Introduction
Mechanical neck pain affects 45 to 54% of the general population at some time in their lives and can lead to severe disability [1]. There is a relationship between the presence of muscle trigger points in the upper fibers of the trapezius muscle and the presence of neck disorders [2]. Myofascial trigger points are hyper-stimulated points in a tight band of skeletal muscle that become painful when stretched or compressed and can trigger sensory-motor and autonomic components. Motor aspects include muscle weakness, stiffness, function, and range of motion (ROM) limitation. Sensory aspects include referred pain, local peripheral sensitivity, and central sensitization [3].
Currently, there are many types of manual and non-manual interventions to deactivate trigger points. Non-manual interventions include botulinum toxin injections, acupuncture, dry needling, spray, stretching techniques, and physical methods, such as ultrasound and transcutaneous electrical nerve stimulation (TENS). Manual interventions include muscle energy technique (MET), strain-counterstrain, manual pressure release, ischemic compression (IC), and neuromuscular facilitation [4]. MET and shock waves are among the new approaches to managing trigger points. There is some evidence about their safety in this field, but we need to try and do more research to clarify the different aspects of their use in this situation. MET is a manual therapy method that uses the muscle’s energy in gentle isometric contractions to relax muscles through automatic or reciprocal inhibition therapy and muscle lengthening. If a stretch of the same muscle follows the submaximal contraction of the muscle, it is called an autogenic inhibition MET, and if a stretch of the opposite muscle follows the submaximal muscle contraction, this is commonly known as a reciprocal MET [5]. MET is recommended as a method for managing trigger points [6, 7]. It is a commonly used method to achieve the release of tone (inhibition) in the muscle prior to stretching by causing an isometric contraction of the overused muscle through the influence of the Golgi tendon organs (autogenic inhibition); relaxation is created after isometric contraction [8]. MET of relaxation type after isometric contraction (Lewit method) has been reported as an effective treatment in reducing muscle stiffness caused by myofascial trigger points [9].
Shockwave is a non-invasive, effective, and harmless tool for musculoskeletal diseases. The main shock wave parameters include energy flux density (EFD) and total acoustic energy. The main therapeutic effects of shock waves refer to the helpful direct pulses in the desired points, and the secondary effects refer to the biological effects that cause tissue repair and regeneration [10]. As a result, it is not known which MET alone, shock wave alone, and a combination of both methods is the most beneficial approach because there is no study on the comparison between these three approaches. More studies are needed to compare the effects of these three therapeutic intervention methods; thus, clinicians will have an objective goal to choose a specific method in myofascial trigger point therapy. Hence, we compared the effects of MET alone, shock wave alone, and the combination of both in minimal sessions on mental pain, Pressure Pain Threshold (PPT), and neck function and movement in people with trigger points in the upper trapezius muscle. 
Materials and methods
A Randomized Controlled Trial (RCT) was designed to treat participants with active trigger points (ATrPs)in the upper trapezius muscle. The ethics committee of Tehran University of Medical Sciences approved this study. All participants (54 people) were selected according to the inclusion and exclusion criteria. Age 18-40 years, the presence of a Taut band palpable, a trigger point in the upper trapezius muscle, and one to three trigger points on one side were the inclusion criteria. Exclusion criteria included fibromyalgia, cervical radiculopathy, facial neuralgia, coagulation problems, cancer, history of neck or shoulder surgery, history of deep vein thrombosis, history of myopathy, history of infiltration in the upper trapezius muscle, anticoagulants, aspirin use during the past three days, having trigger point in other neck muscles, and lack of iron or calcium [11].
Study design: These three groups were compared to evaluate the interventions’ best efficiency. Group A (N=18) received only MET, group B (n=18) received only radial shockwave therapy (RSWT), and group C received both MET and RSWT. All participants entered the study after being diagnosed by an orthopedic specialist, a neurologist, or a rheumatologist regarding the presence of trigger points in the upper trapezius muscle. All participants (33 females and 21 males, 29.38±6.84 years old, body mass index=25.24±3.32 kg/m2) were examined in terms of inclusion and exclusion criteria and received three intervention sessions during a week. Before starting the protocol, the selected participants signed a written consent form to indicate their permission. Before starting the intervention, all possible advantages and disadvantages were explained to the participants in the written consent. In addition, we asked participants to refrain from other treatments or medications at home.
Allocating method to study groups: Participants were randomly assigned to group A (MET only), group B (RSWT only), and group C (combination of MET and RSWT). Randomizing was done using sealed and randomly filled envelopes describing the intervention groups. We prepared three envelopes for each one of the three interventions. We asked the first participant to choose an envelope. We also asked the second participant to choose an envelope; as a result, the third participant was the owner of the third envelope. We repeated the same cycle for the following three participants.
