Introduction
Neck pain is the second common medical problem after back pain [1], which has the fourth grade among disabling diseases. Its incidence is estimated at 30% to 70% of the general population [2, 3]. Acute or chronic neck pain is the frequent cause of disability and job absenteeism [4]. Every year, a large amount of health budget is devoted to treat such pains [5]. Nonspecific neck pain is diagnosed as neck pain (with or without radiation to upper limbs) with the lack of an underlying path basis [6, 7, 8]. Recent studies have indicated that Myofascial Trigger Points (MTrPs) are the prevalent source of pain in patients suffering from nonspecific neck pain [9]. Its prevalence is 65% in women and 35% in men [10]. MTrPs are defined as hyperirritable spots in a palpable taut band of skeletal muscle fibers. They provoke referral pain in a specific pattern during palpation, compression, stretch, or contraction [11, 12]. MTrPs are clinically classified as active and latent [13]. MTrPs may develop in any muscle in the body due to injury, muscle overload, repetitive microtrauma [13], and poor posture [14]. They are more prevalent in upper quarter postural muscles [6], especially upper trapezius muscle because of its important role in stability and movement of neck and shoulder [6]. MTrPs may lead to neck pain, muscle weakness and dysfunction, and decreased range of motion [1]. They have been treated with several therapeutic methods, including ischemic compression techniques [15], strengthening, stretching [16], analgesic modalities [17], and Dry Needling (DN) [18]. Although dry needling is cheap, easy to learn, and with low risks associated, it is an invasive technique used for the treatment of MTrPs. During the last decade, pieces of evidence have shown that MTrPs dry needling improves pain and Pain Pressure Threshold (PPT) in the cervical region in the short term [19]. This improvement is due to mechanical [20], neurophysiological [21], chemical, and microcirculation [22] mechanisms. Nowadays, dry needling is suggested as a standard treatment for MTrPs [19]. The purpose of this study was to investigate the effects of dry needling on pain intensity and PPT in women with MTrPs in the upper trapezius muscle using a two-day follow-up.

Materials and Methods
Study design
A randomized, single-blinded, clinical trial was performed between January and July 2020, at the School of Rehabilitation Sciences of Iran University of Medical Sciences. This study was approved by the Ethics Committee of Iran University of Medical Sciences, Tehran, Iran (Protocol Number: IR. IUMS.REC1398.848). The study was registered at the Iranian Registry of Clinical Trials (IRCT20191208045652N1). 

Clinicians
Two experienced physiotherapists participated in the study. The clinical group met for a minimum of four hours of training/practice sessions before main sessions to train about MTrPs localization and treatment, as well as getting familiar with the measurement tools and verbal instructions for data collection. Assessment and treatment were performed by separate clinicians. The assessor remained blind about the group allocation.

Study population
Participants were women with chronic non-specific neck pain enrolled by public written notifications in the Rehabilitation School of Iran University of Medical Sciences. They were included if they aged between 18 and 35 years, had a history of neck pain more than 3 and less than 6 based on Visual Analogue Scale (VAS) during the last 3 months, and had the presence of active MTrPs in their upper trapezius muscle according to the diagnostic criteria described by Simons et al. in Table 1 [13].

Exclusion criteria were the history of traumatic injuries or surgery in the cervical region or upper limb, any treatment or injection of trigger points during the last 3 months, using anti-coagulant or immunosuppressive drugs, acute psychopathy, radiculopathy signs, rheumatic disorders, fibromyalgia syndrome, coagulopathy, pregnancy, and fear of the needle.
Participants taking part in the study underwent a pre-screening by the blind assessor. The purpose and procedure of the study were fully explained to the eligible participants and they signed consent forms before participating in the study. In the baseline assessment, demographic information was collected, including age, height, and weight (Table 2).
 
Randomization
Participants were randomized using the computer blocking program performed by an independent person to the intervention group that received dry needling with upper trapezius passive stretch (n=15) and the control group that received only a passive stretch (n=15). A power analysis, based on the pilot study information, determined that a sample size of 15 participants per group provided a minimum of 90% power to detect an effect magnitude of 1.4 Standard Deviations (SD) at an alpha of 0.05.

