Surface detected potentials of normal and reinnervated motor units: a simulation study for muscles consisted of short fibres

We aimed to check whether the characteristics used up to now in macro EMG to distinguish between normal and reinnervated motor unit potentials (MUPs), were suitable for surface detected MUPs. MUPs produced by normal and reinnervated MUs were simulated with a fast and precise convolution model. An increased number of fibres in the MU territory enhanced the amplitude, area and RMS of the MUP proportionally irrespective of the MU-electrode distance. An increased scatter of the end-plates and greater desynchronization in the fibres' activation decreased the MUP amplitude and affected the temporal characteristics of the MUP (duration of the negative phase and its area to amplitude ratio). The effects were more pronounced at shorter distances. At larger distances, the effect of the MU-electrode distance on temporal and amplitude characteristics of MUPs was much stronger than that of the parameters changed with reinnervation. We conclude that reinnervated MUs consisting of short fibres can not be distinguished from the normal ones by means of characteristics of MUP used in macro EMG. To discriminate reinnervated MUs non-invasively, the MUP amplitude should be normalized in respect of the MU-electrode distance or other MUP characteristics (independent of MU-electrode distance and sensitive to reinnervation) should be used.     

Comparison of amplitudes of surface macro motor unit potentials recorded from various muscle sites

Peak-to-peak amplitudes and total areas of surface macro motor unit potentials (S-MMUPs) were measured in 19 healthy volunteers. While participants maintained minimal isometric muscle contraction of the left biceps brachii, motor unit potentials (MUPs) were recorded from a needle and surface electrodes. The largest MUP recorded by the needle electrode was designated the trigger source. Electrical activities from the surface electrodes, which emerged synchronously with the trigger-potential, were averaged by the spike-triggered averaging (STA) technique. When the surface electrodes were placed over the muscle belly at a right angle to the muscle fibers, the S-MMUP amplitude and area decreased gradually with the distance of the electrodes from the point of insertion of the needle electrode. In contrast, when the surface electrodes were arranged parallel to the muscle fibers, the S-MMUP amplitude and area did not always decrease. In addition, negative peak positions in individual S-MMUPs showed a time delay along the muscle fibers. The placement and size of the surface electrodes, as well as the depth of the needle electrode, must be carefully considered when MUPs are analyzed by the STA technique. Muscle fiber conduction velocity (MFCV) is measurable by the STA technique combined with surface electrodes.     

Peculiarities of extracellular potentials produced by deep muscles. Part 2: motor unit potentials

The potential fields generated by single fibres far from the sources are non-propagating. This suggests that there should be differences in the features of surface motor unit (MU) potentials (MUPs) detected from deep and superficial muscles. We explored the features using a simulation approach. We have shown that the non-propagating character and similar shapes among surface MUPs recorded over a wide area above deep muscles with monopolar or longitudinal single differential (LSD) electrodes are natural. Contrary to close distances, at large radial distances single differentiation did not emphasize MUP main phase relative weight. The position of the end plate area could be estimated with LSD detections only for MUs with long (123 mm) almost symmetric fibres. With short fibres, the LSD main phase was masked by the outlined terminal phases. This could be misleading in MUP analysis since the terminal phases reflect standing sources. The highly asymmetric fibres could yield peculiar MUP shapes resembling MUPs of two distinct MUs. We have shown that the relative weight of terminal phases at large fibre-electrode distance decreases under abnormal peripheral conditions. However, the changes in membrane depolarization due to fatigue or pathology could be assessed non-invasively also from deep muscles.     

Effect of electrode dimensions on motor unit potentials

Different effects of longitudinal and transversal electrode dimensions on nerve or muscle single fibre action potentials detected monopolarly, were reported in the literature. The results were contradictory. We studied motor unit potentials (MUPs) detected at a large distance (typical of surface recording) on the basis of a mathematical model without source simplification. The MUPs were calculated as a single convolution of the first temporal derivative of a realistic intracellular action potential and MU impulse response. The spatial averaging of the MUPs by rectangular plate electrodes was performed through analytical integration of the MU impulse response over the electrode area. The effects of longitudinal dimension of the electrode were stronger than those of a transversal one. The effects were distance dependent. The longitudinal dimension of the electrode influenced the main phases (that reflected the excitation origin and propagation) more than the terminal phases (that reflected the excitation extinction at the muscle fibers' ends). This was due to differences in the character of the potential fields (quadrupole or dipole) during generation of individual MUP phases. It was shown that the relative weight of the individual MUP phases could be stressed or suppressed by a proper choice of electrode dimensions, position and orientation.     

