Effect of length change in muscle fibers on conduction velocity in human motor units

Length and conduction velocity were determined in muscle fibers belonging to the single motor unit (right m. vastus medialis) in the living human body. A new method was developed for measuring the length of fibers, i.e., the muscle length was defined to be the distance between the starting point of excitation (motor end-plate) and the distal end of muscle fibers belonging to the particular motor unit. Both points were decided by analyzing the shape of the spike potentials of the motor unit recorded by surface electrodes. The length increased by about 70% (ranging from 48% to 97%) when changing the knee angle from fully extended to the fully flexed position, whereas the conduction velocity decreased by about 26% (ranging from 17 to 36%). It can be considered that the decrease of the conduction velocity when increasing the length of the muscle fibers was mainly due to the decrease in fiber diameter.     

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.     

Conduction velocity in human muscle fibers in situ. Study in upper and lower limbs in healthy adults

The muscle fibers of the human biceps brachii and vastus medialis muscles were stimulated with low voltages in their distal portion by means of monopolar needles in 50 healthy adults. The electrical activity was recorded proximally by means of a SFEMG electrode. The conduction velocity of the muscle fibers in situ calculated with this method had a Gaussian distribution for both muscles, showed values slightly higher in males than in females, did not decrease significantly with ageing, and was significantly related to limbs perimeter. The recorded potentials and their propagation velocity were the same before than after curarization. The possible interest of the method for testing the muscle fiber function and size in pathological conditions of the nerve and muscle is suggested.     

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.     

Reduced endplate currents underlie motor unit dysfunction in canine motor neuron disease

Hereditary canine spinal muscular atrophy (HCSMA) is an autosomal dominant degenerative disorder of motor neurons. In homozygous animals, motor units produce decreased force output and fail during repetitive activity. Previous studies suggest that decreased efficacy of neuromuscular transmission underlies these abnormalities. To examine this, we recorded muscle fiber endplate currents (EPCs) and found reduced amplitudes and increased failures during nerve stimulation in homozygotes compared with wild-type controls. Comparison of EPC amplitudes with muscle fiber current thresholds indicate that many EPCs from homozygotes fall below threshold for activating muscle fibers but can be raised above threshold following potentiation. To determine whether axonal abnormalities might play a role in causing motor unit dysfunction, we examined the postnatal maturation of axonal conduction velocity in relation to the appearance of tetanic failure. We also examined intracellularly labeled motor neurons for evidence of axonal neurofilament accumulations, which are found in many instances of motor neuron disease including HCSMA. Despite the appearance of tetanic failure between 90 and 120 days, average motor axon conduction velocity increased with age in homozygotes and achieved adult levels. Normal correlations between motor neuron properties (including conduction velocity) and motor unit properties were also observed. Labeled proximal motor axons of several motor neurons that supplied failing motor units exhibited little or no evidence of axonal swellings. We conclude that decreased release of transmitter from motor terminals underlies motor unit dysfunction in HCSMA and that the mechanisms determining the maturation of axonal conduction velocity and the pattern of correlation between motor neuron and motor unit properties do not contribute to the appearance or evolution of motor unit dysfunction.     

Conduction velocity along muscle fibers in situ in healthy infants

The muscle fibers of the biceps brachii were stimulated distally with low voltages by means of two monopolar needles in twenty-two infants aged 2 to 14 years. The electrical activity was recorded proximally by means of a SFEMG electrode. Conduction velocity of the muscle fibers (MFCV) in situ calculated with this method had a bimodal distribution in the youngest individuals of less than 4 years, and a Gaussian distribution in children aged 5 to 14 years. Propagation velocity along muscle fibers increases with children's age and is significantly slower than in adults, in good correlation with the shorter limb perimeter and with the significantly smaller fiber diameter found in muscle biopsies in infancy. The latency of the evoked potentials was linearly related with the distance between stimulating and recording points, and muscle activity and propagation velocities were the same before and after curarization, that is consistent with the reliability of the method in infancy.     

Conduction velocity of quiescent muscle fibers decreases during sustained contraction

