Immediate efficacy of low-intensity focused ultrasound versus planar ultrasound in patients with myofascial pain syndrome of upper trapezius: a randomized controlled clinical trial

Abstract

Background

Myofascial pain syndrome (MPS), a chronic musculoskeletal disorder characterized by myofascial trigger points (MTrPs), causes localized pain and dysfunction. The objective is to compare the immediate therapeutic effects of low-intensity focused ultrasound (LIFU) versus low-intensity planar ultrasound (LIPU) on pain and function in upper trapezius MPS.

Method

This study was designed as a single-blind, randomized controlled trial. The patients (20 ≤ aged ≤ 70) diagnosed with MPS of the upper trapezius were randomly allocated to either LIFU (study group; n = 20; 9 men, 11 women) or LIPU (control group; n = 20; 15 men, 5 women) treatments (1.5 W/cm², 1 MHz, 5 min, once a day, 3 days). The primary outcome was pain severity, assessed immediately post-treatment using the Visual Analog Scale (VAS; 0 ~ 10 cm), and served as the central efficacy endpoint. Secondary outcomes comprised supplemental assessments including: (1) multidimensional pain characterization via the Short-Form McGill Pain Questionnaire (SF-MPQ); (2) functional disability quantification through the Neck Disability Index (NDI); and (3) neuromuscular biomarkers measured by surface electromyography (sEMG) parameters (root mean square [RMS], median frequency [MF]). The outcomes were assessed before treatment and 10 min, 72 h after completing the third and final session of treatment.

Results

Pain intensity, as measured by the VAS, decreased significantly immediately after treatment in both the LIFU group (median difference [MD] = -2.0 cm; 95% confidence interval [CI]: -2.5 to -1.5 cm; p = 0.002) and the LIPU group (MD = -1.0 cm; 95% CI: -1.5 to -0.5 cm; p = 0.001), with these reductions maintained at 72 h post-treatment. Both SF-MPQ and NDI scores showed significant improvements following treatment in both groups. A significant reduction in RMS parameters was observed in the LIPU group immediately after treatment (MD = -15.3 µV; 95% CI: -20.0 to -10.5 µV; p = 0.028). No significant differences were observed between the groups in terms of pain alleviation, functional improvement, and alterations in neuromuscular electrophysiological activity (p > 0.05); however, moderate to large effect sizes suggest possible clinical relevance. No adverse events were reported.

Conclusion

Both LIFU and LIPU can be considered effective therapeutic options, offering immediate symptom relief in patients with upper trapezius MPS.

Trial registration

The protocol was registered at the Chinese Clinical Trial Register (ChiCTR2500097431) as a clinical trial on 19/02/2025 http://www.chictr.org.cn/.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12891-025-09017-9.

Background

Myofascial Pain Syndrome (MPS) is a chronic musculoskeletal disorder characterized by the presence of one or more myofascial trigger points (MTrPs) [1]. The prevalence of MPS affects up to 85% of the general population in their lifetime [2]. Myofascial trigger points (MTrPs) cause localized pain, stiffness, spasms, and restricted joint motion [3, 4], severely impairing quality of life by contributing to insomnia, depression, and anxiety [5]. Their formation involves increased muscle fiber contraction, local damage, or ischemia [6], explained by the widely accepted “energy crisis” theory [1, 7]. Sustained poor posture during computer use contributes to neck pain in professional populations, as repetitive muscle activation in static positions may trigger localized myofascial pain syndromes [8]. Given that 60% of the U.S. workforce uses computers, and literature confirms MTrPs as a key cause of neck pain (with trapezius commonly involved), these factors are clinically significant [9–11].

Non-invasive interventions for MPS commonly include pharmacotherapy and physical therapy, while invasive treatments encompass dry needling, trigger point injection, and acupuncture [12]. Pharmacotherapy is often associated with gastrointestinal mucosal erosion, ulcers, itching, skin erythema, and other adverse effects [13]. In addition, a study reported that the patients with MPS experienced bleeding or hematoma, and experienced needle pain following acupuncture [14]. Therapeutic ultrasound is one of the physical therapies that is used for pain reduction in clinical settings. It has been shown that ultrasound could inactivate the MTrPs by increasing tissue temperature, blood flow, and extensibility [15, 16]improving membrane permeability, and promoting tissue healing [17].

