Synchronization of motor units in human masseter during a prolonged isometric contraction

Muscle fatigue increases beta-band coherence between the firing times of simultaneously active motor units in the first dorsal interosseous muscle

Synchronization between the firing times of simultaneously active motor units (MUs) is generally assumed to increase during fatiguing contractions. To date, however, estimates of MU synchronization have relied on indirect measures, derived from surface electromyographic (EMG) interference signals. This study used intramuscular coherence to investigate the correlation between MU discharges in the first dorsal interosseous muscle during and immediately following a submaximal fatiguing contraction, and after rest. Coherence between composite MU spike trains, derived from decomposed surface EMG, were examined in the delta (1-4 Hz), alpha (8-12 Hz), beta (15-30 Hz), and gamma (30-60 Hz) frequency band ranges. A significant increase in MU coherence was observed in the delta, alpha, and beta frequency bands postfatigue. In addition, wavelet coherence revealed a tendency for delta-, alpha-, and beta-band coherence to increase during the fatiguing contraction, with subjects exhibiting low initial coherence values displaying the greatest relative increase. This was accompanied by an increase in MU short-term synchronization and a decline in mean firing rate of the majority of MUs detected during the sustained contraction. A model of the motoneuron pool and surface EMG was used to investigate factors influencing the coherence estimate. Simulation results indicated that changes in motoneuron inhibition and firing rates alone could not directly account for increased beta-band coherence postfatigue. The observed increase is, therefore, more likely to arise from an increase in the strength of correlated inputs to MUs as the muscle fatigues.

Synchronization of motor unit firings: an epiphenomenon of firing rate characteristics not common inputs

Synchronous motor unit firing instances have been attributed to anatomical inputs shared by motoneurons. Yet, there is a lack of empirical evidence confirming the notion that common inputs elicit synchronization under voluntary conditions. We tested this notion by measuring synchronization between motor unit action potential trains (MUAPTs) as their firing rates progressed within a contraction from a relatively low force level to a higher one. On average, the degree of synchronization decreased as the force increased. The common input notion provides no empirically supported explanation for the observed synchronization behavior. Therefore, we investigated a more probable explanation for synchronization. Our data set of 17,546 paired MUAPTs revealed that the degree of synchronization varies as a function of two characteristics of the motor unit firing rate: the similarity and the slope as a function of force. Both are measures of the excitation of the motoneurons. As the force generated by the muscle increases, the firing rate slope decreases, and the synchronization correspondingly decreases. Different muscles have motor units with different firing rate characteristics and display different amounts of synchronization. Although this association is not proof of causality, it consistently explains our observations and strongly suggests further investigation. So viewed, synchronization is likely an epiphenomenon, subject to countless unknown neural interactions. As such, synchronous firing instances may not be the product of a specific design and may not serve a specific physiological purpose. Our explanation for synchronization has the advantage of being supported by empirical evidence, whereas the common input does not.

Statistically rigorous calculations do not support common input and long-term synchronization of motor-unit firings

Over the past four decades, various methods have been implemented to measure synchronization of motor-unit firings. In this work, we provide evidence that prior reports of the existence of universal common inputs to all motoneurons and the presence of long-term synchronization are misleading, because they did not use sufficiently rigorous statistical tests to detect synchronization. We developed a statistically based method (SigMax) for computing synchronization and tested it with data from 17,736 motor-unit pairs containing 1,035,225 firing instances from the first dorsal interosseous and vastus lateralis muscles--a data set one order of magnitude greater than that reported in previous studies. Only firing data, obtained from surface electromyographic signal decomposition with >95% accuracy, were used in the study. The data were not subjectively selected in any manner. Because of the size of our data set and the statistical rigor inherent to SigMax, we have confidence that the synchronization values that we calculated provide an improved estimate of physiologically driven synchronization. Compared with three other commonly used techniques, ours revealed three types of discrepancies that result from failing to use sufficient statistical tests necessary to detect synchronization. 1) On average, the z-score method falsely detected synchronization at 16 separate latencies in each motor-unit pair. 2) The cumulative sum method missed one out of every four synchronization identifications found by SigMax. 3) The common input assumption method identified synchronization from 100% of motor-unit pairs studied. SigMax revealed that only 50% of motor-unit pairs actually manifested synchronization.

Interference of tonic muscle activity on the EEG: a single motor unit study

The electrical activity of muscles can interfere with the electroencephalogram (EEG) signal considering the anatomical locations of facial or masticatory muscles surrounding the skull. In this study, we evaluated the possible interference of the resting activity of the temporalis muscle on the EEG under conventional EEG recording conditions. In 9 healthy adults EEG activity from 19 scalp locations and single motor unit (SMU) activity from anterior temporalis muscle were recorded in three relaxed conditions; eyes open, eyes closed, jaw dropped. The EEG signal was spike triggered averaged (STA) using the action potentials of SMUs as triggers to evaluate their reflections at various EEG recording sites. Resting temporalis SMU activity generated prominent reflections with different amplitudes, reaching maxima in the proximity of the recorded SMU. Interference was also notable at the scalp sites that are relatively far from the recorded SMU and even at the contralateral locations. Considering the great number of SMUs in the head and neck muscles, prominent contamination from the activity of only a single MU should indicate the susceptibility of EEG to muscle activity artifacts even under the rest conditions. This study emphasizes the need for efficient artifact evaluation methods which can handle muscle interferences.

