Mandibular movements in response to electrical stimulation of superficial and deep parts of the human masseter muscle at different jaw positions.

Anatomical and electromyographical evidence suggests a compartmentalized function of the human jaw-closing muscles during both static and dynamic motor tasks. However, the voluntary nature of these tasks hampers unequivocal interpretation of this evidence, because it is impossible to activate voluntarily a single part of a muscle exclusively. Activation of discrete, localized regions can be accomplished with electrical stimulation. A previous study confirmed a functional subdivision of the temporalis muscle into at least three parts. Here, differences in the direction of the lower incisal-point (IP) movement in response to electrical stimulation of four different parts of the masseter muscle were examined in five healthy men. The deep masseter muscle and the anterior, middle, and posterior parts of the superficial masseter muscle were stimulated with monopolar wire electrodes in four different jaw positions (resting position; 50% maximum mouth opening; and 10-mm right and left lateral excursions, both with respect to resting position). Electrode-insertion depth was measured from magnetic resonance images. Movement responses to stimulation were recorded with the OKAS-3D jaw-movement analysis system. The variation in the direction of the IP movement in response to stimulation of parts of the masseter was partly explained by the effects of stimulus location and jaw position. The response to stimulation of the deep masseter was mainly laterovertically directed, whereas the response to stimulation of each of the superficial parts had a mainly anterovertical direction, the responses being most pronounced with the mandible in its resting position. These results provide further evidence for a functional subdivision of the masseter into a superficial part and a deep part, but not for a further subdivision of the superficial part into an anterior, middle, and posterior part.

Jaw movement responses to electrical stimulation of different parts of the human temporalis muscle.

Previous EMG studies have provided indications for the differential activation of the human temporalis muscle. However, in these studies the contribution of different parts of the temporalis muscle could not be separated from the contributions of other muscles, since contraction was performed by voluntary activation. Therefore, the aim of this study was to examine functional differences among various parts of the human temporalis muscle by means of recording the incisal point movement response to electrical stimulation of parts of the muscle. Five healthy male subjects participated in this study. Three locations (anterior, middle, and posterior temporalis muscle) were stimulated, by means of monopolar wire electrodes and rectangular pulses. The insertion depths of the stimulation electrodes were determined by means of magnetic resonance images. Stimulation was performed in four jaw positions (resting position, 50% maximum mouth opening, 1 cm to the left, and 1 cm to the right). Movement responses to stimulation of the different muscle parts were recorded with the OKAS-3D jaw movement analysis system. The movement responses were expressed in polar coordinates. The variation in the direction of the jaw movement response was partly explained by the factors 'stimulation location' and 'jaw position' (ANOVA, p < 0.001). When the stimulation location shifted in an antero-posterior direction, the response changed from a vertical-lateral incisal point movement to a lateral-posterior movement with a smaller vertical component. The jaw position during stimulation also influenced the movement response. A functional subdivision of the temporalis muscle into at least three parts is favored.

EMG differences between weak and strong myogenous CMD patients and healthy controls.

Electromyographic and clinical characteristics of 42 myogenous craniomandibular disorder (CMD) patients were related to 40 healthy control subjects. Surface EMG recordings were obtained from the masseter and the temporal muscles. The clinical examination included active maximum mouth opening, endfeel distance, active laterotrusion and lateral endfeel distance, dental abrasion, the presence of a lateral slide (RCP-ICP) and the amount of static pain. Orthopantomographic X-rays were available from 32 CMD patients for measuring the condylar and ramus asymmetry. Compared to the control group lower masseter and temporal EMG amplitudes were found for myogenous CMD patients (P < 0.001). When the activity of the temporal muscle was compared with the activity of the masseter muscle, the CMD patients showed proportionally higher temporal muscle activities than the controls (P < 0.05, 50% clenching level). CMD patients also showed smaller mandibular excursions, larger endfeel distances and more dental abrasion than controls. The temporal muscle asymmetries showed significant negative correlations with the ramus asymmetries. The lower and upper quartile of the distribution of the mean masseter EMG amplitudes were used to distinguish weak and strong muscles in patients and controls. Weak patients showed proportionally high temporal muscle activities, larger masseter and temporal muscle asymmetries, and larger endfeel distances compared to strong CMD patients. These differences were not found between weak and strong controls. In conclusion, it can be said that the electromyographic and clinical findings of the myogenous CMD patients suggest a functional difference between weak and strong patients and indicate the need for more individually designed treatment modalities for functional muscle and joint CMD problems.

Electromyographic evidence of local muscle fatigue in a subgroup of patients with myogenous craniomandibular disorders.

