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INTRODUCTION
Myofascial pain is a common clinical condition that affects all age groups due to the presence of trigger points (TrPs) within the muscles or fascia. The prevalence of myofascial pain ranges from 30% to 93% and can result in sensorimotor and autonomic deficits [1]. Myofascial trigger points (MTrPs) are defined as discrete areas of tenderness in taut bands of skeletal muscles that are painful. The upper trapezius (UT) muscle has been found to be often affected by MTrPs [2], which can result in altered sensory-motor function [3].
MTrPs may be related to pressure pain sensitivity and alterations in the UT muscle activity. The detection of a pressure pain threshold (PPT) specific to TrPs is a reliable and useful parameter for assessing the effects of treatment [4]. A reduction in PPTs and increased electromyographic (EMG) activity of the UT muscle with MTrPs can be respectively interpreted as increased pain receptor sensitization [5] and motor endplate activity [6]. Moreover, MTrPs may also affect the spatial distribution of UT activity. When considering the sampling of surface EMGs from a wide muscle volume, individuals with MTrPs in the UT muscle display a change in the spatial distribution of muscle activity during a shoulder elevation task [7], which can alter the activation pattern of shoulder muscles [8,9]. Therefore, improvement from altered PPT and EMG muscle activity by effective treatment may reveal additional options for the management of MTrPs.
In routine clinical practice, exercises, TrP injections, medications, and alternative therapies such as acupuncture dry needling (DN) are used to treat MTrPs [1]. In recent years, some studies have reported that intramuscularly delivered electrical stimulation is a potential resource for the management of myofascial pain syndrome (MPS) [10,11]. Local trigger point injections (TPI) with or without therapeutic substances act on the peripheral muscular tissues and alleviate pain by peripheral desensitization [1], whereas intramuscular electrical stimulation (IMES) can activate both peripheral and central desensitization processes through its electrical placements at peripheral and paraspinal level. Furthermore, the flow of electrical impulses can cover widespread dermatomal areas and achieve better pain modulation compared to conventional TPI or sham-IMES [10,12]. A study on IMES for MPS suggests that IMES is an effective treatment method for reducing PPT and increasing range of motion (ROM). Evidence from the literature suggests that IMES is effective in conditions such as adhesive capsulitis, MPS, and other non-traumatic musculoskeletal disorders [10,11].
In routine neuromuscular electrical stimulation, conventional electrode placement (CEP) is used, in which an active cathode pole is placed in the peripheral target tissue and a reference anode is placed over the spinal level [12] for peripheral pain modulation. Inverse electrode placement (IEP) is a new electrode placement method in which a cathode pole is placed over the spinal level and an anode pole over the distal parts of the limbs [13]. According to Shanmugam et al. [10], multilevel pain modulation (i.e., peripheral, spinal, and central) achieved using IEP may produce effective pain relief. In their study, IEP model experiments on MPS and non-traumatic shoulder pain disorders achieved significant clinical outcomes for shoulder pain and associated functional disabilities [11]. Although IMES is effective in reducing the pain threshold and changing EMG activity, the difference between the effects of IMES based on IEP and CEP on PPT and EMG activity (%MVIC) remains unknown.
Therefore, the authors aimed to study and compare the effects of IMES using IEP and CEP, and sham-IMES on PPT, EMG activity, UT muscle length, and pain severity in the UT muscle with MTrPs among adults. In line with previous study findings, the authors hypothesized that IMES using IEP would achieve significant changes in the PPT, EMG activity, muscle length, and pain severity of the UT muscle with TrPs compared with IMES using CEP and sham-IMES.
MATERIALS AND METHODS
1. Study design
This randomized clinical trial was conducted from October 2023 to March 2024 after obtaining ethical approval from the Institutional Review Board of Gulf Medical University (Approval Number, IRB-COHS-FAC-28-OCT-2023) and the approved trial protocol was registered in the ClinicalTrials.gov (NCT06604962). Prior to the intervention, all eligible participants were informed about the trial information, possible treatment benefits, and potential adverse effects of IMES and dry needle insertion. Written informed consent was obtained from all participants willing to participate in this study.
2. Participants
Individuals between the ages of 18 and 25 years (only male), with the presence of at least one active/latent TrP in the UT muscle of the dominant hand side, and a reduction of PPT range between 0.5 to 1.5 kg/cm2 were included for this study [14]. Individuals who were obese, had cervical radiculopathy, nutritional deficiencies, diabetes mellitus, migraine, shoulder pathologies, cervical spondylosis, regular participation in strength training, taking medication for major systemic illnesses, having cardiac pacemakers, a history of epilepsy, electrophobia, needle phobia, skin diseases, or undergoing other physiotherapy interventions were excluded [11].
