Objective: We examined the effectiveness of occlusal splints with different hardness on masticatory muscle activity and the autonomic nervous system (ANS) during mental stress. Methods: Healthy adult volunteers (8 females; mean age, 25.0 yrs) participated in the study. A letter identification task over a 2-hour period was employed as a stressor. For convenience sake, the 2-hour period was divided into quarters and they were designated as Section 1 to 4 in order of time (Section 1, the first 30-minute period). Two splints of different hardness were prepared. Three experimental designs were applied to each subject; 1) no splint was worn throughout the whole task period; 2) a hard splint was worn during Section 2 and a soft splint during Section 3; 3) a soft splint was worn during Section 2 and a hard splint during Section 3. Electromyographic records were taken from the temporal and masseteric muscles, and tissue hemoglobin O2 saturation (StO2) was measured from the masseter muscle on the left side noninvasively. Assessment of autonomic function was made by heart rate spectral analysis. Results: StO2 increased throughout the whole task period as compared with that before the task, irrespective of experimental design (p<0.026). The Lo/Hi ratio (a marker of sympathetic activity) increased with the soft splint (p<0.036) when compared to that during an initial 5-minite period of the task. The masseteric EMG activity increased during wearing of the hard splint (p<0.003) during wearing of the soft splint (p<0.001), when compared to that before the task, on the other hand the temporal EMG activity increased only with the soft splint (p<0.017). Conclusion: These results indicate that the hardness of the occlusal splint can produce different effects on masticatory muscle activity and ANS activity during mental stress. Supported by grants from the JSPS (No 12470459).

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