Unveiling dysfunction of the innervation zone in myofascial pain syndrome: high-density surface electromyography evidence of muscle activation heterogeneity

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, providing a theoretical framework to investigate dysfunction in the putative innervation zone associated with MTrPs. 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. In Task 1, Low-energy centroids (consistent with putative innervation zone locations) 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 an association with impaired activation redistribution in MPS patients. 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). HD-sEMG provides a non-invasive perspective to identify spatial features consistent with innervation zone positions and reveals a spatial dissociation between electrophysiologically identified low-energy centroids and clinically palpated MTrPs. This suggests that electrophysiological manifestations of muscle dysfunction may not directly overlay the physical nodules. Clinically, this technique holds potential for identifying dysfunction in putative innervation-zone regions associated with MTrPs and characterizing spatial activation heterogeneity in lumbar musculature, thereby informing future research toward targeted therapeutic interventions.