To examine the association between changes in knee muscle strength (extensor and flexor muscles separately, and mean values) and changes in activity limitations in patients with established knee osteoarthritis at 2 years.
Data from 186 patients with knee osteoarthritis, part of the Amsterdam Osteoarthritis cohort, were gathered at baseline and at 2-year follow-up. Strength of the knee extensor and flexor muscles were assessed using an isokinetic dynamometer. Activity limitations were assessed using the Western Ontario and McMaster University Osteoarthritis Index (WOMAC) - Physical Function subscale, the Get Up and Go test (GUG), and the 12-steps stairs test. Univariate and multivariate linear regression analyses were used to assess the association between changes in muscle strength and changes in activity limitations, adjusting for relevant confounders and baseline activity limitations.
There was an overall 16% increase in mean knee muscle strength (p < 0.001), 19% increase in knee extensor muscle strength (p < 0.001), and 17% increase in knee flexor muscle strength (p < 0.001) at 2 years. Increased mean knee muscle strength and knee flexor muscle strength were associated with better self-reported physical function (WOMAC) (b = -5.7, p = 0.03 and b = -6.2, p = 0.05), decreased time on the GUG (b = -1.2, p = 0.003 and b = -1.4, p = 0.05) and decreased time on the stairs test (b = -4.4, p < 0.001 and b = -6.6, p < 0.001). Increased extensor muscle strength was associated only with decreased time on the stairs test (b = -2.7, p < 0.001).
Increased knee muscle strength, mainly in the knee flexors, was associated with decreased activity limitations in patients with knee osteoarthritis at 2 years. Thus, differences in muscle strength may partially explain the between-patients variability in activity limitations.