Longitudinal Change in Thigh Muscle Strength Prior to and Concurrent With Minimum Clinically Important Worsening or Improvement in Knee Function: Data From the Osteoarthritis Initiative.

OBJECTIVE: 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.
METHODS: 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.
RESULTS: 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.
CONCLUSION: 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.