Low-intensity therapeutic ultrasound can be categorized into two types according to its energy transfer characteristics: low-intensity planar ultrasound (LIPU) and low-intensity focused ultrasound (LIFU). LIPU probes employ piezoelectric crystals that transduce electrical energy into mechanical vibrations via high-frequency alternating currents, with the resultant acoustic energy transcutaneously delivered through a transducer applicator, and the energy distribution is relatively dispersed [18]. Compared to LIPU, LIFU can be tightly focused at a distance from its source, while the ultrasonic beam propagates harmlessly through living tissues. Recent evidence indicates LIPU provides superior neck pain relief versus sham/no treatment [19], while LIFU rapidly alleviates acute muscle soreness [20]. In these studies, no significant adverse events were reported for either of the two treatment methods. However, direct comparisons of both LIPU and LIFU’s immediate effects on MPS remain scarce, particularly regarding neuromuscular electrophysiological activity. The consensus on the immediate therapeutic approaches for MPS remains elusive, and the immediate efficacy between LIFU versus LIPU for MPS has still been unclear [19].

Several studies have shown that neuromuscular electrophysiological activity could be aberrantly elevated at the locations of MTrPs based on the surface electromyography (sEMG) findings [21–23]. Moreover, Sanchez-Infante et al. reported that dry needling reduced the root mean square (RMS) and increased the mean frequency (MF) of sEMG, leading to the effective alleviation of abnormal electromyographic activity, muscle fatigue and pain in the patients with MTrPs of the upper trapezius [24]. Furthermore, the change of neuromuscular electrophysiological activity of MPS after either LIFU or LIPU treatment has not been elucidated using sEMG.

Therefore, the purpose of this study was to analyze the immediate effects of LIFU compared with LIPU in the patients with MPS of upper trapezius by conducting a prospective randomized controlled clinical study, and to further investigate the alterations in neuromuscular activation after either LIFU or LIPU intervention to unveil the associated electrophysiological mechanisms.

Methods

Setting and Patients

This study was designed as a single-blind randomized controlled trial conducted in accordance with the CONSORT guidelines and enrolled 40 patients who received care in the rehabilitation medicine center of West China Hospital, Sichuan University, from March 1 st, 2025, through March 7th.

The patients aged between 20 ~ 70 with a diagnosis of MPS in the upper trapezius would be recruited. They had to present with active MTrPs within the upper trapezius identified according to the Simon criteria: (1) the presence of a palpable taut band in the upper trapezius; (2) the presence of an excessive MTrPs in the taut band; (3) local twitch response elicited by the snapping palpation of the taut band (where palpation is possible); (4) referred pain due to MTrPs compression [25].

The exclusion was performed via history-taking and physical examination of the experienced consultant based on the exclusion criteria. Patients were excluded if they had: (1) clinical signs of cervical radiculopathy; (2) a history of neck trauma (whiplash, vertebral fractures) or prior cervical surgical interventions; (3) fibromyalgia syndrome; (4) coexisting nausea, vomiting, or vertigo; (5) received therapeutic ultrasound treatment within the preceding 12 months; or (6) received therapeutic nerve blocks or botulinum toxin administration within the preceding 12 months [26]. The diagnosis of MTrPs was conducted by a clinician with 15 years of experience in MPS-induced pain.

All the patients confirmed that they had been fully informed about the nature, purpose, potential risks, and benefits of the study, and a consent form had been signed by all the patients. We ensured that they could be provided with detailed verbal and written information about this study. Their participation is entirely voluntary, and they retain the right to withdraw consent at any time without compromising their medical care. All personal and medical data will be anonymized and maintained with strict confidentiality. They consent to the use of de-identified information for publication in scientific journals.

Randomization and Blinding

Patients were randomly assigned (1:1) to the LIFU or LIPU group via a computer-generated block randomization sequence (block size 4) managed exclusively by the second author using a validated electronic system. To ensure allocation concealment, the randomization sequence had been stored in sequentially numbered opaque envelopes, which would be opened only after the recruited patients completed all the baseline assessments.

This study adopted a single-blind design, where patients were blinded to group allocation through the following measures: (1) Patients were escorted to an isolated treatment room with the ultrasound device concealed behind an opaque partition. (2) Patients wore light-blocking goggles throughout the procedure to prevent observation of the device or treatment setup. (3) Patients were seated facing away from the therapist, with verbal reminders to maintain a forward-facing posture and avoid head movement. A trained assistant monitored compliance discreetly. (4) The device was re-concealed before patients were permitted to leave the treatment chair. Therapists administering interventions were unblinded due to technical requirements but followed scripted instructions and standardized protocols (e.g., fixed treatment duration, neutral communication). Outcome assessors were fully blinded to group allocation and had no involvement in treatment delivery.

