Electromyography of lumbar erector spinae muscles — influence of posture,interelectrode distance,strength, and fatigue

Summary The aims of the study were to obtain information (1) on surface electromyograms (SEMG) from the lumbar erector spinae muscles at different interelectrode distances and postures during short isometric contractions with constant force, (2) on the relationships between SEMG and extension force at different postures, and (3) on changes in SEMG during fatiguing isometric contractions at different postures and strengths. Six male subjects developed target forces in prone postures without gravity confounding the measurement of the extension torque. The angles between the constantly horizontal upper trunk and thighs were 90° (P1), 135° (P2), 170° (P3), and 190° (P4). Standard deviations of the distribution of SEMG amplitudes (RMS values), autoregressive (AR) time series models of the 15th order and spectral densities, including mean power frequency (MPF), were computed. Smaller interelectrode distances accompanied smaller RMS values and higher MPF. At a constant extension torque of about 110 Nm, RMS values and MPF increased from P1 to P4. Changes of interelectrode distance were of relatively minor importance, compared with the variation in the posture. With increasing torque, the increase in RMS values was steeper at P3 than at P2. The AR structure and MPF did not exhibit distinct effects of force. During sustained contractions at P2 and P3, only the highest force (mean=140 Nm) at P3 caused a significiant decrease of the MPF at the very beginning of the contraction. Endurance at P2 was greater than at P3. Higher forces and/or shorter muscles (P3) induced more pronounced and earlier relative decreases of the MPF and residual variance of AR models. Up to the “failure point”, RMS values increased slightly, but without significant differences.     

Effects of a 5-h hilly running on ankle plantar and dorsal flexor force and fatigability

This study aimed to examine the effects of a 5-h hilly run on ankle plantar (PF) and dorsal flexor (DF) force and fatigability. It was hypothesised that DF fatigue/fatigability would be greater than PF fatigue/fatigability. Eight male trail long distance runners (42.5 ± 5.9 years) were tested for ankle PF and DF maximal voluntary isokinetic contraction strength and fatigue resistance tests (percent decrement score), maximal voluntary and electrically evoked isometric contraction strength before and after the run. Maximal EMG root mean square (RMS(max)) and mean power frequency (MPF) values of the tibialis anterior (TA), gastrocnemius lateralis (GL) and soleus (SOL) EMG activity were calculated. The peak torque of the potentiated high- and low-frequency doublets and the ratio of paired stimulation peak torques at 10 Hz over 100 Hz (Db10:100) were analysed for PF. Maximal voluntary isometric contraction strength of PF decreased from pre- to post-run (-17.0 ± 6.2%; P < 0.05), but no significant decrease was evident for DF (-7.9 ± 6.2%). Maximal voluntary isokinetic contraction strength and fatigue resistance remained unchanged for both PF and DF. RMS(max) SOL during maximal voluntary isometric contraction and RMS(max) TA during maximal voluntary isokinetic contraction were decreased (P < 0.05) after the run. For MPF, a significant decrease for TA (P < 0.05) was found and the ratio Db10:100 decreased for PF (-6.5 ± 6.0%; P < 0.05). In conclusion, significant isometric strength loss was only detected for PF after a 5-h hilly run and was partly due to low-frequency fatigue. This study contradicted the hypothesis that neuromuscular alterations due to prolonged hilly running are predominant for DF.     

Inter-individual variability among the mechanomyographic and electromyographic amplitude and mean power frequency responses during isometric ramp muscle actions

