Effects of femoral nerve stimulation on the electromyogram and reflex excitability of tibialis anterior and soleus

The present study was undertaken to determine whether femoral nerve stimulation would produce heteronymous reflex responses in tibialis anterior (TA) and soleus, demonstrable by averaging the electromyogram (EMG) produced by a voluntary contraction, and whether the responsible changes in excitability were sufficient to affect the H reflexes of TA and soleus. In both muscles, femoral stimuli produced short-latency, presumably monosynaptic excitation, better defined in poststimulus averages of unrectified EMG, followed by long-lasting inhibition, better defined in averaged rectified traces. The H reflexes underwent changes at appropriate latencies. The thresholds for excitation and inhibition were, respectively, below and above threshold for the quadriceps M wave. The heteronymous responses were largely independent of stimulus rate and, within limits, scaled with the level of background contraction. The ability to define these heteronymous connections using relatively simple methodology extends their utility. Such tests may prove useful in probing pathophysiological mechanisms in individual patients. © 1996 John Wiley & Sons, Inc.     

Quantitative analysis of reflex responses in the averaged surface electromyogram.

The relationship between the averaged surface electromyogram (EMG) and the activity of motoneurones in reflex studies is analysed mathematically. This analysis reveals that, subject to certain conditions being met, the integral of the average of the unrectified EMG is linearly related to the activity of motor units in the muscle. This was tested with experimental data. The activity of 2 motor units, and the surface EMG, were recorded directly in a reflex paradigm. A close match was found between the integral of the average of the unrectified EMG and the summed peristimulus time histograms (PSTHs) of the units' activity. This analysis gives more quantitative measurements of the timing of motoneuronal activity in reflexes than the conventional analysis of the EMG. It also offers the potential for making quantitative measurements of changes in motor unit activity evoked by different stimuli. This analysis avoids the pitfall of the artefactual peaks that can occur in the average of the rectified EMG as a consequence of the full-wave rectification. It is concluded that the integral of the unrectified average offers a valuable adjunct to the conventional analysis in many reflex studies. The analysis can also be applied to quantitative studies of neurograms.     

Habituation of cutaneomuscular reflexes recorded from the first dorsal interosseous and triceps muscle in man

Cutaneomuscular reflexes have been recorded from the first dorsal interosseous muscle during a sustained abduction of the index finger of 20 subjects (25 recordings) following stimulation of the digital nerves at the following frequencies: 2 Hz, 3 Hz, 5 Hz, 7 Hz and 9 Hz, presented in random order. Five hundred stimuli were given at each frequency. EMG was rectified and consecutive batches of 100 sweeps of each set of 500 responses were averaged time locked to the stimulus. All reflex components, E1, I1 and E2, exhibit habituation with the E1 component habituating the most and the I1 component the least. There was considerable variation in the rate of habituation between subjects. The rate of habituation was independent of the frequency of stimulation. Reflex responses were recorded from the triceps brachii muscle in eight subjects; this reflex response habituated at a faster rate than the E2 component recorded from the first dorsal interosseous muscle. These results are discussed in relation to the choice of stimulus parameters for the clinical testing of cutaneous reflexes. We conclude that it is important to consistently average the same number of responses.     

Cutaneous reflexes in Parkinson's disease.

The physiology of rigidity in Parkinson's disease (PD) can be investigated by the study of reflexes. Cutaneous reflexes (CR) were measured in 10 patients with PD and in 10 age- and sex-matched normal volunteers. EMG activity was recorded from the first dorsal interosseous muscle with surface electrodes, rectified and averaged. The index finger was stimulated with an intensity four times the sensory threshold. The subjects abducted the index finger with 20% of maximal force. While the latencies of the different reflex components and the amplitudes of the excitatory peaks were not different in the two groups, the first inhibitory component was less pronounced in patients with PD as compared with normals. This effect is partially reversed with dopaminergic drug treatment. The results are compatible with the loss of an inhibitory spinal mechanism elicited by cutaneous afferents, and can be a partial explanation for increased tone in PD.     

Reflex responses of masseter muscles to sound

Acoustic stimuli can evoke reflex EMG responses (acoustic jaw reflex) in the masseter muscle. Although these were previously ascribed to activation of cochlear receptors, high intensity sound can also activate vestibular receptors. Since anatomical and physiological studies, both in animals and humans, have shown that masseter muscles are a target for vestibular inputs we have recently reassessed the vestibular contribution to masseter reflexes. We found that high intensity sound evokes two bilateral and symmetrical short-latency responses in active unrectified masseter EMG of healthy subjects: a high threshold, early p11/n15 wave and a lower threshold, later p16/n21 wave. Both of these reflexes are inhibitory but differ in their threshold, latency and appearance in the rectified EMG average. Experiments in healthy subjects and in patients with selective lesions showed that vestibular receptors were responsible for the p11/n15 wave (vestibulo-masseteric reflex) whereas cochlear receptors were responsible for the p16/n21 wave (acoustic masseteric reflex). The possible functional significance of the double vestibular control over masseter muscles is discussed.     