Interventions

The recruited patients received either LIFU or LIPU treatment in 3 sessions,, administered once daily. Each session lasted for 5 min. The treatment parameters for both LIFU (FREEMUSL, TY-M100A, China) and LIPU (HANIL-TM, HS-501, South Korea) were set at an intensity of 1.5 W/cm² and a frequency of 1 MHz, delivered in continuous mode [27]. During the treatment, the patients were seated with the ultrasound transducer localized around the MTrPs of the upper trapezius area. The circular motion technique was employed with the transducer moving in a circular motion at a speed of 1–2 cm/second. The medical ultrasonic coupling gel (Tianjin Chengxin Medical Auxiliary Materials Factory, Tianjin, China) was used to ensure the direct contact of transducer with the skin. After each session, the coupling gel was wiped off and the patients were advised to keep the treated area warm.

Outcome assessments

All assessments were conducted by two therapists before treatment, and 10 min, 72 h after completing the third and final session of treatment. All assessors underwent standardized training to avoid leading questions and to maintain neutral body language during the administration of assessments. The operational procedures for each scale were clearly specified, including the use of visual aids for the Visual Analog Scale (VAS) and detailed item explanations for the Short-Form McGill Pain Questionnaire (SF-MPQ) and the Neck Disability Index (NDI). To minimize contextual variability, all assessments were conducted in a controlled environment with a consistent room temperature (22 ~ 24 °C) and minimal ambient noise.

Visual analog scale

The primary outcome was pain intensity which was assessed immediately post-treatment using a standardized VAS (interclass correlation coefficient [ICC] [2,1] = 0.85, 95% confidence interval [CI] = 0.76–0.91), and aligned with the similar internal consistency reported in the literature [28], supporting its internal consistency; this tool quantifies self-reported pain perception through a continuous metric measurement system. The VAS is a 10-cm validated scale on which patients are asked to rate their pain intensity from 0 (no pain at all) to 10 (worst possible pain) [29]. Patients were instructed to mark the point on the line that best represented their pain intensity. The score was determined by measuring the distance from the left end of the line to the point marked by the patient.

Short-form McGill Pain Questionnaire

The SF-MPQ (Cronbach’s α = 0.86) is a multidimensional instrument used to assess pain, comprising 15 descriptors: 11 sensory and 4 affective [30]. Each item is rated on a 4-point scale (0 = no pain, 1 = mild pain, 2 = moderate pain, 3 = severe pain), yielding a total score ranging from 0 to 45. The sum of the scores for all 15 items was calculated and used for analysis. The results aligned with the Cronbach’s α range of 0.81–0.89 reported in the literature [31], supporting its internal consistency.

Neck Disability Index

The NDI (Cronbach’s α = 0.89) is a self-administered questionnaire designed to evaluate the functional status of individuals with neck pain. It consists of 10 items covering pain intensity, personal care, lifting, reading, headaches, concentration, work, driving, sleeping, and recreation. Each item is scored on a scale from 0 to 5, with 0 indicating “no pain” and 5 indicating “the worst pain imaginable,” resulting in a total score ranging from 0 to 50. The total score is obtained by summing the scores of all items. The observed Cronbach’s α coefficients (0.88–0.93) fell within the range reported in prior studies [32], demonstrating strong internal consistency of the scale.

Surface Electromyography

A wireless sEMG system (FREEEMG BTS, Milano, Italy) was used to record the sEMG signals at a sampling rate of 1000 Hz. The FREEEMG BTS system was calibrated prior to each session in accordance with the manufacturer’s guidelines.

The specific sEMG assessment methods are as follows: (1) Before the sEMG assessment, the skin of the shoulder area was shaved lightly and wiped with alcohol pads to provide a good condition for sEMG signal acquisition. (2) Patients were seated comfortably in a relaxed position during electrode placement. (3) Two pairs of Ag/AgCl electrodes (KendallTM 930, Cardinal Health Inc., Dublin, Ohio, USA) were attached to the upper trapezius bilaterally, while the sEMG electrodes were positioned on the surface of the motor endplate of the upper trapezius area, the midpoint between the seventh cervical vertebra (C7) and the acromion (the inter-electrode distance was fixed at 20 mm to ensure the coverage of MTrPs). (4) Patients were instructed to perform bilateral shoulder elevation (shrugging motion) while maintaining neutral arm positioning. (5) Patients conducted six trials, and each trial was a 5-second maximal voluntary contraction and a 5-second rest period between contractions. Only 3-second steady-state contraction sEMG data were used for subsequent data analysis [23]. The raw sEMG signals were digitally filtered by a 50-Hz notch filter and bandpass 3rd order Butterworth filter between 20 Hz and 450 Hz. The RMS (µV) was extracted as the time-domain feature of sEMG using a sliding window method, with the MF (Hz) serving as the frequency-domain feature. The average of each feature was computed. RMS (ICC[2,1] = 0.78, 95% CI: 0.63–0.88) and MF (ICC[2,1] = 0.87, 95% CI: 0.78–0.93) demonstrated reliability consistent with literature reports [33], supporting internal consistency.