On the evaluation of muscle fiber conduction velocity considering waveform properties of an electromyogram in M. biceps brachii during voluntary isometric contraction

The surface myoelectric signal during 20% maximum voluntary contraction was measured in m. biceps brachii using array electrodes for ten subjects in order to evaluate the distribution of muscle fiber conduction velocity (MFCV) in a whole muscle. MFCV was estimated by two calculating methods of the peak maximum method and the cross-correlation method from the myoelectric signals which were processed by techniques of the averaging and the non-averaging. It was found that the values of MFCV depended on the location irrespective of the kind of calculating method used and the kind of processing technique of myoelectric signal. In both the motor end-plate zone and the tendon zone, the values of MFCV showed more than 7.0 m/s. In the regions other than the motor end-plate zone and the tendon zone, the values of MFCV showed about 3.90 m/s which were almost constant. The statistical differences of the values of MFCV were in the same locations measured not recognized between the two calculating methods nor between the two processing techniques. In the cross-correlation method, the relation between MFCV and the electrode location was evaluated by both the maximum correlation coefficient and the amplitude ratio between the different neighboring channels to evaluate the conductive waveform properties of action potentials. The changes of parameters (i.e., MFCV, maximum correlation coefficient and amplitude ratio) depended on the electrode location. The values of MFCV significantly increased in the regions of the motor end-plate zone and the tendon zone, where the maximum correlation coefficient and amplitude ratio significantly decreased. The values of the coefficient of variance (CV) of three parameters in those regions were larger than those in other regions, i.e., the regions other than the motor end-plate zone and the tendon zone. A high maximum correlation coefficient and a high amplitude ratio were necessary for a reliable measurement of the MFCV.     

A morphological analysis of the macro motor unit potential

The technique of macro EMG is used to investigate the motor unit architecture in a number of pathological conditions. Amplitude and area are the most commonly used criteria, but these parameters alone are not sufficient to assess the complexity of the macro MUP morphology. In an attempt to examine the morphology of the macro MUP in more detail, additional measures were investigated including, (i) average power, (ii) duration, and (iii) number of phases. Macro MUP duration was defined as the time parameter that contains a particular fraction (90%) of the total power of the potential. The above mentioned parameters were evaluated for normal subjects and for patients suffering with motor neuron disease (MND), spinal muscular atrophy (SMA), and Becker's muscular dystrophy (BMD). It is shown that high amplitude and average power macro MUPs give shorter macro MUP duration than macro MUPs with normal amplitude. In contrast, in low amplitude macro MUPs there is a tendency towards a higher duration measure, as compared with the duration of the normal amplitude macro MUPs. Also, t–test results for the duration measure gave a significant difference between the NOR–MND, and no significant difference between the NOR–BMD and NOR–SMA groups at P<0.05. Significant difference between the NOR and the three disease groups investigated was obtained for the parameters log amplitude, log area, and log average power. The number of phases was not significantly different between the NOR and the rest of the groups. In conclusion, the average power and duration parameters can possibly be used as additional discriminators to detect abnormalities of the macro motor unit potential in both needle and surface EMG but further investigation is necessary.     

Distribution of motor unit potential velocities in short static and prolonged dynamic contractions at low forces: use of the within-subject’s skewness and standard deviation variables