Distinguishing synchronous and time-varying synergies using point process interval statistics: motor primitives in frog and rat

We present and apply a method that uses point process statistics to discriminate the forms of synergies in motor pattern data, prior to explicit synergy extraction. The method uses electromyogram (EMG) pulse peak timing or onset timing. Peak timing is preferable in complex patterns where pulse onsets may be overlapping. An interval statistic derived from the point processes of EMG peak timings distinguishes time-varying synergies from synchronous synergies (SS). Model data shows that the statistic is robust for most conditions. Its application to both frog hindlimb EMG and rat locomotion hindlimb EMG show data from these preparations is clearly most consistent with synchronous synergy models (p < 0.001). Additional direct tests of pulse and interval relations in frog data further bolster the support for synchronous synergy mechanisms in these data. Our method and analyses support separated control of rhythm and pattern of motor primitives, with the low level execution primitives comprising pulsed SS in both frog and rat, and both episodic and rhythmic behaviors.

Activities of human genioglossus motor units

Upper airway muscles play an important role in regulating airway lumen and in increasing the ability of the pharynx to remain patent in the face of subatmospheric intraluminal pressures produced during inspiration. Due to the considerable technical challenges associated with recording from muscles of the upper airway, much of the experimental work conducted in human subjects has centered on recording respiratory-related activities of the extrinsic tongue protudor muscle, the genioglossus (GG). The GG is one of eight muscles that invest the human tongue (Abd-El-Malek, 1939). All eight muscles are innervated by the hypoglossal nerve (cranial nerve XII) the cell bodies of which are located in the hypoglossal motor nucleus (HMN) of the caudal medulla. Much of the earlier work on the respiratory-related activity of XII motoneurons was based on recordings obtained from single motor axons dissected from the whole XII nerve or from whole muscle GG EMG recordings. Detailed information regarding respiratory-related GG motor unit activities was lacking until as recently as 2006. This paper examines key findings that have emerged from the last decade of work conducted in human subjects. Wherever appropriate, these results are compared with results obtained from in vitro and in vivo studies conducted in non-human mammals. The review is written with the objective of facilitating some discussion and some new thoughts regarding future research directions. The material is framed around four topics: (a) motor unit type, (b) rate coding and recruitment, (c) motor unit activity patterns, and (d) a compartment based view of pharyngeal airway control.

Fatigue related changes in electromyographic coherence between synergistic hand muscles

The aim of this study was to examine coherence between surface electromyographic (EMG) signals from two index finger flexor muscles, the first dorsal interosseous (FDI) and flexor digitorum superficialis (FDS), during and immediately following sustained, fatiguing isometric contraction. Coherence was observed between the FDI and FDS EMG signals in the tremor (8-12 Hz), beta (15-35 Hz) and gamma (35-60 Hz) bands in all subjects. A significant increase in EMG-EMG coherence in the beta and gamma frequency bands was observed immediately following the fatiguing contraction. No significant difference was observed in the tremor band coherence before and after fatigue. Coherence was observed between EMG and force in the tremor band during both the pre- and post-fatigue contractions and a significant increase in the FDI EMG-force coherence post-fatigue was observed. It is suggested that the increase in beta and gamma band coherence with fatigue may be due to increased levels of corticomotoneuronal drive to both muscles. Alternatively, the increased EMG-EMG coherence may reflect an increased contribution of peripheral afferents to coupling across the muscle with fatigue. Although the functional significance is not clear, the increase in coherence may help to overcome reduced motoneuron excitability with fatigue, to bind together different sensorimotor elements or to coordinate force generation across muscles in a more synergistic manner as the force generating capacity of the muscle is decreased.

Influence of motor unit synchronization on amplitude characteristics of surface and intramuscularly recorded EMG signals

The increase in muscle strength without noticeable hypertrophic adaptations is very important in some sports. Motor unit (MU) synchronisation and higher rate of MU activation are proposed as possible mechanisms for such a strength and electromyogram (EMG) increase in the early phase of a training regimen. Root mean square and/or integrated EMG are amplitude measures commonly used to estimate the adaptive changes in efferent neural drive. EMG amplitude characteristics could change also because of alteration in intracellular action potential (IAP) spatial profile. We simulated MUs synchronization under different length of the IAP profile. Different synchronization was simulated by variation of the percent of discharges in a referent MU, to which a variable percent of remaining MUs was synchronized. Population synchrony index estimated the degree of MU synchronization in EMG signals. We demonstrate that the increase in amplitude characteristics due to MU synchronization is stronger in surface than in intramuscularly detected EMG signals. However, the effect of IAP profile lengthening on surface detected EMG signals could be much stronger than that of MU synchronization. Thus, changes in amplitude characteristics of surface detected EMG signals with progressive strength training could hardly be used as an indicator of changes in neural drive without testing possible changes in IAPs.