Selective effects of ageing on vestibular-dependent lower limb responses following galvanic stimulation.

OBJECTIVES: To examine the influence of ageing on electromyographic (EMG) responses to galvanic vestibular stimulation (GVS) in the lower limbs and to define normal values. METHODS: EMG responses to 4 mA/20 ms transmastoid binaural GVS were recorded from the soleus muscles of 70 healthy adults aged 24-85 years. Short (SL) and medium latency (ML) vestibular-dependent reflex latencies and amplitudes were measured from averaged rectified EMG. Side to side differences were examined in 30 subjects, using monaural stimulation, and expressed as an asymmetry ratio (AR). RESULTS: SL reflexes to transmastoid binaural stimulation were absent in 10 of the 25 subjects over the age of 60. ML responses were present in all subjects. SL reflex amplitudes decreased from 24.4+/-12.4% (mean+/-SD) in the 3rd decade to 7.6+/-6.2% in the 8th and 9th decades. The ML reflex amplitudes showed a tendency to increase with age. The mean AR for SL reflex amplitudes increased from 12.1% in the 3rd decade to 80.7% in the 8th and 9th decades but that for ML amplitudes did not change significantly. The average SL and ML response onset latencies were 56.5+/-7.5 and 97.2+/-9.4 ms. SL onset latencies were significantly delayed with age. Both SL and ML responses were highly reproducible between experiments. CONCLUSIONS: SL and ML responses behave differently in response to age. The decrease in SL reflex amplitudes correlates well with previously described age-related morphological changes. Preservation of the ML reflex may reflect central adaptation to reduced afferent input. SL and ML responses are potentially useful measures of vestibular-spinal function, which may have a role in the assessment of older subjects with dysequilibrium. These reflexes need to be interpreted in the context of age.     

Continuous wavelet transform in the evaluation of stretch reflex responses from surface EMG

OBJECTIVE: This is the first reported use of the continuous wavelet transform (CWT) of the surface EMG (sEMG) to extract the reflex response to muscle stretch. We used a modulus-based method to estimate instantaneous amplitude-envelopes from ridges of the CWT (referred in this work as sEMG intensity) to extract the dynamic reflex response from sEMG. We tested the method on tendon reflexes where excellent temporal resolution is required to identify the different latency components, and on the tonic stretch reflex (tonic SR) response to an ongoing perturbation that characteristically has a low signal to noise ratio. METHODS: Eight subjects without neurological impairment were subjected to a series of archilles tendon taps and a 2 min continuous perturbation of the ankle using a pseudo-sinusoidal stretch profile containing frequencies from 0.1 to 8.0 Hz. The tendon reflexes were assessed in the soleus muscle at 10% of MVC and the tonic SR in tibialis anterior while the muscle was relaxed, at 5 and 10% of maximal voluntary contraction. Root mean square (RMS) and wavelet ridge extraction was applied to the sEMG signal to extract sEMG amplitudes (RMS) and intensities for all reflexes. To obtain the tonic SR, these estimates and those from the sEMG-RMS were subsequently cross-correlated with the perturbation record to yield 2 sets of estimates of reflex gain and coherence for comparison. RESULTS: The sEMG intensities were highly correlated with the torques resulting from a ramped voluntary contraction. Following tendon taps, the method resolved the M1, M2, M3 response components at accurate latencies and with more complete reconstruction of the components than RMS-derived estimates. The wavelet ridge estimates extracted the tonic SR from resting and contracting muscles with significantly higher coherence than RMS estimates. Reflex gain, when estimated from sEMG intensity or sEMG-RMS, demonstrated similar relationships to the perturbation frequency and background contraction level. When the sEMG intensity reflex gain estimates from different subjects were pooled, they showed significantly lower variance about the mean than gain estimates derived from the rectified sEMG. CONCLUSIONS: Wavelet-ridge extraction provides a valid approach to reflex evaluation from sEMG that does not depend on the absolute amplitude of the potentials measured at the EMG electrodes. This may have substantial advantages in more directly comparing responses between subjects on an absolute frequency scale without the need for normalisation against maximal contraction levels.     