Behaviour of motor unit potential (MUP) velocities in relation to (low) force and duration was investigated in biceps brachii muscle using a surface electrode array. Short static tests of 3.8 s (41 subjects) and prolonged dynamic tests (prolonged tests) of 4 min (30 subjects) were performed as position tasks, applying forces up to 20% of maximal voluntary contraction (MVC). Four variables, derived from the inter-peak latency technique, were used to describe changes in the surface electromyography signal: the mean muscle fibre conduction velocity (CV), the proportion between slow and fast MUPs expressed as the within-subject skewness of MUP velocities, the within-subject standard deviation of MUP velocities [SD-peak velocity (PV)], and the amount of MUPs per second (peak frequency = PF). In short static tests and the initial phase of prolonged tests, larger forces induced an increase of the CV and PF, accompanied with the shift of MUP velocities towards higher values, whereas the SD-PV did not change. During the first 1.5–2 min of the prolonged lower force levels tests (unloaded, and loaded 5 and 10% MVC) the CV and SD-PV slightly decreased and the MUP velocities shifted towards lower values; then the three variables stabilized. The PF values did not change in these tests. However, during the prolonged higher force (20% MVC) test, the CV decreased and MUP velocities shifted towards lower values without stabilization, while the SD-PV broadened and the PF decreased progressively. It is argued that these combined results reflect changes in both neural regulatory strategies and muscle membrane state.     

Muscle fiber conduction velocity during isometric contraction and the recovery period

Changes of muscle fiber conduction velocity (MFCV) detected by surface array electrodes during an isometric contraction and the recovery period were evaluated. The location on skin measured for action potentials of muscle fiber in m. biceps brachii was a distance of 5 mm and 30 mm from the end-plate to the distal tendon. The MFCV was evaluated by averaging raw EMG waves. The MFCVs at both locations declined gradually during the loads of sustained isometric contractions of 30, 50, and 70% of the maximum voluntary contraction (MVC). The degree of the decrease of the MFCV was extremely intense during a sustained contraction of 70% MVC. The values of the MFCV at the location of 5 mm from the end-plate in the period close to the exhaustion state showed a significant decrease compared with the values in the initial period during the contractions for the above three kinds of loads, while the decrease of the MFCV at the location of 30 mm from the end-plate was not found to be so significant during the contractions. In the recovery period, the contraction of 5% MVC was maintained, and the resultant MFCVs restored gradually to the value of the initial period. At the location of 5 mm from the end-plate, the MFCVs at 15 minutes after the end of the load were significantly higher than those just after the end of the load. At the location of 30 mm from the end-plate, the increases of the MFCVs during the recovery period did not show significant changes. Changes of the MFCV during the isometric contraction and the recovery period depend greatly on the location of the electrodes measured for the action potentials of the muscle fibers.     

The advertisement role of major urinary proteins in mice

The variation of expression of major urinary proteins was studied in laboratory mice to further the understanding of the role of these proteins in social and reproductive contexts. Mouse major urinary proteins (MUPs) are known to carry volatile substances and protect them during their passage from the liver, through the kidneys into the urine. However, most studies on the role of MUPs were carried out on males. Using densitometry analysis of total MUP concentration in the urine, our present study clearly demonstrates that (i) individuals of both sex up-regulate MUPs during social contact, and that (ii) females use these proteins to advertise their reproductive state by varying the concentration of MUPs during the oestrous cycle. As the concentration of MUPs was normalized by the concentration of creatinine -- a marker of glomerular filtration -- the corrected concentration of MUPs represents instantaneous expression on the level of proteins. Cross-correlation analysis between oestrus quantification and MUP expression revealed that the oestrous curve is delayed by 1 day behind the MUP curve so that the expression of MUPs is up-regulated immediately at the beginning of oestrus. To conclude, the regulation of pheromone-carrying MUPs is directly linked to reproduction and, thus, enables female honest signalling.     

