be associated with improvement in function and greater ease with common daily life, emphasizing the importance of preventing loss of strength.
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