Assessment
In this study, the outcomes measured were pain intensity and PP. Each outcome was recorded at baseline and 2 days after the fifth session. The procedures were repeated three times with a time interval of 10 seconds. The mean of these three repetitions was recorded as the main data. It should be noted that for familiarizing the participants with the tests, each test was done 3 times before the main data collection. The assessor remained blind about group allocation.

Myofascial trigger point 
Participants lied in the prone position with hands under the forehead. The possible location of MTrPs was found through pinch pincer palpation of the muscle in a longitudinal direction between the C7 spinous process and acromion. Investigations to identify the exact location of trigger points were performed based on diagnostic criteria proposed by Simons et al. [13]. As all of the assessment and treatment steps were performed on the most painful MTrPs, its precise location was marked on the skin using a water-resistant marker and the participants were asked not to wash the point roughly throughout the study for ease of follow-up identification.

Pain intensity 
Pain intensity was determined by a Visual Analog Scale (VAS). The participants were placed in the prone position with their hands under the forehead and the neck in a neutral position without any rotation. Using an algometer (JTECK USA), the constant vertical pressure of 25 N/cm2 was applied to the identified trigger point for 3 seconds. Participants were then asked to indicate the pain caused by this pressure on a 100-mm-line VAS. The line length was 100 mm, with zero indicating no pain and 100 indicating the worst pain experienced by the participants.

Pain pressure threshold
The PPT was measured using an algometer (JTECK USA). The participants were positioned prone with their hands under the forehead and the neck in a neutral position without any rotation. The Algometer disc with 1 cm2 contact area was placed vertically on the identified trigger point. The constant pressure of 1 N/cm2 per second was applied to the trigger point and the participant was asked to indicate pain onset by saying yes. 

Intervention
Both interventions lasted 3 weeks and were performed twice a week leaving 3 days between sessions, comprising five sessions. All participants were requested not to take any other therapeutic measure, including self-care management, analgesic drugs, or home physical therapy during the follow-up period for pain control. 

Dry needling
The intervention involved dry needling of the most painful MTrPs found in the upper trapezius, using a 0.3×50 mm acupuncture needle with a guided tube (Dong Bang, Korea). After wearing the gloves on both hands, the physiotherapist cleaned the target area with alcohol before treatment.The dry needling was performed in the prone position as described by Simons et al. [13]. The needle was inserted into the active MTrPs with the vertical direction according to Hong’s fast-in and fast-out technique [23]. The dry needling technique was applied until the Local Twitch Responses (LTRs) ended. The LTR indicated the effectiveness of the technique due to its immediate desirable effects [24]. Then, the needle was withdrawn, hemostatic compression was applied, and the passive stretch was performed on the same muscle (Figure 1).  Passive stretch
To stretch the upper trapezius muscle, the participant seated on a chair with their hands on their thighs. The therapist stood behind the participant. Neck contralateral side bending and ipsilateral rotation were applied until tension without any pain elicitation can be sensed at the end of the Range of Motion (ROM) by the therapist. The participant was asked to take a deep breath. The therapist increased the stretch slowly during exhalation. The newly gained ROM was held for 45 seconds. This procedure was repeated three times with 30-second rest intervals between stretches [25] (Figure 2). Results
Figure 3 shows the flow of the participants throughout the study.

The Kolmogorov-Smirnov test was used to evaluate the normality of distribution for the variables before and after treatment. Normal distribution was observed for variables in both groups (Table 2). Paired t-test was used to determine any significant changes in the tested variables 2 days after the last session in the intervention and control groups separately. The results of paired t-test showed significant changes for VAS and PPT (P=0.0001) in both groups (Table 3).

An independent t-test was used to compare score changes between the two groups. The changes for VAS and PPT were statistically significant (P<0.05) (Table 3). 

Discussion
This study was done to examine the effect of upper trapezius active trigger points dry needling on pain and PPT in women with chronic non-specific neck pain. The results showed that the amount of pain intensity decreased and PPT increased significantly after five sessions of dry needling with passive stretch in the intervention group. 

