The role of the biarticular agonist and cocontracting antagonist pair in isometric muscle fatigue

Isometric knee extensions until exhaustion at 30%, 50%, and 70% of maximum voluntary contraction were performed by 18 healthy subjects. During muscle fatigue, surface electromyographic activity was recorded from the knee-extensors vastus lateralis, vastus medialis, and rectus femoris, and the coactive antagonistic biceps femoris. The electromyographic parameter median frequency (MF) served as a measure of fatigue. Coefficients of regression of the MF fatigue changes were analyzed statistically. MF fatigue occurred within the coactive biceps femoris and was significantly more pronounced than in the quadriceps. When the MF fatigue shifts of the coactive biceps femoris were compared with each of the three investigated parts of the quadriceps separately, MF fatigue shifts were similar in shape for the biarticular coactive biceps femoris and the biarticular rectus femoris, but differed significantly between the biceps femoris and the two monoarticular muscles, vastus medialis and vastus lateralis. As both the biarticular agonist and coactive antagonist muscles fatigued at a higher rate than the two monoarticular muscles, it seems likely that this biarticular agonist/antagonist pair determines the time to the limit of endurance. © 1998 John Wiley & Sons, Inc. Muscle Nerve 21: 1706–1713, 1998     

Activation linearity and parallelism of the superficial quadriceps across the isometric intensity spectrum

The purpose of this study was to assess neuromuscular activation of the three superficial portions of the quadriceps femoris muscles during linearly increasing isometric contraction intensities. Thirty healthy volunteers were assessed for isometric electromyographic (EMG) activity of the vastus medialis (VM), vastus lateralis (VL), and rectus femoris (RF) muscles with the knee at 60 degrees of flexion. For 5 s, subjects performed isometric contractions equivalent to 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, and 90% of the average of three maximal voluntary contractions (MVC), in random order. Full-wave rectified and integrated EMG signals over the middle 3 s of each contraction were expressed as a percentage of the activity recorded during the three averaged MVCs. One sample t-tests and 95% confidence intervals were calculated at each relative torque level. A two-factor analysis of variance (muscle by intensity) with repeated measures was performed to evaluate parallel activation across the intensity levels. Activation linearity was assessed via regression analysis for each muscle. VM activation was shown to be significantly lower than expected at 20-70% MVC. VL and RF activations were significantly higher than expected at 10% MVC, and RF EMG was less than expected at 40-70% MVC. EMG of VM was shown to increase significantly more than VL and RF from 80% to 90% MVC. Significant linear and quadratic relations were also demonstrated for all three muscles. Parallel activation of the superficial quadriceps muscles occurred from low to moderate intensities, whereas convergence was noted at near maximal intensities.     

Shifts in EMG spectral power during fatiguing dynamic contractions

Introduction: We examined the etiology of the electromyographic (EMG) spectral shift during dynamic fatigue. Methods: Nineteen subjects (mean ± SD age = 22.4 ± 1.6 years) performed 50 consecutive maximal concentric isokinetic contractions of dominant leg extensors. Surface EMG signals were detected from the vastus lateralis, rectus femoris, and vastus medialis during each contraction, processed with a wavelet analysis, and the resulting spectra were decomposed with a nonparametric spectral decomposition procedure. Results: The results indicated that the decreases in EMG frequency during the 50 contractions were generally due to reductions in high‐frequency power and increases in low‐frequency power. In addition, the spectral shifts were most pronounced for the rectus femoris, followed by the vastus lateralis, and then the vastus medialis. Conclusions: The spectral decomposition procedure is much more sensitive for tracking dynamic fatigue than is EMG mean frequency or median frequency. Muscle Nerve 50 : 95–102, 2014     

A method for positioning electrodes during surface EMG recordings in lower limb muscles

