To assess electromyographic parameters of neuromuscular fatigue in knee extensors and their association with clinical, functional and emotional features in patients with rheumatoid arthritis (RA). Thirty-eight female patients with RA participated. Electromyography parameters (changes in signal amplitude, represented by the root mean square, and frequency content, represented by median frequency-MDF) were assessed during a submaximal (60%) isometric contraction of the knee extensors, sustained for 60 s. Clinical characteristics; the 28-joint Disease Activity Score (DAS-28) in which includes count of swollen joints (out of the 28) and tender joints (out of the 28), the erythrocyte sedimentation rate and global disease activity measured on a visual analogue scale; serum C reactive protein (CRP); information on treatment; the Health Assessment Questionnaire; the Functional Assessment of Chronic Illness Therapy fatigue scale (FACIT-F); the Short Form Health Survey (SF-36) and the International Physical Activity Questionnaire (IPAQ), were also assessed. The mean patient age was 51.0 ± 8.2 years, mean disease activity score was 11.5 ± 7.1, and mean CRP level was 8.0 ± 7.8 mg/dL. There was a moderate correlation between MDF and age (r = 0.5), as well as weak correlations of MDF with FACIT-F (r = 0.3), physical functioning (r = - 0.3) and vitality domains (r = - 0.3) of the SF-36, and IPAQ (r = - 0.3) (p ≤ 0.05 for all). No association was observed between electromyography measurements and clinical or treatment features. The electromyographic parameter MDF was correlated with perception of fatigue, age, physical functioning and vitality domains of SF-36, and physical activity level in this sample. These results indicate that primary muscle factors should also be considered when managing perceived fatigue in patients with RA.
Keywords: Electromyography; Fatigue; Knee extensors; Rheumatoid arthritis.