The purpose of this study was to examine the inter-individual variability in the patterns of responses for mechanomyographic (MMG) and electromyographic (EMG) amplitude and mean power frequency (MPF) of the vastus lateralis (VL) and rectusfemoris (RF) muscles during isometric ramp muscle actions of the leg extensors. Fifteen participants (mean +/- SD age = 24 +/- 4 years) performed two or three 6-s isometric ramp muscle actions with linear increases in torque from 15% to 90% of the highest maximal voluntary contraction (MVC) torque. Surface MMG and EMG signals were simultaneously recorded from the VL and RF muscles. The composite (averaged across subjects) and individual patterns of responses for the normalized MMG and EMG amplitude and MPF vs. isometric torque relationships were analyzed. The composite patterns for MMG amplitude for the VL and RF muscles were fit with cubic models, although, only 87% of the individual patterns (13 of 15 subjects) exhibited the same cubic pattern as the composite. For MMG MPF, the composite relationships were also cubic, but only 27% (4 subjects) and 40% (6 subjects) of the individuals exhibited the same patterns for the VL and RF respectively. 60% (8 subjects) of the individual EMG amplitude patterns of responses matched the same curvilinear composite patterns for the VL and RE, while only 7% (1 subject) and 27% (4 subjects) of the EMG MPF responses were similar to the composite models for the VL and RF muscles, respectively. Therefore, since the individual patterns did not always match the composite relationships (i.e., inter-individual variability), these findings suggested that the MMG and EMG amplitude and MPF vs. isometric torque relationships should be examined on a subject-by-subject and muscle-by-muscle basis during isometric ramp muscle actions.     

Inter-relationship between muscle morphology, mechanical output and electromyographic activity during fatiguing dynamic knee-extensions in untrained females

Changes in mechanical performance and electromyographic activity during fatiguing dynamic knee-extensions were evaluated with respect to muscle fibre type composition of the vastus lateralis muscle in nine sedentary female [23 (3) years] volunteers. The subjects performed 150 repetitive maximum knee-extensions using a Cybex dynamometer at 1.57?rad?·?s^?1. EMG activity was recorded from the vastus lateralis, the vastus medialis and the rectus femoris muscles. For each contraction, mean power frequency (MPF) and the root mean square (RMS) of the EMG were calculated, simultaneously with the peak torque (PT), contractional work (CW) and the mean power (MP). The MPF showed an initial decrease followed by a stable phase. The RMS increased during the initial seven contractions, after which a period of variability was displayed until about the 60th contraction. At the plateau level (last 50 contractions) the relative RMS values were not significantly different from the initial values. The PT, CW and MP increased during the initial five to ten contractions, after which a two-phase pattern was displayed, with a gradual decline followed by a stable phase. The absolute plateau level of MPF for the vastus lateralis muscle showed a significant negative correlation with the area percentage of type-1 fibres (r?=??0.71). Significant correlations were also demonstrated to occur between the absolute plateau levels of PT, CW and MP and the relative proportion of type-1 fibres (r?=?0.80, r?=?0.82 and r?=?0.82 respectively). Thus, in female subjects the mechanical performance and the MPF during fatigue are at least partly determined by muscle morphology.     

Surface EMG amplitude and frequency dependence on exerted force for the upper trapezius muscle: a comparison between right and left sides

Summary Surface electromyographic (EMG) amplitude and mean power frequency (MPF) were used to study the isometric muscular activity of the right versus the left upper trapezius muscles in 14 healthy right-handed women. The EMG activity was recorded simultaneously with force signals during a 10–15 s gradually increasing exertion of force, up to maximal force. Only one side at a time was tested. On both sides there was a significant increase in EMG amplitude (V) during the gradually increasing force from 0% to 100% maximal voluntary contraction (MVC). The right trapezius muscle showed significantly less steep slopes for regression of EMG amplitude versus force at low force levels (0%–40% MVC) compared intra-individually with high force levels (60%–100% MVC). This was not found for the left trapezius muscle. At 40% MVC a significantly lower MPF value was found for the right trapezius muscle intra-individually compared with the left. An increase in MPF between 5% and 40% MVC was statistically significant when both sides were included in the test. The differences in EMG activity between the two sides at low force levels could be due to more slow-twitch (type I fibres) motor unit activity in the right trapezius muscles. It is suggested that this is related to right-handed activity.     