PURPOSE: The aim of this work is to provide information about the degree of inter-subject uniformity of location of innervation zone (IZ) in 13 superficial muscles of the lower limb. The availability of such information will allow researchers to standardize and optimize their electrode positioning procedure and to obtain accurate and repeatable estimates of surface electromyography (sEMG) signal amplitude, spectral variables and muscle fiber conduction velocity. METHODS: Surface EMG signals from gluteus maximus, gluteus medius, tensor faciae latae, biceps femoris, semitendinosus, vastus medialis obliquus, vastus lateralis, rectus femoris, tibialis anterior, peroneus longus, soleus, gastrocnemius medialis and lateralis muscles of ten healthy male subjects aged between 25 and 34 years (average = 29.2 years, S.D. = 2.5 years) were recorded to assess individual IZ location and signal quality. RESULTS: Tensor faciae latae, biceps femoris, semitendinosus, vastus lateralis, gastrocnemius medialis and lateralis showed a high level of both signal quality and IZ location uniformity. In contrast, rectus femoris, gluteus medius and peroneus longus were found to show poor results for both indexes. Gluteus maximus, vastus medialis obliquus and tibialis anterior were found to show high signal quality but low IZ location uniformity. Finally, soleus muscle was found to show low signal quality but high IZ location uniformity. CONCLUSIONS: This study identifies optimal electrode sites for muscles in the lower extremity by providing a standard landmarking technique for the localization of the IZ of each muscle so that surface EMG electrodes can be properly positioned between the IZ and a tendon.     

EMG power spectrum of respiratory and skeletal muscles during static contraction in healthy man

Changes in EMG power spectrum during isometric voluntary contraction maintained until exhaustion in the range of 20–80% MVC were studied in three skeletal muscles (adductor pollicis or AP, vastus lateralis, and medialis) and two respiratory muscles (diaphragm and rectus abdominis). Quantitative EMG analysis consisted of computation of the median frequency (MF) of power spectra and also the continuous measurement of EMG power in two bands of high (EH) and low (EL) frequencies using bandpass filters. This allowed the calculation of the H/L ratio and its time constant of decay rate (TCHδ /L) throughout the sustained static contraction. The main results were: (1) highly significant, positive correlations between TCδH/L and the maximal MF changes and also the endurance time to fatigue; (2) EMG changes were determined early, within the first 10–20 s of contraction; and (3) EL always increased throughout the fatiguing isometric contraction, but EH changes markedly varied within the five muscle groups studied. These observations are discussed in terms of the differences in muscle fiber composition and also the variations in motor unit recruitment. © 1993 John Wiley & Sons, Inc.     

EMG activity and kinematics of human cycling movements at different constant velocities

Surface electromyographic (EMG) activity was recorded from the rectus femoris, vastus medialis, biceps femoris, gastrocnemius and tibialis anterior in the human lower extremity while subjects performed bicycling movements over a range of constant pedalling velocities. Kinematics of knee and hip cyclical movements were analyzed from 16 mm film. The reciprocal pattern of activation in agonist and antagonist muscles and timing of EMG initiation relative to knee joint were studied.

Reciprocal activation of rectus femoris and biceps femoris muscles was generally observed to occur during the mid-extension or mid-flexion phase of knee movements. This timing of activation pattern coincided well the period of peak angular velocity and zero angular acceleration.

As pedalling speeds approached maximum, activation times of the bifunctional, biarticular rectus femoris, biceps and gastrocnemius muscles were considerably advanced in phase relative to knee joint angles, whereas, EMG initiation of monofunctional, single joint, tibalis anterior and vastus medialis muscles maintained a relatively stable knee position-activation time relationship. At higher velocities, biceps femoris EMG activity was characterized as having a double burst pattern of activation. A less distinctive double burst pattern was seen in the rectus femoris EMG at higher cycling speeds.

EMG pattern analysis of the rectus femoris and biceps femoris muscles revealed an earlier onset of activity for both muscles during maximum cycling velocities, relative to cyclical phases of the knee joint angle. Considerable overlapping of the EMG bursts was seen beyond pedalling rates of 1 Hz. Co-contraction between rectus femoris and biceps femoris muscles could be observed during the acceleration period involving an abrupt switch to maximum pedalling performance. When co-contraction was observed, the joint angular acceleration curves observed during the knee flexion period accounted for a larger portion of a single cycle, and were more irregular than the angular accelerations observed during knee extension.     

