M-wave Amplitude Research Articles

Effect of acute hyperoxia during exercise on quadriceps electrical activity in active COPD patients.

This study investigated whether acute hyperoxia improves electrical muscle activity in active chronic obstructive pulmonary disease (COPD) patients with mild hypoxemia (rest PaO(2) = 9.1 +/- 0.4 kPa). Two identical incremental exercise tests were performed by nine patients while breathing either air or 30% oxygen. Pulmonary gas exchanges, venous concentrations of lactate and pyruvate, and the electromyographic signal of the quadriceps muscle (vastus lateralis and vastus medialis) were sampled each minute. Peak working capacity increased significantly in hyperoxia (94.4 +/- 5.2W) compared with normoxia (85.4 +/- 5.8W, P < 0.01). During hyperoxic exercise and for a given work load, oxygen uptake was increased (P < 0.001) and ventilation decreased (P < 0.05). Lactate concentration was significantly decreased (P < 0.01) at isowork level and during recovery (respectively - 26% and at least - 15%). In the quadriceps muscle, M-wave amplitude (P < 0.05), root mean square (P < 0.01) and root mean square/oxygen uptake ratio (P < 0.001) were significantly increased during hyperoxic exercise compared with room air. Although median frequency values did not differ between conditions, the median frequency was significantly decreased for higher exercise intensity in hyperoxic condition. These modifications reflected better aerobic metabolism, later emergence of muscle fatigue, and greater muscle excitability and activation for the same level of exercise under hyperoxic condition. These data suggest that the acute addition of oxygen in active COPD patients improves their muscle electrical activity during dynamic exercise. Hypoxemia-induced skeletal muscle dysfunction most probably acts through mechanisms based on oxygen availability.

Decreased stiffness at the ankle joint in patients with long-term Type 1 diabetes.

To evaluate contractile and reflex properties of the soleus muscle together with ankle joint stiffness in long-term Type 1 patients. Stretch reflex and muscle function of the soleus muscle was tested in 15 Type 1 diabetes mellitus patients and 15 controls. M-wave amplitude and maximal twitch torque was elicited by supramaximal stimulation of the tibial nerve. The stretch reflex was elicited by a 4 degree rotation of the ankle. Rise time and fall time of the maximal twitch torque were increased compared with controls. The amplitude of the short latency stretch reflex was significantly reduced in patients at contraction levels of 5-30 Nm. At a contraction level of 5 Nm the median stretch reflex threshold was 10 degrees/s in patients and 6 degrees/s in controls (P = 0.03). Stiffness was decreased in diabetic patients, especially at lower contraction levels. There was no significant correlation between the clinical neuropathy score and stiffness. In contrast, high neuropathy score was correlated with low amplitude of the reflex amplitude (rho = -0.51, P = 0.05). We demonstrate altered contractile properties, a decrease in stretch reflex of the soleus muscle, and a reduced stiffness at the ankle joint in patients with long-term Type 1 diabetes. These changes may cause delayed muscle contraction and impaired reflex modulation which could contribute to gait disturbances and increased number of falls in diabetic patients.

New method using multi-regression analysis on evoked electromyography during movement to adjust stimulation conditions.

A new method of stimulation of the lower extremities was devised that automatically adjusts the stimulation conditions at any angle of the knee joint. An M-wave is considered to indicate the stimulation conditions, because it is the waveform that results from direct stimulation to the axon of the alpha motor neuron. The present device adjusted the stimulation intensity, using multi-regression analysis to evoke an M-wave of preset amplitude. Participants included five people without any neuromuscular impairment. The subjects sat on a chair during the test. The hip joint was fixed at a flexion angle of 90 degrees, and the ankle joint was fixed at the midposition. During passive knee joint movement ranging from 0 degrees to 135 degrees, M-waves were measured. Electrodes were attached at the popliteal fossa and the patella to stimulate the tibial nerve. Ag-AgCl electrodes were put on the belly of the right soleus muscle for the M-wave measurement. The device was set to give M-waves close to the preset value, 10% Mmax. According to previous research, the allowable limit of M-wave amplitude deviations was reported to be about 5% Mmax. The M-wave amplitudes evoked by the device were in the allowable range (9.2 +/- 2.5% Mmax). The device enabled control of the M-wave amplitude over the entire range of motion of the joint. Using this device, it was possible to examine the excitability of the alpha motor neuron pool more precisely.