Assessment
Pain intensity, PPT, neck function, and range of counter-lateral flexion (CLF) were measured in each group before and after the intervention.
Visual analog scale (VAS): Pain intensity was evaluated using VAS. The VAS is 10 cm long marked at each end with the words “no pain” and “worst pain imaginable”[12]. Participants were asked to indicate the point along the scale that best represented the level of pain before the intervention and after the third session.
The pressure Pain threshold (PPT) is the minimum amount of pressure required to trigger pain. According to Fischer’s recommendation, a digital force gauge was used to evaluate PPT [13]. This method was achieved by placing the surface of the rubber tip (1 cm2) vertically to the known trigger points and pressing at a speed of 1 kg/s, using a gauge (Model SF-500, USA) [14]. Myofascial trigger points were determined by local or referred pain. Participants were asked to report “yes” whenever they felt pain, and their pressure stopped. Three repeated measurements were obtained at the same point with an interval of 30 seconds, and the average value was calculated and used for further analysis.
Range of counter-lateral flexion (CLF): The range of CLF of the cervical spine was measured using a goniometer for CLF [7]. Participants were asked to sit upright. The axis of the goniometer was in the appendage of the first vertebra of the thoracic spine, and the center of the arm of the goniometer was on the occipital protuberance at a right angle. Then, the horizontal arm was fixed by hand, and the vertical arm was placed on the occipital protuberance to measure the lateral flexion angle [15]. Participants were asked to tilt their heads to the opposite side. The movement was stopped by completing the available ROM, and shoulder elevation was prevented. The degree of CLF was recorded.
Neck disability index (NDI) questionnaire, Arabic version [16]: NDI questionnaire was used to measure the neck function of each participant. This scale consists of ten items with zero to five points per item. The higher the score, the more severe the neck disability.
Intervention
Group A (MET only): Participant lies on back in a supine position, arms are together, and therapist stands at the end of the participant’s head. The participant’s head and neck were moved away from the treated side to reach the restraint barrier while the examiner stabilized the shoulder with one hand and cupped the unilateral ear/mastoid region with the other hand. Three bundles of upper trapezius fibers were stretched; posterior fibers with lateral flexion and rotation to the opposite side, middle fibers with lateral flexion and middle range of rotation to the opposite side, and anterior fibers with lateral flexion and slight rotation towards the treatment. The participant was asked to apply mild resistance (20% to 50% of the available force) to the shoulder and ear, moving towards each other. Reciprocal movement is essential to produce a muscle contraction from both ends simultaneously. The degree of effort should be mild, and no pain should be felt. The contraction lasted for 7-10 seconds, and with complete relaxation, the examiner gently relaxed the head and neck to increase the degree of flexion and rotation, simultaneously pulling the shoulder down. As the stretch develops, participants in this phase of the treatment can assist by following the instructions (while breathing, please reach your hands toward the feet). Participation in stretching reduces the likelihood of the onset of the stretch reflex. Once the muscle is stretched, the participant relaxes, and the stretch is held for 30 seconds. This procedure was repeated five times or until no further gain was possible [17, 18].
Group B (RSWT only): The participant sat in a chair near the therapist and could relax their arms on it. Trigger points were determined with a skin marker. Shockwave gel was used on the trigger points of the upper trapezius. Participants were given 2000 shock sessions with low energy flux density (EFD) (0.1 MJ/mm2 per minute) using M.L.R / S.M.R 8300 (Iranian) and PAGANI (Italian) in three sessions per week (total: 6000 shocks).
Group C (combination of MET and RSWT): The intervention started with the shock wave method and was immediately followed by the MET method mentioned above.
Statistical analysis: All statistical analyses were performed by SPSS 24. The Kolmogorov-Smirnov test was used to evaluate the normality of the data. The paired t-test was used to check intra-group impacts, and the one-way ANOVA was used to check between-group impacts. The alpha level was considered 0.05 for all statistical tests.
Results
Fifty-four participants, including 21 males and 33 females, participated in the present study. The mean age in group A was 27.60±6.85 years, in group B was 31.10±6.65 years, and in group C was 29.40±6.94 years. The mean BMI in group A was 24.97±3.30 kg/m2, in group B was 25.53±3.95 kg/m2, and in group C was 25.20±2.95 kg/m2. In terms of age, there was no significant difference between the three groups (F=1.186, P=0.314). Also, there was no significant difference in BMI between the three groups (F=0.190, P=0.828). All variables had a normal distribution based on the results of the Kolmogorov-Smirnov test (P>0.05). According to the results of paired t-test, all groups (Table 1) had significantly decreased pain (P<0.001), decreased muscle sensitivity (P<0.001), increased ROM (P<0.001), and improved neck function (P<0.001). 