The effect of dry needling on the active trigger points of the upper trapezius muscle
In the present study, significant improvement was indicated in pain intensity and PPT after five sessions of dry needling with passive stretch. 
Consistent with the present study, several studies have demonstrated the effects of dry needling on the active trigger points of the upper trapezius muscle. Cerezo-T´ellez et al. in 2016 showed that dry needling with passive stretch of multifidus, levator scapula, upper trapezius, and splenius cervicis muscles leads to a greater reduction in pain intensity than passive stretch alone [19]. The results of a study by Gallego-Sendarrubias et al. in 2020 showed a significant change in pain intensity in dry needle therapy and manual therapy in patients with chronic mechanical neck pain [26]. In this regard, Abbaszadeh et al. in 2017 showed that one session of upper trapezius active trigger points dry needling significantly improved pain intensity compared with before treatment [21]. Kietrys et al. in 2013 also in a systematic review study showed that dry needling can significantly reduce pain intensity compared with the placebo or Sham effect, both immediately and after 4 weeks [27]. Mechanisms proposed to reduce the severity of pain include mechanical, neurophysiological, and chemical effects. From a mechanical point of view, insertion of a dry needle into the trigger points leads to local twitch response, separation of actin fibers from myosin, and return of the sarcomere to its resting length. Following mechanical stimulation, type Ⅱ fibers that are sensitive to changes in fiber length are activated and block pain messages are sent from trigger points by closing the pain gate [28]. From a neurophysiological point of view, dry needle stimulation of A delta receptors leads to the activation of encephalin, production of inhibitory neurons, and activation of the serotonergic and noradrenergic inhibitory system in the posterior horn of the spinal cord and reduces pain following treatment [28]. Studies have shown that the level of chemicals, such as bradykinin, Calcitonin Gene-Related Peptide (CGRP), and substance P is increased at the trigger points’ site. Dry needling and its subsequent local twitch response can return the level of these substances to normal and affect microcirculation [20]. 
In a heterogeneous study in 2018, Martin et al. stated that the pain intensity was aggravated 48 hours after the needling session [29]. The probable reason for the difference between the two studies was due to the different methods of pain intensity assessment.
Researchers believe that performing dry needling following the activation of the descending inhibitory system and reducing the level of chemicals, such as bradykinin and histamine at the trigger points’ site leads to an increase in the PPT [30]. Consistent with the present study, Gerber et al. in 2015 showed that the PPT improved significantly after one dry needling session [31]. These studies and the present study represent the effectiveness of dry needling in patients with MTrPs. 
In a corresponding study in 2016, Martin-Pintado et al. reported that 48 hours after a dry needling session in patients with latent trigger points of the upper trapezius muscle, the PPT was lower than before treatment [32]. The discrepancy between the present study and the Martín-Pintad study was probably due to differences in the nature of the trigger point and the number of treatment sessions in the two studies.

The effect of passive stretch on the active trigger points of UT muscle
In the present study, pain and PPT improved significantly after five sessions of passive stretch. The pain reduction following passive stretch can be due to increased blood flow and muscle metabolism [33]. Consistent with the present study, Jaeger et.al concluded that trigger point sensitivity decreases following passive stretch, and this decrease in sensitivity leads to a decrease in pain intensity [34]. 
Passive stretching also reduces the sensitivity of muscle spindle receptors and increases the PPT [25]. Consistent with the present study, Hong et al. showed that passive stretch with anesthetic spray leads to a significant increase in PPT in people with active trigger points compared with the use of heat [35].
This study showed that dry needling and passive stretching are effective treatments to increase the PPT in women with chronic nonspecific neck pain. 
Comparing the effect of dry needling with passive stretch and passive stretch alone on UT MTrPs
In the inter-group comparison, based on the independent t-test results, in the present study, a significant difference was found between groups in pain intensity and PPT (Figure 4 & 5).