短期筋不活動に伴う示指外転出力レベルと機能低下の特性

Although a large number of studies have been conducted into negative influences on motor function after long-term muscle disuse, little is known about the effect of short-term muscle disuse and its mechanism. Moreover, the aspect of functional decrease during short-term muscle disuse may be different at high and low levels of muscle output. Knowledge of this will be useful for the development of appropriate exercise prescriptions and health-welfare devices that can prevent creeping functional decrease. This study examined the influence of seven days of immobilization on motor function at high- and low-level contraction force with abduction of the index finger.Healthy subjects (12 males, mean 25.9 years) participated in the present study. Their left hands were immobilized with plaster casts. Changes in the cross-sectional area (CSA) of the first dorsal interosseous (FDI) were evaluated with a magnetic resonance imaging system. Subjects performed maximum voluntary contraction (MVC) with isometric abduction of the index finger and they were regulated at the constant force levels of 500 g, 1000 g, 1500 g, and 2000 g. Standard deviation of the force trajectory (F-S.D.) was calculated at each force level as an index of stability of the force regulation. EMG activities were recorded with surface electrodes placed over the FDI. The root mean square (RMS) of the EMG amplitude was calculated at each force level and was normalized with the amplitude of supra-maximum M-wave in order to compare the degree of change by muscle immobilization. Twitch force evoked by supramaximum electrical stimulation at rest was measured to consider factors of the peripheral neuromuscular system. Twitch force at MVC was measured by the twitch interpolation method to consider central factors. Firing rate of motor units at super 80% MVC was then measured. These measurements were taken before and after the immobilization and again in the recovery period.The CSA of the FDI showed no significant changes by short-term immobilization. In contrast, MVC force and muscle activity (RMS) were lost after short-term immobilization. The decline of twitch force at rest suggests that several troubles occurred in neuromuscular function. In addition, the increased twitch force at MVC and the decreased firing rate suggest that the central drive was reduced by short-term immobilization. In the recovery period, these values almost returned to those before immobilization. Therefore, both the peripheral factor and central factor affect the decline of high-level contraction force during short-term disuse before muscular atrophy.The fluctuation (F-S.D.) during force regulation at the same force in the low level increased after immobilization. This result shows that it became more difficult to regulate constant force levels after short-term disuse. The RMS at the same force level also increased after immobilization. In the recovery period, these values almost returned to those before immobilization. Therefore, the decline of neuromuscular efficiency affects the functional decrease of low-level contraction force during short-term disuse.

Alterations of neuromuscular function after prolonged running, cycling and skiing exercises.

It is well known that impairment of performance resulting from muscle fatigue differs according to the types of contraction involved, the muscular groups tested and the exercise duration/intensity. Depending on these variables, strength loss with fatigue can originate from several sites from the motor cortex through to contractile elements. This has been termed 'task dependency of muscle fatigue'. Only recently have studies focused on the origin of muscle fatigue after prolonged exercise lasting 30 minutes to several hours. Central fatigue has been shown to contribute to muscle fatigue during long-distance running by using different methods such as the twitch interpolation technique, the ratio of the electromyogram (EMG) signal during maximal voluntary contraction normalised to the M-wave amplitude or the comparison of the forces achieved with voluntary- and electrically-evoked contractions. Some central activation deficit has also been observed for knee extensor muscles in cycling but central fatigue after activities inducing low muscular damage was attenuated compared with running. While supraspinal fatigue cannot be ruled out, it can be suggested that spinal adaptation, such as inhibition from type III and IV group afferents or disfacilitation from muscle spindles, contributes to the reduced neural drive after prolonged exercise. It has been shown that after a 30 km run, individuals with the greatest knee extensor muscle strength loss experienced a significant activation deficit. However, central fatigue alone cannot explain the entire strength loss after prolonged exercise. Alterations of neuromuscular propagation, excitation-contraction coupling failure and modifications of the intrinsic capability of force production may also be involved. Electrically-evoked contractions and associated EMG can help to characterise peripheral fatigue. The purpose of this review is to further examine the central and peripheral mechanisms contributing to strength loss after prolonged running, cycling and skiing exercises.

Limiting mechanisms of force production after repetitive dynamic contractions in human triceps surae.

