The impulse activity of the motor units of the human soleus muscle in the tonic vibration reflex

The tonic vibration reflex was evoked in the human soleus muscle, the range of vibration frequencies was 30-180 Hz. The potentials of single motor units (MUs) were picked up. At low frequencies (to 70-80 Hz) the potentials of all MUs under study correlated with vibration stimuli. This showed the existence of ripples in the excitatory inflow to motoneurons. With an increase of the vibration frequency the correlation disappeared. In the voluntarily contracted muscle the correlation was revealed at higher vibration frequencies than in the relaxed muscle. Since the vibration is known to depress monosynaptic reflexes from primary spindle endings, it is suggested that the correlation of motoneuron discharges and vibration stimuli at low vibration frequency is, apparently, a result of the activation of secondary spindle endings which via short synaptic pathways evoked discrete EPSPs in motoneurons.     

Information flow from the sensorimotor cortex to muscle in humans.

OBJECTIVES: To investigate the physiologic mechanism of human electroencephalogram-electromyogram (EEG-EMG) coherence, the directed transfer function (DTF) based on a multivariate autoregressive (MVAR) model was computed. METHODS: Fifty-six channel EEG and EMG of the right abductor pollicis brevis muscle during a weak tonic contraction were recorded in 6 normal volunteers. The EEG over the left sensorimotor area and the rectified EMG were used to compute coherence and DTF. RESULTS: EEG-EMG coherence was observed at the peak frequency of 15-29 Hz (mean 18.5 Hz). The peak frequency of DTF from EEG to EMG was 12-27 Hz (mean 17.8 Hz). DTF from EEG to EMG was significantly larger than that from EMG to EEG at 19-30 and 45-50 Hz (P<0.05). CONCLUSIONS: The present findings suggest that the EEG-EMG coupling mechanism for the 19 Hz or higher frequency might differ from that for the lower frequency. Directional information flow from EEG to EMG in the former frequency range likely reflects the motor control command. The finding of the directional information flow from EEG to EMG within the gamma band indicates that 40 Hz EEG-EMG coherence is not specific to the muscle Piper rhythm which is seen only with strong contraction.     

EMG contamination of EEG: spectral and topographical characteristics.

OBJECTIVE: Electromyogram (EMG) contamination is often a problem in electroencephalogram (EEG) recording, particularly, for those applications such as EEG-based brain-computer interfaces that rely on automated measurements of EEG features. As an essential prelude to developing methods for recognizing and eliminating EMG contamination of EEG, this study defines the spectral and topographical characteristics of frontalis and temporalis muscle EMG over the entire scalp. It describes both average data and the range of individual differences. METHODS: In 25 healthy adults, signals from 64 scalp and 4 facial locations were recorded during relaxation and during defined (15, 30, or 70% of maximum) contractions of frontalis or temporalis muscles. RESULTS: In the average data, EMG had a broad frequency distribution from 0 to >200 Hz. Amplitude was greatest at 20-30 Hz frontally and 40-80 Hz temporally. Temporalis spectra also showed a smaller peak around 20 Hz. These spectral components attenuated and broadened centrally. Even with weak (15%) contraction, EMG was detectable (P<0.001) near the vertex at frequencies >12 Hz in the average data and >8 Hz in some individuals. CONCLUSIONS: Frontalis or temporalis muscle EMG recorded from the scalp has spectral and topographical features that vary substantially across individuals. EMG spectra often have peaks in the beta frequency range that resemble EEG beta peaks. SIGNIFICANCE: While EMG contamination is greatest at the periphery of the scalp near the active muscles, even weak contractions can produce EMG that obscures or mimics EEG alpha, mu, or beta rhythms over the entire scalp. Recognition and elimination of this contamination is likely to require recording from an appropriate set of peripheral scalp locations.     

Discharge pattern of single motor units in the tonic vibration reflex of human triceps surae.