VAS changes in the MET group before and after the intervention were estimated as 3.05±2.15 and the changes were significantly different (P<0.001). Also, VAS changes before and after the intervention in the RSWT group were estimated as 3.16±1.29 and the changes were significantly different (P<0.001). In addition, VAS changes in the combined group before and after the intervention were estimated as 3.88±1.49, and the changes were significantly different (P<0.001).
The changes in the PPT in the MET group before and after the intervention were estimated to be 5.37±4.29, and the changes were significantly different (P<0.001). Also, the PPT changes before and after the intervention in the RSWT group were estimated to be 10.50±3.34, and the changes were significantly different (P<0.001). In addition, the PPT changes in the combined group before and after the intervention were estimated to be 11.94±3.68, and the changes were significantly different (P<0.001).
The changes in NDI in the MET group before and after the intervention were estimated as 16.07±10.46, and the changes were significantly different (P<0.001). Also, the compressive NDI before and after the intervention in the RSWT group was estimated to be 17.44±8.61, and the changes were significantly different (P<0.001). In addition, the changes in NDI in the combined group before and after the intervention were estimated to be 22.11±9.74, and the changes were significantly different (P<0.001).
The changes in CLF in the MET group before and after the intervention were estimated to be 6.88±4.22, and the changes were significantly different (P<0.001). Also, the changes in CLF before and after the intervention in the RSWT group were estimated to be 9.05±4.06), and the changes were significantly different (P<0.001). In addition, the changes in CLF in the combined group before and after the intervention were estimated to be 9.88±4.44, and the changes were significantly different (P<0.001).
The one-way ANOVA analysis was used to examine the effects between groups. This study found a significant difference between the three groups regarding PPT (F=14.89, P<0.001). The post hoc test using Bonferroni correction showed a significant difference in the mean improvement between the MET and RSWT groups (P<0.001), and also a significant difference between MET and combined groups (P<0.001). There was no significant difference between the RSWT and the combined groups in terms of mean improvement (P=0.78). In addition, there was no significant difference between the three groups in terms of VAS (F=1.29, P=0.28), NDI (F=1.29, P=0.28), and CLF (F=2.39, P=0.10) (Table 2).


Discussion
This study compared pain intensity, PPT, neck function, and CLF before and after three treatment sessions in three intervention groups (MET only, RSWT only, and both). We found that using all three methods has a beneficial impact on reducing pain intensity (reduction in VAS score), muscle sensitivity (increasing PPT), improving neck function, and increasing neck ROM (increasing CLF) in participants with ATrPs in the upper trapezius muscle, based on the results obtained after the third session of MET (three sessions per week), RSWT (2000 beats, 5 Hz, three sessions per week), and the combination of both (three sessions per week).
The results of this study showed that the VAS and NDI scores decreased significantly, the PPT score increased significantly, and the degree of CLF after the third session in the MET group (P<0.001), RSWT (P<0.001) and combined group (P<0.001) increased in subjects with ATrPs of the upper trapezius muscle.
The findings of this study showed that the changes in PPT scores were significantly different between the three groups, while the changes in the VAS scores, NDI and CLF were not significantly different between the three groups.
To our knowledge, no study has compared MET and RSWT. Therefore, we used articles that compared MET and RSWT with other techniques.
Our results are similar to that of Al-Ghadir et al., who investigated the effectiveness of combined treatments on neck pain and muscle sensitivity in male patients with ATrPs of the upper trapezius muscle. Participants were randomly divided into group A, which received MET and IC method together with conventional intervention; group B, which received all interventions of group A except IC; and group C, which received only conventional treatment. Between-group analysis showed a significant difference in groups A, B, and C in VAS score and PPT. The Cohen’s d showed a significant treatment impact size in all groups except group C. They concluded that the MET plus IC is more effective in reducing neck pain and muscle sensitivity in male patients with ATrPs of the upper trapezius muscle than the MET alone [19]. Our findings are in accordance with the results of Al-Ghadir et al. because changes in VAS score and PPT were significant in the MET group. The method of MET (relaxation after isometric contraction) of the present study was the same as that of this study.
