Modulation of the human soleus H-reflex (Sol. H-R) was studied during voluntary teeth clenching in 11 healthy adult volunteers, to investigate the possible correlation of motor functions between the jaw and body. The following results were obtained; ( 1) Sol. H-R was facilitated during the maximum voluntary teeth clenching more than during the maximum voluntary efforts in the other parts of the body, i. e., Jendrassik maneuver and clenching of both hands. ( 2) The amount of the Sol. H-R

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... tion increased with the masseter EMG activity. ( 3) Facilitation of the Sol. H-R in association with teeth clenching started ca. 60ms before the onset of the masseter EMG activity, reached its peak ca. 100ms after the onset and thereafter decreased to a certain plateau level in accordance with the masseter EMG activity level. ( 4) During the inhibition of the masseter EMG reflexively evoked by electrical stimulation of the lower lip, the amount of the facilitation of the Sol. H-R during teeth clenching decreased compared with that without lip stimulation, though the facilitation was still retained. It was concluded that: 1) The excitability of the Sol. H-R is remarkably elevated during teeth clenching. 2) There is a positive correlation between the amount of the facilitation of the Sol. H-R and the teeth clenching force level. 3) The facilitation is due to both the descending influence from the higher brain and the afferent inputs from the oral structures. It is assumed that the oral motor activity exerts a strong influence on the bodily motor function in general.