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Effect of Sustained Contraction on Motor Unit Action Potentials and EMG Power Spectrum of Human Masticatory Muscles
1 Department of Oral Medicine and Diagnosis, College of Dentistry, Kangnung National University, Korea; and Correspondence: * corresponding author, wdmccall{at}buffalo.edu
The sequelae of a sustained clench are incompletely understood. Our experimental questions were to compare the responses of men and women, to compare masseter and anterior temporalis muscles, and to test hypotheses for the reduction of the median frequency of power spectra. We recorded duration, amplitude, number of phases, and area of the motor unit action potential before and after a sustained clench and the median frequency of the electromyographic power spectrum in 41 subjects. After the clench, the median frequency was lower, the action potential duration longer, the number of phases increased, and the area larger, but the amplitude was not different. Males and females failed to differ. Compared with the temporalis, the masseter had a lower median frequency, longer duration, larger number of phases, and increased area. Our results are consistent with a decrease in the conduction velocity of the muscle action potential as an explanation of the spectral shift.
Key Words: fatigue • duration • amplitude • phases • area
Localized muscle fatigue has been defined as a failure to maintain the expected force (Clark et al., 1984). Fatigue can be induced by several causes, among which is sustained clench (Christensen and Mohamed, 1984). A sustained clench has several sequelae, including muscle pain (Laskin, 1969; Katz et al., 1989; Plesh et al., 1995), and is not completely understood. The evidence on whether men and women respond differently to a sustained jaw clench is sparse (Lee et al., 1998). There is evidence that pain, not neuromuscular failure, ends a sustained clench (Clark et al., 1984; Clark and Carter, 1985; Plesh et al., 1998); that women report more TMD muscle pain than do men (Dworkin et al., 1990); and that women react differently to kappa opioids (Gear et al., 1996). So, one aim of this study was to compare the responses of men and women to a sustained clench. The median frequency of the electromyographic (EMG) power spectrum from the masseter muscle is lower than that from the temporalis muscle (Palla and Ash, 1981; van Boxtel et al., 1983; Lee et al., 1998), which is thought to arise from different filtering characteristics of the tissue between the muscle fibers and the surface electrodes (Lund and Widmer, 1989; Kamen and Caldwell, 1996). This hypothesis assumes that the underlying motor unit action potential parameters do not differ in the two muscles, and so a second aim was to compare the responses of the masseter and temporalis muscles to a sustained clench. The median frequency moves to a lower frequency during sustained masticatory muscle contraction (Palla and Ash, 1981; van Boxtel et al., 1983), and hypotheses to explain this shift include reduction of muscle action potential conduction velocity, recruitment of larger motor units, and increased synchronization of motor units (Palla and Ash, 1981; Lindstrom and Hellsing, 1983; De Luca, 1984). A decreased action potential conduction velocity predicts an increased motor unit action potential duration with no amplitude change after a sustained clench. On the other hand, recruiting larger motor units would predict larger motor unit action potential amplitudes after a sustained clench. So, the third aim of our study was to investigate the motor unit action potential before and after a sustained clench. To investigate these issues, we recorded 4 parameters of motor unit action potential before and after a sustained clench, and surface EMG power spectra during the clench.
Subjects Twenty-nine men (26.6 ± 2.1 yrs) and 12 women (23.6 ± 2.5 yrs), without history or symptoms of pain in the temporomandibular joints or masticatory muscles, participated. The project was approved by an institutional review board, and each subject gave informed consent. A pilot study did not find significant differences between the right and left sides, so data were obtained from the right muscles only.
Protocol
Motor Unit Action Potentials At least 20 motor unit action potentials are recommended for an adequate survey (Kimura, 1989; Engstrom and Olney, 1992; Stålberg and Falck, 1997), and since the needle electrode records only from a restricted area of the muscle, needle repositioning in small steps is necessary. Exploration in various directions from a single insertion minimized the subject’s discomfort. The motor unit action potentials were recorded with a Nicolet Viking IV electrodiagnostic system (Nicolet Biomedical, Madison, WI, USA) at a sensitivity of 100 to 200 µV per division. The low-pass and high-pass filters were set at 10 kHz and 2 Hz. The motor unit action potential features were quantified with the use of the computer-assisted program contained in the Nicolet Viking IV. The motor unit action potentials selected for assessment required a rise-time less than 0.5 ms. A short motor unit action potential rise-time indicates that the recording electrode is very close to at least one muscle fiber of the motor unit, which ensures that the recording electrode is within the motor unit territory (Kimura, 1989; Stålberg et al., 1996).
The Fig.