3. Sample size and randomization
The primary objective was to find the clinically meaningful between group difference on the UT PPT. According to a study, the minimal clinically important difference (MCID) for the PPT of the UT muscle is between 0.45 to 1.13 kg/cm2 [15]. The required sample size was calculated based on the following parameters: 0.5 alpha, 0.8 power, between-group PPT mean difference of 2.5 ± 1.3 kg/cm2, MCID of 1.13 kg/cm2, equal allocation ratio, and 10% anticipated dropout. The estimated sample size was 36 participants, with 12 in each group. Therefore, a total of 36 adults with UT muscle pain with MTrPs were recruited and allocated randomly into three groups at a 1:1:1 ratio using random numbers. Each random number was concealed within a small opaque envelope, later all numbers were concealed within a large opaque envelope. An independent researcher who was not part of the study generated 36 random (computer generated) numbers, and an office clerk was involved in the allotment of the specified interventions. The intervention provider, outcome assessors and statistician were blinded to the allocation of interventions.
4. Interventions
Interventions for the participants across the three groups were performed by the same physiotherapist, who was blinded to the random allocation of participants into the groups. In the prone position, the UT muscle area of the participant was manually palpated, and the maximum tender spot was identified to locate the presence of active/latent MTrPs. Immediately after identification of the location of the MTrPs, the area was cleaned with alcohol swabs (MEDiSOFT, Biotech medical supply), saturated with 70% Isopropyl Alcohol [11]. Prior to the insertion of the needle electrode into the MTrPs of the UT muscle and the associated paraspinal region, the participants were informed about possible pinprick sensations while inserting the needles.
For electrode placement for delivering the IMES (Fig. 1), the first needle electrode was inserted into the paraspinal region, approximately 1–1.5 cm lateral to the tip of the C4 spinous process [10,11]. This needle was inserted inferomedially towards the laminar surface of the vertebra to avoid the direction of the needle into the spinal cord or nerve roots. The second electrode was placed over the TrP of the UT muscle to deliver electrical impulses, following the electrode placement method described by Shanmugam et al. [10], for IMES. A needle of suitable length (Zhongyan Taihe Acupuncture Needle, 25-mm/30-mm/40-mm/50-mm), and thickness (30-mm thickness) was inserted into the UT muscle and cervical spine depending on the depth [11,12]. The neuromuscular electrical stimulator, EN-Stim-4 (Enraf-Nonius B, V.), a four-channel electrical stimulator, was used to deliver electrical impulses with the following parameters: a symmetrical biphasic pulse duration of 500-µsec, frequency of 5 Hz, pulse interval of 1-sec with the stimulus intensity (0–140 mA) as tolerable by participants.
First group (n = 12) received IMES using IEP. The needle electrode inserted into the paraspinal region was connected to the active or cathode pole of the electrical stimulator. The reference or anode pole of the electrical stimulator was connected to the electrode that was inserted into the MTrP area of the UT muscle. The second group (n = 12) underwent IMES with CEP. In this group, the cathode pole of the stimulator was connected to the needle inserted into the UT muscle, and the anode pole was connected to the needles of the paraspinal region of the cervical spine. Needle electrodes were connected using alligator clip connectors. The muscle was stimulated for 10 min with a tolerable intensity in a single session to induce muscle relaxation [10,12]. The third group (n = 12) received sham-IMES. Similar to the experimental intervention groups, the cathode pole of the stimulator was connected to the needle inserted into the UT muscle, and the anode pole was connected to the needles of the paraspinal region of the cervical spine. However, the delivered IMES was of very minimal intensity [11].
5. Outcome measures
1) Pain severity and PPT
The severity of UT muscle pain was assessed using a visual analog scale (VAS: 0–10 cm, where 0 indicates no pain and 10 indicates severe pain), which has high intra- and inter-rater reliability to measure pain severity [16]. Pressure algometer, a device that applies an increasing force over a limited constant surface, allows the quantification of the minimum pressure which induces pain or discomfort, indicated as the PPT. PPT in the UT muscle area was assessed using a pressure algometer (JTECH Medical Commander Echo-Algometer with Console, Serial No-0d219167) by an experienced physiotherapist blinded to the intervention groups. The volunteers were seated in a chair with the torso erect, back supported, feet resting on the ground, and hands resting on the thighs. The algometer, with a rubber disk measuring 1-cm at the edge, was positioned perpendicularly to the fibers of the UT muscle exactly over the MTrPs, and the examiner exerted a gradual compression with a constant speed of approximately 0.5 kg/cm2/sec, controlled by the sound feedback of a digital metronome. These points were pressed until pain was reported, and the value was displayed as the PPT in the UT muscle [14].