Surface EMG crosstalk between knee extensor muscles: experimental and model results

Surface electromyographic (EMG) crosstalk between vastus lateralis, vastus medialis, and rectus femoris muscles was evaluated by selective electrical stimulation of one muscle and recording from the stimulated and another muscle with linear surface arrays of eight electrodes. The ratio between the amplitude of the signals recorded over nonstimulated and stimulated muscles and their correlation coefficient were used as indices to quantify crosstalk. Single-differential and double-differential detection systems were used with interelectrode distances in the range 10-40 mm. The multichannel EMG signals clearly showed that crosstalk is largely due to nonpropagating potentials that correspond in time to the end of the propagation of the action potentials generated by the stimulated muscle. The crosstalk signal increased with increasing interelectrode distance and was statistically higher for single- than for double-differential recordings. The correlation-based indices of crosstalk were poorly correlated with the amplitude-based indices. Moreover, the characteristic spectral frequencies of the signals detected over the nonstimulated muscles were statistically higher than those from the stimulated muscles. A mathematical model of signal generation was used to explain the experimental findings. This study clarifies many controversial findings of past investigations and creates the basis for crosstalk interpretation, simulation, and reduction.     

Knee extensor torque,work, and EMG during subjectively graded dynamic contractions

We examined knee extensor peak torque, work, and electromyogram (EMG) during dynamic contractions to perceived exertion levels in men and women. Thirty subjects performed three maximal effort isokinetic knee extensions (60 deg x s(-1)), followed by three contractions to each of nine separate levels of perceived exertion. Surface EMG of the vastus medialis (VM), vastus lateralis (VL), and rectus femoris (RF), and knee extensor peak torque and work were normalized to a percent of each respective value obtained during the maximal effort contractions. The results demonstrated a significant linear increase in voluntary knee extensor peak torque and work across perceived exertion levels. Knee extensor peak torque and work were less than 70%, 80%, and 90% maximal voluntary contraction (MVC) at perceived exertion levels 7, 8, and 9, respectively. A significant increase in VM, VL, and RF muscle EMG was observed across perceived exertion levels 1 through 9, with EMG increase highest for the VL. The findings demonstrate that dynamic contractions guided by perceived exertion are underproduced at relatively high perceptual intensities, and that reliance on VL activation occurs across submaximal torque levels. The overestimation of knee extensor peak torque and work at relatively high perceptual intensities may suggest the presence of a subconscious mechanism aiming to reduce high muscle and joint forces.     

Wavelet analysis of quadriceps power spectra and amplitude under varying levels of contraction intensity and velocity

Introduction: We investigated the effect of contraction intensity [100%, 75%, 50%, and 25% maximum voluntary contraction (MVC)] and movement velocity (50°, 100°, 200°, and 400°/s) on surface electromyography root mean square amplitude (SEMG_RMS) and median frequency (SEMG_MDF) of rectus femoris (RF), vastus lateralis (VL), and vastus medialis (VM). Methods: SEMGs during knee extension were resolved into their respective frequencies using wavelet transformations. Results: RF, VL, and VM muscles displayed increased SEMG_MDF as contraction intensity increased from 25% to 50% MVC and from 75% to 100% MVC, and each muscle displayed its own unique frequency shifting patterns. The SEMG_MDF was not influenced by movement velocity. SEMG_RMS increased in all 3 muscles as contraction intensity increased and was influenced by movement velocity, with the highest values observed at 400° and 200°/s. Conclusions: We infer that increasing contraction intensity facilitates greater recruitment of fast‐twitch muscle fibers, but there are differing responses in RF, VL, and VM muscles. Muscle Nerve 50 : 844–853, 2014     

The mixed nerve silent period is prolonged during a submaximal contraction sustained to failure

The purpose of this study was to determine the effect of a sustained contraction of vastus lateralis on the silent period (SP) in the surface electromyogram (EMG) following direct neural stimulation. Five men and 5 women performed isometric knee extension at 30% maximal voluntary contraction (MVC) to the limit of endurance. During the contraction, EMG increased, and superimposed twitch amplitude and time to peak tension decreased, but the SP duration did not change. After 10 min of recovery, MVC had returned to its initial value, and the potentiated twitch amplitude was 70% of initial value, but the SP was now 11% shorter. Based on these results, we hypothesize that during a sustained contraction of 30% MVC, the increase in central drive may have been offset by inhibitory input from the periphery, but after 10 min of recovery the SP was shortened because of increased central drive. This aspect of the SP's behavior should be taken into account whenever it is employed as a diagnostic tool.     