Conclusion
Due to the significant reduction in pain intensity and a significant increase in PPT in the dry needling and passive stretch groups compared with the control group, it is recommended to treat the clinical symptoms of women with chronic nonspecific neck pain with dry needling and passive stretching.

Limitations
In this study, there were missing data at follow-up and case allocation limitation, and these data were excluded from the analysis because of the COVID 19 pandemic. Further research is needed to examine the results of dry needling in long term. Due to the short follow-up period, the results of the study should be used with caution in other patients.

Ethical Considerations
Compliance with ethical guidelines
This study was approved by the Ethics Committee of Iran University of Medical Sciences, Tehran (Protocol Number: IR. IUMS.REC1398.848). The study was registered at the Iranian Registry of Clinical Trials (IRCT20191208045652N1).
 
Funding
The paper was extracted from the MSc. thesis of the first author at the Department of Physiotherapy, Faculty of Rehabilitation Science, Iran University of Medical Science, Tehran.

Authors' contributions
Conceptualization, supervision: Marzieh Yassin; Methodology: Reza Salehi, Marzieh Yassin, Fereshteh Navaee; Investigation, writing – review & editing: All authors. Writing – original draft: Fereshteh Navaee; Funding acquisition, resources: Marzieh Yassin.

Conflict of interest
The authors declared no conflict of interest.


