Unveiling motor endplate dysfunction: High-density surface electromyography evidence of muscle activation heterogeneity

Background : Myofascial pain syndrome (MPS), characterized by hyperirritable myofascial trigger points (MTrPs), affects 30–85% of individuals with chronic pain. Current MTrPs localization via palpation suffers from subjectivity and poor reliability. High-density surface electromyography (HD-sEMG) offers non-invasive spatial mapping of muscle activation heterogeneity, potentially identifying MTrPs-related motor endplate dysfunction. Methods : In this cross-sectional study, 36 MPS patients and 35 asymptomatic controls (CTR) performed isometric trunk extension tasks. HD-sEMG (8×8 electrode grid) recorded erector spinae activity during: (1) Task 1: maximal voluntary contractions (MVC), and (2) Task 2: sustained 20% MVC until fatigue. Topographical maps quantified low-energy (≤25th percentile RMS) and high-energy (≥75th percentile RMS) regions. Euclidean distances between palpated MTrPs and low-energy centroids, centroid displacement during fatigue, and spatial agreement (Procrustes analysis) were computed to quantify differences. Non-parametric statistical tests were performed between groups (α=0.05). We hypothesized that the location of tenderness altered the muscle activity distribution in the erector spinae and that the muscle activity distribution change when processing into fatigue is different between MPS patients and CTR participants. Results : In Task 1, Low-energy centroids (indicating motor endplate zones) spatially neighbored palpated MTrPs (mean distance: 11.3 ± 5.2 mm), but lacked significant correlation (r = 0.39, p > 0.05) or shape similarity (Procrustes distance = 0.94). In Task 2, MPS exhibited reduced displacement of high-energy centroids vs. CTR (9.0 ± 4.3 mm vs. 21.7 ± 9.4 mm, p < 0.001), indicating impaired activation redistribution. In addition, low-energy regions in the MPS group showed lower amplitude than the CTR group at all phases (e.g., initial phase: 14.9 ± 5.7 µV vs. 29.5 ± 10.9 µV, p < 0.001) with no significant change over time (p > 0.05), while the CTR group exhibited progressive decline (initial vs. final: p = 0.009). High-energy regions in the MPS group had reduced amplitude vs. the CTR group (e.g., initial: 61.5 ± 14.5 µV vs. 126.6 ± 41.8 µV, p < 0.001) and minimal temporal change (middle vs. final: p = 0.007), whereas the CTR group showed marked reduction (all phase-pairs: p < 0.05). Conclusion : HD-sEMG reliably localizes motor endplate positions and confirms spatial adjacency between electro-physiologically identified low-energy centroids and palpated MTrPs. Clinically, this technique holds potential for identifying MTrPs-related motor endplate dysfunction and characterizing spatial activation heterogeneity in lumbar musculature, thereby facilitating targeted therapeutic interventions.