The effect of atropine on the frog sartorius neuromuscular junction

1. The effect of atropine sulphate and of (+)-tubocurarine chloride (TC) on the amplitude and time course of end-plate potentials (e.p.p.s) and miniature end-plate potentials (m.e.p.p.s) was studied in the sartorius muscle of the frog.2. Atropine sulphate reduces the amplitude of intracellularly recorded e.p.p.s and m.e.p.p.s, in concentrations 100 times higher than TC (6 x 10(-5)M for 50% reduction of amplitude compared with 6 x 10(-7)M for TC).3. Atropine sulphate causes a marked shortening of both e.p.p.s and m.e.p.p.s: when the amplitude of e.p.p.s or m.e.p.p.s is reduced by 50%, their rise-time and half-decay time are both shortened by 40%. The corresponding shortening produced by TC is 15%.4. E.p.p.s prolonged by prostigmine 10(-6) g/ml. undergo a larger shortening (30%) in TC, while atropine-induced shortening related to the corresponding drop of amplitude is the same whether prostigmine is used or not.5. On repeated applications after recovery of amplitude and time course, TC loses its shortening effect on e.p.p.s while the atropine shortening effect remains unchanged.6. Atropine sulphate shortens the rise-time but not the falling phase of brief depolarizations produced by electrophoretic applications of acetylcholine (ACh) to the muscle fibre surface in the end-plate region. It also reduces their amplitude, in the same way that it reduces the amplitude of e.p.p.s.7. Atropine sulphate in concentrations which markedly reduce the amplitude and time course of e.p.p.s has no effect on their quantum content.8. Atropine sulphate at a concentration of 10(-4)M does not change the amplitude and time course of an electrotonic potential produced by a rectangular current pulse passed through the end-plate region of a muscle fibre.9. It is suggested that either enhanced removal of ACh or a spatial gradient of effectiveness of the blocking drugs, or both these mechanisms, participate in shortening the e.p.p. by atropine sulphate and TC.     

The presence of unknown layer of skin and fat is an obstacle to a correct estimation of the motor unit size from surface detected potentials

To overcome problems with a strong distance-dependence of the motor unit potentials (MUPs), different methods to estimate the MU location and size have been proposed. Distance-independence of the exponent of the power function, that describes the MUP distance decline, and homogeneity of the volume conductor, are assumed in all methods. Some of them consider the exponent value as unique, irrespective of persons, muscles and their functional state. One method estimates the current exponent value. We evaluate this method by computer simulation of MUPs in infinite and semi-infinite volume conductor. Our results show that although the first assumption is not fulfilled, it does not affect considerably the estimate of the MU location and size obtained for infinite or semi-infinite homogeneous volume conductor. The errors of the MU location can be insignificant even in inhomogeneous volume conductor with a layer of lower conductivity (skin and fat) between the muscle tissue and electrode. The accurate location of the MU electrical axis is, however, not a sufficient condition for a correct MU size estimation that depends considerably on actual parameters of the layer. Thus, the surface EMG could hardly be considered as non-invasive alternative to macro EMG for detection of the enlarged MUs.     

Distribution of muscle fiber conduction velocity of m. masseter during voluntary isometric contraction

Muscle fiber conduction velocity (MFCV) is the velocity of an interference wave due to muscle fiber action potentials. In general, it has been reported that the value of MFCV in m. masseter is larger than that in limb and trunk muscles. But the values of MFCV in the reports were measured in part of the muscle, and the distribution of MFCV in the whole muscle including the end-plate and the tendon has not been measured. In this study, surface myoelectric signals are recorded in m. masseter during voluntary isometric contractions of 20, 30, and 40% MVC (maximum voluntary contraction) in eleven healthy male subjects with the use of array electrodes. The value of MFCV is found directly using an averaging method. The end-plate zone is concentrated in the lower quarter of the muscle. The distribution of MFCV depends on the location of the measured electrode. The largest value of MFCV of more than 20.0 m/s is obtained in the locations of the end-plate and the tendon. The minimum value of MFCV is obtained at the location of 10 mm measured from the end-plate along the direction of the muscle fiber to the tendon of the upper side. The mean values with the standard deviations during 20, 30, and 40% MVC are 10.3 +/- 0.7, 11.6 +/- 0.7, and 12.2 +/- 0.8 m/s, respectively. The MFCVs between the different neighboring locations are compared and found to be significant by a level of 1% for each contraction level. The MFCVs increase depending on the contraction levels for various electrode locations on the muscle.     

Peripheral effects on the amplitude of monopolar and bipolar H-reflex potentials from the soleus muscle

Summary Variations in the amplitude of mono- and bipolarly measured H-reflex potentials can be influenced by muscle architecture and changes in muscle length. In the passive soleus muscle with the ankle joint fixed at 90°, the maximal-amplitude bipolar H-potentials were obtained along the midline of soleus at a distance of 2.0–4.0 cm below the insertion of the gastrocnemii on the Achilles tendon. In contrast, the optimal location of monopolar H-potentials was 5.0–8.0 cm below the gastrocnemii insertion. Stepwise passive shortening of soleus resulted in an increase in the amplitude of both H- and motor-unit potentials. This correspondence implicates peripheral factors, such as changes in muscle fibre diameter and inclination to the skin surface, as mechanisms mediating the changes in the amplitude of the potentials. Such effects necessitate caution in interpretation of the association between H-potential amplitude and monosynaptic reflex excitability.     