Assessment of muscular fatigue using vibromyography

The purpose of this study was to investigate the behavior of electromyographical (EMG) and vibromyographical (VMG) signals in the time and frequency domains during a fatigue protocol. EMG and VMG records were obtained from the rectus femoris (RF) and vastus lateralis (VL) muscles of 11 adult male subjects during sustained, isometric knee extensor contractions performed at 70% of maximal voluntary contraction (MVC). The average median frequencies of the power density spectra decreased during the fatigue protocol for the EMG (from 73 to 54 Hz for RF, and from 75 to 57 Hz for VL) and the VMG signals (from 40 to 19 Hz for RF, and from 25 to 12 Hz for VL). Raw EMG signals remained the same qualitatively throughout the fatigue protocol, whereas corresponding VMG records appeared to become “smoother.” The results of this study indicate that the pronounced decrease in the high-frequency content of the VMG signal may be observed in the time domain as a “smoothing” of the signal, and thus, that the raw VMG records (which may be displayed readily online) can be used to assess qualitatively the onset and progression of muscular fatigue. © 1994 John Wiley & Sons, Inc.     

A wavelet‐based analysis of surface mechanomyographic signals from the quadriceps femoris

The purpose of this study was to use a wavelet analysis designed specifically for surface mechanomyographic (MMG) signals to examine the MMG responses of the vastus lateralis (VL), rectus femoris (RF), and vastus medialis (VM) muscles. Fifteen healthy men [age (mean ± SD): 26.4 ± 6.1 years] volunteered to perform isometric muscle actions of the dominant leg extensors at 20%, 40%, 60%, 80%, and 100% of the maximum voluntary contraction (MVC). During each muscle action, surface MMG signals were detected from the VL, RF, and VM and processed with the MMG wavelet analysis. The results show that, for the VL and VM muscles, there was compression of the total MMG intensity spectra toward low frequencies for most force levels above 20% MVC. For the RF, however, the peak of the total MMG intensity spectrum occurred at approximately 30–40 HZ for all force levels. Because the VL, RF, and VM are all innervated by the femoral nerve, the discrepancies among the three muscles for total MMG intensity in each wavelet band may have been due to differences in architecture, muscle stiffness, and/or intramuscular pressure. Muscle Nerve 39: 355–363, 2009     

Congenital focal muscle dysplasia in the lower extremities from probable abnormal innervation: a case report.

A two-year-seven-month-old girl with pes equinovarus congenita, muscle hypotonia and weakness limited to the lower extremities is presented. Upon admission to our hospital, she could stand with support but could not walk alone. Serum creatine kinase level was normal and the electromyogram was nondiagnostic. The muscle CT disclosed an almost total absence of bilateral vastus lateralis and medialis, rectus femoris and gastrocnemius muscles. The biopsied vastus lateralis muscle was almost completely replaced by fat tissue, and a small amount of muscle tissue showed uniform type 1 fiber and an aggregate of atrophic fibers in one fascicle. Because of an absence of progressive muscle weakness and neurogenic EMG findings, the authors conclude that the muscle pathology was due to the congenital anomalous condition of probable abnormal innervation to developing muscles.     

EMG spectral differences among the quadriceps femoris during the stretch reflex

Introduction: The purpose of this study was to examine the electromyographic (EMG) spectral characteristics of the quadriceps femoris muscles during tendon tap stretch reflexes. Methods: Sixteen healthy subjects (mean ± SD age = 21.2 ± 2.8 years) performed tendon tap reflexes of the leg extensors as surface EMG signals were detected from the vastus lateralis (VL), rectus femoris (RF), and vastus medialis (VM) muscles of the dominant thigh. All EMG signals were processed with a wavelet analysis, and the resulting spectra were decomposed with nonparametric spectral decomposition. Results: The results showed that the spectra for the VL had significantly more high‐frequency power than those for the RF and VM, with similar spectral shapes for the RF and VM. Conclusions: These findings could be due to differences in the width of the innervation zone, or the fiber type composition of the muscles, although the latter seems to be more likely. Muscle Nerve 52 : 826–831, 2015     

