Masticatory muscle activation patterns manifested by changes in index values

Relevance. Surface electromyography (sEMG) is a method used to record the bioelectrical activity of masticatory muscles both at rest and during movement. This method generates relative metrics (indices) that reflect the relationship between the biopotentials of individual muscles and muscle pairs. The objective of this study was to explore the activation patterns of the temporal and masseter muscles during different test movements, as expressed by variations in test index values, while accounting for correlations among relative metrics.

Materials and methods. Surface electromyography was performed on the temporal and masseter muscles of 165 participants aged 18 to 25 years. The study involved the following tests: “Physiological rest”, “Habitual occlusion”, “Maximal voluntary clenching of the dental arches”, and “Maximal voluntary clenching on cotton rolls”. Indices were calculated to characterize the distribution of bioelectrical activity between homologous muscles (symmetry indices) and muscle pairs (the static stabilizing occlusal index and the mandibular lateral displacement index). Initially, participants were divided into three groups of 20 individuals each based on the mandibular lateral displacement index (TORS) values recorded during the “Physiological rest” test. Data were compared across these groups. Subsequently, the same 165 participants were divided again into three groups of 20 individuals each, based on the TORS values obtained during the “Habitual occlusion” test, and the calculated results were compared across these groups. This procedure was repeated for TORS indices derived from the “Maximal voluntary clenching of the dental arches” and “Maximal voluntary clenching on cotton rolls” tests. The study ultimately examined 12 groups of 20 participants each, categorized by TORS index values (%) calculated for the four tests: ≤80% (groups 1, 4, 7, 10), 95–105% (groups 2, 5, 8, 11), and ≥120% (groups 3, 6, 9, 12). The mean values of the measured indices were compared between groups to determine statistically significant differences. Correlations were evaluated for their presence, strength, and direction both within indices recorded during the same test and across indices obtained from different tests.

Results. The analysis identified a positive correlation between the TORS index and the temporal muscle symmetry index in the “Physiological rest” test, ranging from moderate to strong: groups 1 and 2 (r_s = 0.6, p < 0.001), groups 2 and 3 (r_s = 0.6, p < 0.001), and groups 1 and 3 (rs = 0.8, p < 0.001. In the same test, the TORS index also correlated with the masseter muscle symmetry index, displaying a moderate to strong negative association: groups 1 and 2 а (r_s = -0.57, p < 0.001), groups 2 and 3 (r_s = -0.49, p < 0.001), groups 2 and 3 (r_s = -0.7, p < 0.001). The relationship between TORS index values during dental arch clenching without masticatory muscle tension and temporal muscle symmetry index was characterized as moderate to strong and positive: groups 4 and 5 (r_s = 0.81, p < 0.001), groups 5 and 6 (r_s = 0.41, p = 0.002), groups 4 and 6 (r_s = 0.65, p < 0.001). A strong negative correlation was observed between the TORS index and the masseter muscle symmetry index during the “Maximal voluntary clenching of the dental arches” test: groups 7 and 8 (r_s = -0.7, p < 0.001), groups 8 and 9 (r_s = -0.67, p < 0.001), groups 4 and 6 (r_s = -0.8, p < 0.001. Similarly, a strong negative correlation was found between the TORS index and the masseter muscle symmetry index in the "Maximal Voluntary Clenching on cotton Rolls" test. Further analysis revealed a positive correlation between the TORS indices of the "Maximal Voluntary Dental Arch Clenching" and “Maximal voluntary clenching on cotton rolls” tests.

Conclusion. This study established that the strongest correlations occurred between parameters recorded within the same test. In the “Physiological rest” test, the TORS index was influenced by the symmetrical activity of both the temporal and masseter muscles within the same test. Variations in the TORS index during the “Habitual occlusion” test were predominantly driven by the temporal muscle symmetry index, indicating a symmetrical distribution of bioelectrical activity between the left and right temporal muscles. In static tests involving maximal masticatory muscle contraction, the symmetry index of the masseter muscles strongly influenced the occurrence of torsional (lateral) mandibular movements, both in the "Maximal Voluntary Dental Arch Clenching" test and the “Maximal voluntary clenching on cotton rolls” test.

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