We tested the hypothesis that conduction velocity of quiescent muscle fibers decreases during sustained contraction due to the activity of the active motor units in the muscle. Ten subjects trained for the identification of a target motor unit in the abductor pollicis brevis with feedback on surface EMG signals detected with a two-dimensional array of 61 electrodes. The subjects activated the target motor unit in two 10-s long contractions, before (contraction C1) and after (C3) a 3-min contraction (C2), all in ischemic condition. The target motor unit was not activated during C2. Eight of the 10 subjects (control group) performed a second experimental session identical to the first but with a resting period of 3 min instead of the contraction C2. Exerted force and target motor unit discharge rate were not different between the two subject groups and between C1 and C3 (mean +/- SD, over C1 and C3; C2 group: 15.8 +/- 10.4% maximal voluntary contractions and 13.1 +/- 1.9 pps; control group: 15.6 +/- 22.1% maximal voluntary contractions and 14.5 +/- 1.9 pps, respectively). Muscle fiber conduction velocity of the target motor unit decreased in C3 with respect to C1 in the C2 group (3.59 +/- 0.57 and 3.34 +/- 0.47 m/s for C1 and C3, respectively; P < 0.05) but not in the control group (3.47 +/- 0.68 and 3.46 +/- 0.73 m/s). In the C2 group, the percent decrease in conduction velocity of the target motor unit between C1 and C3 (6.4 +/- 7.1%) was not significantly different from the percent decrease in the average conduction velocity of the motor units active during C2 (9.6 +/- 5.4%). In conclusion, the contraction-induced modifications in electrophysiological membrane properties of muscle fibers are partly independent on fiber activation.     

Fiber density in congenital muscle fiber type disproportion. I. Congenital myopathies

In 15 cases with congenital muscle fiber type disproportion, concentric needle EMG (CNEMG) and single fiber EMG (SFEMG) with fiber density (FD) estimation was performed. The aim of the work was to establish the nature of congenital muscle fiber type disproportion. In 11 cases CNEMG revealed interference pattern on maximal effort and low, polyphasic motor unit action potentials (MUAPs) of short duration indicating a myopathic lesion. In four other cases electromyographic changes were less evident. The FD values were slightly increased in 9 cases. This finding could be explained by pronounced muscle fiber splitting with subsequent ephaptic transmission or even secondary denervation and reinnervation. The most important conclusion from our pilot study is the confirmation of the fact that reinnervation with excessive sprouting is not the only mechanism responsible for muscle fiber type disproportion.     

A method for determination of muscle fiber diameter using single fiber potential (SFP) analysis

We have used computer simulation to study the relationship between the muscle fiber diameter and parameters: peak-to-peak amplitude and duration of the negative peak of the muscle fiber action potential. We found that the negative peak duration is useful in the determination of fiber diameter via the diameter dependence of conduction velocity. We have shown a direct link between the underlying physiology and the measurements characterizing single fiber potential. Using data from simulations, a graphical tool and an analytical method to estimate the muscle fiber diameter from the recorded action potential has been developed. The ability to quantify the fiber diameter can add significantly to the single fiber electromyography examination. It may help study of muscle fiber diameter variability and thus compliment the muscle biopsy studies.     

Facioscapulohumeral muscular dystrophy. A quantitative electromyographic study

BACKGROUND: Quantitative electromyography (EMG) using different needle techniques has not been performed or reported on a relatively large group of patients with facioscapulohumeral muscular dystrophy (FSHD). Purpose: To establish statistically: (1) correlations between clinical features of patients (age, disease duration and degree of weakness) and quantitative needle EMG/SFEMG,; (2) correlations between different EMG parameters in the patient group, and (3) quantitative EMG differences comparing patients with a healthy control group. METHODS: Nerve conduction studies, and needle EMG (motorunit analysis, MacroEMG, SFEMG) were performed on Mm. triceps brachii and Mm. tibialis anterior according to standard techniques on 20 patients with FSHD. RESULTS: Nerve conduction studies were normal. In Mm. triceps brachii and, to a lesser extent, Mm. tibialis anterior motorunit analysis and MacroEMG showed myopathic changes, that correlated with patient clinical parameters. In Mm. triceps brachii (but not in Mm. tibialis anterior) EMG results were statistically different in patients compared to control group data. The most sensitive indicators of a myopathy were MUP duration (motorunit analysis) and MUP area (MacroEMG). In the Mm. triceps brachii SFEMG revealed correlations between worsening pooled MCD data and patient clinical parameters. Pooled MCD results did not correlate with other MUP parameters. SFEMG showed abnormal jitter only in 2 patients with the longest disease duration. CONCLUSION: Quantitative EMG results are compatible with a mild, slowly progressive myopathy. The most sensitive indicators of early muscle disease were MUP duration (motorunit analysis) and MUP area (MacroEMG) that would not be detected on "routine" EMG SFEMG showed subtle, progressive worsening of neuromuscular junction physiology. However, quantitative EMG and SFEMG showed that muscle fiber degeneration and loss followed a course independent of muscle fiber regeneration and reinnervation.     

