Repeatability of maximal voluntary force and of surface EMG variables during voluntary isometric contraction of quadriceps muscles in healthy subjects

The repeatability of initial values and rate of change of EMG signal mean spectral frequency (MNF), average rectified values (ARV), muscle fiber conduction velocity (CV) and maximal voluntary contraction (MVC) was investigated in the vastus medialis obliquus (VMO) and vastus lateralis (VL) muscles of both legs of nine healthy male subjects during voluntary, isometric contractions sustained for 50 s at 50% MVC. The values of MVC were recorded for both legs three times on each day and for three subsequent days, while the EMG signals have been recorded twice a day for three subsequent days. The degree of repeatability was investigated using the Fisher test based upon the ANalysis Of VAriance (ANOVA), the Standard Error of the Mean (SEM) and the Intraclass Correlation Coefficient (ICC).

Data collected showed a high level of repeatability of MVC measurement (normalized SEM from 1.1% to 6.4% of the mean). MNF and ARV initial values also showed a high level of repeatability (ICC>70% for all muscles and legs except right VMO). At 50% MVC level no relevant pattern of fatigue was observed for the VMO and VL muscles, suggesting that other portions of the quadriceps might have contributed to the generated effort. These observations seem to suggest that in the investigation of muscles belonging to a multi-muscular group at submaximal level, the more selective electrically elicited contractions should be preferred to voluntary contractions.     

Do surface electromyograms provide physiological estimates of conduction velocity from the medial gastrocnemius muscle?

Muscle fiber conduction velocity (CV) is commonly estimated from surface electromyograms (EMGs) collected with electrodes parallel to muscle fibers. If electrodes and muscle fibers are not located in parallel planes, CV estimates are biased towards values far over the physiological range. In virtue of their pinnate architecture, the fibers of muscles such as the gastrocnemius are hardly aligned in planes parallel to surface electrodes. Therefore, in this study we investigate whether physiological CV estimates can be obtained from the gastrocnemius muscle. Specifically, with a large grid of 16 × 8 electrodes we map CV estimates over the whole gastrocnemius muscle while eleven subjects exerted isometric plantar flexions at three different force levels. CV was estimated for couples of single differential EMGs and estimate locations (i.e., channels) were classified as physiological and non-physiological, depending on whether CV estimates were within the physiological range (3–6 ms^?1) or not. Physiological CV values could be estimated from a markedly small muscle region for eight participants; channels providing physiological CV estimates corresponded to about 5% of the total number of channels. As expected, physiological and non-physiological channels were clustered in distinct regions. CV estimates within the physiological range were obtained for the most distal gastrocnemius portion (ANOVA, P < 0.001), where occurrences of propagating potentials were often verified through visual analysis. For the first time, this study shows that CV might be reliably assessed from surface EMGs collected from the most distal gastrocnemius region.     

Comparison of electromyographic signals from monopolar current and potential amplifiers derived from a penniform muscle,the gastrocnemius medialis

Electromyograms (EMGs) are measured by bipolar surface electrodes that quantify potential differences. Bipolar potentials over penniform muscles may be associated with errors. Our assumption was that muscle activity can be quantified more reliably and with a higher spatial resolution using current measurements.The purpose of this work is: (a) to introduce the concept of current measurements to detect muscle activity, (b) to show the coherences observed over a segment of a typical penniform muscle, the gastrocnemius medialis where one would expect a synchronicity of the activation, and (c) to show the amount of mixing that is caused by the finite inter electrode resistance.A current amplifier was developed. EMGs were recorded at 40% of maximum voluntary contraction during isometric contractions of the gastrocnemius medialis. EMGs of twelve persons were recorded with an array of four peripheral and one central electrode. Monopolar EMGs were recorded for “all-potential”, “center at current” and “all-current” conditions. Coherence revealed the similarity of signals recorded from neighboring electrodes.Coherence was high for the “all-potential”, significant for the “current at center” condition and disappeared in the “all-current” condition.It was concluded that EMG array recordings strongly depends on the measurement configuration. The proposed current amplifier significantly improves spatial resolution of EMG array recordings because the inter-electrode cross talk is reduced.     

Non-propagated potentials in the electromyogram of mammalian ocular muscles]

Electromyograms of mammalian extraocular muscles were recorded by means of a coaxial electrode. Besides normal extracellular spike potentials (1-2 msec duration), monophasic waves (with a decline lasting up to 7 msec) were recorded. As to the interpretation of these potential changes in terms of a potential drop that is produced by local currents flowing from the resting region of a fibre towards the active region consideration is given to two cases. First, a propagated active region (spike potentials, at least diphasic) and second, a stationary active region (with resulting monophasic waves). In the EMGs spontaneous monophasic potentials recruit at a lower threshold than spike potentials; frequency changes were observed when head position was altered. The latter are interpreted as local depolarizations occurring at neuromuscular junctions of multiple innervated muscle fibres among those fibre types that compose extraocular muscles.     

