Myofascial trigger points and innervation zone locations in upper trapezius muscles

Musculoskeletal impairments are the leading cause of work disability in middle-aged adults, and low back pain is the most common form [1]. There are two broad clinical categories of musculoskeletal impairments: articular and non-articular [2]. The articular group includes joint diseases (intra-articular) involving inflammation or injuries due to trauma or degenerative processes. Typical examples are rheumatoid arthritis or shoulder instability. The non-articular group includes disorders that primarily affect soft tissue (peri-articular), such as muscles, ligaments, and tendons. Examples are fibromyalgia, myofascial pain syndrome, and tendonitis. It is thought that up to 85% of patients that visit pain clinics complain of non-articular musculoskeletal impairments such as the myofascial pain syndrome [3]. The first authors who systematically described the myofascial pain syndrome were Travell and Simons, who theorised that this painful condition is due to the presence of myofascial trigger points (MTrPs) [4].

MTrPs are hyperirritable points located within a taut band (TB) of skeletal muscle that cause referred pain, local tenderness, and sometimes autonomic changes [4, 5]. An active MTrP is characherized by spontaneus pain or pain response to movement, while a latent MTrP is a sensitive spot with pain only elicited in response to compression [5]. The MTrP diagnosis requires detailed history taking and physical examination to confirm the presence or absence of an original set of diagnostic criteria (i.e., taut band, spot tenderness, referred pain, pain recognition, local twitch response) [5].

MTrP pathophysiology appears to be associated with the motor endplate zone [6‐10]. This region, also known as the innervation zone (IZ), is where the α-motor neuron divides into a number of branches and synapses onto target muscle fibres. The IZ is usually described as being in the middle region of the muscle belly [11], but more complex IZ spatial distibutions have been reported in muscles with unipennate or multipennate fibre arrangements (e.g., gastrocnemius and soleus) [12‐14]. Needle electromyography (EMG) studies have demonstrated that MTrPs contain minute loci that produce characteristic low-amplitude electrical activity [10, 15, 16] (i.e. active locus), which is described by in the literature as spontaneous electrical activity (SEA).

The origin of SEA has been extensively debated among experts. Initially, Hubbard and Berkoff attributed the source of SEA action potentials to intrafusal muscle spindles located near MTrPs [15]. Later, Simons considered previous work by Liley [17] and hypothesised that SEA originates from motor endplates and defined it as endplate noise [8].

In support of the last hypothesis, a needle EMG study showed that endplate noise was more prevalent in MTrPs than in adjacent sites [10].

This “motor endplate” hypothesis was also tested by Kuan et al., who injected MTrPs with botulinum toxin type A to block acetylcholine release into the synaptic cleft and found that the injection diminished SEA in MTrPs [18].

Finally, the described electrophysiological findings have been correlated with histological changes [19, 20] and local biochemical alterations [21, 22] (e.g., inflammatory mediators, neuropeptides, catecholamines, and cytokines) to support MTrP pathophysiology.

The described experimental findings of MTrPs suggest that they are located close to the IZ [2, 16]. In a research context aimed to clarify the MTrP etiology a confirmation of the overlapping between MTrP and IZ will help to clarified their interaction (i.e. the active locus and the sensitive locus) [23]. Furthermore manual identification of MTrPs could become useful to optimize treatments addressed to the IZ, like botulinum injections for both cervical dystonia and myofascial pain [24, 25].

A technology useful for detecting the IZ in vivo has recently proposed [26‐29], and to-date, no study has assessed both MTrP and IZ locations. Therefore, the purpose of this work was to describe MTrP and IZ locations in the upper trapezius muscle.

All experimental sessions were conducted at the laboratory of movement analysis of the San Raffaele Scientific Institute in Milan, Italy. The Internal Ethical Committee approved the protocol. All participants signed an informed consent form before enrolling in the study.

All the considered variables showed a normal distribution (VAS: P=0.280, NDI: P=0.70, PPT: P=0.154, X: P=0.595, Y: P=0.106, TrP-IZ: P=0.103). The enrolled neck pain patients showed a mean VAS score of 37.3 ± 15.2 and a mean NDI score of 11 ± 5 out 50. The mean PPT of the MTrPs was 2.6 ± 0.9 kg/cm² (Table 1).

No significant correlations were found among variables except between X and TrP-IZ (P<0,01).

According to the ACS, 45 subjects showed an MTrP medially located with respect to the Y-axis, and all the MTrPs were located in the third quadrant except 3 that were located in second quadrant (Figure 3). No statistically significant difference was found for X (P = 0.6) or Y (P = 0.1) values between active and latent MTrPs.

The IZ was successfully detected in all subjects for each of the matrix columns and was not larger than 8 mm (i.e., the IED) (Figure 2). Typically, the IZ was located medially with respect to the Y-axis and not further than 2.4 cm from the Y-axis (i.e., 3 IED) and in an area that extended from second to the third quadrant. It was occasionally partially included in the first and fourth quadrants (7 out of 48 subjects) (Figure 3).

The mean TrP-IZ was 10.4 ± 5.8 mm, with no statistically significant difference between active and latent MTrPs (P = 0.6). TrP-IZ was significantly different from zero (P<0.01). Figure 3 shows both MTrPs and IZ locations according to the ACS.