Motor unit changes in inflammatory myopathy and progressive muscular dystrophy

The aim of present study was to analyse the motor unit (MU) changes in progressive muscle dystrophy (PMD) and in inflammatory myopathy (IM) and to evaluate eventual neurogenic factors in MU reorganisation. The material consisted of 20 patients with (PMD), 20 patients with (IM) and 20 healthy age-matched volunteers. The shape of concentric needle motor unit potentials (cn MUPs), including their duration, amplitude, area, size index and number of phases, the interference pattern and the amplitude and area of macro MUPs were evaluated. The cn emg data satisfied the classical criteria for myopathy in all examined patients, at least in one of the tested muscles. A decreased amplitude and/or area of macro MUPs, compatible with myopathy, were observed in 32 of the 40 patients. In some cases of chronic IM and PDM the long duration polyphasic potentials were recorded. The size index (SI) value of long polyphasic MUPs was usually decreased or normal. This feature indicated that desynchronisation of "myopathic" MUPs results from a reduced number of muscle fibers and their degeneration and regeneration. The results indicated no difference in MU reorganization between PMD and IM and no evidence of neurogenic factors in MU changes.     

A review of clinical quantitative electromyography

Information regarding the morphology of motor unit potentials (MUPs) and motor unit firing patterns can be used to help diagnose, treat, and manage neuromuscular disorders. In a conventional electromyographic (EMG) examination, a clinician manually assesses the characteristics of needle-detected EMG signals across a number of distinct needle positions and forms an overall impression of the condition of the muscle. Such a subjective assessment is highly dependent on the skills and level of experience of the clinician, and is prone to a high error rate and operator bias. Quantitative methods have been developed to characterize MUP waveforms using statistical and probabilistic techniques that allow for greater objectivity and reproducibility in supporting the diagnostic process. In this review, quantitative EMG (QEMG) techniques ranging from simple reporting of numeric MUP values to interpreted muscle characterizations are presented and reviewed in terms of their clinical potential to improve status quo methods. QEMG techniques are also evaluated in terms of their suitability for use in a clinical decision support system based on previously established criteria. Aspects of prototype clinical decision support systems are then presented to illustrate some of the concepts of QEMG-based decision making.     

Effects of muscle shortening on the responses of cat tendon organs to unfused contractions

1. The discharges from individual Golgi tendon organs of peroneus tertius and brevis muscles were recorded in anesthetized cats. Responses to unfused isometric contractions of single motor units and combinations of motor units were compared with responses to contractions eliciting muscle shortening (i.e., shortening contractions). 2. In 75% of the examined instances, the effect of muscle shortening during unfused contractions was a slight decrease in tendon organ activation, in keeping with the reduction of contractile tension recorded at the muscle tendon. In other instances there was either no change in tendon organ response or, in less than 10% of instances, a slight increase For two motor units eliciting similar activation of a given tendon organ under isometric conditions, the effect of shortening contraction was not necessarily the same. 3. The reductions observed in tendon organ discharges upon muscle shortening were less than proportional to the reductions of contractile tension and difficult to correlate with the properties of motor units, as determined under isometric conditions. The present observations suggest three main reasons for this lack of relation. 4. The first reason depended on the properties of motor units, in that the relation between length changes and tension changes was not the same for all units. Two motor units developing similar isometric tensions did not necessarily produce the same degree of muscle shortening. Some units produced relatively significant shortening without much loss of tension. 5. Second, the dynamic sensitivity of tendon organs is known to exert a major influence on their responses to isometric unfused contractions, accounting for 1:1 driving of discharge during tension oscillations and high frequency bursts upon abrupt increase of tension. Although less tension was produced and the rate of tension development was slower in shortening contractions, similar manifestations of the dynamic sensitivity of tendon organs were observed. In such cases, the responses of tendon organs were the same whether or not the muscle shortened during contraction. 6. Third, when several motor units were stimulated in combination, the unloading influences of in-parallel units were facilitated by muscle shortening so that unloading effects, which were hardly visible under isometric conditions became evident during shortening contractions.     