Motor unit control and force fluctuation during fatigue

During isometric contractions, the fluctuation of the force output of muscles increases as the muscle fatigues, and the contraction is sustained to exhaustion. We analyzed motor unit firing data from the vastus lateralis muscle to investigate which motor unit control parameters were associated with the increased force fluctuation. Subjects performed a sequence of isometric constant-force contractions sustained at 20% maximal force, each spaced by a 6-s rest period. The contractions were performed until the mean value of the force output could not be maintained at the desired level. Intramuscular EMG signals were detected with a quadrifilar fine-wire sensor. The EMG signals were decomposed to identify all of the firings of several motor units by using an artificial intelligence-based set of algorithms. We were able to follow the behavior of the same motor units as the endurance time progressed. The force output of the muscle was filtered to remove contributions from the tracking task. The coefficient of variation of the force was found to increase with endurance time (P < 0.001, R(2) = 0.51). We calculated the coefficient of variation of the firing rates, the synchronization of pairs of motor unit firings, the cross-correlation value of the firing rates of pairs of motor units, the cross-correlation of the firing rates of motor units and the force, and the number of motor units recruited during the contractions. Of these parameters, only the cross-correlation of the firing rates (P < 0.01, R(2) = 0.10) and the number of recruited motor units (P = 0.042, R(2) = 0.22) increased significantly with endurance time for grouped subjects. A significant increase (P < 0.001, R(2) = 0.16) in the cross-correlation of the firing rates and force was also observed. It is suggested that the increase in the cross-correlation of the firing rates is likely due to a decrease in the sensitivity of the proprioceptive feedback from the spindles.

Discharge rate during low-force isometric contractions influences motor unit coherence below 15 Hz but not motor unit synchronization

The purpose of the study was to determine whether pairs of motor units that discharge action potentials at different rates during isometric contractions exhibit different levels of motor unit synchronization or coherence. Twelve subjects (28.6 +/- 6.1 years) performed isometric contractions at target forces slightly above the recruitment threshold (1.02-20.9%) of an isolated motor unit. Based on audio feedback, subjects maintained a relatively constant discharge rate of the isolated unit for about 80 s. Intramuscular electrodes were used to record the discharge of 47 pairs of motor units at rates that ranged from 8.07 to 13.6 pps. Correlated discharge between pairs of motor units was quantified with the common input strength (CIS) index, k' index, and coherence spectrum. Greater discharge rates across pairs of motor units were predicted (R2 = 0.36, P < 0.001) by higher coherence from 8 to 13 Hz (r = -0.52) and lower coherence from 0 to 4 Hz (r = 0.37). Indexes of motor unit synchronization (CIS and k') were strongly associated with motor unit coherence from 16 to 32 Hz (CIS: R2 = 0.63; k': R2 = 0.4; P = 0.001). The CIS index of motor unit synchronization and the motor unit coherence from 16 to 32 Hz did not vary with discharge rate. In contrast, the k' index of motor unit synchronization declined with discharge rate (r2 = 0.20, P = 0.001). Furthermore, greater discharge rates across pairs of motor units were accompanied by higher motor unit coherence in the 8-13 Hz band and lower motor unit coherence in the 0-4 Hz band. These results demonstrate that differences in discharge rate between pairs of motor units in first dorsal interosseus during low-force, isometric contractions were associated with modulation of the correlation in the discharge times of the two motor units at frequencies less than 15 Hz.

Organisation of common inputs to motoneuron pools of human masticatory muscles

OBJECTIVE

To determine the pattern of organization of common inputs to the motoneuron pools of individual muscles in the masticatory system.

METHODS

Six subjects bit on a rubber-coated wooden splint placed between the upper and lower incisor teeth. We recorded the surface electromyogram (EMG) of co-contracting masseter, temporalis and digastric muscles bilaterally during isometric jaw closing at 5%, 10%, 20% and 40% of maximal voluntary masseter EMG.

RESULTS

The cross-correlograms of the EMGs of homologous muscle pairs indicate that there are common synaptic inputs to the motoneuron pools of the left and right masseter, and left and right digastric muscles, but not to left and right temporalis. The amplitude of the central peak in masseter and digastric correlograms increased with bite force. When the activity of ipsilateral muscle pairs was cross-correlated, central peaks were prominent for masseter-digastric and masseter-temporalis muscle pairs, and the peak amplitudes increased significantly with bite force. In contrast, no significant central peak was observed for temporalis-digastric muscle pairs at any level of voluntary biting.