Sample size

Among the outcome measures, sEMG data are continuous variables and generally require a smaller sample size. However, given the clinical relevance and greater variability of the VAS, it was prioritized for sample size estimation. Based on previous studies, a 10% improvement in the VAS is commonly considered the minimal clinically important difference (MCID, Δ = 10%) [34]. Based on preliminary data from our research team and empirical evidence, the standard deviation (SD) of VAS scores was estimated to be 1.0 cm in both the LIFU and LIPU groups (σ_t = σ_c = 1.0 cm). The following statistical parameters were used for the calculation: a two-tailed significance level of α = 0.05, a Type II error of β = 0.15 (power = 85%). Sample size estimation was performed using PASS 2023 software, which indicated that a minimum of 18 patients per group would be required to detect the expected difference. To account for an anticipated 10% dropout rate, the final sample size was adjusted to 20 participants per group.

Adverse events

First, to quantify adverse effects associated with LIFU or LIPU, a standardized protocol was established to ensure consistent interventions. Second, to comprehensively monitor the adverse events, the patients were provided with a contact number to report both immediate and delayed responses to the intervention, including discomfort, swelling, muscular soreness, or any other adverse events.

Statistical analysis

Statistical analyses were performed to examine distribution characteristics and group comparisons for both the LIFU and LIPU groups. Continuous variables were presented as mean ± SD for normally distributed data or as median and interquartile range (IQR) for non-normally distributed data. Categorical variables were reported as frequencies and percentages.

The normality of continuous variables was assessed using the Shapiro–Wilk test. At baseline, between-group differences were assessed using independent samples t-tests for normally distributed continuous variables, Mann–Whitney U tests for non-normally distributed data, and Fisher’s exact tests for categorical variables. Within-group comparisons of outcomes over time (baseline vs. immediate post-treatment and immediate post-treatment vs. 72-hour post-treatment) were analyzed using the Wilcoxon signed-rank test. Between-group differences in treatment effects were examined using the Mann–Whitney U test. All tests were two-sided, and a p-value < 0.05 was considered statistically significant. To control for multiple comparisons across time points, Bonferroni correction was applied where appropriate.

Effect sizes (ES) for ordinal and non-normally distributed variables were calculated using Cliff’s Delta, a distribution-free measure of effect size [35]. Cliff’s Delta does not make any assumptions on the distributions of the 2 variables and is easily interpretable. Values under 0.147 are considered “negligible,” values between 0.147 and 0.330 are considered “small,” values between 0.330 and 0.474 are considered “medium,” and values larger than 0.474 are considered “large.” All statistical analyses were conducted using SPSS Statistics Version 25.0 (IBM^®, US).

Results

Characteristics of enrolled subjects

From March 1 st, 2025, through March 10th, A total of 46 patients were enrolled, and subsequently, 1 declined participation, 4 were ineligible, and 1 failed to provide a final decision. Ultimately, 40 patients successfully completed both the therapeutic regimen and post-treatment follow-up (Fig. 1). No patients dropped out during the follow-up period. The demographics and characteristics of enrolled patients were summarized in Table 1. There were no significant differences in the baseline characteristics between the LIFU and LIPU groups.

Fig. 1.

The trial profile of CONSORT. A total of 46 patients have been consecutively enrolled from the rehabilitation medicine center, of which 6 patients were excluded according to exclusion criteria. The recruited 40 patients were randomly assigned at 1:1 to either the LIFU group or the LIPU group using the block randomization. Abbreviation: CONSORT, Consolidated Standards of Reporting Trials; LIFU, low-intensity focused ultrasound; LIPU, low-intensity planar ultrasound

Table 1.

Patient characteristics at the time of enrolment

Characteristic	LIFU (n = 20)	LIPU (n = 20)	p value a
Age (years)	34.87 ± 8.7	35.5 ± 4.7	0.78
Gender (F/M)	9/11	15/5	0.07
Disease course (months)	6.3 (6.0)	7.5 (7.0)	0.55
BMI (kg/m²)	22.2 (4.0)	21.5 (3.0)	0.42
Affected side (L/R)	7/13	11/9	0.22

Data are presented as mean ± SD or median (IQR)

Abbreviation: L Left; R Right; LIPU Low-intensity planar ultrasound, LIFU Low-intensity focused ultrasound, T Total, F Female, M Male, BMI Body mass index; mon: month, SD Standard deviation

^aGroup differences were analyzed by using either Mann-Whitney U test, Fisher’s exact test, or Independent t-test; the statistical significance level was set at p-value < 0.05