The influence of repetitive dynamic fatiguing contractions on the neuromuscular characteristics of the human triceps surae was investigated in 10 subjects. The load was 50% of the torque produced during a maximal voluntary contraction, and the exercise ended when the ankle range of motion declined to 50% of control. The maximal torque of the triceps surae and the electromyographic (EMG) activities of the soleus and medial gastrocnemius were studied in response to voluntary and electrically induced contractions before and after the fatiguing task and after 5 min of recovery. Reflex activities were also tested by recording the Hoffmann reflex (H reflex) and tendon reflex (T reflex) in the soleus muscle. The results indicated that whereas the maximal voluntary contraction torque, tested in isometric conditions, was reduced to a greater extent (P < 0.05) at 20 degrees of plantar flexion (-33%) compared with the neutral position (-23%) of the ankle joint, the EMG activity of both muscles was not significantly reduced after fatigue. Muscle activation, tested by the interpolated-twitch method or the ratio of the voluntary EMG to the amplitude of the muscle action potential (M-wave), as well as the neuromuscular transmission and sarcolemmal excitation, tested by the M-wave amplitude, did not change significantly after the fatiguing exercise. Although the H and T reflexes declined slightly (10-13%; P < 0.05) after fatigue, these adjustments did not appear to have a direct deleterious effect on muscle activation. In contrast, alterations in the mechanical twitch time course and postactivation potentiation indicated that intracellular Ca(2+)-controlled excitation-contraction coupling processes most likely played a major role in the force decrease after dynamic fatiguing contractions performed for short duration.

Reliability of a new measure of H-reflex excitability.

This study examined the intraclass reliability of different measures extracted from Hoffmann reflex (H-reflex) stimulus-response curve that are used to assess neuromuscular excitability. The following measures were compared: (1) the peak-to-peak amplitude of the H-reflex at a stimulus intensity associated with 5% of the maximum M-wave; (2) the slope of the regression line of the H-reflex stimulus-response curve; and (3) the peak of the first derivative of the H-reflex stimulus-response curve, a new measure introduced in this paper. The H-reflex was elicited in the soleus for 24 subjects (12 males and 12 females) on 5 separate days. Vibration was applied to the Achilles tendon prior to stimulation to test the sensitivity of the measures on test day 4. The stimulus intensity was gradually increased from below the threshold for an H-reflex response to above the maximum M-wave (Mmax) response. The means of 5 evoked potentials at each intensity level were used to create both the H-reflex and M-wave stimulus-response curves for each subject across test days. Determination of reliability involves the consideration of both the stability and consistency of the measures. A repeated measures analysis of variance evaluated the stability of the group means across test sessions. The consistency of scores within individuals was determined by calculating the intraclass correlation coefficient (ICC). Calculation of the 95% confidence interval of estimation was used to assess significant differences between ICCs. The H-reflex measures were both stable and consistent across the first 3 test days. Achilles tendon vibration resulted in a profound reduction (59-70%) on test day 4, and then there was a return to baseline levels on test day 5. The ICC for H-reflex at a stimulus intensity associated with 5% of the maximum M-wave was 0.85. The ICC for the slope of the regression line of the H-reflex stimulus-response curve was 0.79, while it was 0.89 for the peak of the first derivative of the H-reflex stimulus-response curve. However, there was no statistical significance (P>0.05) between the 3 EMG measures of the H-reflex arc. Maximum M-wave amplitude had an ICC of 0.96 attesting to careful methodological controls. The peak of the first derivative of the H-reflex stimulus-response curve was shown to have comparable sensitivity and reliability as other more established measures. The first derivative of the H-reflex stimulus-response curve provides the rate of change, rather than amplitude, making it a robust measure of reflex arc excitability. The higher ICC for the first derivative offers greater statistical power, which is of practical significance.

Hypercapnic impairment of neuromuscular function is related to afferent depression.

Acetazolamide (ACZ), a carbonic anhydrase inhibitor, results in altered neuromuscular function secondary to depressed afferent transmission in intact humans. One effect of ACZ is hypercapnia. Thus, to test if the neuromuscular depression observed following ACZ treatment is related to elevated CO(2), human subjects ( n=10) were exposed to 15 min of room air (0% CO(2)) or hypercapnia (7% inspired CO(2)), and neuromuscular function was evaluated. Isometric force (36.8 to 31.1 N) and peak-to-peak electromyographic amplitude (EMG, 1.5 to 1.0 mV) associated with an Achilles tendon tap, and soleus H(max):M(max) ratio (69.0 to 62.2%) were depressed, while EMG latency (34.8 to 39.8 ms) was increased by hypercapnia. Reflex recovery profiles (following a conditioning tap to the contralateral Achilles tendon), motor nerve conduction velocity, amplitude of the maximum M-wave, and peak twitch tension at M(max) were unaltered by hypercapnia. We conclude that elevated CO(2) impairs neuromuscular function through effects on afferent transmission or synaptic integrity between type Ia fibers of the muscle spindle and the alpha motor neuron, without affecting the muscle spindle, efferent conduction or skeletal muscle force-generating capacity.