Automatic segmentation of surface EMG images: Improving the estimation of neuromuscular activity

Surface electromyograms (EMGs) recorded with a couple of electrodes are meant to comprise representative information of the whole muscle activation. Nonetheless, regional variations in neuromuscular activity seem to occur in numerous conditions, from standing to passive muscle stretching. In this study, we show how local activation of skeletal muscles can be automatically tracked from EMGs acquired with a bi-dimensional grid of surface electrodes (a grid of 8 rows and 15 columns was used). Grayscale images were created from simulated and experimental EMGs, filtered and segmented into clusters of activity with the watershed algorithm. The number of electrodes on each cluster and the mean level of neuromuscular activity were used to assess the accuracy of the segmentation of simulated signals. Regardless of the noise level, thickness of fat tissue and acquisition configuration (monopolar or single differential), the segmentation accuracy was above 60%. Accuracy values peaked close to 95% when pixels with intensity below ～70% of maximal EMG amplitude in each segmented cluster were excluded. When simulating opposite variations in the activity of two adjacent muscles, watershed segmentation produced clusters of activity consistently centered on each simulated portion of active muscle and with mean amplitude close to the simulated value. Finally, the segmentation algorithm was used to track spatial variations in the activity, within and between medial and lateral gastrocnemius muscles, during isometric plantar flexion contraction and in quiet standing position. In both cases, the regionalization of neuromuscular activity occurred and was consistently identified with the segmentation method.     

Muscle functional state assessment by estimation of muscle fiber conduction velocity

In 20 healthy subjects (10 males and 10 females) aged between 24 and 45, the conduction velocity (CV) of excitation along muscle fibers was measured in m. biceps brachii muscle using level averaging of interference electromyograms (EMGs) recorded by branched and monopolar surface electrodes. The measured CV, recorded with both monopolar and branched electrodes, were essentially equal (p < 0.05) at all different muscle tensions (10, 25, 50, and 75% of maximal voluntary contractions (MVCs)) and for all investigated healthy subjects. The CV was significant higher (p < 0.05) at 50 and 75% compared to 10 and 25% of MVC. In branched averaged potentials (AvPs), the terminal positive phase was suppressed. The monopolar AvPs were with waveform similar to those of motor units potentials and the parameters of the phases of different potentials (including the terminal positive one) may give additional information on functional state of muscles.The level-triggered averaging of the interference EMG recorded from the skin surface by branched electrodes is an adequate noninvasive method for measuring the propagation velocity of excitation along muscle fibers and for evaluating changes in the muscle functional state.     

Sensitivity of the cross-correlation between simulated surface EMGs for two muscles to detect motor unit synchronization.

The purpose of the study was to evaluate the use of cross-correlation analysis between simulated surface electromyograms (EMGs) of two muscles to quantify motor unit synchronization. The volume conductor simulated a cylindrical limb with two muscles and bone, fat, and skin tissues. Models of two motor neuron pools were used to simulate 120 s of surface EMG that were detected over both muscles. Short-term synchrony was established using a phenomenological model that aligned the discharge times of selected motor units within and across muscles to simulate physiological levels of motor unit synchrony. The correlation between pairs of surface EMGs was estimated as the maximum of the normalized cross-correlation function. After imposing four levels of motor unit synchrony across muscles, five parameters were varied concurrently in the two muscles to examine their influence on the correlation between the surface EMGs: 1) excitation level (5, 10, 15, and 50% of maximum); 2) muscle size (350 and 500 motor units); 3) fat thickness (1 and 4 mm); 4) skin conductivity (0.1 and 1 S/m); and 5) mean motor unit conduction velocity (2.5 and 4 m/s). Despite a constant and high level of motor unit synchronization among pairs of motor units across the two muscles, the cross-correlation index ranged from 0.08 to 0.56, with variation in the five parameters. For example, cross-correlation of EMGs from pairs of hand muscles, each having thin layers of subcutaneous fat and mean motor unit conduction velocities of 4 m/s, may be relatively insensitive to the level of synchronization across muscles. In contrast, cross-correlation of EMGs from pairs of leg muscles, with larger fat thickness, may exhibit a different sensitivity. These results indicate that cross correlation of the surface EMGs from two muscles provides a limited measure of the level of synchronization between motor units in the two muscles.     

