Introduction. Subjective assessments, such as the Numerical Rating Scale (NRS), are widely used in clinical settings to assess low back pain (LBP); however, there is a clear need for more objective pain assessment methods. We seek to establish surface electromyography as a new objective method for pain assessment. In this study, we examined the correlation between integrated electromyography (iEMG) of the erector spinae muscles and NRS scores during sustained postures.
Methods. We assessed the correlation between iEMG, muscle fatigue, and NRS scores for the erector spinae muscles in three postures: seated, standing, and 30˚ forward flexion. Data were collected from seven healthy participants (age. Ave. 22.57±1.18 yo) without chronic LBP, with each posture maintained for 10 minutes, yielding 14 samples (left and right sides). Data with unchanging pain levels were excluded. Electrodes were positioned 3 cm lateral to the L4 level, based on prior studies and orthopedic guidance. NRS scores were recorded every minute, with iEMG values derived from the 10-second windows immediately after each NRS recording. Muscle fatigue was quantified by calculating the median frequency from the iEMG power spectral density. To assess left-right asymmetry, the Wilcoxon signed-rank test was used, and ANOVA with Tukey’s HSD test applied for posture comparisons.
Results. For the seated posture, the correlation between iEMG and NRS was low (mean: -0.032; absolute mean: 0.475), indicating minimal association with NRS scores. Muscle fatigue and NRS exhibited a weak correlation (mean: 0.027; absolute mean: 0.304). For the standing posture, the correlation between iEMG and NRS was slightly higher than in the seated posture (mean: 0.186; absolute mean: 0.476), while the correlation between muscle fatigue and NRS remained low (mean: 0.113; absolute mean: 0.287). For the 30˚ forward flexion posture, iEMG and NRS (mean: -0.054; absolute mean: 0.801), muscle fatigue and NRS (mean: -0.443; absolute mean: 0.818) both showed strong correlations. The Wilcoxon test showed significant left-right asymmetry in muscle activity across all postures. ANOVA and Tukey’s HSD test identified significant differences in muscle activity among postures. Standing and forward flexion particularly induced muscle imbalances, increasing the risk of LBP and muscle fatigue. Variability in asymmetry among participants, even in the same posture, suggests individual differences in stability and muscle strength affect LBP risk.
Conclusion. The present study indicates that specific postures, particularly standing and forward flexion, may induce asymmetric muscle activity and an imbalanced lumbar load, potentially elevating the risk for LBP and muscle fatigue. Future studies should explore the effects of these asymmetries on self-perceived LBP (NRS) to further elucidate these relationships.