The Effects of Pulse Configuration on Magnetic Stimulation

A study is presented in which the authors have examined the effects of pulse configuration, stimulation intensity, and coil current direction during magnetic stimulation. Using figure-8 and circular coils, the median nerve was stimulated at the cubital fossa and at the wrist of 10 healthy volunteers, and the response amplitude and site of stimulation were determined. The key findings of this study are in agreement with other researchers' findings and confirm that biphasic stimulating pulses produce significantly higher M-wave amplitudes than monophasic stimulating pulses for the same stimulus intensity. Mean response amplitudes for biphasic stimuli applied by both coils at the elbow and wrist are consistently higher for the normal current direction. Mean response amplitudes for monophasic pulses are almost always higher for reversed currents. The site for effective stimulation (the position of the virtual cathode) cannot be defined within a fixed distance from the center of the coil (3 to 4 cm), as has been suggested by other researchers, but was found to vary depending on the coil current amplitude and direction as well as the degree of inhomogeneity of the tissues surrounding the nerve. There is a statistically significant relationship between virtual cathode shift and stimulus intensity for biphasic and monophasic pulses. Reversing the coil current direction has no statistically significant effect on the virtual cathode position. Virtual cathode shifts can be measured for biphasic and monophasic stimulations using a figure-8 coil at the wrist and the elbow. However, such a shift is difficult to determine with a circular coil.

Decreased cortical excitability during motor imagery after disuse of an upper limb in humans.

The present study investigated the effect of joint immobilization on corticomotoneuronal excitability to only intracortical input from a hierarchical level above the primary motor cortex. Motor evoked potentials (MEPs) and H-reflexes in the flexor carpi radialis muscle were elicited from 8 orthopedic patients with splints and 8 healthy volunteers. Each patient was examined on the day of splint removal (disuse stage) and 2 months after that day (recovery stage). Both potentials were recorded under 3 conditions: at rest, while imagining motor movement (during motor imagery), and during 10% of maximum voluntary contraction (10% MVC). In the patient group, the amplitude of surface electromyography during voluntary maximum wrist flexion was lower at the disuse stage than at the recovery stage, although the supra-maximum M-wave amplitude did not change between stages. Compared to both the patient group at the recovery stage and the control group, patients at the disuse stage recorded significantly lower MEPs, but only during motor imagery. In contrast, the H-reflex amplitudes were not significantly changed under any of the 3 conditions for both patients and control. The present results indicated a strict parallelism between motor execution (the reduction of electromyography during mvc after immobilization) and motor imagery (the reduction of MEP-amps after immobilization). This parallelism suggests that a functional reorganization or decreased excitability in the cerebral cortex area involved in executing movement likely decreases the motor capability to produce voluntary muscular output after immobilization.

F-waves in neonates: increased spinal anterior horn motor neuron excitability

We analyzed characteristics of F-waves in neonates to establish normal parameters for this age. F-waves recorded from the median nerve in 26 normal neonates were analyzed and compared to those in 10 adults. F-wave parameters measured included occurrence rate (Fpersistence), latency (Flatency), latency corrected for subject height (normalized Flatency), duration (Fduration), amplitude (Famplitude) and its ratio to M-wave amplitude (F/M ratio), F-wave conduction velocity (FCV), and Fchronodispersion. Mean Fpersistence in neonates was 100%. Famplitude and the F/M ratio were higher in neonates than in adults. Similar F-wave waveforms were observed repeatedly in neonates, while waveforms varied in adults. These results in neonates suggest increased excitability of motor neurons in the spinal anterior horn reflecting immaturity of inhibition by the central nervous system.