Using a single fibre EMG electrode the firing pattern of 46 motor units in the triceps surae has been studied during vibration of the Achilles tendon at frequencies of 25--200 Hz. Potentials activated in the tonic vibration reflex (TVR) were phase-locked to the vibration cycle but tended to become somewhat less so with continued vibration. The firing pattern of voluntarily activated motor units became locked to the waveform by the application of the vibrator. The discharges of 21 motor units were studied during low threshold (sub-M wave) tetanic stimulation of the tibial nerve at 25--100 Hz. No evidence was found of synchronization of potentials activated in the resulting tonic contraction. During weak voluntary contractions, stimulation also failed to regularize voluntarily activated motor units. The findings can be reconciled by postulating that, in normal man, vibration activates monosynaptic and polysynaptic pathways, the latter circuit being adequate to generate reflex contraction, while the former merely affects the temporal patterning of the motor outflow.     

Characteristics of physiologic tremor in young and elderly adults.

OBJECTIVE: To determine the prevalence of tremor-related motor-unit entrainment in young and elderly adults. METHODS: Postural hand tremor in neurologically healthy people, age ranges 20-42 (59 women and 41 men) and 70-92 (50 women and 50 men) years, was studied with accelerometry and forearm electromyogram (EMG). Tremor was recorded with and without a 300 g load distributed over the distal half of the horizontally extended hand and was analyzed with Fourier spectral techniques. RESULTS: No tremor-related spectral peak was found in the EMG of 59 young and 65 elderly controls, and inconsistent EMG peaks were observed in 29 young and 21 elderly. Twelve young and 14 elderly people exhibited a well-defined tremor-coherent EMG peak with and without 300 g loading, and the frequency of the EMG peak decreased less than 1 Hz in 8 young and 7 elderly. The EMG peak frequency was 9-12 Hz during mass loading in all 8 young adults but in only two elderly adults. The other 5 elderly people had peak frequencies at 5-7 Hz. Age had no significant effect on the frequency and amplitude of the mechanical-resonant component of hand tremor. CONCLUSIONS: Approximately 8% of young and elderly adult controls have an EMG-acceleration pattern that is indistinguishable from mild essential tremor. SIGNIFICANCE: These results provide a framework for the interpretation of electrophysiologic studies in patients with suspected essential tremor.     

Effect of vibration of the ankle stretch reflex in man

Vibration at frequencies above 50 Hz applied to the tendon of the extensor muscles of the ankle joint produce the tonic vibration reflex (TVR) which increases when the vibration frequency is increased. The TVR affects a joint's mechanical response to sinusoidal and random oscillations in a manner similar to that seen with tonic voluntary contraction. Although the myotatic reflex is suppressed by vibration, repeated stretches of sinusoidal oscillation produce an average EMG response which is not different in magnitude from the no vibration case. Either polysynaptic mechanisms at the spinal cord level of mechanisms involving higher centers (and possibly both) are able to overcome the inhibitory mechanisms at the Ia-alpha motoneuron level in producing a stretch evoked resonance near 6 Hz. The degree of inhibition of the myotatic component of the stretch reflex is proportional to the vibration frequency. This is in contrast to the facilitation of the myotatic reflex produced by tonic voluntary contraction. Vibration does not seem to influence the post-myotatic component (> 100 msec) of the stretch reflex. These results indicate that the post-myotatic responses to limb perturbation are not only different in their latency but also in their functional dependence upon peripheral influences.     

Levodopa reversible loss of the Piper frequency oscillation component in Parkinson's disease