Power Spectral Analyses Surface EMG signals for the power spectral analysis were obtained with surface electrodes connected to the Nicolet system. During the sustained isometric contraction, EMG activity from the anterior temporalis or masseter muscle was recorded for post-clench analysis. Fifteen median frequencies were calculated automatically by the electrodiagnostic system during the contraction. The first and last (start and end of the sustained contraction) median frequencies were analyzed.
Statistical Analyses
Descriptive results are given in Table 1  , and analysis of variance results are given in Table 2.
Motor Unit Action Potentials The duration showed a significant interaction among sex, muscle, and time (Table 2  ). Comparison of pairs of cell means (t tests and paired t tests) with a Bonferroni correction for the 28 tests revealed that only the largest cell (women, masseter, post-contraction) differed from the smallest cell (men, anterior temporalis, pre-contraction). This was not predicted, does not have a ready interpretation, and does not seem to contaminate the main effects. The main effect for muscle was that the masseter durations were larger than the anterior temporalis durations (Tables 1, 2). The main effect for time was that the durations after the sustained clench were longer than the durations before the sustained clench. For amplitude, none of the tests was significant.
For the number of phases, none of the interaction terms reached statistical significance. The main effect for time was that the number of phases was increased after the sustained clench. The main effect for muscle was that the number of phases of the masseter was more than that of the temporalis muscle (Tables 1, 2
For the area, none of the interaction terms reached statistical significance. The area (Tables 1, 2
Median Frequencies of Power Spectra
Our main findings were: (1) The median frequency was decreased at the end of the clench compared with the beginning, as predicted; (2) motor unit action potential duration was longer, the number of phases increased, the area was larger, but the amplitude was not significantly different after the sustained contraction; (3) there were no significant differences between men and women; and (4) compared with the anterior temporalis, the masseter motor unit action potential had a lower median frequency, and longer duration, larger number of phases, and greater area.
Differences between "Before" and "After" Sustained Contraction
Recruitment of new motor units as smaller motor units cease firing could increase the average duration of the motor unit action potentials and therefore produce the power spectral shift to lower frequency range (Palla and Ash, 1981; Hägg, 1991). This larger-motor-unit hypothesis also predicts larger motor unit action potential amplitudes. However, the observed increase of the motor unit action potential amplitude was not significant in our study. From our data, we estimate that a sample size of 113 would be required to make this effect statistically significant, with Synchronization of motor units is a potential mechanism causing the power spectrum to shift to lower frequencies (Kamen and Caldwell, 1996). Increased synchronization of motor units after the sustained contraction might be expected to be associated with increased duration, increased amplitude, and increased number of phases. In our data, the duration and number of phases increased significantly, and the amplitude increased but not significantly. Thus, our data are not inconsistent with this hypothesis. Decrease of action potential conduction velocity along the muscle fiber is thought to be a main cause of the shift in the median frequency (Palla and Ash, 1981). A lowering of the intracellular pH, caused by production of acid metabolites, results in a lowering of the excitability of the membrane (Lindstrom and Hellsing, 1983), so a decrease in the propagation velocity of action potentials and consequent spectral changes might develop. However, Plesh et al. (1995) found no specific metabolic changes associated with exertion from exercise for the masseter muscle and only small pH changes. In this study, there was an increase of motor unit action potential duration after sustained contraction, and we suggest that this increase in motor unit action potential duration resulted from decreased muscle fiber conduction velocity after muscular fatigue. Previous studies also report prolonged duration of action potentials after muscular fatigue (Sandercock et al., 1985; Celichowski et al., 1991; Enoka et al., 1992). Those studies also suggested that an increased duration of action potential was related to a decreased muscle fiber conduction velocity after muscular fatigue. Our results are consistent with a decrease in the conduction velocity of the muscle action potential as an explanation of the spectral shift in the surface EMG during sustained contraction of masticatory muscles. The increase in area after sustained contraction found in this study may follow from the significant increase in duration and the non-significant increase in amplitude.
Differences between Men and Women
Differences between Muscles We speculate that the greater duration and area of the motor unit action potential in the masseter muscle compared with the anterior temporalis muscle may contribute in part to the lower median frequency in the masseter muscle.
This study was supported in part by the Postdoctoral Fellowship Program of Korea Science & Engineering Foundation (KOSEF). A preliminary report was presented at the Society for Neuroscience Annual Meeting, abstract 168.15, 2001. Received for publication December 21, 2001. Revision received June 28, 2002. Accepted for publication July 8, 2002.
Journal of Dental Research, Vol. 81, No. 9,
646-649 (2002) This article has been cited by other articles:
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ABSTRACT
INTRODUCTION
= 0.05 and β = 0.80, and, conversely, the estimated power of the present data is only 24%. While not conclusive, the lack of a strong amplitude effect would seem to mitigate against the larger-motor-unit hypothesis. 