Presynaptic effects of cholecystokinin octapeptide on neuromuscular transmission in the frog

Intracellular recordings were obtained from the frog sartorius muscle end-plate to investigate the effects of cholecystokinin octapeptide (CCK-8) on cholinergic transmission at the neuromuscular junction. A brief bath-application of CCK-8 (1 microM) produced a depression, followed by a long-lasting facilitation, of the amplitude and the quantal content of the end-plate potential (epp). CCK-8 had a biphasic effect, an initial depression followed by an augmentation of the frequency of the miniature epps. CCK-8 did not affect the sensitivity of the nicotinic receptor at the end-plate. These results suggest a significant role for CCK-8 in cholinergic transmission, possibly as a modulator of the evoked release of acetylcholine from motor nerve terminals.     

Correlation between nerve terminal size and transmitter release at the neuromuscular junction of the frog

1. End-plate potentials were recorded intracellularly at the frog neuromuscular junction bathed in a solution containing a low concentration of calcium and a high concentration of magnesium.2. The muscle was subsequently subjected to ;cholinesterase staining', and the area of the individual end-plates, studied with intracellular electrodes, was measured.3. A positive correlation was found between the end-plate area and the diameter of muscle fibres.4. The mean quantum content (m) showed a positive correlation with the size of end-plates.5. The frequency of spontaneous miniature end-plate potentials was positively correlated with m as well as with end-plate area.6. It is concluded that the amount of transmitter released following nerve stimulation is related to the size of nerve endings.     

Intra- and extracellular measurements of frog neuromuscular transmission upon stretch of the muscle at different stimulus frequencies

Flexible intracellular micro-electrodes were used to study the effect of changes in muscle length on the end-plate potential in the isolated m. cutaneus pectoris for different frequencies of stimulation (1/60-5Hz). A 20% step-wise increase in muscle length within the physiological range increases the end-plate potential immediately by about 50% (range 0-120%) at all frequencies tested. At stimulus frequencies lower than 1/5 Hz this increase is sustained during a period of 15 min stretch. At 1 Hz, however, the initial increase in the end-plate potential amplitude on the average declines within a few minutes to a steady-state value about 35% higher than the steady-state end-plate potential before stretch. At 5 Hz, the initial amplitude increase is followed by a decline of about 15 min duration and the final amplitude is not increased in comparison with the pre-stretch amplitude. The amplitude of the compound muscle action potential of the gastrocnemius muscle with intact circulation shows a similar time dependent increase upon stretch at different stimulus frequencies. It is concluded that stretch of a frog muscle gives both an immediate and a sustained increase in transmitter release from the nerve terminals during prolonged stimulation at frequencies up to about 5 Hz. This effect of stretch on transmitter release can improve in vivo neuromuscular impulse transmission.     

Anatomic variations of the musculotendinous junction of the soleus muscle and its clinical implications

The soleus muscle, like the gastrocnemius, is a powerful plantarflexor muscle in the lower limb. The soleus muscle joins the aponeurosis of the gastrocnemius muscle to form the calcaneal (Achilles) tendon. While the basic anatomy of the soleus muscle has been previously described, no study has addressed the anatomical variations of its distal attachment. We found considerable anatomic variation in the distance between this musculotendinous junction and the most proximal point of the proximal edge. This distal measuring point was defined as the most proximal point of the proximal edge of the posterior surface of the calcaneal tuberosity. Eighty human cadaver specimens were preserved according to Thiel's method; we examined one limb from each cadaver, studying 80 lower extremities in total. Following careful dissection of the lower limb, we measured the distance between the distal point of attachment of the soleus muscle fibers (the musculotendinous junction) and the designated measuring point. Our findings were divided into three groups: Group 1 (10 cases, 12.5%), where the distance between the musculotendinous junction and the designated point on the calcaneal tuberosity was between 0 and 1 inches; Group 2 (56 cases, 70%), where the distance was between 1 and 3 inches; and Group 3 (14 cases, 17.5%), where the distance was greater than 3 inches. Detailed knowledge of the anatomic variations of the soleus muscle at its insertion point onto the calcaneal tendon has clinical implications in calcaneal tendon repair following rupture and in the planning of reconstructive surgery using soleus muscle flaps.