NEURAL EXCITABILITY FOLLOWING RESISTANCE TRAINING STUDIED WITH TRANSCRANIAL MAGNETIC STIMULATION

An increase in the excitability of the central nervous system during resistance training has been implicated through observations of increases in the surface EMG during maximal contractions. PURPOSE The purpose of the study was to test this hypothesis by measuring changes in the amplitude of motor evoked potentials (MEP) elicited by transcranial magnetic stimulation of the cortex before and after short-term resistance training. METHODS Ten subjects participated in a resistance training program and 10 subjects were matched as controls. The training group performed six sets of 10 maximal isometric voluntary contractions (MVCs) of the tibialis anterior muscle three times a week for four weeks. Each contraction was held for 5 seconds. The data recorded during the experiment were: the average of three maximal voluntary contractions at the start of each training day, mean maximal surface EMG (EMGmax) during three MVCs, maximal M-waves elicited by supramaximal stimulation of the peroneal nerve and the average of ten MEPs elicited during a baseline contraction of 10% MVC. RESULTS There was a progressive increase in all parameters over the course of the experiment. By the end of the twelfth training session the pooled data showed that the force of the maximal voluntary contraction increased by 18.9 ± 1.8%. The peak-to-peak amplitude of the maximal M-wave, RMS of the maximal surface EMG normalized to the maximal M-wave and the peak-to-peak amplitude of the MEP normalized to the maximal M-wave all increased significantly (p <0.05). There were no changes in any of these parameters measured in the control group over the same period. CONCLUSION These data indicate that there may be an increase in the excitability of the entire motor pathway, from the cortex to the sarcolemma, following a short, intense period of isometric resistance training. However, the extent to which this increase in excitability actually functions to increase maximal force output needs to be determined. Supported by NSERC

Somatosensory graviception inhibits soleus H-reflex during erect posture in humans as revealed by parabolic flight experiment.

The purpose of this study was to investigate how gravity level affects the excitability of the soleus muscle (SOL) motoneuron pool to Ia afferent input while erect posture is maintained in humans. Three healthy male subjects participated in an experiment whereby three different gravity conditions [microgravity (MG), normal gravity (NG), and hypergravity (HG)] were imposed using a parabolic flight procedure. The SOL H-reflex was evoked every 2 s while the subjects kept an erect posture. The stimulus intensity was controlled automatically on a real-time basis by personal computer to induce the constant amplitude of M-wave (10+/-5% of maximal M-wave amplitude). The background electromyographic activity (BGA) of the SOL was largest during HG, while it was almost absent during MG. The SOL H-reflex amplitude was significantly larger during HG and MG than during NG ( P<0.05). During NG and HG, there was a linear relationship between the BGA and the H-reflex amplitude; the difference in the SOL H-reflex amplitude between both gravity conditions could be explained in terms of the BGA level. However, during MG, despite the absence of BGA, the SOL H-reflex amplitude was larger than that during NG. Furthermore, when the subjects voluntarily activated the SOL by applying a load to the lower limb joints and spine by pulling a handle upward, this H-reflex enhancement almost disappeared. These results suggest that the somatosensory systems detecting a load at the lower limbs and/or vertebral column might play a role in reducing the excitability of the SOL motoneuron pool to Ia afferent inputs by presynaptic inhibition.

Propofol Impairs the Central but Not the Peripheral Part of the Motor System

Propofol provides some degree of muscle relaxation. Previous studies have investigated the effects of propofol on either the central or peripheral parts of the motor system. In this study, we simultaneously assessed both central (spinal) and peripheral effects. In 15 patients, general anesthesia was induced and maintained with fentanyl and midazolam. Neuromuscular blocking drugs were not administered. To investigate the central portion of the motor system, we monitored spinal F waves, an electrophysiologic variable of α-motoneuron excitability. Direct electrophysiologic muscle responses (M waves) and mechanomyography were studied to detect the peripheral effects of propofol on neuromuscular transmission or muscle contraction strength. After baseline recordings, 3 IV boluses of propofol (2 times 1 mg/kg followed by 2 mg/kg) were administered at 5-min intervals. Mean F-wave amplitudes were significantly reduced compared with baseline measurements (mean ± sd, 0.22 ± 0.13 mV) after the first (0.13 ± 0.08 mV;P < 0.05), second (0.08 ± 0.09 mV;P < 0.05), and third (0.03 ± 0.04 mV;P < 0.01) propofol injections. M-wave amplitudes and mechanomyography signals remained unchanged. Our data suggest that the central part, but not the peripheral part, of the motor system is impaired after bolus administration of propofol.

