Shpuntoff and Shpuntoff (1956) found that there was an absence of electric muscular activity in the mandibular postural position. In addition, Carlson and Jarabak state that there is electromyographic silence at-rest position. The study of Yemm and Berry (1969) also noted that there is a coincidence between the rest position and electrical silence. On the contrary, in the present study EMG activity was recorded at rest position and slight position. EMG findings agree with several studies that found masticatory muscle activity was not minimal or absent( Garnik and Ramfjord,1962; Drago,1979; Wessberg et al,1982;Moller,1976,Michelotti,1997).
However, a recent review by Miles(2007) found low activity in jaw closing and opening mucles which does not contribute to the rest postion and he states that the mandible is supported by passive viscoelastic forces in peroral soft tissues. Variables such as gender shown in the previous study that did not have any effect on the EMG activity at rest postion (Ferrario et al,)
The muscle activities in a slightly open position were less than the activity at rest position similar results were obtained in other studies. these studies showed that the activity in the muscles diapered, immediately after the contact between the teeth is open ( Carlsoo 1956; Jarbak 1957Garnick; Ramfjord 1962).
EMG recording was performed during hypnotic stat and the rest position has been also measured between two spots one on the tip of the nose and the other one on the chin. The divider has been used instead of the Willis gague because it will not interfere with the subject during hypnotic state also it dsent displace the soft tissue in the submental area. the measurements showed a decrease in muscle activity, together with a great increase in the rest position. This results in agreement with Cranio (1990) who found when the subject was under hypnosis a significant reduction of the tonic EMG activity was observed (43 to 50%), together with a great increase of the inclusion space. this observation suggests that the muscle activity is not a result of the subject’s emotional reaction to the experiments as Yemm,(1969) claimed in his study.
This observation could be due to the subject being relaxed under hypnosis, the jaw muscles also relaxed, and then the mandible dropped. Therefore the opening of the mouth is increased and consequently, the EMG decrease. this is in agreement with Manns et al (1981) who found decreases in electrical activity in three muscles Masseter, anterior and posterior temporal muscles when the subjects open his mouth. this has been explained that the action of opening the mouth implies a mechanism of reciprocal innervations with nervous impulses that excite the motor neurons of the mandibular depressor mucles and inhibit those of the elevators muscles (Kawamura, 1967).
Final measurement for the rest position and the slightly open position was performed after hypnosis to confirm that the decrease in the muscle activity is due to hypnosis not due to other reasons. The results showed that the activities of the muscles rose again but still less than before hypnosis this confirms that the muscle activity was decreased inactivity was due to hypnosis. These results disagree with what other researchers state about that EMG activity at rest position was a result of the subjects emotional stress to the experimental environment; clinical rest postion was passively maintained by gravity. Yemm.
Hypnosis findings
The breathing rate was monitored during the experiment and the results showed that if the subject is hypnotized, breath differently. Hypnotize subject breath slowly and deeper breathes compare to their normal state respiratory rate. Whorwell et al (1992)
stat that hypnosis could change the breathing rate and in most cases, he found that the breathing rate becomes longer and deeper breathes compare with waking state respiratory rate. he also reports that sometimes people have a higher respiratory rate when hypnotized, but this occurs only if the person has an exciting or upsetting experience in hypnosis.
Hypnotic susceptibility was measured to test how easily a person can be hypnotized by using the eye roll test, which was first proposed by Herbert Spiegel. Most of the subjects scored between two and three. This is surprising because the majority of the subjects were not English native speakers despite that they showed highly susceptibility be hypnotized. However, It has been argued that no scale can be seen as completely reliable and no person can be hypnotized if they do not want to be; therefore, a person who scores very low may not want to be hypnotized, making the test scores invalid.
Limitation
Further research on the effect of hypnosis on the EMG activity would obviously be strengthened by a large sample. Because of the sample size (n=17)in this study it was impossible to confirm that there wasn’t a correlation between increase in rest position and decrease in EMG activity. The results would be further explained if a three measurement was performed in waking state for all the subjects not only for the last 11
Most researches including this study have utilized surface elctrodes on the masseter muscles. There are many possibilities for erroeonus interpretation.increased skin resistance due to oil,the presence of hair,and the movment of the electrode can cause false negative EMG response (Weinberg,1982). The limitation to EMG recording lies in the difficulty of distinguishing a very low level of activity from none at all.it is difficult to determine how much of an increase is required in the background ‘noise’ of the recording system to indicate an increase muscle activity.Also,the stress induced in a situation such as an experimental set up influences the electrical activity of the masticatory muscles (Yemm,1969).
Implication
In summary, the results of this study add more evidence to previous studies which suggest that the physiological basis of the postural position of the mandible is maintained actively. And significant effects were found on the effect of hypnosis on the EMG muscle activity.
Further research is needed to study the effectiveness of hypnosis on the masseter muscle activity at rest position and rest vertical dimension.