Association between interleukin-6 and lower extremity function after hip fracture - The role of muscle mass and strength

OBJECTIVES: To examine whether an effect on muscle mass or strength explains the association between interleukin-6 (IL-6) and lower extremity function in the year after hip fracture. DESIGN: Analysis of data from a longitudinal cohort study. SETTING: Two Baltimore-area hospitals. PARTICIPANTS: Community-dwelling women aged 65 and older admitted to one of two hospitals in Baltimore with a new, nonpathological fracture of the proximal femur between 1992 and 1995. MEASUREMENTS: At 2, 6, and 12 months postfracture, serum IL-6, appendicular lean muscle mass (aLM), and grip strength were measured, and the Lower Extremity Gain Scale (LEGS), a summary measure of performance of nine lower extremity tasks was calculated. Generalized estimating equations were used to model the longitudinal relationship between IL-6 tertile and LEGS. Whether muscle mass or strength explained the relationship between IL-6 and LEGS was examined by adding measures of aLM, grip strength, or both into the model. RESULTS: Subjects in the lowest IL-6 group performed better on the LEGS than those in the highest tertile by 4.51 (95% confidence interval (CI)=1.50-7.52) points at 12 months postfracture. Adjusting for aLM and grip strength, this difference was 4.28 points (95% CI=1.14-7.43) and 3.81 points (95% CI=0.63-7.00), respectively. Adjusting for both aLM and grip strength, the mean difference in LEGS score was 3.88 points (95% CI=0.63-7.13). CONCLUSION: In older women, after hip fracture, reduced muscle strength, rather than reduced muscle mass, better explains the poorer recovery of lower extremity function observed with higher levels of the inflammatory marker IL-6.