CONCLUSIONS

We conclude that there is synchronous modulation of input bilaterally to the masseter muscles and to the digastric muscles but not to the temporalis muscles. There is synchronous modulation of input to ipsilateral masseter-digastric and masseter-temporalis muscle pairs but not to temporalis and digastric muscles.

SIGNIFICANCE

The extent of common input to motoneuron pools of muscles acting around a common joint varies for different muscle pairs, and is not simply a function of whether the muscles of the pair are synergists or antagonists.

Coherence at 16-32 Hz Can Be Caused by Short-Term Synchrony of Motor Units

Time- and frequency-domain measures of discharge times for pairs of motor units are used to infer the proportion of common synaptic input received by motor neurons. The physiological mechanisms that can produce the experimentally observed peaks in the cross-correlation histogram and the coherence spectrum are uncertain. The present study used a computational model to impose synchronization on the discharge times of motor units. Randomly selected discharge times of a unit that was being synchronized to a reference unit were aligned with some of the discharge times of the reference unit, provided the original discharge time was within 30 ms of the discharge by the reference unit. All time-domain measures (indexes CIS, E, and k′) were sensitive to changes in the level of imposed motor-unit synchronization ( P < 0.01). In addition, synchronization caused a peak between 16 and 32 Hz in the coherence spectrum. The shape of the cross-correlogram determined the frequency at which the peak occurred in the coherence spectrum. Further, the magnitude of the coherence peak was highly correlated with the time-domain measures of motor-unit synchronization ( r2 > 0.80), with the highest correlation occurring for index E ( r2 = 0.98). Thus the peak in the 16- to 32-Hz band of the coherence spectrum can be caused by the time that individual discharges are advanced or delayed to produce synchrony. Although the in vivo processes that adjust the timing of motor-unit discharges are not fully understood, these results suggest that they may not depend entirely on an oscillatory drive by the CNS.

Effects of experimental muscle pain on mechanical properties of single motor units in human masseter

OBJECTIVE

Muscle pain is known to influence muscle activity but the details of its effects on the mechanical properties of single motor units (SMU) have not been described. We have recently reported a decreased firing rate of SMU in the human masseter muscle during painful contractions with a constant force output. Force output can be modulated by the SMU discharge rate in relation to the contractile properties of SMU. Therefore, the objective of the present study was to measure the mechanical properties of SMU in the masseter to clarify the mechanism which underlies the decrease in SMU firing rate during jaw-muscle pain.

METHODS

A spike-triggered averaging (STA) technique was used to determine the mechanical properties of low-threshold SMU in the masseter muscle recorded with fine wire electrodes during a voluntary isometric contraction. The twitch amplitude, contraction time, and half-relaxation time were determined from the averaged force records before and during experimental jaw-muscle pain induced by injection of 0.2 ml (100 microg/ml) capsaicin in 8 healthy subjects. Injections of 0.2 ml isotonic saline served as a non-painful control in 11 healthy subjects.

RESULTS

The twitch amplitude was significantly increased during capsaicin-evoked muscle pain (P<0.001) without significant changes of half-relaxation time and contraction time. No significant changes in SMU twitch properties were observed during the control injections.

CONCLUSIONS

Potentiation of twitch force could be a possible compensatory mechanism to maintain a constant force output during painful isometric contractions when SMU firing decreases. This finding therefore provides new information on the adaptation of motor function by muscle pain.

Properties of human motor units after prolonged activity at a constant firing rate

The primary purpose of this study was to examine if there are changes in the intrinsic properties of spinal motoneurons after prolonged submaximal contractions. To do this, we assessed whether or not the synaptic drive to motoneurons needs to increase in order to maintain a constant firing rate of a motor unit. Recruitment of new units and an increase in total electromyographic (EMG) activity of the muscle of interest were taken as estimates of an increase in synaptic drive. Subjects were asked to maintain a constant firing rate of a clearly identifiable (targeted) motor unit from the first dorsal interosseous muscle for approximately 10 min, while surface EMG and force were recorded simultaneously. For the 60 units studied, the duration of the constant-firing-rate period ranged from 73 to 1,140 s (448 +/- 227 s; mean +/- SD). There was a significant increase ( t-test, p<0.001) in the magnitude of mean surface EMG, and DC force while the targeted motoneuron maintained a constant rate suggesting an increase in the net excitatory input to the motoneuron pool. Changes occurring simultaneously in other parameters, namely, variability in interspike interval, magnitude of force fluctuations, the duration of motor unit action potentials, and the median power frequency of surface EMG were also computed. The firing rates of 16 concurrently firing motoneurons, not controlled by the subject, remained constant. The key finding of this study is that after prolonged activity, a motoneuron requires a stronger excitatory input to maintain its firing rate. Additional results are indicative of significant changes in the characteristics of the synaptic inputs, changes at the neuromuscular junction (both pre- and postsynaptic regions) and the sarcolemma.

