Purpose: We investigated the relationship between clenching occurring during the day under natural conditions and psychological attributes.
Method: Subjects were 37 dentistry students and staff (14 women, 23 men; mean age 27.4 ± 4.7 years). A portable EMG recording device was used to record muscle activity in the temporal muscles for a continuous 5-h period under subjects’ natural conditions, including having lunch. Clenching was identified by electromyography, and subjects were categorized into clenching and non-clenching groups. Psychological testing was performed during the same period using the Modified Taylor Manifest Anxiety Scale (MAS), the Self-rating Depression Scale (SDS), and the Cornell Medical Index (CMI), and the psychological attributes of clenching and non-clenching groups were compared.
Results: Of the 37 subjects, 23 were assigned to the clenching group and 14 to the non-clenching group. Thirteen of the 23 subjects of the clenching group and none of the 14 subjects of the non-clenching group were determined to have psychological problems according to the MAS, with a significant difference observed between the clenching and non-clenching groups in terms of anxious tendency (p < 0.001; Fisher's exact test). Total muscle activity in the clenching group, excluding eating and talking, was 3.5 times greater compared with the non-clenching group, and a significant difference between the groups was observed (p < 0.05; Mann–Whitney U-test).
Conclusions: Daytime clenching was shown to be associated with anxious tendency. Total muscle activity in the clenching group was 3.5 times greater compared with the non-clenching group.

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