单纤维肌电图(SFEMG)的研究进展

单纤维肌电图(single fiber electromyogram,SFEMG)研究的主要内容是骨骼肌同一运动单位相邻两条肌纤维在重复兴奋过程中,电位对时间间隔的变异,即颤抖(jitter)。两根肌纤维的放电间在2—20mS之间,而人体骨骼正常jitter值一般为5-55uS,因此在引导、测量、分析方面难度较大。随着科学技术的发展,SFEMG已在生理学科的研究和临床医学诊断等领域中占有重要地位。本文阐述了单纤维肌电图jitter的产生、引导、测量、分析的方法及应用价值。     

Isometric contractions of motor units in a fast twitch muscle of the cat

1. Isosmetric contractions of cat flexor digitorum longus whole muscles and of functionally isolated motor units have been measured under conditions similar to those used by Buller & Lewis (1965a).2. Motor unit twitch time to peak was inversely related to axonal conduction velocity. The logarithm of tetanic tension was directly related to conduction velocity. These relationships suggest that each motoneurone has an influence on the muscle fibres which it innervates.3. The ratio of twitch to tetanic tension was directly related to the time to peak of the motor unit. This fact might be explained by variation between motor units of the duration of ;active state'.4. The muscle length at which tension was maximal varied between motor units and the optima were found over the range of muscle lengths which could occur in the body. Slow motor units had longer optimal lengths.5. The sample of motor units was considered to be unbiased because the distribution of axon conduction velocities was compatible with reported motor fibre diameter spectra of the muscle nerve. The mean motor unit tetanic tension gave a reasonable estimate of the number of alpha-motor axons in the muscle nerve. Twitch tensions gave a value that was 40% higher.6. Motor unit and whole muscle data were in good agreement for length-tetanus tension curves, for times to peak and for twitch-tetanus ratios at long muscle lengths.     

Muscle fiber conduction velocity studied by the multi-channel surface EMG.

Muscle fiber conduction velocity in single motor unit (MU) during voluntary contraction was measured by using the multi-channel surface EMG. The subjects examined were 28 normal controls (5-40 years old), four patients with myopathy and three patients with neuropathy. The tibialis anterior and the biceps brachii muscles were investigated at weak and moderate contraction levels. In normal muscles, the mean muscle fiber conduction velocity increased with muscle force (P less than 0.01). In three of four cases with myopathy, the conduction velocities were reduced compared to normal subjects (P less than 0.01), and there was little or no correlation between the conduction velocity and muscle force in myopathy. The conduction velocities were within the normal range in the muscles of patients with neuropathy. However, high-amplitude MUAPs with fast conduction velocities were detected during weak voluntary contraction in one case with neuropathy.     

Effects of muscle fibre shortening on the characteristics of surface motor unit potentials

Traditionally, studies dealing with muscle shortening have concentrated on assessing its impact on conduction velocity, and to this end, electrodes have been located between the end-plate and tendon regions. Possible morphologic changes in surface motor unit potentials (MUPs) as a result of muscle shortening have not, as yet, been evaluated or characterized. Using a convolutional MUP model, we investigated the effects of muscle shortening on the shape, amplitude, and duration characteristics of MUPs for different electrode positions relative to the fibre–tendon junction and for different depths of the MU in the muscle (MU-to-electrode distance). It was found that the effects of muscle shortening on MUP morphology depended not only on whether the electrodes were between the end-plate and the tendon junction or beyond the tendon junction, but also on the specific distance to this junction. When the electrodes lie between the end-plate and tendon junction, it was found that (1) the muscle shortening effect is not important for superficial MUs, (2) the sensitivity of MUP amplitude to muscle shortening increases with MU-to-electrode distance, and (3) the amplitude of the MUP negative phase is not affected by muscle shortening. This study provides a basis for the interpretation of the changes in MUP characteristics in experiments where both physiological and geometrical aspects of the muscle are varied.     

Myopathy in acromegaly. A case study

The muscle findings in a patient with acromegaly of 20 years duration are presented. The patient demonstrated mild muscular weakness and atrophy. Standard needle electromyography was normal and single fiber electromyography showed normal motor unit densities. Muscle biopsy revealed normal size type-I muscle fibers, but three populations of type-II fibers - hypertrophied, atrophied, and normal-sized fibers. Hypertrophy of type-II fibers may be the direct result of the excess growth hormone, while the atrophic fibers may result from the disturbance of other endocrine function.     

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.     

EMG dynamics in polymyositis and dermatomyositis in adults

In order to analyze the EMG dynamics in acute and chronic polymyositis 44 patients were examined. Thirty-four were seen in the acute stage, 28 in the chronic stage and 18 serially. Investigations included quantitative electromyography using the Polish minicomputer "ANOPS 105" connected to a DISA electromyograph. Additionally fiber density was analyzed by single fiber electromyography in the chronic stage only. The acute stage findings confirmed the observations of earlier authors with the classical expression of excessive spontaneous activity, polyphasic potentials of short duration and low amplitude. In the chronic stage, motor unit potentials with increased duration and amplitude and with late components of the type seen in satellite potentials were noted. This was compared with the increased fiber density found at this stage. Additionally, in some muscles in the chronic stage, motor unit potentials were seen with increased duration, but also a reduction in the mean amplitude of motor unit potentials counted by the automatic analysis method. The decreased amplitude of the motor unit potentials in the chronic polymyositis may be the result of the smaller size of regenerating muscle fibers.     