Interpretation of EMG changes with fatigue: facts, pitfalls, and fallacies

Failure to maintain the required or expected force, defined as muscle fatigue, is accompanied by changes in muscle electrical activity. Although studied for a long time, reasons for EMG changes in time and frequency domain have not been clear until now. Many authors considered that theory predicted linear relation between the characteristic frequencies and muscle fibre propagation velocity (MFPV), irrespective of the fact that spectral characteristics can drop even without any changes in MFPV, or in proportion exceeding the MFPV changes. The amplitude changes seem to be more complicated and contradictory since data on increased, almost unchanged, and decreased amplitude characteristics of the EMG, M-wave or motor unit potential (MUP) during fatigue can be found in literature. Moreover, simultaneous decrease and increase in amplitude of MUP and M-wave, detected with indwelling and surface electrodes, were referred to as paradoxical. In spite of this, EMG amplitude characteristics are predominantly used when causes for fatigue are analysed. We aimed to demonstrate theoretical grounds for pitfalls and fallacies in analysis of experimental results if changes in intracellular action potential (IAP), i.e. in peripheral factors of muscle fatigue, were not taken into consideration. We based on convolution model of potentials produced by a motor unit and detected by a point or rectangular plate electrode in a homogeneous anisotropic infinite volume conductor. Presentation of MUP in the convolution form gave us a chance to consider power spectrum (PS) of MUP as a product of two terms. The first one, PS of the input signal, represented PS of the first temporal derivative of intracellular action potential (IAP). The second term, PS of the impulse response, took into account MFPV, differences in instants of activation of each fibre, MU anatomy, and MU position in the volume conductor in respect to the detecting electrode. PS presentation through product means that not only changes in MFPV could be responsible for PS shift as is usually assumed. Changes in IAP duration and IAP after-potential magnitude, affecting the first term of the product, influence the product and thus MUP PS. Moreover, the interrelations between the two spectra and thus sensitivity of spectrum to different parameters change with MU-electrode distance because the second term depends on it. Thus, we have demonstrated that theory does not predict a linear relation between the characteristic frequencies (maximum, mean and median) and MFPV. IAP duration and after-potential magnitude are among parameters affecting MUP or M-wave PS and thus, EMG PS detected by monopolar and bipolar electrodes. Usage of single fibre action potential models instead of MUP ones can result in false dependencies of frequency characteristics. The MUP amplitude characteristics are determined not only by amplitude of IAP, but also by the length of the IAP profile and source-electrode distance. Due to the IAP profile lengthening and an increase in the negative after-potential, surface detected EMG amplitude characteristics can increase even when IAP amplitude decreases considerably during fatigue. Increase in surface detected MUP or M-wave amplitude should not be attributed to a weaker attenuation of the low-frequency components with distance. Simultaneous decrease and increase in amplitude of MUP and M-wave detected with indwelling and surface electrodes are regular, not paradoxical. Corner frequency of the high pass filter should be 0.5 or 1 Hz when muscle fatigue is analyzed. The area of MUP or M-wave normalized in respect of the amplitude of the terminal phase (that is produced during extinction of the depolarized zones at the ends of the fibres) could be useful as a fatigue index. Analysing literature data on IAP changes due to Ca 2+ increasing, we hypothesised that the ability of muscle fibres to uptake Ca 2+ back into the sarcoplasmic reticulum could be the limiting site for fatigue. If this hypothesis is valid, IAP changes are not a cause of fatigue; they are due to it.

Evaluation of therapeutic electrical stimulation to improve muscle strength and function in children with types II/III spinal muscular atrophy

The study aimed to evaluate the effect of low‐intensity nighttime therapeutic electrical stimulation (TES) on arm strength and function in children with intermediate type spinal muscular atrophy (SMA). The design was a randomized controlled trial with a 6‐month baseline control period. Children were evaluated at baseline, 6, and 12 months. TES was applied from 6 to 12 months to the deltoid and biceps muscle, of a randomly selected arm with the opposite arm receiving a placebo stimulator. Thirteen individuals with SMA between 5 to 19 years of age were recruited into the study and eight completed the 12‐month assessment. No statistically significant differences between the treatment and control arm were found at baseline, 6, and 12 months for elbow flexors, or shoulder abductors on quantitative myometry or manual muscle testing. There was no significant change in excitable muscle mass assessed by M‐wave amplitudes, nor function on the Pediatric Evaluation of Disability Inventory (self‐care domain). Therefore, in this study there was no evidence that TES improved strength in children with SMA.