Yeganeh et al. in 2015 investigated the combined effect of dry needling and MET on hidden trigger points of the upper trapezius muscle in women, who were randomly divided into three groups: Group 1 received dry needling and MET, group 2 received only MET, and group 3 received only dry needling. All three treatment groups had a reduction in pain and an increase in PPT as well as an increase in lateral neck flexion. However, the group receiving dry needling and MET showed significant improvement in VAS score, PPT, and ROM compared to the other two groups. No significant difference was found between the MET-only group and the dry-needling-only group. Finally, they concluded that all three treatment methods used were effective for treating myofascial trigger points. This study suggested dry needling and MET as a new way to treat myofascial trigger points [11]. The results of our study are in accordance with the findings of Yaganeh et al. as changes in VAS score, PPT, and ROM were significant in the MET group.
Another study by Kumar et al. investigated the effectiveness of MET, IC, and strain-counterstrain on trigger points of the upper trapezius muscle. The participants were assigned to three groups, including males and females, with unilateral upper trapezius muscle trigger points; group A was given MET, group B was given IC, and group C was given the strain-counterstrain technique along with conventional physical therapy. The results showed a significant difference between the groups after four weeks regarding VAS score, CLF, and NDI. Finally, this study concluded that a four-week intervention with IC, strain-counterstrain, and MET effectively treated upper trapezius muscle trigger points. However, MET was superior in treating upper trapezius trigger points than IC and strain-counterstrain techniques [20]. The results of our study are similar to the findings of Kumar et al. as changes in VAS score, NDI, and lateral neck flexion were significant in the MET group. 
The results of the present study were in line with those of Luan et al., who conducted a randomized trial comparing the effect of the shock wave and dry needling on trigger points. The participants were randomly divided into the two shock wave and dry-needling groups. This study showed a significant improvement in VAS and NDI scores and PPT at all times after treatment in both treatment groups. The shear modulus of trigger points decreased immediately after the first treatment in both dry needling and shock wave groups. Both groups maintained a significant reduction in shear modulus up to three months after the treatment. There was no significant difference between the shock wave and dry needling groups. Finally, they concluded that shock wave is as effective and safe as dry needling for pain relief, function repairing, and muscle tightness reduction for the treatment of the upper trapezius trigger points over three months. The shock wave can be recommended for clinical and study use for myofascial pain syndrome.
Further study is needed to explore the cost-effectiveness of the two interventions and the optimal shock wave parameters for myofascial trigger points [21]. The results of our study are similar to those of Luan et al.; changes in VAS and NDI scores and PPT were significant in the shock wave group. The protocol of the present study (2000 shocks per session, with an EFD of 0.10 mJ/mm2/min) was the same as that of this study.
Another study by Lee et al. was consistent with ours. They studied the effects of proprioceptive neuromuscular facilitation (PNF), shock wave, and trigger point injection (TPI) on pain and functional effectiveness on myofascial pain syndrome. They showed no obvious difference between the two groups in PPT, but the scores of the Constant-Murley Scale (pain, range, and activities of daily living) and NDI had significant differences. Finally, they concluded that PNF treatment improves neck function, ROM in the shoulder joint, and activities of daily living compared to another method. Shock wave reduces pain and increases performance. Injection therapy in the trigger point reduces pain but is not effective in increasing functional activities [22]. The results of our study were similar to those of Lee et al.; changes in VAS and NDI scores and PPT were significant in the shock wave group. The protocol of the present study (2000 shocks per session) was different compared to that of this study (1000 shocks).
Jeon et al. called patients with myofascial pain syndrome in the upper trapezius muscle and randomly divided them into two shock wave and TPI+transcutaneous electrical nerve stimulation (TENS) groups. They found a significant increase in pain threshold (one pound per square centimeter) and a significant decrease in VAS score after the first and third treatments in both groups (shock wave and TPI + TENS). Changes between groups were significantly different in terms of PPT and VAS. Changes in the McGill pain questionnaire, CLF, and pain rating scale were not significantly different between the two groups. Finally, they concluded that shock wave produces pain and increases CLF more effectively than TPI and TENS in patients with myofascial pain syndrome in the upper trapezius muscle [23]. The results of our study are in accordance with their findings as changes in the VAS score, PPT, and CLF were significant in the shock wave group. The current study protocol (2000 shocks/session, three sessions per week, EFD 0.10 MJ /mm2/min) differed only in the number of strokes compared to the study by Jeon et al. (1500 shocks/session, three sessions per week, EFD 0.10 MJ /mm2/min). 