The effects of the innervation zone and interelectrode distance on the patterns of responses for electromyographic amplitude and mean power frequency versus isometric torque for the vastus lateralis muscle

The purpose of the present study was to examine the effects of the innervation zone (IZ) and differences in interelectrode distance (IED) on the patterns of responses and mean values for absolute and normalized electromyographic (EMG) amplitude and mean power frequency (MPF) versus isometric torque for the vastus lateralis. Ten men performed submaximal to maximal isometric muscle actions of the dominant leg extensors, and four separate bipolar surface EMG signals were detected from the vastus lateralis using a linear electrode array. Two electrode arrangements had IEDs of 10 and 30 mm and were located directly over the IZ. Two other electrode arrangements also had IEDs of 10 and 30 mm, but were distal to the IZ. The results indicated that in 50% of the cases, the IZ affected the patterns of responses for absolute and normalized EMG amplitude and MPF versus isometric torque. In 80% of the cases, however, there were no differences between the electrode arrangements with 10 and 30 mm IEDs for the absolute and normalized EMG amplitude and MPF versus isometric torque relationships. Both electrode placement over the IZ and differences in IED affected the mean values for absolute EMG amplitude, but not those for absolute EMG MPF In addition, there were no mean differences among the four electrode arrangements for normalized EMG amplitude and MPF values. Thus, these findings supported normalization, and indicated that it is useful for reducing the influences of the IZ and differences in IED on absolute EMG amplitude and MPF data.     

EMG power spectra of trunk muscles during graded maximal voluntary isometric contraction in flexion-rotation and extension-rotation

The purpose of this study was to determine the electromyographic (EMG) power spectral characteristics of seven trunk muscles bilaterally during two complex isometric activities extension-rotation and flexion-rotation, in both genders to describe the frequency-domain parameters. Eighteen normal young subjects volunteered for the study. The subjects performed steadily increasing isometric extension-rotation and flexion-rotation contractions in a standard trunk posture (40° flexed and 40° rotated to the right). A surface EMG was recorded from the external and internal oblique, rectus abdominis, pectoralis, latissimus dorsi, and erector spinae muscles at the 10th thoracic and the 3rd lumbar vertebral levels, at 1?kHz and 25%, 50%, 75% and 100% of maximal voluntary contraction (MVC). The median frequency (MF), mean power frequency (MPF), frequency spread and peak power were obtained from fast Fourier transform analysis. The MF and MPF for both extension-rotation and flexion-rotation increased with the grade of contraction for both males and females. The EMG spectra in flexion-rotation were different from those of extension-rotation (P?P?相似文献   

Mechanomyographic time and frequency domain responses of the vastus medialis muscle during submaximal to maximal isometric and isokinetic muscle actions

The purpose of this study was to examine the patterns for the mechanomyographic (MMG) amplitude and mean power frequency (MPF) versus torque relationships during isometric and isokinetic muscle actions. Ten adults (mean age +/- SD = 22 +/- 1 y) volunteered to perform isometric and isokinetic leg extension muscle actions at 10, 20, 30, 40, 50, 60, 70, 80, 90, and 100% of peak torque on a Cybex II dynamometer. A piezoelectric crystal contact sensor was placed on the vastus medialis to detect the MMG signal. Regression analyses indicated that for the isometric muscle actions, the relationships for MMG amplitude (R2 = 0.998) and MPF (R2 = 0.987) versus torque were cubic. For the isokinetic muscle actions, the relationships for MMG amplitude (r2 = 0.927) and MPF (r2 = 0.769) versus torque were linear. The different patterns for MMG amplitude and frequency may reflect differences in the motor control strategies that modulate torque production for isometric versus dynamic muscle actions.     

Effect of mechanical compression due to load carrying on shoulder muscle fatigue during sustained isometric arm abduction: an electromyographic study