Within-train neuromuscular propagation varies with torque in paralyzed human muscle.

Electromyographic (EMG) recordings may serve an important role in predicting torque during repetitive activation of paralyzed muscle. We compared the initial M-wave to the subsequent M-waves of the same train under fatigued and recovered conditions in the paralyzed human soleus muscle. Sixteen individuals with chronic (n = 13) or acute paralysis (n = 3) had the tibial nerve activated before and after a repetitive supramaximal stimulation protocol. The mean within-train M-wave amplitude and median frequency increased approximately 20%, whereas the duration decreased approximately 15% compared with the initial M-wave of each train. During fatigue, there was a linear decrease in the difference between the initial M-wave amplitude and subsequent train ( approximately 20% to 8%). Following fatigue, this difference recovered to approximately 12%. The difference between the M-wave train average and the initial M-wave for amplitude, duration, and median frequency closely followed torque (Pearson correlations = 0.99, 0.94, and 0.98, respectively) during fatigue. We conclude that the difference between the later-occurring M-waves (average of the train) and initial M-wave is large when muscle torque is high and less when torque is low and, therefore, predicts torque during activation of paralyzed muscle. This difference in the within-train M-wave amplitude, duration, and median frequency may reflect a mechanical change, such as muscle shortening and increased muscle cross-sectional area during isometric contractions. Electromyographic feedback may assist in the optimization of neuromuscular electrical stimulation of paralyzed muscle.

Propofol Reduces Spinal Motor Neuron Excitability in Humans

Propofol Reduces Spinal Motor Neuron Excitability in Humans

Human neuromuscular fatigue is associated with altered Na+-K+-ATPase activity following isometric exercise.

The purpose of this study was to investigate the hypothesis that reductions in Na+-K+- ATPase activity are associated with neuromuscular fatigue following isometric exercise. In control (Con) and exercised (Ex) legs, force and electromyogram were measured in 14 volunteers [age, 23.4 +/- 0.7 (SE) yr] before and immediately after (PST0), 1 h after (PST1), and 4 h after (PST4) isometric, single-leg extension exercise at ~60% of maximal voluntary contraction for 30 min using a 0.5 duty cycle (5-s contraction, 5-s rest). Tissue was obtained from vastus lateralis muscle before exercise in Con and after exercise in both the Con (PST0) and Ex legs (PST0, PST1, PST4), for the measurements of Na+-K+-ATPase activity, as determined by the 3-O-methylfluorescein phosphatase (3-O-MFPase) assay. Voluntary (maximal voluntary contraction) and elicited (10, 20, 50, 100 Hz) force was reduced 30-55% (P < 0.05) at PST0 and did not recover by PST4. Muscle action potential (M-wave) amplitude and area (measured in the vastus medialis) and 3-O-MFPase activity at PST0-Ex were less than that at PST0-Con (P < 0.05) by 37, 25, and 38%, respectively. M-wave area at PST1-Ex was also less than that at PST1-Con (P < 0.05). Changes in 3-O-MFPase activity correlated to changes in M-wave area across all time points (r = 0.38, P < 0.05, n = 45). These results demonstrate that Na+-K+- ATPase activity is reduced by sustained isometric exercise in humans from that in a matched Con leg and that this reduction in Na+-K+-ATPase activity is associated with loss of excitability as indicated by M-wave alterations.

Neuromuscular fatigue of elbow flexor muscles of dominant and non-dominant arms in healthy humans

The purpose of this study was to assess differences in fatigue-related changes in variables related to structures within the neuromuscular system, between the dominant and non-dominant elbow flexor muscles of right-handed individuals. Two experimental sessions were performed on the right arm and one on the left arm. For each session, maximum voluntary torque, level of voluntary activation, M-wave amplitude, twitch/train or twitch/doublet torque ratio and EMG median frequency were obtained before and up to 20 min after a sustained maximum isometric fatigue task. Our main results were: 1) reproducible fatigue-induced changes in all variables of interest between the two sessions performed with the right arm, 2) significantly greater failure in voluntary activation and neuromuscular propagation with sustained activity for the non-dominant compared with dominant side, and 3) no effect of dominance on MVC torque, endurance time, and fatigue-induced changes in EMG median frequency and elicited torques. These results suggest that the preferential use of elbow flexor muscles with the dominant arm leads to more fatigue resistance in certain structures/mechanisms of the neuromuscular system, but not in others.