Factors affecting the stability of the spike-triggered averaged force in the human first dorsal interosseus muscle

The reproducibility of motor unit twitches obtained using spike-triggered averaging (STA) was examined in the human first dorsal interosseus. For each motor unit (30 total) a series of STA twitches was derived using a 30 s averaging window. Within each averaging window, eight independent measures characterizing motor unit discharge and whole muscle force properties were recorded. These included the mean and standard deviation (S.D.) of the interspike interval (ISI), the mean and S.D. of pre and post-trigger ISIs used in averaging, and mean and S.D. of whole muscle force. To determine the relative importance of the independent variables on twitch reproducibility, the variables were used in a multiple regression analysis performed on STA twitch peak force (PF), time to peak force (TTP) and time of half-force decay (HFD). It was found that PF was significantly correlated to the mean and S.D. of whole muscle force, and mean post-trigger ISI. TTP was significantly correlated to the S.D. of the post-trigger ISI and mean whole muscle force while HFD was related to the mean and S.D. of the pre-trigger ISI and the mean post-trigger ISI. It was concluded that by minimizing whole muscle force variability and the mean and S.D. of acceptable ISIs used in the STA process, the reproducibility of the STA twitch is improved.

Synchronization analysis using joint peri-stimulus time histograms for human motor units

The concurrence of action potentials from two motor units (MUs) happens more frequently in human than what one would expect by chance. It can be visualized as the central peak of the Cross-Correlogram of spike trains from two MUs. It is widely believed that a common pre-synaptic input to motoneurons from the cortical tract generates the concurrence. A number of synchronization indices have been proposed in order to evaluate the quantity of the common input, however, these indices are dependent on the firing rates of the MUs. To resolve this issue, we introduce a new synchronization index beta using a joint peri-stimulus time histograms (JPSTH). In this paper, we discuss the dependence of synchronization indices on firing rates using a spike model, and the application of JPSTH and the index beta to experimental data for the quantitative analysis of MU synchronization. We confirm that the novel index beta represents the synchronization state independently from firing rates, and that it also accurately traces event-related temporal modulation of synchronization. In conclusion, we consider the index beta to be a valuable index for the analysis of MU synchronization.

Electromyographic analysis of repeated bouts of eccentric exercise

The repeated bout effect refers to the protective effect provided by a single bout of eccentric exercise against muscle damage from a similar subsequent bout. The aim of this study was to determine if the repeated bout was associated with an increase in motor unit activation relative to force production, an increased recruitment of slow-twitch motor units or increased motor unit synchronization. Surface electromyographic (EMG) signals were recorded from the hamstring muscles during two bouts of submaximal isokinetic (2.6 rad x s(-1)) eccentric (11 men, 9 women) or concentric (6 men, 4 women) contractions separated by 2 weeks. The EMG per unit torque and median frequency were analysed. The initial bout of eccentric exercise resulted in strength loss, pain and muscle tenderness, while the repeated eccentric bout resulted in a slight increase in strength, no pain and no muscle tenderness (bout x time effects, P < 0.05). Strength, pain and tenderness were unaffected by either bout of concentric exercise. The EMG per unit torque and median frequency were not different between the initial and repeated bouts of eccentric exercise. The EMG per unit torque and median frequency increased during both bouts of eccentric exercise (P < 0.01) but did not change during either concentric bout. In conclusion, there was no evidence that the repeated bout effect was due to a neural adaptation.

A review of recent applications of cross-correlation methodologies to human motor unit recording

This article reviews some recent applications of time and frequency domain cross-correlation techniques to human motor unit recording. These techniques may be used to examine the pre-synaptic mechanisms involved in control of motoneuron activity during on-going motor tasks in man without the need for imposed and artificial perturbations of the system. In this review we examine, through several examples, areas in which insights have been gained into the basic neurophysiological processes that bring about motoneuron firing in man and illustrate how these processes are affected by central nervous system pathology. We will demonstrate that synchronization and coherence may be revealed between human motor unit discharges and give examples that support the hypothesis that these phenomena are generated by activity in a focused common corticospinal input to spinal motoneurons. Disruption of central motor pathways due to diseases of the nervous system leads to pathophysiological alterations in the activity of these pre-synaptic motoneuron inputs that can be revealed by cross-correlation analysis of motor unit discharges. The significance of these studies and outstanding questions in this field are discussed.