The purpose of this work was to describe the location of MTrPs and IZs in the upper trapezius muscle. It is unclear whether the locations of the MTrPs and the IZs are closely related [2, 16, 23]. A study that investigates both these locations in the same subjects has not previously been undertaken. Previous studies have shown that the MTrP region includes active loci where it is possible to identify low-amplitude electrical activity (i.e., SEA), which was attributed to motor endplate dysfunction termed endplate noise. Additionally, high correlation between MTrP site irritability and endplate noise prevalence has been demonstrated by inserting an EMG needle [34]. The endplate noise searching procedure included the insertion of an intramuscular needle at the MTrP region identified during palpation [10]. As stated by the authors, this suggests an immediate proximity between the MTrP and the IZ [8, 10, 16, 18].

Results indicated that MTrPs were located in a well-defined area of the upper trapezius with a PPT that was clearly lower than in normal subjects [35] and showing values similar to those reported in previous studies [10, 34, 36].

In the current study, the investigated area was the inferior part of upper trapezius and a portion of the upper part of the mid trapezius where fibres were described to be horizontally oriented [33]. This anatomical region is where patients with neck pain typically report tenderness and where both active and latent MTrPs are frequently observed [37]. The data describe an area for MTrP location in upper trapezius that is similar to the MTrP chart proposed by Simons and Travell, which also matches the results of a recent study that used the same ACS [38]. MTrPs in the upper trapezius appear to have a stereotyped location, and clinicians could use our ACS to guide their palpatory examinations.

No significant correlations were found among the considered variables except between X and TrP-IZ, that obvious considering how TrP-IZ was computed. Additionally, no significant differences were observed for either X or Y values between active and latent MTrPs. A similar location for active and latent MTrP would support, as described by Simons, a natural course for myofascial pain that includes a subclinical stage in which the MTrPs are not spontaneously painful (i.e., latent) [5]. We screened 42 neck pain-free subjects with a negative history for arm/shoulder complains, and just 16 were negative for TBs with SPs. The common presence of TBs in pain-free subjects [37, 39, 40] and the similar location for active and latent MTrPs seem to suggest that TBs could be considered as a necessary precursor to the development of MTrP. It is likely that stress factors (e.g., muscle overload or emotional distress) could be involved in progression from latent to active [5]. This was recently confirmed by Shah et al., who demonstrated that active and latent MTrPs contain the same biochemical substances [22] (bradykinin, substance P and serotonin), and that their concentration is lower in latent MTrPs compared with active MTrPs. In the current study, SP compression failed to evoke complaints (i.e., a negative PR criteria) in just 6 of 24 subjects with neck pain, suggesting that MTrPs in the upper trapezius frequently contribute to neck pain. The presence of MTrPs should not be overlooked when examining subjects with painful conditions; moreover, high MTrP prevalence has been reported in several selected patient populations [41‐44].

The electrode matrix covered a 30.72 cm² area (9.6 cm × 3.2 cm), and the IZ can be approximately drawn on the skin as a straight line that runs orthogonally to the upper trapezius fibres and tends to curve medially towards the spine in its caudal part. A similar distribution for the IZ in upper trapezius was previously reported by Saito el al. in three healthy subjects [29]. However, this is the first time that IZ location was investigated in a large group of subjects considering the area that extends over the muscle surface. The same experimental setting was applied a in previous study that did not focused on the IZ morphology [26]. It should be noted that there was limited variability for the IZ location in the upper trapezius, and our results support the generally accepted principle that muscles with parallel fibres contain IZs in the midbelly [11].

Our findings confirm that the MTrPs in the upper trapezius are located in proximity of the IZ but do not overlap; rather, they are about 10 mm apart. In contrast with previous investigations, we observed distinct locations for IZs and MTrPs, but it is important to note that we investigated a different region of the upper trapezius, from previous studies. Interestingly, MTrPs were not equally distributed along the IZ and only affected specific groups fibres in the upper trapezius muscle.

The described close spatial relationship between IZ and MTrPs can be potentially useful to guide treatments targeting the IZ. As for example botulinum toxin injection in various pain conditions including muscle spasticity, cervical dystonia, headache and myofascial pain [24, 45].

A few limitations need to be taken into account when interpreting the results of this study.

We identified MTrP locations using SPs located on TBs using palpation, similar to previous studies [10, 34]. Although this method has been demonstrated to reliably locate MTrPs in the upper trapezius, we are aware that this only provides an approximation ranging from a few millimetres to 1.5 centimetres [36]. Also our detection methodology for the IZs and MTrPs gives bi-dimensional locations on the skin, rather than a 3-dimensional location. Finally, it is important to note that the electrode array with 8-mm IED provides an approximation of about 4 mm when locating the IZ [26].

MTrP and IZ locations were described according to the ACS in all enrolled subjects. MTrPs were located in well-defined areas of the upper trapezius, showing a typical location with a mean TrP-IZ of 10 mm. MTrPs in upper trapezius are proximally located to the IZ but not overlapped. These results provide an interesting insight for future research regarding the mechanism underlying the MTrP iperalgesia. Moreover, the anatomical reference system proposed in this study may help clinicians identify these areas.