OBJECTIVES: Although Parkinson's disease is typically characterised by bradykinesia, rigidity, and rest tremor, the possibility that two additional motor deficits are manifest during small hand muscle activity was explored-namely, weakness and abnormal physiological tremor. METHODS: A paradigm previously used in normal subjects reliably records the strength, tremor and surface EMG of index finger abducting contractions against a compliant (elastic) resistance. In addition to the well known physiological tremor at around 10 Hz, there are other co existing peak tremor frequencies at around 20 and 40 Hz; the last of these frequencies corresponds to the range of EMG Piper rhythm. The same technique was used to study parkinsonian patients while on and off dopaminergic medication. RESULTS: The maximum strength of finger abduction produced by first dorsal interosseous contraction was considerably lower when patients were off medication (mean (SD) 6.27 (1.49) N when off v 12.33 (3.64) N when on). There was also a marked reduction in the power of Piper frequency finger tremor (p<0.0005) and EMG (p<0.0005) oscillations that did not simply result from weaker contraction. CONCLUSION: As the components of physiological tremor at higher frequencies are thought to derive from CNS oscillations important in motor control, their loss in parkinsonism in association with severe off symptoms may represent an important pathophysiological link between dopaminergic depletion and parkinsonian motor deficits.     

Exaggerated 16-20 Hz motor cortical oscillation in patients with positive or negative myoclonus.

BACKGROUND: Muscle jerks and their cortical correlates usually occur abruptly and arrhythmically in patients with positive or negative myoclonus. However, there have been several reports in which oscillatory pre-myoclonic cortical discharges occur in patients with repetitive myoclonus (Mov Disord 6 (1994) 633; Brain 119 (1996) 1307; Brain 124 (2001) 2459; Brain 126 (2003) 326). In the present paper, we describe 4 patients with non-repetitive positive or negative myoclonus whose jerks were preceded by 16-20 Hz oscillatory EEG potentials over the contralateral sensorimotor cortex. METHODS: Jerk-locked averaging (JLA) was performed in 3 patients with positive myoclonus two of which had benign familial myoclonus epilepsy whilst the other had tuberculous meningitis. Silent period locked averaging (SPLA) was performed in a patient with hepatic encephalopathy and asterixis. RESULTS: In the 3 patients with positive myoclonus, JLA revealed a sequence of 20 Hz EEG potentials preceding the myoclonus. The positive peak of the last oscillation preceded the onset of myoclonus by the cortico-muscular latency. In the patient with hepatic encephalopahty, SPLA showed that 16 Hz EEG oscillations over the contralateral motor cortex were associated with small EMG oscillations, and that the largest EEG wave of the oscillation preceded the onset of a large EMG discharge just prior to the EMG silence. These oscillatory activities are similar in frequency to the motor cortical oscillations seen in monkeys and humans during voluntary contraction. CONCLUSIONS: Abnormally enhanced rhythmic (16-20 Hz) activities in the motor cortex are associated with arrhythmic positive or negative myoclonus. The rhythmic activities may also be responsible for the generation of the tremulousness (or mini-asterixis) in metabolic encephalopahty.     

Electrocorticogram-electromyogram coherence during isometric contraction of hand muscle in human.

OBJECTIVE: To clarify how the primary sensorimotor and supplementary motor areas are involved in the generation of the rhythmicity of electromyogram (EMG) activity during continuous muscle contraction. METHOD: We analyzed the coherence between subdurally recorded cortical electroencephalograms (EEG) and EMGs of the contralateral wrist extensor muscle during continuous isometric contraction in 8 patients with medically intractable epilepsy. RESULTS: In all subjects, a significant coherence between the primary motor area (M1) and EMG was observed at the peak frequency of 15+/-3 Hz (means+/-SD). In the primary somatosensory area (S1) of 7 subjects and the supplementary motor area proper (SMA proper) of 4 subjects, significant coherence with EMG was observed at 12+/-5 and 15+/-4 Hz, respectively. The time lags revealed by cross-correlogram were 10+/-3, 7+/-1 and 22+/-8 ms in the M1, S1 and SMA proper, respectively, with the EMG lagging in all areas. CONCLUSION: These findings suggest that the rhythmic activity in the SMA proper, as well as in the S1 and M1, is related to the generation of the rhythmicity of EMG activity.     

Electroencephalographic measurement of motor cortex control of muscle activity in humans.

