Lower-extremity muscle strength was associated with functional improvements measured by self-reported physical disability scores in patients with systemic lupus erythematosus (SLE), according to a study conducted by James Andrews, MD, and colleagues at the University of California, San Francisco, and published in Arthritis Care & Research.1

An increase in fat mass and a decrease lean mass is noted in patients with SLE, even after controlling for other variables.2   This study sought to address whether muscle strength or muscle mass was associated with differences in physical function, as quantified by measures of self-reported disability.

Lower extremity muscle strength was measured by chair-stand times and peak isokinetic knee torque, as these are common proxy measures for muscle strength.  As part of a strength assessment, participants were instructed to stand up after sitting in a chair 5 times. Peak isokinetic torques of knee extension and flexion at 120° per second were also recorded. 


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Whole-body dual x-ray absorptiometry (DXA) was used to identify bone, muscle, and fat to determine the distribution of body muscle.  Physical disability was quantified by 2 self-reported scores.  The first was the Short Form-36 (SF-36) survey of physical function, which assessed the subjective functional ability to complete specific actions such as lifting and carrying, walking, and climbing stairs.  The second score was the Valued Life Activities (VLA) disability scale, which quantifies the level of difficulty associated with performing 21 specific activities. 

Using these self-reported measures of strength, researchers found that reduced lower extremity muscle strength correlated with worse SF-36 and VLA scores.

“While muscle mass correlated with muscle strength, only muscle strength was significantly associated with disability in adjusted models. These findings make a unique contribution to the current literature on physical disability in SLE”, the authors write. 

Summary and Clinical Applicability

Proinflammatory cytokines,such as tumor necrosis factor, have been shown to interfere with muscle fiber contraction in prior studies.3 As a result, many diseases that are inflammatory in nature present with musculoskeletal derangements.  In this study, loss of muscle strength, rather than loss of muscle mass, is associated with self-reported physical disability in patients with SLE.  This study was limited by its observational and cross-sectional design, restricting the extent of extrapolation of the data found.  Additionally, this study included only women with SLE and did not have a healthy control comparison group.   With future studies examining the long-term effect of muscle strengthening, physical disability in SLE can be better addressed.

Reference

1.       Andrews JS, Trupin L, Schmajuk G, et al. Muscle strength, muscle mass, and physical disability in women with systemic lupus erythematosus. Arthritis Care Res (Hoboken). 2015;67(1):120-127.

2.       Katz P, Gregorich S, Yazdany J, et al. Obesity and its measurement in a community-based sample of women with systemic lupus erythematosus. Arthritis Care Res (Hoboken). 2011;63:261-268.

3.       Lundberg IE, Nader GA. Molecular effects of exercise in patients with inflammatory rheumatic disease. Nat Clin Pract Rheumatol. 2008;4:597-604.