Teeth Grinding Muscle Movement: The Masseter Muscle
The teeth-grinding muscle movement is called bruxism, and it is characterized by clenching or grinding of the teeth due to contraction of the masseter, temporalis, and other jaw muscles. 1
Primary Muscles Involved in Bruxism
The masseter muscle is the principal muscle responsible for the clenching action during bruxism, along with the temporalis and pterygoid muscles, which together generate the powerful forces characteristic of this parafunctional activity. 1, 2
These masticatory muscles can generate extremely strong forces during bruxism that exceed those produced during normal functional chewing, and these forces can be sustained for prolonged periods. 3
Muscle dysfunction of the masticatory muscles (masseter, temporalis, pterygoids) accounts for approximately 45% of all temporomandibular joint region symptoms, making it the most common pathogenic mechanism in TMD. 2, 4
Clinical Manifestations of Masseter Activity in Bruxism
Chronic bruxism leads to masticatory muscle hypertrophy, particularly visible enlargement of the masseter and temporalis muscles, which can be detected on physical examination. 2, 1
Palpation tenderness of the masseter and other masticatory muscles, along with trigger points, are reliable physical examination findings in patients with bruxism-related muscle dysfunction. 2
Dental wear facets on occlusal surfaces indicate chronic masseter and temporalis muscle activity from repetitive clenching and grinding. 2
Bruxism Classification and Muscle Activity Patterns
Bruxism is classified into two types: awake bruxism (AB) and sleep bruxism (SB), both involving repetitive jaw-muscle activity but potentially having different etiologies and pathophysiology. 5
Sleep bruxism is now recognized as a centrally controlled condition with various systemic risk factors, rather than simply a dysfunctional movement or pathological condition. 1
During sleep bruxism, masseter and temporalis muscle activity occurs primarily during non-REM sleep in most patients. 2
Relationship to Stress and Central Nervous System
Bruxism represents a parafunctional clenching and grinding action that serves as a stress management mechanism, with evidence suggesting that aggressive biting activity attenuates stress-induced neurochemical changes in the brain. 3
The clenching function of the masticatory muscles may serve as an emergency outlet during periods of psychological overload, contributing to an individual's ability to manage stress. 3
Lesions of the cortico-basal ganglia-thalamo-cortical loops are thought to be the most likely neurological substrate underlying bruxism in neurological disorders. 6
Clinical Relevance: Mouth Guard Study Demonstrating Masseter Function
When participants were instructed to clench their masseter muscles tightly, statistically significant increases in masseter and sternocleidomastoid muscle activity were observed, with corresponding decreases in head acceleration during ball heading in football players. 7
The effect of masseter clenching was strongest when players wore a mouth guard, demonstrating that active engagement of the masseter through teeth clenching enhances neck muscle stabilization. 7
Common Pitfall to Avoid
- Do not assume that bruxism is a direct cause of pain—available scientific evidence does not support this view, and bruxism should be understood as a risk factor that may contribute to pain in susceptible individuals rather than a primary pain generator. 5