Mushtaq et al. compared shock waves and IC and randomized participants into two groups. Participants in group A received shock waves with MET, and participants in group B received IC with MET. They found a significant reduction in pain perception in terms of VAS score increase in PPT and CLF in the shock wave and IC groups in participants with trigger points in the upper trapezius muscle. No difference in VAS score reduction was observed between the two groups; however, the increase in PPT and ROM in the shock wave group was significantly higher than in the IC group. Finally, they showed that shock wave therapy was superior to IC in improving PPT and CLF in participants with upper trapezius trigger points when combined with MET. However, in terms of pain reduction, shock wave therapy and IC are equally effective [17]. The results of the present study are in accordance with the findings of Mushtaq et al. as changes in VAS score, PPT, and CLF were significant in the combined group. The method used for group A was the same as the combined group in the present study.
Various factors can be attributed to the reduction of pain in the intervention groups. Short sarcomere length and hypoxia in the region are the sources of pain in trigger points. In general, two factors need special attention in trigger point therapy [24]: increasing blood supply to myofascial trigger points and increasing the length of sarcomeres.
The possible mechanism of pain reduction in the MET group can be attributed to the analgesic effects that can be activated by the inhibitory reflex of the Golgi tendon during isometric contraction, which leads to the relaxation of the muscle reflex. Sympathetic stimulation is due to the activation of muscle and joint mechanical receptors, which are stimulated by the activation of somatic and local afferents of the periaqueductal gray matter, which plays a role in the downward modulation of pain. The increase in ROM by the MET can be explained based on the physiological mechanisms behind changes in muscle flexibility, reflex relaxation, and viscoelastic and tensile changes. A combination of contraction and stretch (as used in MET) may be more effective for producing viscoelastic change than passive stretch, as higher forces can increase viscoelastic change and passive stretch [25].
The mechanism of shock waves to reduce pain can be explained based on general theories. The biological basis of the analgesic effect of shock wave therapy is the interaction with sensory neuropeptides, such as substance P or the peptide related to the calcitonin gene, which contributes to musculoskeletal pain [26,  27]. RSWT modulates pain by activating A-beta fibers in the muscle and inhibiting pain through GABA interneurons in the posterior horn. When shock wave therapy is applied repeatedly, this phenomenon is more evident, associated with desensitization of the exposed area, and explains the analgesic effects of shock wave therapy. This issue confirms the findings of Travel, who observed years ago that pain relief and trigger points disappear after pressure and stretching [28] [29]. Mechanical dissociation of permanent actin/myosin contractions with the help of shock waves is done through local transverse stretching of sarcomeres using shock waves perpendicular to the fiber direction to destroy abnormally contracted sarcomeres.
Conclusion
According to the present study, all three intervention groups (the MET group, the RSWT group, and the combined group) were given appropriate treatments for ATrPs in the upper trapezius muscle. However, the combined group showed increased pain intensity (reduction in VAS score), increased PPT and CLF, and improved neck function with reduced NDI after three treatment sessions. Therefore, the combination of MET with RSWT is recommended as a more effective and suitable method for treating these patients. It is assumed that more sessions will lead to more progress.
Some limitations of this study should be mentioned. First, some referred participants were hesitant to participate in the study after being explained the potential discomfort of the RSWT. Second, some eligible participants did not attend treatment sessions during the study due to the COVID-19 crisis.

Ethical Considerations
Compliance with ethical guidelines
The Tehran University of Medical Sciences Ethics Committee approved the study protocol (IR.TUMS.MEDICINE.REC.1399.917), and its IRCT code is IRCT20130121012210N8. The participants were fully informed about the study objectives. In addition to obtaining written consent, they were assured that their information would remain confidential. Participation in the present study was voluntary, and participants who did not want to continue cooperation were excluded from the study.

Funding
This article is extracted from the MA. thesis of the first author from the Department of Physiotherapy, Faculty of Rehabilitation, Tehran University of Medical Sciences. The present study was carried out with the support of the Tehran University of Medical Sciences (grant number: 99-163-3-51135).

Authors' contributions
Conceptualization: Karar Hassan AlboMahmoud, Azadeh Shadmehr; Methodology and analysis: Karar Hassan AlboMahmoud, Azadeh Shadmehr, Mohammad Reza Hadian, Shohreh Jalai, Sara Faridounnia; Research: Karar Hassan AlboMahmoud, Jamil Tahsin; Editing and finalization: Karar Hassan AlboMahmoud, Azadeh Shadmehr, Shohreh Jalai, Sara Fereydonnia; Supervision: Azadeh Shadmehar

Conflict of interest
According to the authors, this study has no conflict of interest.

Acknowledgments
We want to thank the Research Vice-Chancellor of Tehran University of Medical Sciences for supporting and financing this study and the participants.


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