The use of surface electromyography (EMG) for studying the effect of mechanical compression of occupational origin on muscle fatigue has been the subject of poor attention in ergonomic research. This study examined the effect of backpack carrying on fatigue of two shoulder muscles during sustained low force static contraction: the middle deltoid (MD) muscle and the upper trapezius (UT) muscle on which the backpack strap exerted direct compressive force. EMG activities of MD and UT muscles, of the dominant and non-dominant sides, were studied on eight subjects during two tasks, a maximal and an exhausting submaximal bilateral isometric 90° arm abduction, which were performed while carrying a backpack load of 0, 10, and 20 kg, respectively. EMG amplitude (root mean square, RMS) and spectral (mean power frequency, MPF) parameters were computed from the recorded signals. No significant differences between the dominant and non-dominant sides were found for none of the parameters whatever the load-carrying mass. Load-carrying masses which were tested did not influence significantly the maximal bilateral arm abduction performance contrary to the time to exhaustion during the submaximal task, which decreased significantly with increasing load-carrying mass. A significant increase in fatigability, defined by the slope of MPF decrease, was observed for both muscles when load-carrying mass increased; but only UT presented a significant increase in muscle fatigue level, defined by the MPF value with respect to its initial value, at the end of the exhausting submaximal task. Furthermore, the increase in muscle activation of UT, quantified by RMS, during the exhausting task was not significantly higher with increasing load-carrying mass. So, the increased signs of local fatigue of UT may be interpreted by a localised blood flow impairment resulting from the direct compressive force exerted by backpack on this muscle.     

Myo-electric signals from two extrinsic hand muscles and force tremor during isometric handgrip

Summary The objective of the present study was to investigate the myo-electric signs of muscle fatigue and the isometric force tremor of two extrinsic hand muscles, extensor digitorum communis and flexor digitorum superficialis, during isometric power grip. In addition, the synergy between flexor and extensor muscles and hand differences in a right-handed population have been studied. During isometric hand-dynamometry the myo-electric signal was recorded using surface electrodes and isometric force tremor was recorded using a special load cell. Eight subjects participated in this study and contractions were performed at 20%, 40%, 60% and 80% of maximal voluntary contraction (MVC) with left and right hands. The decrease of mean power frequency (MPF) with duration of contraction was greater in the left extensor as compared to the ipsilateral flexor muscle. No differences in the decrease in MPF with the duration of the contraction were found between the right extensor and flexor muscles. Isometric force tremor root mean square did not change during contractions at a given contraction level. Isometric tremor amplitude increased from 20% to 60% MVC and decreased at higher contraction levels. Tremor amplitude was higher in the left hand at all contraction levels but 60% MVC. These data would suggest differences in fatiguability and muscle fibre composition between the dominant and nondominant hand, which may be due to preferred use. The significance of force tremor for the evaluation of recruitment order and muscle fatigue is discussed.     

Mechanomyogram and force relationship during voluntary isometric ramp contractions of the biceps brachii muscle

The aim of the present study was to examine the non-stationary mechanomyogram (MMG) during voluntary isometric ramp contractions of the biceps brachii muscles using the short-time Fourier transform, and to obtain more detailed information on the motor unit (MU) activation strategy underlying in the continuous MMG/force relationship. The subjects were asked to exert ramp contractions from 5% to 80% of the maximal voluntary contraction (MVC) at a constant rate of 10% MVC/s. The root mean squared (RMS) amplitude of the MMG began to increase slowly at low levels of force, then there was a slight reduction between 12% and 20% MVC. After that, a progressive increase was followed by a decrease beyond 60% MVC. As to the mean power frequency (MPF), a relatively rapid increase up to 30% MVC was followed by a period of slow increment between 30% and 50% MVC. Then temporary reduction at around 50% MVC and a further rapid increase above 60% MVC was observed. The interaction between amplitude and MPF of the MMG in relation to the MU activation strategy is discussed for five force regions defined on the basis of their inflection points in the RMS-amplitude/force and MPF/force relationships. It was found that the MMG during ramp contractions enables deeper insights into the MU activation strategy than those determined during traditional separate contractions. In addition, this contraction protocol is useful not only to ensure higher force resolution in the MMG/force relationship, but also to markedly shorten the time taken for data acquisition and to reduce the risk of fatigue. Accepted: 31 August 2000     

The influence of muscle fiber type composition on the patterns of responses for electromyographic and mechanomyographic amplitude and mean power frequency during a fatiguing submaximal isometric muscle action

