Quantitative sensory response of the SCM muscle on sustained low level activation simulating co-contractions during bruxing (2023)


During the past decades it has been hypothesized that prolonged clenching periods might be a significant factor in the development of painful myogenic temporomandibular disorders (TMD).

In the context of TMD, epidemiological surveys consistently document comorbidity between masticatory and cervical muscle pain (De Laat, Meuleman, Stevens, & Verbeke, 1998; de Wijer, Steenks, de Leeuw, Bosman, & Helders, 1996; Visscher, Lobbezoo, de Boer, van der Zaag, & Naeije, 2001), which may infer mechanisms based on pathophysiologic interactions between both body segments. In particular, biomechanical connections, i.e. co-contractions of cranio-cervical muscles are hypothesized in this context. The neck musculature including the sternocleidomastoid muscle (SCM), semispinalis capitis, semispinalis cervicis, splenius capitis, levator scapulae, trapezius, multifidi, suprahyoidal and infrahyoidal muscles co-contract during jaw clenching activities (Ehrlich, Garlick, & Ninio, 1999; Ferrario, Tartaglia, Galletta, Grassi, & Sforza, 2006; Forrester, Allen, Presswood, Toy, & Pain, 2010; Lodetti, Mapelli, Musto, Rosati, & Sforza, 2012; Rodriguez et al., 2011). However, the co-contraction levels in experimental studies range between 7 and 9% of maximum voluntary contraction (MVC) only (Giannakopoulos et al., 2013), at jaw-clenching activities in the range of 50–70% also commonly observed in sleep bruxism (Gallo, Gross, & Palla, 1999; Lavigne et al., 2001). Heterotopic muscle activation in adjacent musculoskeletal body segments has also been reported in the trapezius muscle (Zennaro, Laubli, Krebs, Klipstein, & Krueger, 2003; Zennaro, Laubli, Krebs, Krueger, & Klipstein, 2004) provoked by repetitive stereotypical finger movement patterns (Sjogaard & Sogaard, 1998). These data might support the hypothesis, that tonic neck muscle activation at low levels, triggered by jaw clenching, can overload neck muscles in susceptible patients.

Studies on the effect of parafunctions like clenching usually focused on maximal clenching activity or clenching to exhaustion, controlled by electromyographic (EMG) muscle activity of masseter and temporalis muscle (Christensen, 1989, Clark and Carter, 1985; Clark, Jow, & Lee, 1989). During such experiments, the participants reported increasing pain, which finally led to the termination of the contraction task when the pain got unbearable (Christensen, 1989). Immediately after cessation of the task, the jaw muscles completely recovered and showed no significant pressure pain hyperalgesia. In contrast, EMG signs for fatigue could be detected after low-level clenching (Fernandez-de-las-Penas et al., 2010). Based on these data, maximum clenching does not seem to be an appropriate model for the research on the etiology of myofascial pain. In contrast, submaximal clenching evolved as a relevant factor in the multifactorial pathogenesis of myofascial pain, for it has been shown that long-lasting low-level clenching provokes delayed onset muscle soreness or long-lasting sensitization. Low-level clenching can be sustained for much longer times and the continual stress may provoke ultrastructural tissue damage (Yamasaki, Harada, Higuchi, Osame, & Ito, 2005).

As these pathophysiological relationships have so far been elicited for the masseter muscle only, there is a need to characterize the sensitization behavior of neck muscles during and following sustained low level muscle contraction experimentally (Arendt-Nielsen & Graven-Nielsen, 2008). Quantitative sensory testing (QST) provides an appropriate measure to detect sensitization phenomena (Arendt-Nielsen & Graven-Nielsen, 2008; Michelotti, Farella, Steenks, Gallo, & Palla, 2006), measured in a standardized manner as psychophysical response to defined mechanical and thermal stimuli (Rolke, Magerl et al., 2006). This allows for conclusions regarding the involvement of peripheral or central sensitization (Cruccu et al., 2004, Graven-Nielsen and Arendt-Nielsen, 2010, Pavlakovic and Petzke, 2010). To our knowledge, there has been no approach so far to systematically collect comprehensive QST data of the neck muscles during sustained low-level loading, simulating realistic neck muscle co-contractions during bruxing activity.

The purpose of this study was to analyse the sternocleidomastoid muscle with the QST battery in healthy subjects (according to the protocol by the German Research Network for Neuropathic Pain (Rolke, Magerl et al., 2006)) in the course of sustained low-level contraction, reflecting the co-contraction exposure time for a single night of bruxing. Following previous results on this subject for the masseter (Takeuchi et al., 2015), special focus was laid on the expected decrease of PPTs post exercise.

Section snippets


In total, 21 female subjects, all dental students aged between 20 and 25 (mean: 22.57) years were tested. Exclusion criteria were orofacial pain or functional disorders (including self-reported bruxism) according to the RDC/TMD (Research Diagnostic Criteria for Temporomandibular Disorders) and neck pain. The test subjects suggested comparable body perception controlled by identical sex and similar age. The tests were administered during the early follicular phase of the menstrual cycle (day 5)

Low level contraction of the SCM

All subjects performed the muscle activation task without interruption. No data were lost or missing. In the verum condition, the pain ratings varied between 0 and 82mm (mean: 13.79mm), in the sham condition between 0 and 35mm (mean: 1.90mm), view Fig. 3. Differences between the verum and sham experiment with regard to pain perception were significant (ANOVA, P<0.001). The pain ratings increased during the motor task, which was also statistically significant for both conditions (ANOVA, P<0.001).

Test protocol

Our results indicate that there were significant sensitization effects on SCM provoked by the testing procedure. Surprisingly, differences were found only in the sham condition: SR function and PPT showed small, but significant alterations towards increased sensitivity. On this basis the initially stated hypothesis has to be rejected.

Since there was no muscle loading in the sham condition, changes can be attributed only to the prolonged testing period or strong focusing effects. No such effects


This investigation was supported by the Deutsche Forschungsgemeinschaft, grant No. RA 1737/2-1. The authors wish to express their gratitude to Dr. Verena Kelter for her assistance during the QST experiments.

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