Diminished vagal tone and altered muscle biomechanics in clinically based bruxism: An exploratory study of the autonomic-myogenic relationship

Background Bruxism is a complex condition with a multidimensional aetiology, wherein it is cross-sectionally associated with dysfunction of the autonomic nervous system (ANS); however, the relationships among ANS activity, muscle biomechanics, and pain remain unclear. Objective This study examined links among ANS activity, masticatory biomechanics, and pain in clinically based bruxism using quantitative methods. Methods This observational case-control study included 44 individuals with clinically based bruxism and 45 controls. Pain intensity (at rest and during chewing via Numeric Pain Scale), ANS activity (heart rate variability, heart rate, and respiratory rate via Polar H10), and masticatory muscle properties (tone, stiffness, and elasticity via MyotonPro) were measured. Results Compared to controls, the bruxism group showed impaired sympathovagal balance (LF/HF ratio: p < .001), reduced parasympathetic activity (HF power: p = .014), and altered muscle biomechanics (increased tone and stiffness, decreased elasticity; all p < .001). Correlation analyses found that lower HF power was negatively correlated with masseter stiffness ( r = –.373, p = .043) and pain intensity (rest: r = –.387, p = .039; chewing: r = –.572, p = .018). Increased muscle tone and stiffness were positively correlated with pain intensity ( p < .05). Conclusion This exploratory study suggests a significant relationship among ANS dysfunction (specifically reduced parasympathetic activity), impaired muscle biomechanics, and pain in clinically based bruxism. These findings may provide a conceptual basis for multidisciplinary treatment strategies targeting ANS regulation in bruxism management.