甲状腺机能亢进伴发重症肌无力患者电生理研究

目的：探讨甲状腺机能亢进（甲亢）伴发重症肌无力（MG）患者的电生理检测特点，和与慢性甲状腺机能亢进性肌病（CTM）的关系。方法：对7例甲亢伴发MG患者进行神经传导速度（NCV）、重复电刺激（RNS）、针极肌电图（EMG）和单纤维肌电图（SFEMG）检测。结果：7例NCV均正常，RNS5例异常。EMG2例发现有肌病的表现，SFEMG均出现jitter增宽。结论：甲亢病人可伴发MG，还可能同时伴发CTM，进行RNS、EMG、SFEMG检测是有必要的。     

Changes in properties of the medial gastrocnemius motor units in aging rats

1. The properties of motor units were investigated in the medial gastrocnemius (MG) of old rats [27.5 +/- 1.6 (SD) mo old, n = 18]. Individual motor units were functionally isolated by ventral root fiber splitting and grading stimulus intensity. The muscle-unit portion of the motor unit was identified by the glycogen depletion method. The physiological properties of 77 motor units in 6 animals and the histological results of 7 slow-twitch (type S) muscle units were compared with data from motor units in the same muscle of middle-aged rats (12.8 +/- 1.6 mo old, n = 33). 2. The motor units were classified into four types of categories [FF (fast-twitch motor units with a fatigue index less than or equal to 0.5), FI (fast-twitch motor units with a fatigue index greater than 0.5 but less than 0.75), FR (fast-twitch motor units with a fatigue index greater than or equal to 0.75), S (slow-twitch motor units with a fatigue index greater than 0.75)] using the same criteria (i.e., presence or absence of the "sag" property and fatigability) used for middle-aged rats. No significant difference in the relative distributions of these unit types was detected, although the MG muscle in old rats exhibited a relatively high proportion of type S units and fewer type FR units. 3. The mean tetanic tensions for type FF + FI and FR units were significantly smaller than those in the middle-aged rats. On the other hand, type S motor units produced more tension than in the middle-aged rats. 4. The conduction velocity of motor axons was considerably slower in any unit type of old motor units, and the most marked change was found in type FR units. 5. The general morphological features of the old rat MG were fiber-type grouping, disseminated atrophic or angulated fibers, a decrease in the total number of muscle fibers, and an increase in the number of type I muscle fibers. The major distribution patterns of fibers of different types were the same as those in the middle-aged MG. 6. Seven type S units that produced large tetanic tension were depleted of glycogen in the muscle-unit portions. These units had a large innervation ratio compared with those in the middle-aged rats, whereas the mean cross-sectional area of muscle fibers and the calculated specific tension remained unaltered.(ABSTRACT TRUNCATED AT 400 WORDS)     

Proportional enlargement of motor units after partial denervation of cat triceps surae muscles.

1. To determine the capacity of motoneurons to increase their motor unit (MU) size by collateral sprouting and to assess this capacity in relation to the size of the motor nerve, we partially denervated soleus, lateral gastrocnemius (LG), and medial gastrocnemius (MG) muscles in adult and neonatal cats. Isometric force and extracellular nerve potentials were recorded from > or = 7% of the remaining MUs, 2.5-18 mo later. S1 or L7 roots were sectioned unilaterally and the number of remaining MUs was quantified by use of charge and force measurements. 2. The mean unit force increased inversely with MU number in partially denervated muscles, but the increase was less than predicted for extensive denervations (> or = 90%). The same enlargement of MU size occurred whether muscles were partially denervated in neonatal or adult animals. 3. The force distribution of MUs in partially denervated muscles was similar to normal but was shifted to larger force values in direct proportion to the extent of partial denervation (PD). All MUs increased in size by the same factor to preserve the normal force distribution. 4. Normal size relationships among force, contractile speed, and axon potential amplitude were observed for MUs in partially denervated muscles. Because changes in muscle fiber size could not account for the increase in unit force, these data show that increase in MU size, with respect to unit force and innervation ratio (muscle fibers per motoneuron), is proportional to the size of the motor nerve. 5. Enlargement of MU size in partially denervated muscles did not have a retrograde effect on nerve fiber caliber because axon potential amplitude and conduction velocity were not changed after PD. 6. Under conditions of extensive PD (> 85%), regenerated nerves from the cut spinal root reinnervated the gastrocnemius muscles. It is likely that nerves supplied muscle fibers that were not innervated by sprouts from nerves in the uncut root as well as displacing sprouts from terminals in extensively enlarged MUs. 7. We conclude that all motoneurons within a motor pool compensate for partial nerve injuries by collateral sprouting and that enlargement of MU size is a function of motor nerve size, consistent with Henneman's size principle.