Methods: Twenty normal subjects (mean age 26.8 years, sd 2.0 years; 9 females and 11 males) were recruited from our department. Exclusion criteria were: 1) engaging occupational mental arithmetic duties; 2) missing posterior teeth (excluding third molars); and 3) having other dental or medical problems including orofacial pain. They were told to refrain from smoking and consuming caffeine or alcohol for three hours before the test.
Surface EMG was recorded from the left masseter muscle by a portable EMG system (Bagnoli-2 Handheld, Delsys, USA). EMG data were sampled at 512 Hz by a digital data acquisition system (UAS-108S, Unique Medical, Japan). Sampled data were rectified and integrated.
100-square calculation of addition of two-digit numbers in three minutes was adopted for the mental arithmetic task. Four sets of calculation were performed. The first set was conducted for familiarization. The second was conducted in usual jaw condition. The third and fourth sets were randomly conducted with no contact of teeth or rhythmical clenching under a conscious effort during calculation. Intermission of one minute was taken between each set of calculation. The number of right answers was counted as the measure of accuracy and quickness of mental arithmetic.
Results: EMG activities were almost doubled during conscious clenching. No significant change of calculation outcome was observed between no tooth contact condition and clenching condition (no contact: 68.8, sd 12.2; clench: 66.5, sd 12.7).
Conclusions: In normal subjects, tooth clenching showed no beneficial effect such as improved calculation outcome. Although this study measured only short term effects of clenching on a simple calculation task, the results show that clinicians can encourage their patients to keep free from clenching of teeth.