2) UT’ muscle activity
The surface EMG recording during the experimental condition and maximum voluntary isometric contraction (MVIC) of the UT muscle was performed using a BTS FREEEMG 1000 system (BTS Bioengineering) in three sessions: day one, before and after treatment, and a day three follow-up. EMG data collected during the MVIC of the right-side UT muscle was used as a reference value to normalize the EMG activity of the experimental condition. In the context of EMG, MVIC refers to the peak EMG signal recorded during the maximal level of muscle activation that a person can voluntarily generate during an isometric contraction. In the experimental protocol, the participants were initially seated in a comfortable chair with their knees and hips flexed at 90°, and the skin was lightly rubbed with a swab containing 70% isopropyl alcohol before electrode placement [17]. Following the SENIAM guidelines [18], bipolar surface electrodes (Fcc id; YQH-BTSWEMG2, wireless EMG probes: 41.5 × 24 mm mother electrode and 16 × 12 mm satellite electrode) were placed over the mid-point of the UT muscle belly with a 20 mm interelectrode distance. The electrode was placed approximately 2 cm lateral to the midpoint between C7 and the acromion, with the poles parallel to the direction of the muscle fibers [8].
For UT contraction, in a comfortable seated position, the participants were instructed to lift the arm actively to the side until 90° shoulder abduction was reached through the frontal plane, with the hand facing down and the neck in a neutral position. Once 90° shoulder abduction was reached, the resistance to the movement was measured by wrapping a 3-kg weight cuff around the arm, proximal to the elbow joint, and the EMG activity of the UT muscle was recorded. The participants were also instructed not to move the shoulder past 90° abduction and were asked to maintain it at 90° against the resistance. MVIC was measured during resisted isometric abduction. Participants sat with the right shoulder abducted at 90°, elbow and wrist extended, and manual resistance was applied to the distal upper arm [19]. Each contraction at 90° shoulder abduction was held for five seconds, and EMG data for the middle three seconds were recorded and averaged for analysis. To ensure the objectivity of the data, measurements were taken three times, and the average values were used for statistical analysis. To prevent fatigue, participants took a three-minute break after each five-second UT muscle contraction interval [8].
This EMG unit provided a differential input impedance greater than 100 MΩ, a gain of 1,000, a band-pass filter of 20–500 Hz, and a common-mode rejection ratio greater than 100 dB at 50–60 Hz. The sampling rate was set to 1,000 Hz. All raw EMG signals were transferred to a Windows computer through an analog/digital converter at 1,000 Hz and a 16-bit analog-to-digital board. EMG signals were collected during three trials of experimental and MVIC tests specified for the muscle of interest, as described by SENIAM, to allow for the normalization of EMG measurements. Then, the root mean square (RMS) was used to quantify the amplitude of the surface EMG collected from the UT muscle. The basal noise was eliminated for the RMS of the experimental condition and MVIC before the normalization. The ratio between the RMS of the experimental condition with the reference RMS of the MVIC (RMS of experimental condition/RMS of the MVIC) was calculated and used for the statistical analysis.
3) UT’ muscle length
The UT length test was used to assess the UT muscle length. This involves gently lifting the patient's head off the table, flexing the neck slightly to take the UT to a lengthened position, and assessing the ROM and any restrictions or discomfort experienced by the patient during this maneuver. This test is a valid assessment tool for identifying restrictions or tightness in the UT muscle. The reliability coefficients for muscle length were generally higher for the healthy group compared to the neck pain group, with intraclass correlation coefficient values ranging from 0.19 to 0.93 for the neck pain group and 0.40 to 0.93 for the healthy group [20].
The primary (PPT and RMS) and secondary (UT muscle length and pain severity) outcomes were measured on day one before the treatment, day one post treatment and in a day three follow-up to determine the immediate and short-term effectiveness of IMES in participants with UT muscle pain due to the presence of MTrPs.
6. Statistical methods
Quantitative data such as age, body height, body weight, body mass index (BMI), duration of pain, and number of TrPs were presented in mean and standard deviation. Other clinical outcome data are described using the median and interquartile interval (IQI). The test of normality results showed no normal distribution for the UT-PPT, UT-RMS, UT-muscle length, and UT pain severity scores. The Friedman test was used to find out the differences within groups clinical data measured at different timelines, whereas the Kruskal–Wallis test was used to find out the overall differences between groups in the change scores (pre-post) of the clinical data. Post-hoc analysis was conducted using the Mann–Whitney U-test to determine which specific groups were statistically significantly different from each other. SPSS version 21 was used to perform the statistical analysis, performed at 5% alpha, 80% power, and 95% confidence interval.