OBJECTIVE: To detect and measure correlation between cortical and muscle activities, coherence analysis was used. METHODS: The electroencephalogram (EEG) and electromyogram (EMG) were recorded in 9 normal volunteers during tonic contraction of upper and lower limb muscles on the right side. Coherence between EEG and EMG was computed to analyze their linear association. RESULTS: EEG over the contralateral sensorimotor area was coherent with EMG, with peak coherence at 11-36 Hz (mean, 22 Hz). For the abductor pollicis brevis (APB) muscle, peak coherence, as determined by functional brain mapping with focal transcranial magnetic stimulation (TMS), was over or slightly posterior to the hand area on the primary motor cortex determined by focal transcranial magnetic stimulation (TMS). Peak coherence over the scalp was somatotopically organized. The temporal relation between EEG and EMG was analyzed with a new model for interpreting the phase shift ('constant phase shift plus constant time lag' model). For the APB muscle, the phase relation between cortical and muscular oscillations differed in the frequency ranges of 3-13 Hz and 14-50 Hz, respectively, suggesting that different coupling mechanisms operate in different bands. Only the phase shift between cortical and motoneuronal firing at 14-50 Hz was reliably estimated by a linear model. At 14-50 Hz, motoneuronal firing was led by surface-negative cortical activity with a constant time lag that depended on the cortical-muscular distance. For the APB muscle, the time lag was slightly shorter than the cortical-muscular conduction time determined by TMS. Vibratory stimulation (100 Hz) of a muscle tendon during tonic contraction had no significant effect on cortical-muscular coherence, indicating that cortical oscillation reflected motor rather than sensory activity. CONCLUSIONS: The present findings suggest temporal coding of the oscillatory motor control system (3-13 Hz vs. 14-50 Hz), and confirm the functional importance of cortical beta and gamma rhythms in the motor efferent command. Cortical-muscular synchronization is most likely mediated by the direct corticospinal pathway within the frequency range of 14-50 Hz.     

Assessment of muscular fatigue using vibromyography

The purpose of this study was to investigate the behavior of electromyographical (EMG) and vibromyographical (VMG) signals in the time and frequency domains during a fatigue protocol. EMG and VMG records were obtained from the rectus femoris (RF) and vastus lateralis (VL) muscles of 11 adult male subjects during sustained, isometric knee extensor contractions performed at 70% of maximal voluntary contraction (MVC). The average median frequencies of the power density spectra decreased during the fatigue protocol for the EMG (from 73 to 54 Hz for RF, and from 75 to 57 Hz for VL) and the VMG signals (from 40 to 19 Hz for RF, and from 25 to 12 Hz for VL). Raw EMG signals remained the same qualitatively throughout the fatigue protocol, whereas corresponding VMG records appeared to become “smoother.” The results of this study indicate that the pronounced decrease in the high-frequency content of the VMG signal may be observed in the time domain as a “smoothing” of the signal, and thus, that the raw VMG records (which may be displayed readily online) can be used to assess qualitatively the onset and progression of muscular fatigue. © 1994 John Wiley & Sons, Inc.     

TVR and vibration-induced timing of motor impulses in the human jaw elevator muscles.

