Objectives: We investigated the effect of occlusal splint in the muscle fatigue of masseter muscle on motor evoked potentials (MEPs) using transcranial magnetic stimulation (TMS). Methods: A double cone coil driven by a Magstim model 200 magnetic stimulator was placed on the skull overlaying the right temporal area. The stimulus intensity was set to 50% value of its maximum output. We compared the MEPs of the three condition (without splint, with splint1, with splint2) in the continuous voluntary contraction (as determined by random assignment). Two types occlusal vertical dimension of splints were fabricated (splint1 was about 2.0 mm, splint2 was about 10.0 mm) for 10 individuals who reported no history, signs or symptoms of myofascial pain or arthralgia. Subjects were instructed to establish 50 % maximal voluntary contraction without splint. Two types of MEPs were measured. One was in the ipsilateral masseter muscle with short latency (about 3.0 ms) as root MEPs (R-MEPs) and the other was in the contralateral masseter with long latency (about 7.0 ms) as cortex MEPs (C-MEPs). The data were analysed using repeated measured two way ANOVA and statistical significance was inferred if P value less than 0.05 (SPSS Ver.11.0J). Results: There were no changes in the latency of the R-MEPs or the C-MEPs during experiment. The amplitudes of both MEPs were decreased during continuous voluntary contraction in every conditions (P<0.01). Ratio between the amplitude of the C-MEPs and the R-MEPs (C-MEPs/R-MEPs) was decreased during continuous voluntary contraction without splint (P<0.05). There were no changes in the other conditions. Conclusions:These findings indicated that the occlusal splint reduced reflex inhibition of peripheral feedback into the motor neuron of the masseter muscle during fatigue.

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