Results from a study recently published in the journal Arthritis Care and Research showed that the lower extremity strength or weakness of women with systemic lupus erythematosus (SLE) was predictive of later higher or lower functional status but was unrelated to grip strength.
Evidence showed that reduced muscle strength is associated with decreased physical function in various study populations, including the elderly and individuals with osteoarthritis. Among populations with rheumatic diseases, data are emerging that describe similar associations between muscle structure and function and physical function. Prior observational studies have demonstrated strong associations between muscle density, a measure of muscle integrity and fatty infiltration, and physical function among individuals with rheumatoid arthritis (RA).
To examine whether reduced upper and lower extremity muscle strength could predict declines in function over time among adult women with SLE, in the study titled “Muscle Strength and Changes in Physical Function in Women With Systemic Lupus Erythematosus”, Patricia P. Katz, PhD from the Department of Medicine, University of California and colleagues assessed 146 women with systemic lupus erythematosus (SLE).
The sample was drawn from participants in the University of California, San Francisco (UCSF), Lupus Outcomes Study (LOS), who had formerly enrolled in a study of genetic risk factors for SLE outcomes and were recruited from both clinical and community-based sources, including UCSF-affiliated clinics (22%), non-UCSF rheumatology offices (11%), lupus support groups and conferences (26%), and through newsletters, web sites, and other forms of advertising (41%). The researchers confirmed an SLE diagnose using the American College of Rheumatology criteria.
All patients were assessed at baseline and at follow-up with the International Physical Activity Questionnaire (IPAQ), the Short Physical Performance Battery (SPPB) and the Systemic Lupus Activity Questionnaire (SLAQ). A Lunar Prodigy (GE Healthcare) whole-body, dual-energy, X-ray absorptiometry (DXA) scan was used to assess body composition and muscle distribution. Knee torque was measured to evaluate lower body strength using a Biodex unit (Biodex Medical Systems) to assess peak isokinetic torques of knee extension and exion at 120 per second with adjustments for body weight.
Results showed that at baseline the mean peak knee torque extension was of 44.5 Nm and flexion was of 29.9 Nm, with the mean grip strength at 22.7 Nm and a mean SPPB score of 8.8. A total of 24 patients did not complete the knee torque test, 24 patients did not complete the baseline grip test, and 7 patients did not complete any of the assessments.
Regression analyses assessed associations of baseline upper and lower extremity muscle strength with followup SPPB scores controlling for baseline SPPB, age, SLE duration, SLE disease activity (Systemic Lupus Activity Questionnaire), physical activity level, prednisone use, body composition, and depression. Secondary analyses tested whether associations of baseline muscle strength with followup in SPPB scores differed between intervals of varying baseline muscle strength.
Results revealed that lower extremity muscle strength strongly predicted changes over 2 years in physical function even when controlling for covariates. The association of reduced lower extremity muscle strength with reduced future physical function was higher among the weakest women. “We observed that, among adult women with SLE, reduced lower extremity muscle strength predicted subsequent declines in physical function even when adjusting for covariates,” the researchers explained in a recent news release. “In addition, we demonstrated that the weakest women are the ones at increased risk of future declines.”
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