Methods: Forty-five experiments were performed on 8 subjects. Electrical stimuli were applied to hair-bearing skin of the lip, vermilion skin, labial and gingival mucosae, across the alveolar bone and to the crown of an upper central incisor. At the start of each experiment, the minimum stimulus required to elicit a sharp sensation was determined for a given site while the subject was relaxed. This stimulus was taken as the nociceptive threshold. During subsequent sequences the subjects, using visual feedback, maintained electromyographic activity in the masseter muscle at 12.5% of the voluntary maximum for 70 seconds while further stimuli were applied to the same site. The nociceptive threshold was determined again from the subjects' descriptions of these stimuli.
Results: In 38 of the 45 experiments, the nociceptive thresholds were higher for stimuli applied during the biting task than when the subjects were relaxed. In the other 7 cases, the thresholds were the same. The mean increases in threshold were 97%, 59%, 82%, 121%, 72% and 39% for labial and vermilion skin, labial and gingival mucosae, transalveolar and dental stimulation respectively. For the first four of these stimulation sites, the rises in threshold were significant (Paired t-tests, P<0.05).
Conclusions: These results are in line with previous findings in other laboratories that biting tasks - usually much stronger ones than in the present study - can suppress sensations from the oro-facial region. This should be taken into account when analysing concomitant sensations and reflexes in active jaw muscles.