Correlated changes in the firing rate of human motor units during voluntary contraction

Spike trains of long duration were recorded from concurrently active pairs of motor units in the human masseter and tibialis anterior muscles. An innovative analysis technique was used to investigate functional coupling between the motoneurons by plotting the discharge frequency of one motor unit with respect to the firing times of the other (peri-spike frequencygram). Conventional cross-correlograms of discharge times were also constructed for each pair to detect synchronous firing and to compare them with the peri-spike frequencygrams (PSFs). The PSFs were examined with the hypothesis that, if the net common input of the two motoneurons was excitatory, the firing frequency of both units should increase around the time of the synchronous discharge of both units (i.e., time zero in the cross-correlogram). Conversely, if the net common input was inhibitory, the firing frequency of both units should fall around time zero. In 24 out of 37 masseter pairs tested, either one (n = 20) or both (n = 4) units of the pair displayed a statistically significant increase (P < 0.001) in the firing frequency around time zero of the PSF. No significant decrease in the discharge frequency was ever detected in any of the units of the 37 pairs tested. The probability of occurrence of a significant increase in firing rate was found to be significantly higher X2 = 5, P < 0.05) in the pairs with significant synchronous firing (13 out of 15) than in the pairs without (11 out of 22). Moreover, the percentage increase in the discharge frequency was found to be significantly higher in the pairs with significant synchronous activity (1.4%) than in those without (0.74%). In 24 out of 56 tibialis anterior pairs tested, either one (n = 19) or both (n = 5) units of the pairs displayed significant changes (P < 0.001) in discharge frequency around time zero of the PSF. A reduction in the mean firing rate starting about 20 ms before time zero and ending around time zero was most commonly detected in the PSF, the probability of this occurrence being significantly higher (X2 = 6.7, P < 0.01) in the pairs with significant synchronous discharges (18 out of 41) than in the pairs without (1 out of 15). However, the mean percentage fall in the firing frequency during this "inhibitory" phase did not correlate in amplitude with the presence or the absence of significant synchronous activity (-1.4% and -1.1% respectively). In many of the cases, this "inhibitory" period was followed by an "excitatory" period where the firing frequency displayed a significant increase (from about 6 ms to 20 ms past time zero). Such dual changes in the firing rate were only found in the pairs with significant synchronous activity (13 of the 41 pairs). The mean percentage increase in the firing frequency during the "excitatory" phase was significantly higher in the pairs with synchronous firing than in the pair without (1.4% and 0.5% respectively). The specificity of the frequency changes was assessed by performing control analyses where the discharge from one unit was cross-correlated with the discharge from the other unit at a different time (time-shuffled pairs). Synchronisation peaks were never observed in the cross-correlograms of the time-shuffled pairs. Furthermore, none of the 18 time-shuffled pairs in the masseter showed significant frequency changes in the same time period as that seen with the non-shuffled pairs. In the tibialis, only 1 out of 31 time-shuffled pairs showed a significant frequency change in one motor unit, and that was in the opposite direction to the frequency changes found in the non-shuffled pairs. It is suggested that the PSF analysis is an important tool for investigating the functional relationship between individual motoneurons as they fire voluntarily.

Physiology and interpretation of the electromyogram

The purpose of this review is to consider some issues in the interpretation of the electromyogram (EMG) and to discuss current areas of controversy regarding use of the EMG. We consider the underlying physiology and origin of the EMG signal and offer an abbreviated discussion of measurement issues and selected factors that affect the characteristics of the EMG signal. We discuss many of the problems affecting interpretation, including normalization, crosstalk, and issues specific to contraction. In the final section, we consider topics of current interest in electromyography, such as muscle fatigue, task specificity, multichannel representations, and muscle fiber conduction velocity. We present, in addition, alternative analysis techniques. This review should interest researchers and clinicians who seek to obtain the valuable information inherent in the EMG while respecting the potential sources of variance and misinterpretation.

Limitations of the surface electromyography technique for estimating motor unit synchronization

Motor unit synchronization was estimated from the surface electromyograms (EMG) of the first dorsal interosseus muscle of human volunteers by a simplified surface-EMG technique (Milner-Brown et al. 1973, 1975). Single motor units were identified from intramuscular recordings and were used to obtain a spike-triggered average of the surface-EMG. The discharge rate of a reference motor unit was controlled at two levels (high and low), and the effect of motor unit activity on the surface-EMG estimate of synchronization was studied in 56 motor units. The surface-EMG estimate of motor unit synchronization was significantly higher when the reference motor unit discharged at the high rate than when it discharged at the low rate. A regression analysis indicated that the synchronization ratio calculated from the surface EMG was significantly correlated with the level of EMG activity in the muscle. Motor unit synchronization was also estimated from surface-EMG measurements that were derived by computer simulation. The simulation permitted manipulation of motor unit activity (discharge rate and recruitment) with a complete absence of synchrony among the units in the pool. The stimulated surface-EMG index was influenced by an artifact associated with signal rectification, and this effect changed non-monotonically with motor unit activity. Furthermore, the increase in the motor unit activity reduced the signal-to-noise ratio of the spike-triggered surface EMG average, and consequently decreased the sensitivity of the surface-EMG index as an estimate of motor unit synchronization. We conclude that the simplified surface-EMG method (Milner-Brown et al. 1973, 1975) does not provide a useful index of motor unit synchronization due to its inability to accurately distinguish the synchronization from methodological effects related to a rectification artifact and variation in the signal-to-noise ratio.