Comparison of electromyographic responses for the superficial quadriceps muscles: Cycle versus knee‐extensor ergometry

The purpose of this study was to compare the electromyographic (EMG) amplitude and mean power frequency (MPF) versus power output relationships for the three superficial quadriceps muscles during incremental cycle (CE) and knee‐extensor (KE) ergometry in the same subjects. Eight men performed incremental CE and KE tests to exhaustion. Surface EMG signals were recorded simultaneously from the vastus lateralis (VL), rectus femoris (RF), and vastus medialis (VM). Polynomial regression analyses on a subject‐by‐subject basis indicated that the relationship between EMG amplitude versus power output was best‐fit with either a linear, quadratic, or cubic model for CE, whereas the relationship was best‐fit with a linear model for all subjects for the KE test. No consistent relationship was found for EMG MPF within subjects and between muscle groups for CE or KE. Compared with CE, however, the EMG amplitude during KE exercise was, on average, approximately 87% and 30% higher for the RF and VM muscles, respectively. These results suggest that KE exercise may be a better mode of examining EMG amplitude in the quadriceps muscle during incremental exercise than traditional CE. Muscle Nerve, 2009     

Effects of fatigue on intermuscular common drive to the quadriceps femoris

ABSTRACT The purpose of this investigation was to examine intermuscular common drive to motor units in the vastus lateralis and vastus medialis during isometric contractions of the leg extensors. A secondary purpose was to investigate the effects of fatigue on the common drive. Fourteen healthy subjects (mean ± SD, age?=?21.7 ± 1.5?years) volunteered to perform two separate isometric contractions of the leg extensors at 50% of the maximum voluntary contraction (MVC). These contractions were performed before (PRE) and immediately after (POST) a fatigue protocol of 10, 10?s isometric MVCs with 10?s of rest in between (i.e., 10?s on, 10?s off). During the PRE and POST isometric contractions, surface electromyographic (EMG) signals were detected from the vastus lateralis and vastus medialis and decomposed into individual motor unit action potential trains. A mean firing rate curve was then calculated for each motor unit, and the curves for the vastus lateralis were cross-correlated with those from the vastus medialis. The results showed that the average peak cross-correlation coefficients were always between 0.39 and 0.55, with coefficients for individual motor unit comparisons occasionally as high as 0.93. However, these coefficients were not affected by fatigue. These findings demonstrated a significant level of intermuscular common drive that was generally unaffected by fatigue.     

Time-frequency analysis and estimation of muscle fiber conduction velocity from surface EMG signals during explosive dynamic contractions

Time-frequency analysis of the surface electromyographic (EMG) signal is used to assess muscle fiber membrane properties during dynamic contractions. The aim of this study was to compare the direct estimation of average muscle fiber conduction velocity (CV) with instantaneous mean frequency (iMNF) of surface EMG signals in isometric and explosive dynamic contractions. The muscles investigated were the vastus lateralis and medialis of both thighs in 12 male subjects. The isometric contractions were at linearly increasing force (0-100% of the maximal voluntary contraction in 10s). The explosive contractions were performed on a multipurpose ergometer-dynamometer (MED). The subject, sitting on the MED, performed six explosive contractions, separated by 2min rest, by pushing against two force platforms and thrusting himself backwards with the maximum possible speed, while completely extending his legs. The estimated CV significantly increased with force in both the isometric (mean+/-S.D., from 3.24+/-0.34 to 5.12+/-0.31m/s for vastus lateralis and from 3.17+/-0.26 to 5.11+/-0.34m/s for vastus medialis, with force in the range 10-100% of the maximal voluntary contraction level) and explosive contractions (from 4.36+/-0.49 to 5.00+/-0.47m/s for vastus lateralis, and from 4.32+/-0.46 to 4.94+/-0.44m/s for vastus medialis, with force in the range 17.5-100% of maximal thrusting force). Moreover, estimated CV was not significantly different at the maximal force in the two exercises. On the contrary, iMNF, computed from the Choi-Williams time-frequency transform, was significantly lower in the explosive (57.7+/-8.2 and 66.5+/-10.3Hz for vastus laterialis and medialis, respectively) than in the isometric exercises (73.7+/-9.2 and 75.0+/-8.5Hz for vastus laterialis and medialis, respectively) and did not change with force in any of the conditions. It was concluded that EMG spectral features provide different information with respect to average muscle fiber CV in dynamic contractions. Thus, in general, they cannot be used to infer CV changes during the exertion of a dynamic task. A joint analysis of CV and EMG spectral features is necessary in this type of contractions.     

