Author(s): KALC, M., MAGDIC, M., MURKS, N., KRAMBERGER, M., HOLOBAR, A. , Institution: SCIENCE AND RESEARCH CENTRE KOPER, Country: SLOVENIA, Abstract-ID: 2211

INTRODUCTION:
The interconnected nature of human musculature and the neural system suggests that localised activities in the neck and masticatory systems could significantly impact spinal excitability throughout the body. For example, teeth occlusion, occurring voluntarily or non-voluntarily during sports activities, has an immediate facilitatory effect (1). However, the effect of stretching and relaxation of masticatory muscles is not well studied. This study aims to investigate the effects of neck and masticatory musculature mobilisation exercises, as well as specific chewing exercises, on masticatory muscle stiffness and the soleus (SOL) H reflex.
METHODS:
Ten participants (age 27.2 ± 6.8 years, 4 females) engaged in two interventions: relaxing exercises for the neck and masticatory musculature (EX) and heavy chewing (CW). These interventions were separated by a 30-minute washout period. Masseter muscle tone and stiffness, as well as SOL H-reflex and D1 presynaptic inhibition, were assessed before and after each intervention using a handheld myotonometric device and a 64-channel matrix electrode (GR08MM1305, OT Bioelettronica, Italy). A bipolar version of the signal was computed to extract peak-to-peak amplitudes of electrically elicited responses (Global EMG). Additionally, signals were decomposed to analyse contributions from single motor units (2). A total of 8400 firings from 376 distinct MUs were categorised based on low, medium, and high firing thresholds. Data were analysed using repeated measures nested linear mixed-effect models.
RESULTS:
The H-reflex amplitude decreased post-EX intervention, in contrast to the CW intervention, where it remained unchanged, resulting in a significant interaction effect (p < 0.001). No statistically significant changes were observed in D1 presynaptic inhibition (p = 0.2). Analysis at the single MU level reflected results consistent with the global EMG, but interventions did not selectively affect MUs of different threshold categories. Masseter muscle stiffness and tone increased post-EX and decreased post-CW, showing a significant interaction effect (p < 0.001).
CONCLUSION:
Neck and masticatory muscle stretching and mobility exercises significantly reduced SOL H-reflex amplitude, indicating a potential global reduction in spinal excitability and enhanced relaxation. This effect does not appear to be mediated by D1 presynaptic inhibition mechanisms and could involve other spinal ionotropic and neuromodulatory mechanisms (3). In contrast, an increase in muscle tone and stiffness was observed during EX, possibly due to increased blood flow in the active muscles, while the decrease post-CW might be attributed to fatigue-induced mechanisms.

REFERENCES:

1. Yamada et al., Health Science Reports, 2023
2. Kalc et al., IEEE TNSRE, 2023
3. Mesquita et al., J Physiol, 2022