References

1. Ferrari R, Russell AS. Neck pain. Best Pract Res Clin Rheumatol. 2003;17(1):57-70. [DOI:10.1016/S1521-6942(02)00097-9]
2. Hoy D, March L, Woolf A, Blyth F, Brooks P, Smith E, et al. The global burden of neck pain: Estimates from the Global Burden of Disease 2010 study. Ann Rheum Dis. 2014;73(7):1309. [DOI:10.1136/annrheumdis-2013-204431] [PMID]
3. Côté P, van der Velde G, Cassidy JD, Carroll LJ, Hogg-Johnson S, Holm LW, et al. The burden and determinants of neck pain in workers: Results of the bone and joint decade 2000-2010 task force on neck pain and its associated disorders. J Manipulative Physiol Ther. 2009; 32(2 Suppl):S70-86. [DOI:10.1016/j.jmpt.2008.11.012] [PMID]
4. Côté P, Cassidy JD, Carroll L. The saskatchewan health and back pain survey. The prevalence of neck pain and related disability in Saskatchewan adults. Spine (Phila Pa 1976). 1998; 23(15):1689-98. [DOI:10.1097/00007632-199808010-00015] [PMID]
5. Ribeiro DC, Belgrave A, Naden A, Fang H, Matthews P, Parshottam S. The prevalence of myofascial trigger points in neck and shoulder-related disorders: A systematic review of the literature. BMC Musculoskelet Disord. 2018; 19(1):252. [DOI:10.1186/s12891-018-2157-9] [PMID] [PMCID]
6. Sciotti VM, Mittak VL, DiMarco L, Ford LM, Plezbert J, Santipadri E, et al. Clinical precision of myofascial trigger point location in the trapezius muscle. Pain. 2001; 93(3):259-66. [DOI:10.1016/S0304-3959(01)00325-6]
7. Borghouts JAJ, Koes BW, Bouter LM. The clinical course and prognostic factors of non-specific neck pain: A systematic review. Pain. 1998; 77(1):1-13. [DOI:10.1016/S0304-3959(98)00058-X]
8. Walton DM, Macdermid JC, Nielson W, Teasell RW, Chiasson M, Brown L. Reliability, standard error, and minimum detectable change of clinical pressure pain threshold testing in people with and without acute neck pain. J Orthop Sports Phys Ther. 2011; 41(9):644-50. [DOI:10.2519/jospt.2011.3666] [PMID]
9. Cerezo-Téllez E, Torres-Lacomba M, Mayoral-del Moral O, Sánchez-Sánchez B, Dommerholt J, Gutiérrez-Ortega C. Prevalence of myofascial pain syndrome in chronic non-specific neck pain: A population-based cross-sectional descriptive study. Pain Med. 2016; 17(12):2369-77. [DOI:10.1093/pm/pnw114] [PMID]
10. Drewes AM, Jennum P. Epidemiology of myofascial pain, low back pain, morning stiffness and sleep-related complaints in the general population. J Musculoskeletal Pain. 1995; 3(suppl 1):68.
11. Martín-Rodríguez A, Sáez-Olmo E, Pecos-Martín D, Calvo-Lobo C. Effects of dry needling in the sternocleidomastoid muscle on cervical motor control in patients with neck pain: A randomised clinical trial. Acupunct Med. 2019; 37(3):151-63. [DOI:10.1177/0964528419843913] [PMID]
12. Kietrys DM, Palombaro KM, Azzaretto E, Hubler R, Schaller B, Schlussel JM, et al. Effectiveness of dry needling for upper-quarter myofascial pain: A systematic review and meta-analysis. J Orthop Sports Phys Ther. 2013; 43(9):620-34. [DOI:10.2519/jospt.2013.4668] [PMID]
13. Simons DG, Travell JG, Simons LS. Travell & Simons’ myofascial pain and dysfunction: Upper half of body. Pennsylvania: williams & wilkins; 1999. https://www.google.com/books/edition/Travell_Simons_Myofascial_Pain_and_Dysfu/sU0XupX7DGsC?hl
14. Venâncio Rde A, Alencar FG, Zamperini C. Different substances and dry-needling injections in patients with myofascial pain and headaches. Cranio. 2008; 26(2):96-103. [DOI:10.1179/crn.2008.014] [PMID]
15. Fryer G, Hodgson L. The effect of manual pressure release on myofascial trigger points in the upper trapezius muscle. J Bodyw Mov Ther. 2005; 9(4):248-55. [DOI:10.1016/j.jbmt.2005.02.002]
16. Corbetta D, Agazzi S, Gatti R, Barbero M. Efficacy of low-level laser therapy on myofascial trigger point: a systematic review and meta-analysis of randomized clinical trials. Physiotherapy. 2015; 101:e268-9. [DOI:10.1016/j.physio.2015.03.455]
17. Kroeling P, Gross A, Graham N, Burnie SJ, Szeto G, Goldsmith CH, et al. Electrotherapy for neck pain. Cochrane Database Syst Rev. 201; (8):CD004251. [DOI:10.1002/14651858.CD004251.pub5] [PMID]
18. Emshi ZA, Okhovatian F, Kojidi MM, Zamani S. The effects of instrument-assisted soft tissue mobilization on active myofascial trigger points of upper trapezius muscle. J Clin Physiother Res. 2018; 3(3):133-8. [DOI:10.1002/14651858.CD004251.pub5]
