Properties of tonic episodes of masseter muscle activity during waking hours and sleep in subjects with and without history of orofacial pain

Purpose: To provide a scientific data related to the tonic activity of masseter muscle in subjects with and without history of orofacial pain during their normal daily life. Methods: Thirty-three subjects were divided into two groups, a pain history group (PHG) and a non-pain history group (non-PHG), based on their responses to the Research Diagnostic Criteria for Temporomandibular Disorders questionnaire. After excluding four subjects with incomplete recordings, full-day masseter muscle surface EMGs of 29 subjects (10 men, 19 women; mean age 24.1 years) were analyzed. Tonic episode (TE) was defined as continuous EMG activity with a duration at least 2 s with intensities above twice the baseline noise level. TEs were classified into 6 strength categories (<7.5%, 7.5–10%, 10–15%, 15–25%, 25–40% and >40% of the maximum voluntary clenching (MVC)). The mean duration of activity observed in the non-PHG + 2 SD was adopted as a cutoff for identifying sustained TE. Results: During waking hours, the incidence of sustained TEs was significantly higher in the PHG than in the non-PHG (p < 0.05). The incidence and total duration of sustained TEs were significantly higher in the PHG than in the non-PHG at intensities of 7.5–10% MVC, 10–15% MVC, and 15–25% MVC (p < 0.05). No significant difference was observed during sleep. Conclusions: Within the limitations of this study, it would be concluded that sustained TEs may have a correlation with orofacial pain and the intensity range of 7.5–25% MVC would be an important range for future clenching studies