Serum biomarkers of inflammation and muscle strength among women with systemic lupus erythematosus

Objectives Muscle strength is an important determinant of physical function in women with systemic lupus erythematosus (SLE). Serum biomarkers of inflammation, including interleukin-6 (IL-6) and C-Reactive Protein (CRP), are associated with differences in muscle strength among individuals without rheumatologic disease. We examined whether serum levels of IL-6 and CRP are associated with upper and lower extremity muscle strength among adult women with SLE. Methods One hundred thirty-six women with SLE participated in this cross-sectional study. High-sensitivity CRP was analyzed by nephelometry. IL-6 serum levels were analyzed by high sensitivity enzyme-linked immunosorbent assay. Upper and lower extremity muscle strength were assessed by grip strength and peak torque of knee extension and flexion, respectively. Regression analyses modeled associations of CRP and IL-6 with upper and lower extremity muscle strength controlling for age, SLE duration, physical activity, prednisone use, BMI, plaquenil use, and pain. Results Higher serum levels of IL-6 and CRP were associated with significantly weaker upper and lower extremity muscle strength even when controlling for covariates. Conclusions Increased serum IL-6 and CRP are associated with clinically significant differences in upper and lower extremity muscle strength and may be useful in identifying those at risk for weakness and decreased physical function.