RESULTS
Among 68 subjects with complaints of UT pain screened, 12 did not meet the inclusion criteria and 20 declined to participate due to fear of DN or electrical stimulation. Thirty-six eligible subjects participated in the study with a zero percent drop-out rate (Fig. 2).
1. Demographic characteristics
The mean (standard deviation) scores of the participants’ demographic characteristics, including age, height, weight, BMI, duration of pain, and number of TrPs, are provided in Table 1. One-way analysis of variance test results showed that there are no significant differences in the demographic characteristics between the groups (P > 0.05).
2. Comparison of pain intensity, muscle activity and length
1) Within-group effects
The Friedman test results show significant within-group differences (P < 0.001) on the outcome variables, including UT-PPT, UT-RMS, UT-muscle length, and UT pain severity across the three treatment groups (IEP, CEP, and sham) (Table 2).
2) Between-group effects
The Kruskal–Wallis test was used to compare the change scores of the UT-PPT, UT-RMS, UT-muscle length, and UT pain severity variables across the three groups (IEP, CEP, and sham) at two different time points (day one before the treatment to post-treatment and day one before the treatment to day three follow-up). Across the three groups, from day one before the treatment to post-treatment, the median (IQI) change score of PPT, RMS, UT muscle length, and pain severity were significantly different between the groups. Similarly, from day one before the treatment to day three follow up, the median (IQI) score changes of PPT, RMS, UT muscle length, and pain severity also showed significant (P < 0.05) differences between groups (Table 2).
Table 3 presents the results of post-hoc comparisons using the Mann–Whitney U-test to examine the between-group differences in the change scores for all outcome variables. When comparing the IEP group to the sham group, the IEP group showed significantly (P < 0.05) greater improvements in UT-PPT, UT-RMS, UT-muscle length, and UT pain severity from day one before the treatment to post-treatment (median difference of 1.25 kg/cm2, 0.08 mV, 6 degrees & 1 point respectively), as well as from day one before the treatment to day three follow-up (median difference of 2.18 kg/cm2, 0.10 mV, 8 degrees, and 1 point respectively).
Similarly, when comparing the CEP group to the sham group, the CEP group showed significantly (P < 0.05) greater improvements in UT-PPT, UT-RMS, UT-muscle length, and UT pain severity from day one before the treatment to day one post-treatment (median difference of 0.88 kg/cm2, 0.10 mV, 5 degrees, and 1 point respectively), as well as from day one before the treatment to day three follow-up (median difference of 1.68 kg/cm2, 0.15 mV, 6.5 degrees, and 1 point respectively) (Table 3).
However, when comparing the IEP and CEP groups, there were no statistically significant differences (P > 0.05) in the changes of UT-PPT, UT-RMS, UT-Muscle length, and UT pain severity from day one before the treatment to post-treatment (median difference of 0.37 kg/cm2, –0.02 mV, 1 degree, and 0 points respectively), or from day one before the treatment to day three follow-up (median difference of 0.50 kg/cm2, –0.05 mV, 1.5 degrees, and 0 points respectively) (Table 3).
DISCUSSION
The authors hypothesized that IMES using IEP would achieve significant changes in the PPT, EMG activity, pain severity, and muscle length of the UT with TrPs compared with IMES using CEP and sham-IMES. The study results showed that both types of IMES produced significant changes in PPT, EMG activity, pain severity, and muscle length compared with sham-IMES. However, IMES using the IEP and CEP methods had similar effects on reducing pain severity and EMG activity, as well as improving the PPT and UT-muscle length among individuals with UT TrPs. The similar effects observed in this study can be attributed to the consistent electrode placement sites and electrical stimulation parameters used across all groups. While the results on PPT and muscle length suggest minimal differences favoring IMES using IEP, these differences are relatively insignificant. The slight advantage of IEP may be due to the method's ability to deliver higher electrical currents at the spinal level through the active electrode. Despite this minor distinction, the potential physiological mechanisms appear to be largely the same for both electrode placement methods. These mechanisms likely include normalization of muscle blood flow, reduction of endplate noise at TrPs, and elicitation of antinociceptive effects through peripheral, spinal, and supraspinal pain inhibitory systems [12]. The uniformity of these underlying processes across both electrode placement techniques explains the overall similarity in outcomes observed in the study.