The purpose of this investigation was to examine the influence of muscle fiber type composition on the patterns of responses for electromyographic (EMG) and mechanomyographic (MMG) amplitude and mean power frequency (MPF) during a fatiguing submaximal isometric muscle action. Five resistance-trained (mean +/- SD age = 23.2 +/- 3.7 yrs) and five aerobically-trained (mean +/- SD age = 32.6 +/- 5.2 yrs) men volunteered to perform a fatiguing, 30-sec submaximal isometric muscle action of the leg extensors at 50% of the maximum voluntary contraction (MVC). Muscle biopsies from the vastus lateralis revealed that the myosin heavy chain (MHC) composition for the resistance-trained subjects was 59.0 +/- 4.2% Type IIa, 0.1 +/- 0.1% Type IIx, and 40.9 +/- 4.3% Type I. The aerobically-trained subjects had 27.4 +/- 7.8% Type IIa, 0.0 +/- 0.0% Type IIx, and 72.6 +/- 7.8% Type I MHC. The patterns of responses and mean values for absolute and normalized EMG amplitude and MPF during the fatiguing muscle action were similar for the resistance-trained and aerobically-trained subjects. The resistance-trained subjects demonstrated relatively stable levels for absolute and normalized MMG amplitude and MPF across time, but the aerobically-trained subjects showed increases in MMG amplitude and decreases in MMG MPE The absolute MMG amplitude and MPF values for the resistance-trained subjects were also greater than those for the aerobi-cally-trained subjects. These findings suggested that unlike surface EMG, MMG may be a useful noninvasive technique for examining fatigue-related differences in muscle fiber type composition.     

Comparison of the EMG power spectrum of the human soleus and gastrocnemius muscles

The purpose of this study was to compare the behaviour of electromyographic (EMG) power spectrum statistics, mean power frequency (MPF) and median frequency (MF), across increasing force levels of the soleus (SO), gastrocnemius medialis (GM) and gastrocnemius lateralis (GL) muscles. Surface EMG signals of these three muscles were recorded in 12 men and 10 women during both (1) ramp (single ongoing contractions with the force increasing linearly from 0 to 100% of the maximum voluntary contraction (MVC); and (2) step (steady force levels: 10, 20, 30, 40, 60 and 80% MVC) static (isometric) plantar flexions. Power spectral analysis of these signals was performed on single 256-ms windows at all of the above-mentioned force levels, for both types of contraction. The MF and MPF were calculated from each of the obtained spectra. A less pronounced increase in the MF or MPF was expected for the SO because of its higher type I fibre content. The main results are as follows: (1) similar behaviours were found in the value of MPF and MF across increasing force for the SO and GL muscles, while the GM gave rise to a different behaviour; (2) no difference was found between ramp and step contractions in the behaviour of either MF or MPF across force levels; and (3) different behaviours were observed between the MF and MPF across increasing force levels, for both ramp and step contractions. Our initial expectations were thus not confirmed. It is concluded that the present results support the hypothesis that the EMG power spectrum may be more sensitive to the diameter of the fibres than to the fibre type proportion of the triceps surae muscles. Furthermore, the sensitivity of the power spectrum statistics of a given muscle to the low-pass filter effect of its skin layer was also emphasized.     

Development of muscle fatigue as assessed by electromyography and mechanomyography during continuous and intermittent low-force contractions: effects of the feedback mode

The goal of the present study was to investigate the significance of low-force continuous or intermittent static contraction and feedback mode (visual or proprioceptive) on the development of muscle fatigue as assessed by electromyography (EMG) and mechanomyography (MMG). Visual (force control) and proprioceptive (displacement control) feedback was investigated during intermittent (6 s contraction, 4 s rest) and continuous static contractions at 10% and 30% of the maximum voluntary contraction (MVC). Mean force, force fluctuation, rating of perceived exertion and root mean square (RMS) and mean power frequency (MPF) of the EMG and MMG signals were analysed. The general pattern for MMG RMS and EMG RMS values and the rating of perceived exertion was an increase with contraction time, while the EMG MPF values decreased (P<0.05). The increase in RMS values was generally more pronounced for the MMG compared with the EMG, while the decrease in MPF values was more consistent for the EMG compared with the MMG signal. During the intermittent contractions, the main effect was on MPF for both EMG and MMG. Lower force fluctuation and larger rating of perceived exertion (P<0.05), greater slopes of EMG and MMG RMS and MPF values versus time were observed with proprioceptive feedback compared with visual feedback. The findings suggest that (1) the EMG and MMG signals give complementary information about localised muscle fatigue at low-level contraction: they responded differently in terms of changes in the time and frequency domain during continuous contraction, while they responded in concert in the frequency domain during intermittent contractions, and (2) the different centrally mediated motor control strategies used during fatiguing contraction may be dependent upon the feedback modality. Electronic Publication     