Analysis of the interference electromyogram of human soleus muscle after exposure to vibration

The properties of m. soleus surface EMG recorded under conditions of voluntary contraction against vibrational stimulation were studied using vibration-triggered averaging and spectral estimates. The averaging procedure indicated EMG bursts locked to the vibration cycle. Narrow peaks appeared in the EMG spectrum at vibration frequency and harmonics. These effects were more pronounced in rectified EMG at low vibration frequencies (30-70 Hz) and in EMG at high frequencies (70-120 Hz). The disappearance of the peak after ischemic blockade preceded that of the tendon reflex. The peak normalized to the EMG power decreased when the force was enlarged. The peak augmented with prolonged contraction under vibration. The results are suggested to reflect alterations of the relative weight of the excitatory inflow through short spindle-motoneuron connections in the overall motoneuron inflow.     

Electromyography of the larynx with skin surface electrodes]

A new method of skin surface electromyography (EMG) was evaluated. Twelve electrodes were placed symmetrically in the cricothyroid and thyroid lamina region. Raw EMG signals were continuously recorded, filtered, and rectified. Phonation of the vowel /a/ triggered a computer, which thereafter stored a 625-ms EMG recording, starting 375 ms before and ending 250 ms after onset of the audible phonation. Sixteen to 64 of these recordings were averaged to eliminate random noise. Twenty-six healthy subjects and 9 patients with unilateral laryngeal paralysis were studied. EMG activity exhibited great intra- and interindividual variation. Often, even in healthy subjects, EMG levels were different between the left and right side of the larynx. Thus, this method is not suitable for diagnosis or follow-up of laryngeal paralysis. Sources of variation in EMG activity are discussed.     

Tendon reflex variability and method of stimulation

Human tendon reflexes show large fluctuations, even when accurate mechanical instruments are used for stimulation. To examine the contribution of stimulus factors to reflex variability two methods of stimulation were compared in healthy subjects. One method consisted of a normal hand-held reflex hammer, with a transducer to measure stimulus intensity. The results with this hammer, both after maximal stimuli and after stimuli over a wide range of intensities, showed a similar variability of reflexes as with a more accurate instrumental stimulator. In previous studies in human subjects an equal or larger variability of reflexes was found. It is concluded that for many applications a simple hand-held reflex hammer gives equally reliable results as more complicated instrumental stimulators.     

Low‐frequency component of rectified EMG is temporally correlated with force and instantaneous rate of force fluctuations during steady contractions

Introduction: The usefulness of surface EMG for assessing motor output variability during steady contraction is unclear. The purpose of this study was to examine the temporal correlation in signal characteristics between low‐frequency components of full‐wave rectified EMG (with or without high‐pass filtering before rectification) and force fluctuations or the instantaneous rate of force fluctuations (dF/dt) during steady contractions. Methods: Fourteen young adults produced steady force with the first dorsal interosseus muscle at various intensities. Cross‐correlation functions were calculated between the following signals: <5 Hz: force, dF/dt, and rectified EMG with or without high‐pass filtering at 300 Hz before rectification. Results: Rectified EMG correlated better with dF/dt compared with force itself, and high‐pass filtering of interference EMG before rectification improved the temporal correlation. Conclusions: The low‐frequency component of rectified EMG following high‐pass filtering may provide temporal information on the instantaneous rate of force fluctuations during steady contractions. Muscle Nerve 47: [?show $1534v]–[?show $1535v], 2013     

Comparison of conventional and decomposition-enhanced spike triggered averaging techniques.

OBJECTIVE: Spike triggered averaging (STA) is a technique to extract an estimate of a recurring motor unit potential from a complex electromyographic (EMG) signal. In conventional STA (C-STA), potentials related to the discharges of single intramuscular motor units are isolated and used to trigger an averager to obtain an individual surface-detected motor unit potential (S-MUP) from an EMG signal. In decomposition-enhanced STA (DE-STA), EMG signal decomposition algorithms determine discharges of a number of different motor units (4 to 10) that can be used to trigger an averager to obtain their corresponding S-MUPs. We tested the accuracy of extracting and averaging S-MUPs using DE-STA compared to C-STA for the same EMG signals. METHODS: We compared the intramuscular potentials used for triggering and the resultant averaged S-MUPs that were common in both techniques. RESULTS: We found no statistically significant differences in the metrics used to describe the triggering potentials and S-MUPs. CONCLUSIONS: We conclude that DE-STA is an accurate and efficient method to obtain a large number of intramuscular motor unit potentials and their corresponding S-MUP in proximal and distal muscles.     

Different responses to auditory and somaesthetic stimulation in patients with an excessive startle: a report of pediatric experience.