These findings on pain severity and PPT support the findings of a previous study, in which Hadizadeh et al. [21], reported that IMES has a significant effect on reducing TrP sensitivity. Similarly, a study on adhesive capsulitis with MTrPs reported a similar improvement in pain threshold and disability [11]. Lee et al. [22] suggested that increasing the microcirculation leads to a reduction in the pain sensitizers and improvement in the clinical outcomes in the muscles with TrPs immediately after the needle IMES on cervical MPS. Moro et al. [23] employed electroacupuncture at TrPs and the motor point of the trapezius muscle which produced a significant reduction in pain.
A study conducted by Perreault et al. [12] found that IMES within a TrP region normalized muscle blood flow, decreased the endplate noise of the TrP, and elicited antinociceptive effects by engaging the supraspinal descending pain inhibitory systems. Another study on the effects of a single-session IMES on pain and dysfunction following active TrPs in the UT muscle showed that IMES effectively improved pain, PPT, ROM, and neck disability [21]. Additionally, a randomized controlled trial (RCT) comparing DN alone with DN combined with IMES demonstrated improvement and maintenance of disability and pain for six weeks [24]. Although these studies employed IMES with different electrode placement methods, they achieved clinical outcomes in terms of pain severity. This supporting evidence clearly suggests that IMES using either the IEP or CEP method has potential effects on MPS.
One study found that percutaneous electrical nerve stimulation (PENS) was effective in providing short-term relief for individuals with neck pain and active TrPs [25]. A recent systematic review and meta-analysis of five RCTs involving 342 patients found that patients treated with DN-electrical stimulation experienced a greater decrease in pain than those treated with conventional physiotherapy (CPT) alone [26]. Lara-Palomo et al. [27] offer insights into the efficacy of electrical DN and CPT in addressing chronic low back pain associated with MTrPs.
According to previous studies, application of IMES [11,21,24], PENS [25], electroacupuncture, and DN-electrical stimulation [26] to specific points of the UT muscle (either motor point or TrPs) might improve microcirculation [22,23], descending pain modulation, muscle relaxation, and muscle length. Also reported to reduce pain sensitivity and EMG activity. Even though the previous studies highlighted various conventional and minimally invasive techniques that are useful in managing the TrPs, 15-minutes single session IMES focused within a TrP region of the UT muscle or paraspinal region achieved greater improvement in PPT, pain severity, muscle length, and EMG activity in the UT muscle with MTrPs. Therefore, the authors suggest that IMES can be employed to deactivate MTrPs as an immediate treatment option, which is a potential and effective nonpharmacological intervention for achieving faster clinical outcomes.
In addition to various methods of IMES, needle injections into the TrPs with or without pain relieving agents are also considered as one of the standard cost-effective interventions for MPS [1,28,29]. Even though the findings of the present study showed a significant effect on the clinical outcomes, it may be inappropriate to compare them with the published literature on TPI effects because of the methodological variations. In fact, further studies can compare these treatments to find out their therapeutic effectiveness. While both IMES and TrP injections are effective low-cost treatment methods, IMES may be preferred over TPI in specific clinical scenarios, particularly when patients require multiple injections [30]. IMES is often more suitable for managing chronic pain conditions, as it can provide longer-lasting relief without the need for repeated injections, whereas TPI has shown greater effectiveness in acute and subacute pain management [31]. In cases where a patient presents numerous TrPs, IMES can be a superior alternative, targeting a larger area simultaneously and potentially offering greater efficiency than multiple TPIs. This approach works by normalizing muscle blood flow, reducing endplate noise at TrPs, and eliciting antinociceptive effects through supraspinal descending pain inhibitory systems [12]. Patients with allergies or sensitivities to local anesthetics or corticosteroids commonly used in TPIs are better candidates for IMES [28]. Importantly, IMES can be seamlessly integrated into a comprehensive physical therapy program, potentially offering improved long-term outcomes for certain musculoskeletal conditions such as non-specific upper and lower back pain [32]. The versatility and minimal invasive nature of IMES make it an alternate option for patients who may benefit from ongoing treatment without the risks associated with repeated injections.
While previous studies focused on IMES using CEP, the present study is the first to compare the effects of different methods of IMES and sham-IMES and provide additional valuable insights into non-pharmacological approaches for MPS management. Importantly, none of the study participants experienced adverse effects from IMES, with zero percent dropout. The few limitations experienced in this study were the limited number of samples and the fact that only male subjects were recruited because females were unwilling to participate. Furthermore, it was found that a single session of IMES stimulation for 15 minutes may be beneficial for providing immediate relief from pain and improving PPT.
In conclusion, this study’s findings suggest that pain modulation can be effectively achieved using IMES regardless of electrode placement method, with different electrode configurations for clinical practice. Future studies can determine the add-on effects of IMES when combined with other conventional physical therapies to sustain long-term therapeutic benefits.
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