Mechanomyographic and electromyographic time and frequency domain responses during submaximal to maximal isokinetic muscle actions of the biceps brachii

The purpose of this investigation was to determine the mechanomyographic (MMG) and electromyographic (EMG) amplitude and mean power frequency (MPF) versus torque relationships during isokinetic muscle actions of the biceps brachii. Twelve adults [mean (SD) age, 22.2 (2.7) years] performed submaximal to maximal isokinetic muscle actions of the dominant forearm flexors. Following determination of isokinetic peak torque (PT), the subjects randomly performed submaximal muscle actions in 20% increments from 20% to 80% PT. Polynomial regression analyses indicated linear increases in both MMG (r²=0.984) and EMG (r²=0.988) amplitude to 100% PT. There were no significant (P>0.05) relationships, however, for MMG and EMG MPF versus isokinetic torque. The results demonstrated similar responses for MMG and EMG in both the time and frequency domains. These findings suggested that simultaneous examination of MMG and EMG amplitude and MPF may be useful for describing the unique motor control strategies that modulate dynamic torque production. Furthermore, the results indicated that dynamic muscle actions can be used when applying techniques that require a linear EMG amplitude versus torque relationship.     

Signal Characteristics of EMG with Special Reference to Reproducibility of Measurements

Reliability and constancy of recordings of EMG signal characteristics were investigated from the measurements taken with minitature size surface electrodes during submaximal and maximal contraction of the rectus femoris muscle. The following EMG variables were studied: integrated EMG (IEMG) various band-widths of the power spectral density function, mean power frequency (MPF), and rise time, amplitude and number of spikes of the averaged motor unit potential (AMUP). The results indicated that for most of the variables studied the reproducibility of measurements was better within the test session (reliability) than between the different test days (constancy). The reliability values for IEMG, MPF and AMUP amplitude were rather high (r = 0.77–0.92). MPF and number of spikes in AMUP showed good constancy values (r=0.73–0.93) and it is suggested that these parameters can be recommended for use in EMG studies where recordings are repeated over a period of several days.     

Mechanomyographic and electromyographic amplitude and frequency responses during fatiguing isokinetic muscle actions of the biceps brachii

The purpose of this investigation was to examine the mechanomyographic (MMG) and electromyographic (EMG) amplitude and mean power frequency (MPF) patterns during fatiguing isokinetic muscle actions of the biceps brachii. Ten adults [three women (mean +/- SD age = 20 +/- 2 yrs) and seven men (mean +/- SD age = 23 +/- 3 yrs) ] volunteered to perform 50 consecutive maximal, concentric isokinetic muscle actions of the biceps brachii at 180 degrees x s(-1). The percent decline (mean +/- SD) in isokinetic peak torque (PT) was 70 +/- 17% and polynomial regression analyses indicated a cubic relationship (R2 = 0.994) between PT and repetition number. Both MMG amplitude and MMG MPF decreased linearly (r2 = 0. 774 and 0.238, respectively) across repetitions. The results for EMG amplitude demonstrated a cubic (R2 = 0.707) pattern across repetitions, where EMG amplitude increased during repetitions 1-20, remained stable during repetitions 20-40, and increased during repetitions 40-50. There was a quadratic (R2 = 0. 939) reduction in EMG MPF throughout the test. The decreases in MMG amplitude and MMG MPF may have been due to de-recruitment of fast fatiguing motor units, a reduction in muscular compliance, or the effects of "muscle wisdom." The results for EMG amplitude may have reflected nonmaximal efforts by the subjects and/or peripheral fatigue. The factor(s) determining the decrease in EMG MPF are unclear, although a reduction in muscle fiber action potential conduction velocity may have been partially responsible.     