OBJECTIVES: Children with cerebral injury often exhibit brief muscle contraction to a variety of stimuli. However, it remains to be determined whether or not the pattern of the reaction is stereotypical irrespective of the site stimulated. To answer this question, we studied electromyographic (EMG) responses to three types of stimuli in children. METHODS: The EMG responses of cranial and limb muscles were recorded after acoustic or somaesthetic stimulation in 6 patients and 23 control subjects. RESULTS: Acoustic stimuli evoked patterned motor activity with a rostrocaudal progression. Nose-tapping stimuli elicited reflex EMG activity in the VIIth cranial muscles that was similar to the R1 component of the electrical blink reflex. Sternum-tap stimuli evoked motor activity in the sternocleidomastoid and arm muscles, and this reflex was probably mediated through the cervical cord (H-reflex). Moreover, late reflexes were evoked following these early reflexes in the patients. In particular, atypical forms of myoclonic jerks were evoked on sternum-tap stimuli. CONCLUSIONS: Many types of primitive reflexes were evoked following three types of stimuli. These reflexes included startle reflex, trigeminomotor reflex, H-reflex and atypical forms of myoclonus, and they were enhanced in the patient group. There are many startle-mimicking reflexes.     

Nociceptive EMG suppression in triceps brachii muscle in humans.

OBJECTIVE: Inhibitory spinal reflexes in human hand muscles mediated by cutaneous afferents (=cutaneous silent periods, CSPs) serve to protect the hand from injury. Proximal muscles are included in a complex protective pattern implementing both excitatory and inhibitory reflexes. METHODS: We investigated the functional organization of CSPs in the triceps brachii muscle (TB) - the prime elbow extensor - assessing different stimulus intensities and elbow positions. Thirteen healthy subjects underwent recurrent electrical index finger tip stimulation with different intensities while volitionally activating TB against resistance. The rectified and averaged electromyogram (EMG) was recorded from TB with surface electrodes while the elbow joint was held in full flexion, full extension, and at an angle of 90 degrees. RESULTS: Digit II stimulation resulted in distinct CSPs in TB. Increasing stimulus intensities caused a more profound EMG suppression. Different elbow positions had a significant influence on CSP onset and end latency, and duration. CONCLUSIONS: The present findings are consistent with a functional organization of protective reflexes in the proximal human upper limb across several metameric segments. Spinal inhibitory neurons serving to rapidly suppress basic muscle synergies may compete with neural circuitry involved in postural control.     

Origin of the low-level EMG during the silent period following transcranial magnetic stimulation

ObjectiveThe cortical silent period refers to a period of near silence in the electromyogram (EMG) after transcranial magnetic stimulation (TMS) of the motor cortex during contraction. However, low-level EMG of unknown origin is often present. We hypothesised that it arises through spinal reflexes. Sudden lengthening of the muscle as force drops during the silent period could excite muscle spindles and facilitate motoneurones.MethodsSubjects (n = 8) performed maximal isometric, shortening and lengthening contractions of the elbow flexors during which TMS (90–100% output) was delivered over the motor cortex. The rate of flexion during shortening contractions reduced muscle lengthening caused by muscle relaxation. Surface EMG was recorded from biceps brachii and brachioradialis, and the low-level EMG during silent periods produced by TMS was measured.ResultsLow-level EMG activity was reduced on average by 68% in biceps and 63% in brachioradialis in the shortening contraction compared to all other contraction conditions (p < 0.001). Levels of pre-stimulus EMG were similar between conditions.ConclusionsMuscle lengthening contributes to low-level EMG activity in the silent period, through spinal reflex facilitation by muscle spindle afferents.SignificanceThe silent period depth is not only dependent on cortical output but also reflex effects evoked by muscle lengthening.     

Observations on the variability of the H reflex in human soleus

H reflexes were evoked in human soleus by stimulating the tibial nerve at a constant intensity. Each trial was then assigned to one of three groups on the basis of the amplitude of its H reflex; all trials in each group were then full-wave rectified and reaveraged. There was a strong positive relationship between the amplitude of the H reflex and the level of electromyographic activity in the muscle at the time of onset of the H reflex, which reflects the activity of the motoneuronal pool when the afferent volley arrived. Thus, much of the variability of the H reflex is due to small changes in the level of activation of the motoneuronal pool during repeated trials. The steady torque preceding the H reflex was a poor predictor of the H-reflex amplitude, presumably because of the delay between the changes in the electrical activity of motoneurons and the mechanical outcome thereof.     