In order to investigate myotatic reflex involvement in jaw muscle control, an analysis was made of the motor responses induced by mechanical vibration (120-160 Hz) of the jaw elevator muscles in healthy subjects. As seen in torque measurements and mean-voltage electromyographic (EMG) recordings, the vibration caused involuntary reciprocal changes in jaw muscle tone, the contraction force increasing in jaw elevators and decreasing in antagonistic jaw opening muscles. This tonic vibration reflex (TVR) elicited from the jaw elevators exhibited many characteristics similar to those previously described for limb muscle tonic vibration reflexes: it varied in strength from one subject to the next independently of the briskness of the jaw elevator tendon jerks; it had a gradual onset with successive recruitment of jaw elevator motor units firing largely out of phase with one another and at rates much lower than the vibration frequency; it was susceptible to voluntary control--when allowed visual feed-back from the torque meter all subjects were able to suppress the TVR and keep mean contraction force constant. The results indicate that with respect to the tonic motor response to sustained inflow in the Ia afferent nerve fibres, the jaw elevators do not differ markedly from other skeletal muscles. Independently of whether a TVR was present or not, the vibration caused a timing of the motor unit discharges in the jaw elevators that could not be controlled voluntarily and that showed up in gross EMG recordings as a marked grouping of discharges synchronous with each wave of vibration. A similar but less distinct grouping of the gross EMG pattern was seen in limb muscles exposed to vibration, the dispersion increasing with the peripheral conduction distances of the reflex arcs. It is suggested that contrary to the TVR, which depends on the sustained mean level of the Ia afferent input, the timing phenomenon depends, like the tendon jerk, on the degree of synchrony in the afferent Ia volleys. Monosynaptic projections may well be involved in the dynamic timing of motor discharges during tonic firing, but this does not imply that the TVR or the tonic stretch reflex is dependent upon such projections.     

Inhibition of electromyographic activity in human triceps surae muscles during sinusoidal rotation of the foot.

Torque and electromyographic (EMG) responses to sinusoidal rotations of the foot were measured. The frequency range of the movements was 0.5 Hz to 15 Hz at amplitudes ranging between 1 and 10 degrees. At frequencies above 7 Hz, the EMG activity did not follow individual foot rotation cycles. The EMG activity was inhibited whenever the peak torque was large with respect to the first cycle peak torque. Dantrolene sodium reduced the torque developed in triceps surae, allowing the EMG activity to follow individual stretch cycles. As the drug was metabolized, the EMG activity returned to the character seen in the pre-drug control--that is, inhibition on alternate stretch cycles. It is concluded that the EMG inhibition phenomenon can be attributed in part to force receptors in muscle but that these receptors are not the sole contributors to the inhibition.     

Coherence of EMG activity and single motor unit discharge patterns in human rhythmical force production

The purpose of this study was to examine the modulation of the motor neuronal pool as a function of task dynamics. Specifically, we investigated the effects of task frequency on the single motor unit discharge pattern, electromyogram (EMG) activity and effector force output. Myoelectric activity and effector force were recorded while young adults isometrically abducted their first dorsal interosseus at five sinusoidal targets (0.5 Hz, 1 Hz, 2 Hz, 3 Hz and 4 Hz) and at two force levels (5% and 25% maximum voluntary contraction (MVC)). Individual motor unit spike trains were isolated from the EMG. Auto-spectral and coherence analyses were performed on the force output, EMG and motor unit spike trains. The frequency of maximal coherence between the EMG and force output closely corresponded to the target frequency in all conditions. There was a broadband distribution of power with multiple peaks in the EMG and motor unit spectrums in the 0.5 Hz and 1 Hz targets. However, the EMG and motor unit spectrums in the 2 Hz, 3 Hz and 4 Hz targets were characterized by an increasingly narrower band of activity with one dominant peak that closely corresponded to the target. There is high coherence between EMG output and target force frequency, but the relative contribution of the fast and slow neuromuscular bands are differentially influenced by the task frequency. The rhythmical organization of neuromuscular output in the 0.5 Hz task is relatively broadband and similar to that shown previously for constant level force output. The frequency structure of neuromuscular organization becomes increasingly more narrowband as the frequency of the target increases (2-4 Hz). The modulation of the motor neuronal pool is adaptive and depends on the relative contribution of feedback and feedforward control processes, which are driven by the task demands.     

