Electromyography tension and frequency spectrum analysis of some masticatory muscles at rest, isotonic and isometric contractions.

On a population of 52 subjects surface electromyographic recordings of temporals and masseters, simultaneously with mandible dynamic of closure and clenching, were performed, in order to study tension and frequency behaviour in three postural conditions: rest, isotonic and isometric contractions. Frequency was studied using the median resulting from FFT calculation, and a new computing method, which presents the proportion of frequencies making up the whole EMG signal, by steps of 50 Hz. Tension was calculated as well. The results permit us to draw the following conclusions: 1. a period of EMG silence was present in 51 of 52 subjects at mandible closure (SPA--Silent Period Area); 2. SPA onset was before teeth contact (22.5 msec., during the motion of the mandible), while its end was after closure (10.2 msec., during motionless phase of clenching). This allowed to use the SPA as a tool to clearly distinguish isotonic from isometric contraction; 3. the comparison of tension and frequency, expressed as median, showed that at rest a muscle presents low frequency and low tension. In active contraction both increase their values. Nevertheless, in active contraction, while no differences were found in frequency behaviour, tension showed a difference: although higher than at rest, isotonic contraction presented lower values than during isometric contraction; 4. the study performed by the new program showed that the low frequency at rest was due to the high proportion (30-40%) of frequencies of less than 50 Hz, while the increase at function was due to the parallel increase of frequencies comprised between 100 and 250 Hz. Because it is known that muscles are composed of fibers at low frequency and at high frequency of discharge, which play different functional roles, the last finding suggests that the mathematical analysis of the spectrum of frequencies, could provide a functional-histological image of the muscle.

Mandible position and head posture: electromyography of sternocleidomastoids.

A study was performed to evaluate relationships between mandible position, dynamics, muscle activity and head posture while swallowing by use of surface EMG and mandible kinesiograph on two population groups (118 pathologics and 31 controls). The study produced the following: 1. specific mandible dynamics with a very fast rising phase (0.3 sec) and longer phase of stabilization (1.5 sec); 2. more than 60% of the subjects presented deglutition at occlusion level, the others swallowing at a distance of 0.1-4.6 mm; 3. the whole muscle activity (temporals, masseters, digastrics, sternocleidomastoids) lasted 1.5 sec with no correlation of duration to age; 4. sternocleidomastoids fired at swallowing with an effort of one-half of temporals or masseters; 6. firing order presented a particular pattern: digastrics more often first, sternocleidomastoids more often last. No differences were found between the pathologics and controls. Findings suggest that the oral phase or mandible dynamic, and the stabilization phase or oropharyngeal phase of swallowing, has an individual role that is important in head postural equilibrium.

Electromyography tension and frequency spectrum analysis at rest of some masticatory muscles, before and after TENS.

On a population of 52 subjects surface electromyographic recordings were performed of 13.5 sec. of duration before and after ULF (Ultra Low Frequency)-TENS relaxing procedure, while they were holding their mandible at rest. For each recording the average of tension (IEMG) and the median of frequency was calculated. To compute the median of frequencies a Fast Fourier Transformer (FFT) was applied. In order to compare modifications induced by the 45' ULF-TENS relaxing procedure, so that the influence of ULF-TENS could be well isolated from any influence due simply to the time passing between one recording and another, three recordings were performed at different times: the first at time 0', the second at time 0' + 20', and only the third after TENS, time 0' + 20' + 45'. The results of the study permit us to draw the following conclusions: 1. it is confirmed that ULF-TENS can decrease muscle IEMG; 2. the study of the IEMG and frequency of the electromyographic signal at rest can be carried out starting from a window whose size and position in the 13.5 sec. of recording is arbitrary; 3. there is no connection between IEMG and frequency: in other words, at rest, there is no necessary correspondence between high or low IEMG and a high or low frequency values; 4. on application of the neurodiagnostic test of ULF-TENS, the IEMG of the electromyographic signal decreases, while the frequency of the signal remains unchanged. These last two observations permit us to hypothesize that the IEMG and the frequency of the electromyographic signal reflect some different and independent characteristics of the electrical activity of the muscle at rest.