Comparison of spinal myotatic reflexes in human adults investigated with cross-correlation and signal averaging methods

A cross-correlation method for recording spinal myotatic reflexes has been developed to meet the need for brief test periods in babies and children and subjects with central neurological pathology. In normal adult subjects the method has been validated by comparing excitatory and inhibitory reflexes obtained with cross-correlation with those obtained with conventional signal averaging. In the cross-correlation method a pseudo-random binary sequence of 64 brief tendon taps was delivered in <1.5 s, and in the averaging method 20-150 taps at one per second. The reflexes were expressed as unit impulse responses to enable direct, quantitative comparisons to be made. With cross-correlation the responses were slightly expanded in time, had lower peak amplitudes, and onset latencies advanced by 10 ms, the clock period of the pseudo-random binary sequence. The amplitude of biceps phasic stretch reflex increased with muscle contraction in a similar manner with both methods. In tests for stationarity the amplitude of biceps phasic stretch reflex varied <10% in the first six repeats of the pseudo-random binary sequence. The tap force required at threshold for cross-correlation was approximately half that for averaging, but with both methods the magnitude of biceps phasic stretch reflex varied linearly with tap force over the range of one to two times threshold. The validity of responses obtained with cross-correlation was assessed by a statistical procedure. In conclusion, the cross-correlation method is robust and gives similar results to those obtained with averaging.     

The recorded corneo-mandibular reflex

Corneo-mandibular reflexes were studied electrophysiologically in 27 patients with cerebral lesions and normal consciousness. EMG demonstrated that the reflex is absent in 40 controls and may appear with lesions which involve the precentro-bulbar tract. The reflex is considered to be a pathological exteroceptive reflex. The response fell into 4 categories of decreasing spread. In 2 controls and in 2 patients with hemiplegia clinical corneo-mandibular reflexes turned out electromyographically to be corneo-mental reflex responses.     

Relationship between the degree of inhibited stretch reflex activities of the wrist flexor and reaction time during quick extension movements

It has been reported that stretch reflex responses, including the long latency component, are modulated by motor preparation for the direction and type of movement. In the present study, human subjects were required to make a reaction movement in the direction of the wrist extension following a muscle stretch to the wrist flexor, and we investigated the relationship between the modulation of reflex activities of the wrist flexor and the length of reaction time (premotor time) of the wrist extensor. Twenty-five healthy males, ranging in age from 20 to 28, participated in the experiments. A DC torque motor was used to evoke the reflex EMG responses on the flexor. Averaging the rectified EMG, recorded with the surface electrodes over the flexor, showed short and long latency reflexes (M1 and M2 components) in response to the muscle stretch. For all subjects, the amplitudes of the reflex components during the extension reaction movement decreased, compared to those amplitudes in the non-reaction tasks. The decrease in the M2 component, which is considered a transcortical reflex, was significantly larger than the decrease in the M1 component, which is a spinal reflex. Moreover, there were correlations between reaction time to muscle stretch and the degree of decrease in reflex activities with the extension reaction (r=0.652 for M1, r=0.813 for M2, P<0.01). It became clear that the subjects with shorter reaction times inhibited their reflex activities of the flexor, particularly the M2 component which prevents the extension movement, to a greater degree than the subjects with longer reaction times. Therefore, our results suggest that the degree of M2 modulation directly reflects the individual motor control required to perform quick movements.     

Neurophysiology of myoclonus

Myoclonus may be generated by any area in the central nervous system. Finding its generator is helpful in the diagnostic process. Although clinical features have to be carefully analyzed as they may give a first idea, neurophysiologic study of myoclonus provides the most important clues for the determination of the generator. Surface electromyography (EMG) allows analyzing the recruitment order in generalized myoclonus, thereby suggesting either a cortical, brainstem, or spinal origin. It also reveals whether myoclonus is positive (jerks that are caused by muscle activation) or negative (jerks that are caused by brief muscle inhibition). In non-generalized myoclonus the EMG burst duration gives an idea of the level of the generator. Repetitive peripheral nerve stimulation is required to record somatosensory evoked potentials (SEPs) as well as long latency reflexes (LLR), especially the C reflex. The presence of giant cortical SEPs is an indirect argument for cortical myoclonus. Similarly the existence of LLR at rest orientates towards cortical reflex (sensitive to sensory stimuli) myoclonus. Finally EEG-EMG polygraphy is the only test which is able to prove directly the cortical origin of myoclonus. This is the case when focal cortical events precede myoclonus with a fixed delay. These premyoclonic cortical potentials may either be seen directly on raw recordings or require the use of jerk-locked back averaging (JLBA). This technique allows the averaging of the EEG prior to myoclonus onset (as determined by EMG) in order to reveal a premyoclonic spike that otherwise would remain undetected in the global EEG.