Muscle stiffness in rheumatoid arthritis is not altered or associated with muscle weakness: a shear wave elastography study

Objective: To investigate muscle stiffness and strength in rheumatoid arthritis patients compared to healthy controls. Methods: A sample of 80 RA patients from three discrete groups: 1-newly diagnosed treatment-naïve RA (n = 29), 2-active RA for at least 1 year (n = 18) and 3-in remission RA for at least 1 year (n = 33), was compared to 40 healthy controls. Shear wave velocity (SWV) was measured using shear wave elastography as a surrogate for tissue stiffness in multiple muscles. All participants performed isometric grip strength, timed get-up-and-go test, 30-sec chair stand test and isokinetic knee extension/flexion(60°/sec). The difference in SWV amongst the groups was tested using one-way ANOVA, and the correlation between SWV and muscle strength results were calculated using Pearson's coefficients. Results: The mean age ± SD was 61.2 ± 12.8 for RA patients and 61.5 ± 10.5 years for controls. SWV was not significantly different amongst the groups on all muscles (p > 0.05). In comparison to controls, the new and active RA groups showed a significantly lower isokinetic strength by -29%(p = 0.013) and -28%(p = 0.040), fewer chair stands by -28%(p = 0.001) and -44%(p < 0.001), longer walking times by -25% (p = 0.025) and -30% (p = 0.001) respectively, and weaker grip strength by -45% for both (p < 0.001). The muscle strength in the remission RA groups was not significantly lower, except in the isokinetic knee strength (-21%; p = 0.027). The correlations between SWE and the muscle assessment results were weak and insignificant (r < 0.30; p > 0.05). Conclusions: Significant muscle weakness was demonstrated in patients with RA disease. However, muscle stiffness was normal and not associated with muscle strength.