Alleviation of pain intensity

Both the LIFU and LIPU groups demonstrated significant immediate reductions in pain intensity, as measured by VAS and SF-MPQ scores (Table 2; Fig. 2A and F). In the LIFU group, VAS scores decreased significantly immediately after intervention (MD = −2.0 cm; 95% CI: −2.5 to −1.5 cm; p = 0.002; ES = 0.6; Table 2; Fig. 2A), with further reduction at 72 h. The LIPU group also showed significant immediate improvement in VAS (MD = −1.0 cm; 95% CI: −1.5 to −0.5 cm; p = 0.001; ES = 0.45; Table 2; Fig. 2B), with sustained effects at 72 h. SF-MPQ scores in both groups followed a similar trend, with significant immediate and sustained reductions (Table 2; Fig. 2D and E). Importantly, there were no statistically significant differences between the two groups in pain alleviation at any time point (p > 0.05; Table 2; Fig. 2C and F). The observed ES ranged from medium to large, suggesting the possibility of clinically meaningful differences.

Table 2.

 Baseline, follow-up values, change scores, and effect size for pain intensity and physical function outcomes

Measures (median and IQR)	Group	Evaluation time			Within-Group (MD, 95% CI, ES) P value b
		Baseline	Post im	Post 72 h	Baseline vs. Post im	Baseline vs. Post 72 h	Post im vs. Post 72 h
VAS (cm)	LIFU	4.5 (3.0)	2.0 (4.0)†	2.0 (3.0)†‡	−2.0, −2.5 to −1.5, 0.60 0.002	−3.0, −3.5 to −2.5, 0.65 0.000	−1.0, −1.5 to −0.5, 0.40 0.018
	LIPU	4.0 (3.0)	3.0 (2.0)†	2.5 (3.0)†‡	−1.0, −1.5 to −0.5, 0.45 0.001	−1.5, −2.0 to −1.0, 0.50 0.000	−0.5, −1.0 to 0.0, 0.30 0.021
	P value a	0.162	0.891	0.933
Between-Group	MD		−1.0	−2.0
	95% CI		−1.5 to −0.5	−2.5 to −1.5
	ES		0.40	0.55
SF-MPQ	LIFU	7.0 (5.0)	4.5 (4.0)†	3.0 (3.0)†‡	−5.0, −6.0 to −4.0, 0.60 0.000	−7.0, −8.0 to −6.0, 0.70 0.000	−2.0, −3.0 to −1.0, 0.35 0.001
	LIPU	8.0 (10.0)	6.0 (9.0)†	4.5 (10.0)†‡	−4.0, −5.0 to −3.0, 0.55 0.001	−5.0, −6.0 to −4.0, 0.55 0.000	−1.0, −2.0 to 0.0, 0.20 0.046
	P value a	0.644	0.157	0.063
Between-Group	MD		−2.0	−3.0
	95% CI		−3.0 to −1.0	−4.0 to −2.0
	ES		0.45	0.50
NDI	LIFU	7.0 (5.0)	4.0 (4.0)†	3.0 (2.0)†‡	−6.0, −7.0 to −5.0, 0.70 0.000	−9.0, −10.0 to −8.0, 0.75 0.000	−3.0, −4.0 to −2.0, 0.50 0.009
	LIPU	8.5 (11.0)	7.0 (10.0)†	5.5 (10.0)†‡	−5.0, −6.0 to −4.0, 0.65 0.003	−7.0, −8.0 to −6.0, 0.60 0.001	−2.0, −3.0 to −1.0, 0.40 0.002
	P value a	0.158	0.062	0.051
Between-Group	MD		−3.0	−4.0
	95% CI		−4.0 to −2.0	−5.0 to −3.0
	ES		0.55	0.65

Note: Values are expressed as median and IQR. Effect sizes (Cliff’s Delta) were interpreted as negligible (< 0.147), small (0.147–0.330), medium (0.331–0.474), and large (> 0.474). Post im: immediately after treatment (after 3 session baseline)

Abbreviations: CI Confidence interval, ES Effect size, im immediately, IQR Interquartile range, LIFU Low-intensity focused ultrasound, LIPU Low-intensity planar ultrasound, MD Median difference, MF Median frequency, NDI Neck disability index, RMS Root mean square, SF-MPQ Short-form McGill pain questionnaire, VAS Visual analog scale

^aThe statistics are analyzed using the Mann-Whitney U test

^bThe statistics are analyzed using the Wilcoxon test

†P < 0.05,vs. Baseline; ‡ P < 0.05, Post im vs. Post 72 h

Fig. 2.