Changes in soleus motoneuron pool reflex excitability and surface EMG parameters during fatiguing low- vs. high-intensity isometric contractions

The fatigue-related changes in soleus motoneuron pool reflex excitability and surface electromyography (EMG) parameters, and maximal voluntary contraction (MVC) force of the plantarflexor (PF) muscles during repeatedly sustained low- (30% MVC) vs. high-intensity (70% MVC) isometric contractions were evaluated Twelve young men with mean (+/- SE) age of 22.4 +/- 0.3 years participated in two fatigue tasks on separate days with at least 1-week interval. The fatigue task consisted of three sustained isometric contractions of PF muscles at a target force level until exhaustion separated with 2-min pause between contractions. M-wave (muscle compound action potential) amplitude (M(max)), Hoffmann reflex maximal amplitude (H(max)) to M-wave amplitude ratio (H(max)/M(max)), and root mean square amplitude (RMS) and median frequency (MF) of EMG power spectrum were recorded from the soleus muscle. The M(max) remained constant immediately post-fatigue and during recovery for low- and high-intensity fatigue tasks, whereas H(max)/M(max) was significantly (p < 0.05) reduced only after high-intensity fatigue task. The increase in RMS and decrease in MF during isometric contractions, and reduction in MVC force immediately after the exercise was greater (p < 0.05) for low-intensity fatigue task. We conclude that low-intensity isometric contractions, repeatedly sustained to fatigue, resulted in a marked increase in the EMG amplitude and spectral compression without a significant post-fatigue reflex inhibition of soleus motoneuron pool. High-intensity contractions, however, resulted in post-fatigue reflex inhibition of soleus motoneuron pool and less pronounced EMG spectral compression during fatiguing contractions. A failure of neuromuscular transmission-propagation was not evident after repetitive fatiguing isometric contractions.     

Relation between EMG power spectrum shifts and muscle fibre action potential conduction velocity changes during local muscular fatigue in man

Summary Muscular fatigue was studied in the human m. biceps brachii by contracting the muscle as long as possible at 50% of maximum voluntary strength. For 8 subjects changes in the muscle fibre action potential conduction velocity could successfully be measured using the cross-correlation technique between two surface EMG signals. Shifts in the EMG power spectrum were quantified using the mean power frequency (MPF) of the EMG power spectral density function.During fatigue the EMG power spectrum gradually shifted to lower frequencies. The mean value and standard deviation of the MPF value decreased from (115±20) Hz at the beginning of the experiment to (60±18) Hz at the end. For 4 of the 8 subjects the decrease in MPF value was accompanied by a substantial decrease in conduction velocity (mean decrease was 33% of the initial velocity value). For the other 4 subjects, despite the great MPF changes, there was hardly any change in conduction velocity (mean decrease was 3%).The present study shows that great EMG power spectral shifts during muscular fatigue may occur without a concomitant change in muscle fibre action potential conduction velocity.     

Effect of muscular ultrasound stimulation on power spectrum electromyography during a strengthening training

The purpose of this study is to describe the electromyographic (EMG) power spectral evolution during a muscle strengthening training based on ultrasound stimulation applied to the right rectus femoris muscle simultaneously with its maximum voluntary isometric contraction (MVIC) in ten healthy subjects. From the surface EMG of rectus femoris the Zero Crossing (ZC) and integrated EMG (IEMG) were calculated. The Mean Power Frequency (MPF) and Median Frequency (MF) from the EMG power spectrum were obtained using a maximum entropy method. An Analysis of Variance (ANOVA) for repeated measures showed significant changes for IEMG and ZC throughout sessions and for MPF, MF and ZC between the pretest and posttest. Immediate increases in MPF, MF and ZC were observed (paired t-test, p < 0.05) after training and an increment of IEMG during sessions (one-way ANOVA, p = 0.01). The ultrasound pulsed stimulation applied simultaneously with a MVIC produced the development of the muscle force. The mechanism of action of this application could be the mechanical effect, compression and rarefaction, produced by the ultrasound stimulation during the muscle contraction process.