19. Cerezo-Téllez E, Torres-Lacomba M, Fuentes-Gallardo I, Perez-Muñoz M, Mayoral-Del-Moral O, Lluch-Girbés E, et al. Effectiveness of dry needling for chronic nonspecific neck pain: A randomized, single-blinded, clinical trial. Pain. 2016; 157(9):1905-17. [DOI:10.1097/j.pain.0000000000000591] [PMID]
20. Ziaeifar M, Arab AM, Karimi N, Nourbakhsh MR. The effect of dry needling on pain, pressure pain threshold and disability in patients with a myofascial trigger point in the upper trapezius muscle. J Bodyw Mov Ther. 2014; 18(2):298-305. [DOI:10.1016/j.jbmt.2013.11.004] [PMID]
21. Abbaszadeh-Amirdehi M, Ansari NN, Naghdi S, Olyaei G, Nourbakhsh MR. The neurophysiological effects of dry needling in patients with upper trapezius myofascial trigger points: Study protocol of a controlled clinical trial. BMJ Open. 2013; 3(5):e002825. [DOI:10.1136/bmjopen-2013-002825] [PMID] [PMCID]
22. Cagnie B, Barbe T, De Ridder E, Van Oosterwijck J, Cools A, Danneels L. The influence of dry needling of the trapezius muscle on muscle blood flow and oxygenation. J Manipulative Physiol Ther. 2012; 35(9):685-91. [DOI:10.1016/j.jmpt.2012.10.005] [PMID]
23. Hong CZ. Considerations and recommendations regarding myofascial trigger point injection. J Musculoskelet Pain. 1994; 2(1):29-59. [DOI:10.1300/J094v02n01_03]
24. Hong CZ. Lidocaine injection versus dry needling to myofascial trigger point. The importance of the local twitch response. Am J Phys Med Rehabil. 1994; 73(4):256-63. [DOI:10.1097/00002060-199407000-00006] [PMID]
25. Kostopoulos D, Nelson AJ, Ingber RS, Larkin RW. Reduction of spontaneous electrical activity and pain perception of trigger points in the upper trapezius muscle through trigger point compression and passive stretching. J Musculoskelet Pain. 2008; 16(4):266-78. [DOI:10.1080/10582450802479594]
26. Gallego-Sendarrubias GM, Rodríguez-Sanz D, Calvo-Lobo C, Martín JL. Efficacy of dry needling as an adjunct to manual therapy for patients with chronic mechanical neck pain: A randomised clinical trial. Acupunct Med. 2020; 38(4):244-54. [DOI:10.1080/10582450802479594]
27. Kietrys DM, Palombaro KM, Azzaretto E, Hubler R, Schaller B, Schlussel JM, et al. Effectiveness of dry needling for upper-quarter myofascial pain: A systematic review and meta-analysis. J Orthop Sports Phys Ther. 2013; 43(9):620-34. https://www.jospt.org/doi/10.2519/jospt.2013.4668
28. Dommerholt J. Dry needling in orthopedic physical therapy practice. Orthop Phys Ther Practice. 2004; 16(3):15-20. https://www.researchgate.net/publication/309376176_Dry_needling_in_orthopedic_physical_therapy_practice
29. Martín-Pintado-Zugasti A, Fernández-Carnero J, León-Hernández JV, Calvo-Lobo C, Beltran-Alacreu H, Alguacil-Diego I, et al. Postneedling soreness and tenderness after different dosages of dry needling of an active myofascial trigger point in patients with neck pain: A randomized controlled trial. PM R. 2018; 10(12):1311-20. [DOI:10.1016/j.pmrj.2018.05.015] [PMID]
30. Chen JT, Chung KC, Hou CR, Kuan TS, Chen SM, Hong CZ. Inhibitory effect of dry needling on the spontaneous electrical activity recorded from myofascial trigger spots of rabbit skeletal muscle. Am J Phys Med Rehabil. 2001; 80(10):729-35. [DOI:10.1097/00002060-200110000-00004] [PMID]
31. Gerber LH, Shah J, Rosenberger W, Armstrong K, Turo D, Otto P, et al. Dry needling alters trigger points in the upper trapezius muscle and reduces pain in subjects with chronic myofascial pain. PM R. 2015; 7(7):711-8. [DOI:10.1016/j.pmrj.2015.01.020] [PMID] [PMCID]
32. Martín-Pintado-Zugasti A, Rodríguez-Fernández ÁL, Fernandez-Carnero J. Postneedling soreness after deep dry needling of a latent myofascial trigger point in the upper trapezius muscle: Characteristics, sex differences and associated factors. J Back Musculoskelet Rehabil. 2016; 29:301-8. [DOI:10.3233/BMR-150630] [PMID]
33. Phadke A, Bedekar N, Shyam A, Sancheti P. Effect of muscle energy technique and static stretching on pain and functional disability in patients with mechanical neck pain: A randomized controlled trial. Hong Kong Physiother J. 2016; 35:5-11. [DOI:10.1016/j.hkpj.2015.12.002] [PMID] [PMCID]
34. Jaeger B, Reeves JLJP. Quantification of changes in myofascial trigger point sensitivity with the pressure algometer following passive stretch. Pain. 1986; 27(2):203-10. [DOI:10.1016/0304-3959(86)90211-3]
35. Hong CZ, Chen YC, Pon CH, Yu J. Immediate effects of various physical medicine modalities on pain threshold of an active myofascial trigger point. J Musculoskelet Pain. 1993; 1(2):37-53. [DOI:10.1300/J094v01n02_04]