Ten female patients with myogenous craniomandibular disorders who had complete dentitions and mainly unilateral muscle pain and tenderness to palpation, and 10 healthy female controls, participated in the study. The surface electromyographic signals of masseter and anterior temporal muscles were recorded during 30-s test contractions at 50% of the maximum voluntary clenching effort. A force transducer placed between the central incisors recorded the maximum voluntary bite force. The patients exerted a lower maximum voluntary bite force than the controls (p less than 0.05). In the patients the electromyographic signals of the painful muscles were weaker than those of the painless muscles (p less than 0.01). The signals of the control subjects were stronger than those of the painless muscles of the patients (p less than 0.01). The mean power frequency of the signals, recorded at the start of the 30-s contractions, showed no differences between the painful and painless muscles and between the muscles of the patients and of the controls. The rate of mean power frequency shift in the electromyographic signal, as a response to the 30-s test contraction, was normalized with respect to the amplitude of that signal to account for its amplitude dependency. The painful muscles showed a higher normalized rate of mean power frequency shift than the painless muscles and the muscles of the control subjects (p less than 0.01). These results confirm the presence of an impaired condition in the painful muscles of this group of patients with myogenous craniomandibular disorders.

Recovery of the human biceps electromyogram after heavy eccentric, concentric or isometric exercise.

Five men performed submaximal isometric, concentric or eccentric contractions until exhaustion with the left arm elbow flexors at respectively 50%, 40% and 40% of the prefatigued maximal voluntary contraction force (MVC). Subsequently, and at regular intervals, the surface electromyogram (EMG) during 30-s isometric test contractions at 40% of the prefatigued MVC and the muscle performance parameters (MVC and the endurance time of an isometric endurance test at 40% prefatigued MVC) were recorded. Large differences in the surface EMG response were found after isometric or concentric exercise on the one hand and eccentric exercise on the other. Eccentric exercise evoked in two of the three EMG parameters [the EMG amplitude (root mean square) and the rate of shift of the EMG mean power frequency (MPF)] the greatest (P less than 0.001) and longest lasting (up to 7 days) response. The EMG response after isometric or concentric exercise was smaller and of shorter duration (1-2 days). The third EMG parameter, the initial MPF, had already returned to its prefatigued value at the time of the first measurement, 0.75 h after exercise. The responses of EMG amplitude and of rate of MPF shift were similar to the responses observed in the muscle performance parameters (MVC and the endurance time). Complaints of muscle soreness were most frequent and severe after the eccentric contractions. Thus, eccentric exercise evoked the greatest and longest lasting response both in the surface EMG signal and in the muscle performance parameters.

Recovery following exhaustive dynamic exercise in the human biceps muscle.

The aim of the study was to investigate the recovery of the maximum voluntary contraction force (MVC), the endurance time and electromyographical (EMG) parameters following exhaustive dynamic exercise of the m. biceps brachii. EMG recordings were made in ten healthy subjects using bipolar surface electrodes placed over the common belly of the left arm biceps muscle. Up to 25 h post-exercise, the maximum contraction force and the EMG signal were recorded alternately at regular intervals. The EMG signal was recorded during 30-s contractions at 40% of the pre-fatigued MVC. Four hours and 25 h post-exercise, the endurance time of a 40% pre-fatigued MVC was recorded. Up to 25 h after the exercise the maximum contraction force, the endurance time and the EMG parameters were significantly different from the pre-exercise values. Nine out of ten subjects complained that muscle soreness had developed. Thus, long-lasting changes are found after exhaustive dynamic exercise, not only in the MVC and the muscle's endurance capacity, but also in the EMG signal.

Electromyographic power-spectrum changes during repeated fatiguing contractions of the human masseter muscle.

During 7 sessions at weekly intervals, 10 healthy males performed the same endurance test three times by isometrically contracting the masseter muscles at 20, 30, 40, 50, 60, 75 or 90 per cent of maximal electromyographic activity. The interval between the tests in any one session was 30 s. Changes in the surface electromyogram were monitored by calculating the changes in mean power frequency (MPF) of the signals. With the higher contraction levels, endurance times were shorter, the MPF shifted to lower frequencies more rapidly and the MPF value was lower at the end of the test. At these levels, the subjects reported lack of power as the sole reason for stopping the test but at low levels, pain was the important reason. The endurance time and the rate of MPF shift for the second and third test of each experiment were the same. For the first test, the endurance time was longer (p less than 0.025) and the rate of MPF shift was lower (p less than 0.05). Thus the analysis of specific electromyographic (MPF) indicators provides information about the development of muscle fatigue.

The influence of stimulus type and stimulus strength on the silent period in human masticatory muscles.

The silent period in the jaw-closing muscles of nine healthy subjects was investigated. The stimulus types used were the open-close-clench movement (OCC), the mechanical chin tap and electrical skin stimulation. Irrespective of the jaw-closing speed for the OCC movements only the unmeasurable and single type silent periods were observed. Increasing the chin tap strength resulted in a decreasing percentage of single type silent periods and an increasing number of depressed, double and merged silent periods. The duration of the short and combined silent period was shorter for the OCC movements than for the chin taps. For the chin taps an increasing stimulus strength resulted in longer silent periods. Only the transition from hard to painful chin tap showed a slight, but significant decrease in short silent period duration. The beginning of the late inhibitory period, which could sometimes be evoked by electrical skin stimulation, coincided with the beginning of the second inhibitory phase of the chin-tap evoked double silent period. The long silent period as shown by one of the subjects cast doubt on the validity of the relationship between silent period duration and TMJ dysfunction.