Background Surface electromyography (sEMG) has been used in several studies to assess muscle activity in patients with low back pain (LBP). It has also been analyzed the lumbar flexion-relaxation phenomenon (FRP) defined by reduced lumbar erector spinae (ES) muscle myoelectric activity during full trunk flexion in healthy individuals, through the flexion relaxation ratio (FRR). In LBP patients, this relaxation, compared to the peak reached at the flexion phase, is smaller and even non-existent. There are very few studies that analyze this effect in patients with Axial Spondyloarthritis (axSpA).
Objectives To evaluate muscular activity at the erector spinae using sEMG in patients with axSpA to analyze FRP.
Methods 39 subjects were included: 25 patients with axSpA (49.3±5.6 years, 75% men) and 14 healthy subjects (46.7±8.7 years, 71% men) as control group. Demographic data, conventional metrology, advanced metrology using motion capture (UCOTrack) and PRO questionnaires were collected. Electrodes were placed on left and right side, at L4-L5 level and separated 2cm from the spinous process, on the ES. Muscle activity values were obtained in 4 phases (standing, flexion, relaxation and extension). With the values of flexion and relaxation, the FRR index and its inverse 1/FRR were calculated. Student t tests were used for differences between groups.
Results There were no significant differences between the right and left sides of the measurements at ES, so mean values were considered for the analysis. There were also no significant differences in age and gender between the control group and patients. Results obtained in each of the phases are shown in the graph. The FRP appeared in healthy individuals, but not in patients, as show the FRP line in the graph: in axSpA, sEMG values at the Flexion phase are above values at the stand phase, and in control group is the opposite, so there is a truelly relaxation. There were significant differences between patients and control group in flexion, relaxation, and in the FRR and 1/FRR ratios. 1/FRR presented better correlations with several parameters (age, lateral flexion, Schöber, BASMI, BASFI, UCOASMI, BASG, all with p<0.01), but not with activity indexes (BASDAI, ASDAS). Distribution of 1/FRR values and results of sEMG between group (** p<0.01, * p<0.05) are shown at the chart. The area under a ROC curve to discriminate healthy patients using 1/FRR was 0.85 (95% CI 0.72–0.96 p<0.001).
Conclusions In our study, there were differences between healthy and patients with axSpA in the FRP, as in other studies with LBP patients. The 1/FRR index shown the best results in correlation with other parameters and it was where major differences between groups appeared. There was good correlation with the patient global score (BASG) and with the functional BASFI index, so assessment with sEMG could be an objective and quantitative test to evaluate the patient's functional status. It would be very interesting to analyze, in future studies, the sensitivity to change to treatments which would give us a good indicator to assess their effectiveness.
Acknowledgements This study was supported by the XXI Research Promotion Program of the University of Cόrdoba (Spain).
Disclosure of Interest None declared