Amitriptyline enhances the central component of physiological tremor

OBJECTIVES: Postural tremor is a regularly encountered side effect of amitriptyline which can be strong enough to cause discontinuation of therapy. The aim was to characterise amitriptyline induced tremor and to assess if the central or reflex component of physiological tremor was modulated by this drug. METHODS: The postural hand tremor was measured in 15 patients on a clinical rating scale, by power spectral analysis of accelerometer, forearm flexor, and extensor EMG before and after the beginning of amitriptyline treatment for major depression or chronic pain syndrome. A coherence analysis between flexor and extensor muscles on the same side was performed. RESULTS: There was a clinically visible increase in postural tremor in a third of these patients. The tremor amplitude measured by accelerometer total power increased in every patient under amitriptyline. The EMG synchronisation as reflected by significant peaks in the flexor or extensor spectrum generally occurring at higher frequencies (8-18 Hz) than the accelerometric tremor frequencies (6-11 Hz) did not change. The number of patients with a significant flexor-extensor coherence in the 7-15 Hz range increased significantly under amitriptyline, the frequency bands of significant coherence corresponded with the EMG frequencies, and both were independent of changes to the hand's resonant frequency by added inertia. CONCLUSIONS: An enhancement of postural tremor under amitriptyline is a common phenomenon although not always clinically apparent. The increase in EMG-EMG coherence indicates an increased common central drive to the motor units as its frequency is not influenced by peripheral resonance or reflex mechanisms. This is the first account of a drug induced enhancement of the central component of physiological tremor.     

Reliability of EMG determinism to detect changes in motor unit synchrony and coherence during submaximal contraction

The determinism (DET) is a parameter used in nonlinear analysis to quantify the occurrence of recurrent patterns in a signal. Applied to the electromyographic activity (EMG), DET has been proposed as an index of motor unit synchrony in human. We have recently shown that the amount of motor unit synchronous firings above chance level was enhanced with stronger submaximal muscle contraction. Using these data, we aimed at determining if (1) EMG DET and motor unit synchrony varied in the same way and (2) EMG DET was more specifically related to the degree of oscillatory coupling between motor unit discharges. Cross-correlation and coherence analyses were applied to the discharges of 30 motor unit pairs tested at various force levels to assess the amount of synchronous impulses and the strength of oscillatory coupling in the time and frequency domains, respectively. Recurrent quantification analysis was applied to EMG activity to extract its DET. Overall, changes in EMG DET were poorly explained by changes in motor unit synchronous impulse probability (6%) and frequency (5%), and by changes in motor unit coherence in the 6-12Hz (5%) and 25-40Hz (8%) bands. Moreover, the comparison of the data obtained at the weakest and the strongest contraction levels tested with each motor unit pair showed that EMG DET remained unaltered with stronger contraction despite the occurrence of consistent changes in motor unit synchrony in both time and frequency domains. This speaks strongly against the reliability of DET in evaluating changes in motor unit synchronization during submaximal muscle contraction.     

Analysis of preganglionic nerve evoked cholinergic contractions of the guinea pig bronchus.

We compared cholinergic bronchial muscle contractions induced by vagus nerve (preganglionic) stimulation (VNS) with those induced by electrical field (postganglionic) stimulation (EFS). When normalized to their respective maximum response, the frequency-response curves (10 s trains) between 4 and 16 Hz were similar between VNS and EFS; however, at frequencies of 0.1-2 Hz, and at frequencies greater than 32 Hz, the VNS contractions were significantly less than EFS. When contractions elicited by 100 pulses were examined, it was found that the responses to VNS were maximal at 10-30 Hz then declined significantly to 82-35% of maximal between 40 and 200 Hz, whereas the response to EFS was essentially unchanged at frequencies up to 60 Hz and declined only to 72% of maximal up to 200 Hz. At frequencies as low as 20 Hz, the contractions evoked by VNS faded to 45 +/- 9% of the peak contraction during 60 sec of continuous stimulation, whereas those evoked by 60 sec continuous EFS remained constant. This fade observed during prolonged VNS was not blocked by the antagonists, pirenzepine and AFDX-116, at concentrations selective for M1 and M2 muscarinic receptors, respectively; nor was the fade blocked by pre-treatment with indomethacin, propranolol, phentolamine, or choline. At frequencies greater than 10 Hz, the amplitude of the preganglionic compound action potential also faded during repetitive stimulation. The results support the hypothesis that the airway ganglion neurons innervating guinea pig bronchial smooth muscle effectively filter preganglionic stimuli, especially at low and relatively high frequencies. During continuous vagus nerve stimulation, preganglionic mechanisms may also play a role in limiting the ultimate output of airway ganglia.     

