Comparison of two repetitive peripheral magnetic stimulation protocols for upper trapezius myofascial pain syndrome: A randomized trial

Background Upper trapezius myofascial pain syndrome (MPS) is a common contributor to chronic neck pain and functional limitations. Repetitive peripheral magnetic stimulation (rPMS) has emerged as a non-invasive neuromodulatory intervention; however, the optimal stimulation duration remains uncertain. Clarifying duration-dependent effects is important for informing parameter standardization in rehabilitation practice. Methods This single-centre, assessor-blinded randomized trial enrolled adults aged 18–55 years with chronic (≥ 3 months) upper trapezius MPS and baseline Visual Analog Scale (VAS) scores ≥ 4. Participants were randomly assigned in a 1:1 ratio using permuted blocks to receive either a 15-minute or 10-minute rPMS protocol once weekly for 4 weeks, in addition to standardized stretching exercises. The primary outcome was pain intensity measured using the VAS. Secondary outcomes included Pressure Pain Threshold (PPT) and the Neck Disability Index (NDI). Assessments were conducted at baseline, week 2, and week 4 by a blinded assessor. Analyses followed the intention-to-treat (ITT) principle using linear mixed-effects models (LMMs) with fixed effects for group, time (baseline, week 2, week 4), and the group-by-time interaction. Results Of 83 individuals screened, 66 were randomized (33 per group) and included in the ITT analysis. Baseline characteristics were comparable between groups. LMMs demonstrated significant improvements over time in VAS, PPT, and NDI (all p < 0.001), with no significant group-by-time interactions. At the prespecified primary endpoint (week 4), VAS decreased from baseline by 4.87 points in Protocol A and 5.02 points in Protocol B. Across both groups, PPT increased by 2.51 kg/cm² and NDI decreased by 9.15 points. The adjusted between-group difference in VAS at week 4 was −0.27 (95% CI −0.95–0.40; p = 0.43), indicating no significant difference between protocols. Similarly, no significant between-group differences were observed for PPT (0.17, 95% CI −0.70–1.03; p = 0.71) or NDI (−0.64, 95% CI −3.29–2.02; p = 0.64). Conclusions Both treatment durations were associated with significant short-term improvements in pain intensity, pressure pain threshold, and neck-related disability. No significant between-protocol differences were detected within the present sample and follow-up period. These findings suggest that a 10-minute rPMS protocol may be a practical option for clinical rehabilitation settings. Trial registration This trial was prospectively registered with the Thai Clinical Trials Registry (Ref: TCTR20250929011; https://www.thaiclinicaltrials.org/show/TCTR20250929011 ).