Whenmightisn’t right: the impact of muscle weakness on physical function in systemic lupus erythematosus

Abstract

Pascal, the renowned 17th century French philosopher and mathematician, was admittedly concerned less with physical weakness, than with spiritual or moral weakness when he penned this statement of surprise. Nevertheless, the sense of astonishment underlying his words remains apropos to our discussion of physical weakness among individuals with systemic lupus erythematosus (SLE). Reduced physical functioning is common among individuals with SLE and is frequently associated with significantly reduced health-related quality of life [1,2]. Differences in body composition, and in particular differences in muscle strength, represent an important etiology of reduced physical functioning among individuals with SLE. What is truly surprising is that despite the considerable burden of physical disability and muscle weakness in SLE, published data addressing these relationships remain limited. There exists, however, a body of literature examining relationships between body composition, muscle strength and physical disability among other chronic conditions, both rheumatic and nonrheumatic. Among elders and individuals with osteoarthritis (OA), muscle strength and muscle mass (both regional and total body muscle mass) are inversely related to physical disability [3,4]. However, muscle strength is more closely related to the degree of disability than is muscle mass among the elderly [5]. Among individuals with rheumatoid arthritis (RA), similar relationships exist between muscle mass, muscle strength and physical disability [6]; and, similar to in OA, in RA measures related to muscle function are more strongly associated with differences in physical functioning than are measures of muscle mass [7]. Taken together, these observations suggest that muscle strength makes an important contribution to an individual’s physical functioning in various populations. Until recently it was not known whether similar relationships exist among individuals with SLE. Our group recently demonstrated that among women with SLE muscle strength is directly associated with differences in physical functioning even when adjusting for differences in muscle mass; and differences in muscle strength at baseline predict changes in physical functioning 2 years later [8,9]. In this cohort of adult women with SLE, reduced lower extremity muscle strength (measured by knee strength and chair stand time) but not reduced muscle mass was associated with reduced self-reported physical functioning on the SF-36 Physical Functioning subscale and on the Valued Life Activities assessment, when adjusting for covariates such as disease activity, medication use and depression. Reduced knee strength also predicted significant decreases in physical functioning approximately 2 years later, as measured by the Short Physical Performance Battery When might isn’t right: the impact of muscle weakness on physical function in systemic lupus erythematosus