Automatic analysis of EMG during clonus

Clonus can disrupt daily activities after spinal cord injury. Here an algorithm was developed to automatically detect contractions during clonus in 24 h electromyographic (EMG) records. Filters were created by non-linearly scaling a Mother (Morlet) wavelet to envelope the EMG using different frequency bands. The envelope for the intermediate band followed the EMG best (74.8-193.9 Hz). Threshold and time constraints were used to reduce the envelope peaks to one per contraction. Energy in the EMG was measured 50 ms either side of each envelope (contraction) peak. Energy values at 5% and 95% maximal defined EMG start and end time, respectively. The algorithm was as good as a person at identifying contractions during clonus (p = 0.946, n = 31 spasms, 7 subjects with cervical spinal cord injury), and marking start and end times to determine clonus frequency (intra class correlation coefficient, α: 0.949), contraction intensity using root mean square EMG (α: 0.997) and EMG duration (α: 0.852). On average the algorithm was 574 times faster than manual analysis performed independently by two people (p ≤ 0.001). This algorithm is an important tool for characterization of clonus in long-term EMG records.     

Descending control of muscles in patients with cervical dystonia.

It was reported recently that specific features in the frequency analysis of electromyographic (EMG) activity in the sternocleidomastoid (SCM) and splenius (SPL) muscles were able to distinguish between rotational idiopathic cervical dystonia (CD) and voluntary torticollis in individual subjects. Those with CD showed an abnormal drive to muscles at 5 to 7 Hz and an absence of the normal 10 to 12 Hz peak in the autospectrum of SPL. We sought to determine whether the same abnormalities in the frequency domain are found in complex CD, in which the head is displaced in more than two planes. EMG activity was recorded in the SCM, SPL, trapezius, and levator scapulae muscles bilaterally in 10 patients with complex CD. Frequency analysis of EMG was compared with conventional clinical and polymyographic assessment. The autospectrum of SPL during free dystonic contraction showed an absence of a significant peak at 10 to 12 Hz in 8 of the 10 patients. The presence of a 5 to 7 Hz frequency drive showed a significant association with muscle pairs determined as dystonic by means of polymyography (P < 0.005). The neck posture predicted blindly, based on the low-frequency drive, correlated significantly with the clinical assessment of posture (P < 0.01). Conventional assessment and the results of frequency analysis correlated, suggesting that a low-frequency drive to neck muscle may be a general feature of simple rotational and more complex cervical dystonia. The pattern of coherence between the EMG in different neck muscles may provide a means of identifying leading dystonic muscles, especially in patients with complex cervical dystonia.     

Motor unit size estimation: confrontation of surface EMG with macro EMG

Surface EMG (SEMG) is little used for diagnostic purposes in clinical neurophysiology, mainly because it provides little direct information on individual motor units (MUs). One of the techniques to estimate the MU size is intra-muscular Macro EMG. The present study compares SEMG with Macro EMG. Fifty-eight channel SEMG was recorded simultaneously with Macro EMG. Individual MUPs were obtained by single fiber triggered averaging. All recordings were made from the biceps brachii of healthy subjects during voluntary contraction at low force. High positive correlations were found between all Macro and Surface motor unit potential (MUP) parameters: area, peak-to-peak amplitude, negative peak amplitude and positive peak amplitude. The MUPs recorded with SEMG were dependent on the distance between the MU and the skin surface. Normalizing the SEMG parameters for MU location did not improve the correlation coefficient between the parameters of both techniques. The two measurement techniques had almost the same relative range in MUP parameters in any individual subject compared to the others, especially after normalizing the surface MUP parameters for MU location. MUPs recorded with this type of SEMG provide useful information about the MU size.