Night‐time autonomic nervous system ultradian cycling and masticatory muscle activity

OBJECTIVES To test if there was a correlation between night-time masticatory muscle activity, as measured by duty factors, and ultradian cycling of autonomic nervous system (ANS) spectral powers in subjects without temporomandibular disorder (TMD)-related pain. SETTING AND SAMPLE POPULATION The University of Missouri-Kansas City School of Dentistry. Three women and four men of average ages 38 ±8 and 56 ± 17 years, respectively, gave informed consent to participate. MATERIAL AND METHODS Investigators taught subjects to record heart (electrocardiography, ECG) and masticatory muscle activities (electromyography, EMG). ECG recordings were analysed for ANS ultradian cycling by a polynomial fit to the ratio of sympathetic and parasympathetic spectral powers (ms$^{2}$ ). Masseter and temporalis EMG recordings were analysed over 20-minute epochs around peaks and valleys in the ANS ultradian cycles. Duty factors (% time of masticatory muscle activity/20-minute epoch) were determined relative to average threshold EMG (T$_{EMG}$ ) required to produce a given bite force (N). Regression analyses quantified relationships between normalized muscle duty factors and ANS spectral powers. RESULTS Subjects made a total of 27 sets of night-time ECG and EMG recordings that averaged 6.6 ± 1.1 hours per recording. Highest average duty factors were associated with T$_{EMG}$ of 1-2 N and showed cumulative masseter and temporalis activities of 9.2 and 8.8 seconds/20-minute epoch, respectively. Normalized masticatory muscle duty factors showed non-linear relationships with normalized sympathetic (R$^{2}$ = +0.82), parasympathetic (R$^{2}$ = -0.70) and sympathetic/parasympathetic spectral powers (R$^{2}$ = +0.75). CONCLUSIONS Night-time ANS spectral powers showed ultradian cycling and were correlated with masseter and temporalis muscle activities in adult subjects without TMD.