Comparison of immediate effect between LIFU versus LIPU on pain and physical function of MPS. A-C The VAS scores have been reduced after either LIFU or LIPU. D-F The SF-MPQ scores have been decreased after either LIFU or LIPU. G-I The disability index, NDI scores have been improved post-treament of either LIFU or LIPU. Abbreviation: LIFU, low-intensity focused ultrasound; LIPU, low-intensity planar ultrasound; NDI, neck disability index; SF-MPQ, McGill pain questionnaire; VAS, visual analog scale. Data presented as median and interquartile range. Post im: 10 min after treatment (after 3 session baseline); Post 72 h: 72 h after treatment (after 3 session baseline). *p < 0.05; **p < 0.001

Improvement of physical function

Physical function, assessed by NDI scores, improved significantly in both groups immediately after treatment (LIFU: MD = −6.0; 95% CI: −7.0 to −5.0; p < 0.001; ES = 0.7; Table 2; Fig. 2G; LIPU: MD = −5.0; 95% CI: −6.0 to −4.0; p = 0.003; ES = 0.65; Table 2; Fig. 2H), with further decreases observed at 72 h post-intervention. The magnitude of improvement was maintained over time, and no significant differences were found between the LIFU and LIPU groups at any assessment point (p > 0.05; Table 2; Fig. 2I); however, the ES were large, suggesting the potential for clinically meaningful differences.

Alteration in Neuromuscular Electrophysiological Activity

Regarding neuromuscular electrophysiological activity, the LIPU group exhibited a significant immediate reduction in RMS parameters after treatment (MD = −15.3 µV; 95% CI: −20.0 to −10.5 µV; p = 0.028; Table 3; Fig. 3B), with further decline at 72 h. No significant changes in MF parameters were detected in the LIPU group (Table 3; Fig. 3E), and the LIFU group showed no significant post-treatment changes in either time- or frequency-domain parameters (p > 0.05; Table 3; Fig. 3A and D). At 72 h, a significant difference in MF parameters was observed between groups (MD = 3.9 Hz; 95% CI: 0.8 to 6.5 Hz; p = 0.033; Table 3; Fig. 3F), but no other intergroup differences were found at other time points (Table 3; Fig. 3C and F).

Table 3.

 Baseline, follow-up values, change scores, and effect size for surface electromyography outcomes

Measures (median and IQR)	Group	Evaluation time			Within-Group (MD, 95% CI, ES) P value b
		Baseline	Post im	Post 72 h	Baseline vs. Post im	Baseline vs. Post 72 h	Post im vs. Post 72 h
RMS (µV)	LIFU	79.8 (74.2)	80.4 (99.2)†	79.4 (61.8)‡	−25.0, −30.0 to −20.0, 0.55 0.940	−16.7, −22.0 to −11.5, 0.45 0.709	8.3, 5.0 to 12.0, 0.25 0.601
	LIPU	75.0 (71.2)	58.0 (65.1)	79.5 (61.6)	−15.3, −20.0 to −10.5, 0.40 0.028	−10.8, −15.0 to −6.5, 0.30 0.502	4.5, 0.0 to 9.0, 0.10 0.044
	P value a	0.685	0.745	0.957
Between-group	MD		−32.1	−15.2
	95% CI		−38.0 to −26.0	−20.0 to −10.5
	ES		0.5	0.3
MF (Hz)	LIFU	75.1 (16.9)	74.1 (25.3)	78.7 (17.6)	8.1, 6.0 to 10.0, 0.45 0.391	5.6, 3.0 to 8.0, 0.35 0.681	−2.5, −4.5 to −0.5, 0.20 0.823
	LIPU	72.0 (24.6)	66.8 (30.7)	66.1 (23.5)	5.2, 3.0 to 7.5, 0.30 0.935	3.4, 1.0 to 5.8, 0.25 0.232	−1.8, −3.5 to 0.0, 0.15 0.171
	P value a	0.417	0.099	0.033
Between-group	MD		6.7	3.9
	95% CI		4.0 to 9.3	0.8 to 6.5
	ES		0.35	0.2

Note: Values are expressed as median and IQR. Effect sizes (Cliff’s Delta) were interpreted as negligible (< 0.147), small (0.147–0.330), medium (0.331–0.474), and large (> 0.474). Post im: immediately after treatment (after 3 session baseline)

Abbreviations: CI Confidence interval, ES Effect size, im immediately, IM Immediately, IQR Interquartile range, LIFU Low-intensity focused ultrasound, LIPU Low-intensity planar ultrasound, MD Median difference, MF Median frequency, RMS Root mean square

^aThe statistics are analyzed using the Mann-Whitney U test

^bThe statistics are analyzed using the Wilcoxon test

†P < 0.05, vs. Baseline; ‡ P < 0.05, Post im vs. Post 72 h

Fig. 3.

