Abstract
Axial spondyloarthritis (axSpA) is a chronic rheumatic disease characterized by the presence of inflammatory back pain. In patients with chronic low back pain, the lumbar flexion relaxation phenomenon measured by surface electromyography (sEMG) differs from that in healthy individuals. However, sEMG activity in axSpA patients has not been studied. The purpose of this study was to analyze the flexion relaxation phenomenon in axSpA patients. A study evaluating 39 axSpA patients and 35 healthy controls was conducted. sEMG activity at the erector spinae muscles was measured during lumbar full flexion movements. sEMG activity was compared between axSpA patients and the controls, as well as between active (BASDAI ≥ 4) and non-active (BASDAI < 4) patients. The reliability (using intraclass correlation coefficients (ICC)), criterion validity and discriminant validity using the area Under the curve (AUC) for the inverse flexion/relaxation ratio (1/FRR) were evaluated. Significant differences (p < 0.05) were observed between axSpA patients and the control group in lumbar electric activity, especially during flexion, relaxation, and extension and in FRR and 1/FRR (0.66 ± 0.39 vs. 0.25 ± 0.19, respectively). In addition, significant differences were found between active and non-active but also between non-active and healthy subjects. The sEMG showed good reliability (ICC > 0.8 for 1/FRR) and criterion validity. ROC analysis showed good discriminant validity for axSpA patients (AUC = 0.835) vs. the control group using 1/FRR. An abnormal flexion/relaxation phenomenon exists in axSpA patients compared with controls. sEMG could be an additional objective tool in the evaluation of patient function and disease activity status.
Highlights
When patients were grouped into active axSpA (BASDAI ≥ 4, n = 22) vs. non-active axSpA (BASDAI < 4, n = 17) (Table 2), we found that active patients showed lower values of EMG signals in standing (6.59 ± 2.18 vs. 10.47 ± 5.19) and extension (23.38 ± 12.40 vs. 46.49 ± 23.28), lower FRR (1.51 ± 1.05 vs. 3.11 ± 2.11), and a higher score in 1/FRR
These results showed that an increment in 1/FRR in axSpA patients is associated with poorer mobility
We demonstrated that the variability of FRR and 1/FRR in axSpA patients is directly associated with disease activity, functionality, and mobility, as measured by BASDAI, Bath Ankylosing Spondylitis Functionality InIndex (BASFI) and Bath Ankylosing Spondylitis Metrology Index (BASMI), respectively
Summary
The concept of mechanical stress in the pathogenesis of axSpA has recently been revitalized, with the theory that interactions between biomechanical factors and the innate immune response may lead to the development of enthesitis [3,4]. This activation of metabolic pathways and cytokines would not be confined exclusively to entheses but would involve tissues immediately adjacent to this organ: bone, fascia, extra enthesis, and the Synovio-Entheseal Complex (SEC) [5]. Some authors have reported pathological changes in the paravertebral muscles in axSpA patients (such as muscle fiber atrophy and cytoarchitectural abnormalities) [6,7], which could be associated with overactivity through their paravertebral muscles and altered load-sharing capability of the tissues [8]
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