To investigate whether a clinically relevant change in knee function in patients with or at risk for osteoarthritis (OA) of the knee is associated with concurrent or prior change in thigh muscle strength.
Participants in the Osteoarthritis Initiative (n = 2,675 [1,485 women and 1,190 men]) with available measurements of isometric muscle strength at baseline, 2-year follow-up, and 4-year follow-up were grouped into those with greater than minimum clinically important worsening (score of ≥6 [of a possible 68] on the Western Ontario and McMaster Universities Osteoarthritis Index function subscale) between 2-year follow-up and 4-year follow-up, those with greater than minimum clinically important improvement, and those without relevant change. Changes in isometric muscle strength concurrent with function change (between 2-year follow-up and 4-year follow-up) and preceding function change (between baseline and 2-year follow-up), with 95% confidence intervals (95% CIs), were determined, and differences between groups were assessed by analysis of covariance.
Concurrent loss in extensor muscle strength in participants with worsening knee function during the 2-year follow-up-4-year follow-up period differed significantly from that in participants without change in knee function (-4.6% [95% CI -6.8, -2.4] and -2.2% [95% CI -3.0, -1.4], respectively; P = 0.03), as did the concurrent increase in strength among those with functional improvement (2.2% [95% CI -0.3, 4.7]; P < 0.0001). This increase in strength among subjects with improved function remained significantly different from the change in subjects with no change in function after adjustment for covariates, but was preceded by a greater loss in strength (-7.7% [95% CI -10.3, -5.0], P = 0.02) during the baseline-year 2 period compared to those without change in function during the year 2-year 4 period (-4.3% [95% CI -5.2, -3.4]). The decrease in strength during the baseline-year 2 period in those with worsening knee function during the year 2-year 4 period (-4.5% [95% CI -6.9, -2.2]) did not differ significantly from that in patients without a change in function (P = 0.87). No differences in changes in flexor muscle strength were observed between groups.
These findings suggest that there is a positive concurrent longitudinal association between change in extensor muscle strength and worsening/improvement in knee function in patients with knee OA. However, a corresponding change in thigh muscle strength preceding the change in function was not observed.