Acute effects of orofacial, neck, and shoulder relaxation exercises and chewing on soleus H-reflex and motor unit discharge patterns

The interconnected nature of orofacial, neck musculature, and the neural system suggests that localized activities, such as teeth clenching, can influence remote spinal excitability. Although stretching exercises are known to have both local and remote effects, the specific impact of orofacial muscle stretching remains underexplored. This study investigates the effects of two interventions: 25 guided orofacial and neck stretching and mobility exercises (exercises), and chewing six chewing gums for six minutes (chewing), on the soleus H-reflex and D1 presynaptic inhibition. Ten volunteers (mean age: 28.75 ± 9 yr) participated, with H-reflex measurements collected using high-density electromyography (HDsEMG) before and after each intervention. Latency (H_LAT), duration (H_DUR), peak-to-peak (H_P2P, D1_P2P), and positive peak (H_POS) amplitudes were extracted from unconditioned and conditioned H-reflexes. The ratio (D1_P2P/H_P2P) between conditioned (D1_P2P) and unconditioned (H_P2P) H-reflex was calculated to study the D1 presynaptic inhibition mechanisms. In addition, 8,400 firings from 376 distinct motor units (MUs), categorized by firing threshold were analyzed for latency, firing ratio, and inhibition probability (D1_PROB). H_P2P, H_POS decreased and H_DUR was significantly increased after the exercise intervention, whereas the chewing intervention had no effect on these parameters. The D1_P2P/H_P2P ratio and D1_PROB remained unchanged, suggesting that the observed drop in H_P2P is not mediated by presynaptic inhibition mechanisms. Single MU analysis confirmed the H-reflex findings. The results of this study suggest that stretching and mobility exercises targeting the neck and orofacial region can reduce neuromuscular excitability, offering potential for nonpharmacological management of conditions associated with motoneuron hyperexcitability and general whole body relaxation.<b>NEW & NOTEWORTHY</b> This study provides the first evidence that orofacial and neck mobility exercises can acutely reduce spinal excitability in remote lower-limb muscles. By combining high-density surface EMG with both global and single motor unit H-reflex analyses, we demonstrate a decrease in soleus H-reflex amplitude independent of presynaptic inhibition. These findings suggest potential nonpharmacological applications for managing motoneuron hyperexcitability and promoting whole body relaxation in individuals with cervical and orofacial constraints.