Reliability of the log‐transformed EMG amplitude‐power output relationship for incremental knee‐extensor ergometry

Introduction: The purpose of this investigation was to determine the reproducibility of the log‐transformed model for electromyography (EMG) amplitude during incremental single‐leg knee‐extensor exercise. Methods: Eight healthy college‐aged men performed 3 incremental tests on separate occasions on a knee‐extensor ergometer. EMG amplitude was analyzed for each participant on each occasion for the rectus femoris and vastus medialis muscles at 4 different exercise power outputs (30%, 50%, 70%, and 90%) corresponding to each participant's maximal power output. Intraclass correlation coefficients (ICC) were determined for the slope and y‐intercept terms derived from the log‐transformed EMG amplitude‐power output relationship for each muscle. Results: The ICC values for the rectus femoris (slope = 0.779; y‐intercept = 0.787) and vastus medialis (slope = 0.756; y‐intercept = 0.763) muscles were high. Conclusions: The log‐transformed EMG amplitude‐power output relationship is a reliable index for measuring motor unit activation. Muscle Nerve 52:428–434, 2015     

MMG and EMG responses during fatiguing isokinetic muscle contractions at different velocities

The purpose of this study was to determine mechanomyographic (MMG) and electromyographic (EMG) responses of the superficial quadriceps muscles during repeated isokinetic contractions in order to provide information about motor control strategies during such activity, and to assess uniformity in mechanical activity (MMG) between the investigated muscles. Ten adults performed 50 maximal concentric muscle contractions at three randomly selected contraction velocities (60, 180, and 300 degrees.s(-1)) on different days. Surface electrodes and an MMG sensor were placed on the vastus lateralis (VL), rectus femoris (RF), and vastus medialis (VM). EMG and MMG amplitude and peak torque (PT) were calculated for each contraction, normalized, and averaged across all subjects. The results demonstrated that MMG amplitude more closely tracked the fatigue-induced decline in torque production at each velocity than did EMG amplitude. This indicates that MMG amplitude may be useful for estimating force production during fatiguing dynamic contractions when a direct measure is not available, such as during certain rehabilitative exercises. MMG amplitude responses of the VL, RF, and VM were not uniform for each velocity or across velocities, indicating that it may be possible to detect the individual contribution of each muscle to force production during repeated dynamic contractions. Therefore, MMG amplitude may be clinically useful for detecting abnormal force contributions of individual muscles during dynamic contractions, and determining whether various treatments are successful at correcting such abnormalities.     

Inhibition of dynamic thigh muscle contraction by electrical stimulation of the posterior cruciate ligament in humans.

We investigated the influence of electrical stimulation of the posterior cruciate ligament (PCL) on the motoneuron pool of the thigh muscle during voluntary static and dynamic muscle contraction. The study group comprised nine young men with no history of injury to the knee joints. Multistranded Teflon-insulated stainless-steel wires were inserted into the PCL guided by ultrasound. In three subjects wires were also inserted into the fat pad of the knee. The PCL was electrically stimulated during static, concentric, or eccentric muscle contraction with a constant load of 20% of the maximal voluntary contraction of either the quadriceps or the hamstrings. Electromyographic signals were recorded with bipolar surface electrodes placed over the vastus medialis, rectus femoris, vastus lateralis, biceps femoris caput longum, and semitendinosus muscles. The stimuli consisted of four pulses delivered at 200 HZ; the stimulus amplitude was two to three times the sensory threshold. The electrical stimulation of the PCL inhibited the ongoing muscle activity in both the quadriceps and hamstrings with latencies of 114-150 ms and 99-130 ms, respectively. Stimulation of the fat pad of the knee did not influence the muscle activity. The study suggests that the mechanoreceptors in the PCL are involved in controlling muscle activity during both static and active muscle contractions. The relative long latency of the reflex makes it unlikely that it can serve as a directly protective reflex for the cruciate ligaments.