Human spinal lateralization assessed from motoneurone synchronization: dependence on handedness and motor unit type

1. Motoneurone synchronization as a means of investigating synaptic connectivity was studied in the extensor carpi radialis muscles of the preferred and non-preferred arms of healthy right- and left-handed human subjects. The activities of pairs of motor units recorded during voluntary isometric contractions were analysed by cross-correlation to detect any synchronous motor unit firing in the form of central peaks in the cross-correlation histograms. 2. The synchronization peaks were compared first in the case of 273 motor unit pairs tested in the preferred and non-preferred arms of two left-handed subjects and two right-handed subjects. The percentage of synchronized motor unit pairs was found to be significantly higher in the preferred arm with synchronization peaks significantly larger and broader than in the non-preferred arm. The narrow peaks (< 7.5 ms) likely to reflect the activity of common inputs to motoneurones were also found to be significantly larger in the preferred arm of all four subjects. 3. The handedness-related differences in synchronization were definitely confirmed in a total of 275 pairs of motor units tested in the left extensor carpi radialis muscles of fourteen right-handed subjects using their non-preferred arm and six left-handed subjects using their preferred arm. In order to determine whether the differences in synchronization were dependent on the motor unit type, each motor unit was characterized on the basis of its recruitment threshold and on the basis of the contraction time of its twitch extracted from the overall muscle force using the spike-triggered averaging method. Two populations of motor units were distinguished, namely the 'slow' motor units (recruitment thresholds < 0.4 N, contraction times > 40 ms) and the 'fast' motor units (recruitment thresholds > 0.6 N, contraction times < or = 40 ms). 4. In the non-preferred arm, the synchronization peaks were always fairly narrow, whatever the motor unit's biomechanical properties; whereas in the preferred arm, broad peaks were found to be particularly common among the pairs including one or two fast motor units, which also showed the largest rate of synchronization occurrence. 5. The narrow peaks (< 7.5 ms) were found to be consistently larger in the preferred than the non-preferred arm whatever the categories of motor unit pairs. In both arms, however, the amplitude of the narrow peaks tended to increase as the recruitment threshold of the motor unit decreased and as their contraction time increased.(ABSTRACT TRUNCATED AT 400 WORDS)

The control of human motor units

1. The motor unit, consisting of a single motor neuron and the skeletal muscle fibres that it innervates, is the final output pathway of the motor system. 2. Much is now known about the way that human motor neurons are recruited and controlled during voluntary and reflex movements. This review briefly summarizes some of the recent experimental data that has contributed to our present understanding. The review is largely limited to data obtained in human experiments. While much of what we know about the organization of the nervous system has come from studies of the anatomy and physiology of experimental animals, there are some questions that cannot be addressed in reduced animal preparations. The development of new techniques has made it possible to investigate the human nervous system at a level of detail that has not hitherto been possible.

Rate modulation of jaw-elevator motor units as revealed from the low-frequency power spectrum of the surface electromyogram in myogenous CMD patients

The firing pattern of the motor units (MUs) in jaw-elevator muscles was studied within a wide range of isometric contraction levels by means of changes in the frequency and broadness of the primary peak in the low-frequency (5-40 Hz) power spectral density function of the surface EMG. EMG was recorded from both masseter and anterior temporal muscles in 11 myogenous CMD patients as well as in 11 gender- and age-matched controls who clenched in intercuspal occlusion under the control of visual feedback at various levels (0.5-67% MVC for the various muscles studied). The EMG was digitized for 12 periods of 1.6 s per condition; the power spectrum was averaged and smoothed for the various clenching levels. Linear regression analysis showed that the positive slope in the peak frequency (PF)/% MVC relationship, a measure of rate modulation of the MUs, did not differ significantly between patients and controls. At a low clenching level, PF was smaller (p < 0.01) for the anterior temporal muscles of the patients, suggesting lower firing rates for a wide range of clenching levels of the patients because of a similar rate modulation for patients and controls. Furthermore, the variance in the slope values was larger (p < 0.05) for the masseteric muscles of the patients, which may be explained by more heterogeneity of the masseteric rate modulation in the patient group. The broadness of the primary peak was smallest at a low clenching level (p < 0.001) for the anterior temporal muscles of the patients, suggesting a more uniform firing rate or more synchronization between MUs.

