Objectives: This study investigated the cooperative function of the digastric muscle during unilateral clenching. Methods: The subjects were three healthy persons with normal dentition. The measured muscles were the right and left temporal muscle, masseter muscle and digastric muscle. The subjects requested to clench resin blocks, between the lower first molar central pit and the upper first molar mesial palatal cusp, placed unilaterally at eight sites from the right and left first premolar to second molar. The resin blocks had a height of 5, 10 and 15mm respectively. The subjects performed submaximal clenching for three seconds under the 24 assumptions and electromyographic measurements were recorded. RMS values were calculated from EMG data. Statistical analysis in was performed by two-way ANOVA and multiple comparisons were made to determine differences between the measured muscles and clenching sites. Results: When subjects clenched resin blocks unilaterally, the digastric muscle activity was shown. On two-way ANOVA, the influence of clenching 5mm resin blocks unilaterally was recognized as significant by differences in muscle measurements(p<0.01), differences by clenching sites were not significant. On multiple comparisons, the digastric muscle which is the opening muscle showed a significantly different aspect from the masseter muscle and the temporal muscle which are the closing muscles (p<0.01). These findings were also true using 10 and 15mm resin blocks. As the assumption increased, the less variation in the influence of clenching resin blocks unilaterally was recognized at the masseter muscle and the temporal muscle. In contrast, an increasing influence of resin blocks was recognized at the digastric muscle (p<0.05). Conclusions: From these results, the digastric muscle was recognized as having cooperative function during unilateral clenching.

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