Comparison of immediate effect between LIFU versus LIPU on neuromuscular electrophysiological activity. (A-C) The abnormal time-domain features (RMS) induced by MTrPs were immediately readjusted following either LIFU or LIPU treatment. (D-F) The abnormal frequency-domain features induced by MTrPs were also immediately readjusted following either LIFU or LIPU treatment. Abbreviation: LIFU, low-intensity focused ultrasound; LIPU, low-intensity planar ultrasound; MF, median frequency; RMS, root mean square. Data presented as median and interquartile range. Post im: 10 min after treatment (after 3 session baseline); Post 72 h: 72 h after treatment (after 3 session baseline). *p < 0.05

Adverse events

No adverse events were observed or documented during the study period.

Discussion

The present single-blinded randomized controlled trial was conducted to evaluate the immediate efficacy of LIFU versus LIPU in patients with MPS of the upper trapezius. Both the LIFU and LIPU groups demonstrated significant improvements in pain alleviation and physical function. However, there were no statistically significant differences between the two groups in pain alleviation, functional improvement, or alterations of neuromuscular electrophysiological activity. Nonetheless, potential clinical differences between the groups cannot be ruled out. No adverse events were observed or documented during the study period.

Pain intensity is the agreed core outcome for neck pain research [36]. The majority of patients in our study initially presented with moderate pain. Among these, most experienced a reduction in pain intensity from moderate to mild levels following treatment. In this study, the immediate within-group changes in pain intensity exceeded the minimal clinically important difference (MCID, > 10%) [34] for VAS, indicating clinically meaningful improvements. These results suggest that both modalities are capable of immediately and effectively reducing pain intensity. Our findings are consistent with previous studies: Xia et al. [20] reported that LIFU alleviated exercise-induced delayed-onset muscle soreness within 24 ~ 48 h, while Aguilera et al. [37] found that LIPU provided immediate alleviation of myofascial pain of the upper trapezius at latent MTrPs. In addition, previous studies have reported findings that are inconsistent with our results. According to the results of a review by Luke et al., [38] there is moderate evidence indicating that conventional ultrasound is no more effective than a placebo for MTrPs in the neck or upper back. However, the authors acknowledged that due to the heterogeneity among the included trials, the review’s conclusions may be susceptible to change with the addition of a few high-quality studies. Although the present study did not directly investigate the underlying mechanisms, previous research suggests that the analgesic effects of therapeutic ultrasound may be related to the suppression of inflammatory mediators [20, 39, 40].

Both methods demonstrated immediate improvements in physical function. The observed intragroup changes in physical function exceeded the established MCID (> 10%) [41] threshold for the NDI, indicating clinically meaningful enhancement. Although most previous studies have evaluated cervical function using cervical range of motion (ROM) rather than the NDI, improvements in ROM are generally associated with better neck function and reduced disability. These findings are consistent with previous studies reporting that ultrasound therapy can enhance cervical mobility in patients with upper trapezius MTrPs [42] and improve quality of life in individuals with MTrPs-induced neck pain [43]. However, contradictory evidence exists; for example, Cho et al. [44] found no additional benefit in immediately increasing hamstring muscle flexibility or improving hip joint proprioception when ultrasound treatment was combined with myofascial stretching, and Ilter et al. [45] reported that therapeutic ultrasound was comparable to sham interventions in terms of pain alleviation and mobility improvement. These inconsistencies are likely attributable to heterogeneity in study protocols, such as differences in ultrasound intensity, frequency, treatment sites, and coupling agents. Moreover, most previous studies have primarily focused on analgesic outcomes rather than functional improvement, with limited standardization across trials, making direct comparison of functional outcomes between studies challenging.

In the present study, abnormal time-domain and frequency-domain features induced by MTrPs were immediately readjusted following either the LIFU or the LIPU treatment, as evidenced by a decrease in RMS parameters and an increase in MF, although no significant difference between these two groups. These trends are consistent with previous studies, which have reported that the down-regulation of RMS and the up-regulation of MF are associated with the improved neuromuscular function following MTrPs therapy. For example, De et al. [46] demonstrated that a single dry needling session significantly decreased upper trapezius sEMG activity after a typing task in office workers with trapezius myalgia. Another study also showed the reductions of neuromuscular electrophysiological activity after ischemic compression treatment for the latent MTrPs [37]. These findings suggest that the readjustment of motor unit recruitment might contribute to the improvement of motor endplate function [24, 47, 48]. In addition, a decrease in MF has been recognized as an indicator of muscle fatigue during isometric muscle action [49, 50] Notably, in this study the observed electrophysiological readjustments were accompanied by significant improvements in both pain intensity and physical function.

According to Witt et al. [14]acupuncture may lead to adverse events, with 54% of patients developing bleeding or hematoma and 17% reporting needle-related pain. Nonsteroidal anti-inflammatory drugs (NSAIDs) are frequently prescribed for soft tissue injury management, but their prolonged use is linked to a range of side effects, including gastrointestinal mucosal damage, ulceration, pruritus, and erythema affecting both the digestive tract and skin [13]. No skin lesions or gastrointestinal adverse events (commonly associated with NSAID administration) were observed in either LIFU or LIPU groups during follow-up, underscoring the favorable safety profiles of both methods.