Short-term synchronization of motor units in human extensor digitorum communis muscle: relation to contractile properties and voluntary control

Synchronous activity was studied in relation to the contractile properties of pairs of motor units (MUs) recorded with independent microelectrodes in the right extensor digitorum communis muscle (EDC) of human subjects during isometric finger extension. MU contractile properties were characterized in terms of the rise time and amplitude of twitch tensions extracted by spike-triggered averages of the extension force. Synchronization of MU discharges appeared in the form of narrow central peaks in the cross-correlograms of 35 of 50 pairs of MUs, suggesting the contribution of common last-order neurons. Synchronization peaks tended to be briefer and higher among pairs of MUs with slower and smaller twitches than among pairs of MUs with faster and larger twitches. The higher peaks of slow-contracting MUs suggest a greater effectiveness of the common synaptic inputs. The broader peaks of fast-contracting MUs might reflect an additional synchronization of the inputs to fast MUs at high force levels. The areas of the cross-correlogram peaks were similar for both groups and suggest that under our conditions, about three motoneurons would discharge synchronously for a given motoneuron spike. To test whether the amount of MU synchronization could be altered voluntarily, four subjects attempted to increase or decrease synchrony, using as feedback clicks triggered by coincident firings of the recorded MUs. In nine of 15 conditioning sessions, the magnitudes of the synchronization peaks showed significant changes in the intended direction. These results imply that supraspinal centers can control the relative amount of inputs that contribute to the synchronization of motoneuron discharges during voluntary contraction of EDC.

Estimating the strength of common input to human motoneurons from the cross-correlogram

1. The relationship between the motor unit discharge pattern (rate and variability) and synchronization of motor unit pairs was studied in the first dorsal interosseus muscle of human subjects. In separate trials of up to 4 min duration, subjects voluntarily controlled the mean discharge rate of an identified motor unit at one of several prescribed rates (range 7.5-17.5 Hz). 2. The effect of discharge rate on the synchronous peak in the cross-correlogram was examined in eighty motor unit pairs from six subjects. Five commonly used synchronization indices were used to quantify synchrony in the cross-correlograms constructed from different discharge-rate trials. For each synchronization index, the apparent magnitude of synchrony increased at lower motor unit discharge rates. The synchronization indices were not equally sensitive to discharge rate; increases in the different indices ranged from 72 to 494% between the highest and lowest discharge rates. 3. A model of the membrane potential trajectory underlying rhythmic motoneuron discharge was used to explain the observed increase in the magnitude of the synchronization indices at lower discharge rates. The essential feature of this model is that the probability of a common-input EPSP causing a synchronous discharge in two motoneurons is independent of discharge rate. This means that the number of synchronous action potentials in excess of chance in any trial depends on the properties of the common-input EPSPs and the duration of the trial, but is not related to motor unit discharge rates. The model also demonstrated that when the excess synchronous counts are normalized to motor unit discharge rate, or baseline counts in the histogram (as in the conventional synchronization indices), the magnitude of the index increases when the motor unit discharge rates are low. 4. The strength of common input to motoneurons could be misinterpreted if conventional synchronization indices are used because of discharge-rate effects. The model was used to derive an index of the strength of common input to motoneurons (CIS) that was independent of motor unit discharge rate. CIS is the frequency of synchronous action potentials in the motor unit pair in excess of those expected due to chance (calculated during periods of tonic discharge in both units). The mean CIS in first dorsal interosseus motor unit pairs ranged from 0.052 to 1.005 extra synchronous action potentials per second across subjects. 5. Discharge variability was correlated with each of the synchronization indices and the CIS.(ABSTRACT TRUNCATED AT 400 WORDS)

Instability of motor unit firing rates during prolonged isometric contractions in human masseter

The firing patterns of up to 4 concurrently active masseter motor units were studied with intramuscular electrodes during a continuous isometric contraction of 15 min duration, in which the subject maintained the mean firing rate of one selected unit at 10 Hz. With this paradigm the net excitation (i.e. mean firing rate) of one unit in the muscle was controlled. This served as the reference for the functional state of other active units during the prolonged contraction. With the mean firing rate of one unit in the muscle fixed, 58% of other active units showed a slow, statistically-significant change in mean firing rate over the 15 min. The initial firing rate of the units did not influence the change in rate. The original firing rate hierarchy, which in short-term contractions reflects the recruitment order, was altered during the prolonged contraction. The explanation for these differential changes in motoneuron net excitation is not clear; they could be intrinsic to the motoneurons or perhaps mediated by reflex pathways. The selective facilitation or suppression of some motor units with continuous activation means that the original size-structured combination of motor units can be modified during a prolonged contraction.

Discharge variability and physiological properties of human masseter motor units

Intramuscular electrodes were used to study discharge variability in motor units of human masseter whose physiological properties were determined using spike-triggered averaging. Subjects voluntarily controlled the mean firing rate of a selected motor unit at 10 Hz for 15 min of continuous activation. Discharge variability was assessed at the beginning and end of this period. In 81% of units, the discharge variability at a mean interspike interval (ISI) of 100 ms increased after 15 min of continuous activity. There was a wide range of discharge variability within the population of masseter units studied, but no significant correlations were found between initial discharge variability and recruitment threshold, twitch tension or time-to-peak tension (TTP). There was, however, a significant correlation between motor unit fatigability and its initial discharge variability. This represents a link between the motoneuron and the functional properties of the muscle fibers in innervates.