This study is among the few to systematically evaluate the immediate efficacy of LIFU versus LIPU for patients with MPS. Our findings have important clinical implications and could potentially be translated into clinical practice to improve patient care. First, the comparable and immediate pain alleviation and functional improvement observed with both LIFU and LIPU treatments suggest that clinicians can select either modality based on practical considerations such as cost, availability, or technical resources without sacrificing treatment efficacy. Second, our results support the use of ultrasound therapy as a non-pharmacological option that provides immediate pain alleviation for patients with MPS, particularly for those who are unwilling or unable to use medication. Third, the effectiveness of a single-session LIFU or LIPU protocol in achieving immediate and sustained analgesia highlights the potential for shorter treatment courses, which may reduce both patient burden and healthcare costs. Finally, in the present study, most patients were classified as having moderate pain intensity according to VAS assessments, suggesting that the findings are likely generalizable to individuals with comparable pain levels in routine clinical practice and supporting the broader application of ultrasound therapy in the management of MPS.

Limitations

This investigation has several methodological constraints. First, the single-center design may limit the generalizability of the findings to broader populations. Second, the limited sample size and short-term follow-up period necessitate a cautious interpretation of therapeutic durability, and it might also be necessary to incorporate a long-term follow-up of at least 3 months in the study design. Third, unmeasured multidimensional confounders, including heterogeneous patient expectations, cultural health beliefs, pretreatment clinical trajectories, and psychosocial comorbidities, may have influenced outcome assessments. Fourth, the majority of enrolled patients presented with moderate pain intensity according to VAS assessments, which may limit the applicability of our findings to individuals with more severe pain; future studies should therefore intentionally recruit more patients with severe pain for subgroup analysis. Fifth, the absence of a control group limits our ability to attribute the observed effects solely to the intervention, as improvements may also be due to the natural course of symptom resolution. Finally, although no statistically significant differences were observed between the two groups, the moderate to large effect sizes suggest that the study may have been underpowered to detect significant differences, indicating the need for further research with larger sample sizes to validate these findings.

Conclusion

This study demonstrated that short-term LIFU and LIPU therapy sessions could be significantly effective in alleviating pain and improving the physical function of patients with MPS of the upper trapezius immediately. No adverse effects have been reported. The comparison of immediate efficacy has shown no significant difference between LIFU and LIPU treatments. The MTrPs-induced abnormal time-domain and frequency-domain features could be readjusted after ultrasound intervention. The changes in sEMG parameters may provide objective targets for monitoring treatment response, complementing traditional pain and function scales. Current evidence supports LIFU and LIPU as promising first-line therapies for MPS, providing rapid symptom alleviation and demonstrating a favorable safety profile. These approaches contribute meaningfully to improving the quality of life in patients with MPS. Further studies with a placebo-controlled or better variable-controlled design with longer follow-up periods are needed to ratify and elucidate the results.

Abbreviations

CI

Confidence interval

CONSORT

Consolidated standards of reporting trials

COX-2

Cyclooxygenase-2

DOMS

Delayed-onset muscle soreness

ES

Effect size

IL-6

Interleukin-6

LIFU

Low-intensity focused ultrasound

LIPU

Low-intensity planar ultrasound

MCID

Minimal clinically important difference

MD

Median difference

MF

Median frequency

MPS

Myofascial pain syndrome

MTrPs

Myofascial trigger points

NSAIDs

Nonsteroidal anti-inflammatory drugs

NDI

Neck disability index

RMS

Root mean square

SF-MPQ

Short-form McGill pain questionnaire

sEMG

Surface electromyography

TNF-α

Tumor necrosis factor-alpha

VAS

Visual analog scale

Authors’ contributions

Chen: Conceptualization, Methodology, Formal analysis, Writing – Original Draft. Sun: Data Curation, Formal analysis, Visualization. Li: Data Curation, Investigation, Visualization. Lin: Data Curation, Investigation. He: Data Curation, Investigation. Wang: Supervision, Project administration, Funding Acquisition, Writing – Review & Editing.

Funding

This research was supported by the 1·3·5 project for disciplines of excellence–Clinical Research Fund, West China Hospital, Sichuan University (024HXFH013).

Data availability

The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request.

Declarations

Ethics approval and consent to participate

This study design was approved by the Ethics Committee for Biomedical Research of West China Hospital of Sichuan University (Approval number: 2023 − 251). All procedures performed in this study were in accordance with the ethical standards of the 1964 Helsinki Declaration. The consent was obtained from all of the participants prior to the study.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.