Adaptation to fatigue of long duration in human wrist movements.

Subjects made fast, accurate, consistent wrist flexions under normal conditions and under conditions of low-frequency fatigue. Movements made 1 h after fatiguing exercise were indistinguishable from those made before exercise, even though twitch tensions were only approximately 60% of their fresh values. Electromyograms (EMGs) recorded from the fatigued muscles were, however, different from those recorded before exercise. EMGs during unfatigued movements showed multiple bursts typical for rapid movements. In the presence of low-frequency fatigue, the duration of the first burst was longer than that under normal conditions, and its onset occurred earlier relative to the initiation of movement. The area of the second agonist burst and, in some cases, the antagonist burst, was increased, although changes in their timings were unclear. We conclude that subjects adapted to low-frequency fatigue by changing the neural patterns controlling their muscles and present a simple model of excitation-contraction coupling that demonstrates how the observed changes in excitation can produce the same kinematics.     

Inhomogeneous response of expiratory muscle activity to cold block of the ventral medullary surface.

We assessed the effects of cooling the ventral medullary surface (VMS) on the activity of chest wall and abdominal expiratory muscles in eight anesthetized artificially ventilated dogs after vagotomy and denervation of the carotid sinus nerves. Electromyograms (EMGs) of the triangularis sterni, internal intercostal, abdominal external oblique, abdominal internal oblique, and transversus abdominis muscles were measured with EMG of the diaphragm as an index of inspiratory activity. Bilateral localized cooling (2 x 2 mm) in the thermosensitive intermediate part of the VMS produced temperature-dependent reduction in the EMG of diaphragm and abdominal muscles. The rib cage expiratory EMGs were little affected at 25 degrees C; their amplitudes decreased at lower VMS temperatures (less than 20 degrees C) but by significantly fewer degrees than the diaphragmatic and abdominal expiratory EMGs at a constant VMS temperature. With moderate to severe cooling (less than 20 degrees C) diaphragmatic EMG disappeared, but rib cage expiratory EMGs became tonic and resumed a phasic pattern shortly before the recovery of diaphragmatic EMG during rewarming of the VMS. These results indicate that the effects of cooling the VMS differ between the activity of rib cage and abdominal expiratory muscles. This variability may be due to inhomogeneous inputs from the VMS to expiratory motoneurons or to a different responsiveness of various expiratory motoneurons to the same input either from the VMS or the inspiratory neurons.     

江豚鼻道肌的解剖和构筑研究

江豚的鼻部肌共分为后外肌、前外肌、后内肌、前内肌和深肌5层,无间肌和大小内肌较退化,无对角膜肌。通过测定各肌的肌重、平均肌纤维长、平均肌小节长以及肌纤维角度,计算了各肌的生理横截面积,估计最大强直张力和肌鲜重对估计最大强直张力之比值等指标。鼻部肌各肌的相对肌纤维长度相似。各鼻部肌的肌纤维角度均为零。前部肌比后部肌具有较大的收缩速度和收缩位移优势,后部肌则具有较强的张力产生能力。着于额隆和唇部吻肌的张力产生能力很强。     

Innervation zones of the upper and lower limb muscles estimated by using multichannel surface EMG

The distribution of innervation zones was investigated in 3 subjects for 17 muscles and 8 muscle groups in the upper and lower limb, by detecting bi-directional propagation of motor unit action potentials (MUAPs) with the multichannel surface electrode array. Clarification of the distribution of innervation zones depended on the ease in detecting the propagation of MUAPs and the actual scattering of innervation zones, which were closely related with muscle morphology with respect to the arrangements of muscle fibers. In muscles having fibers running parallel to each other, such as the biceps brachii, intrinsic hand muscles, vastus lateralis and medialis, tensor fasciae latae, peronei, soleus, tibialis anterior, and hypothenar muscles in the foot, it was relatively easy to detect the propagating MUAPs, and the innervation zones were distributed in a relatively narrow band around muscle belly. On the other hand, in muscles with a complicated structure including pinnation of muscle fibers, in-series muscle fibers and aponeurotic tissues, such as the deltoid, flexors and extensors in the forearm, rectus femoris, sartorius, hamstrings and gastrocnemius, it was more difficult to detect the propagating MUAPs and to identify the innervation zones, which were widely scattered or distributed in complex configurations. The distribution of the innervation zones clarified in the present study can be used to find the optimal location of electrodes in surface EMG recordings and of stimulus electrodes in the functional and therapeutic electrical stimulations. It may also be useful in motor point biopsy for diagnosis of neuromuscular diseases as well as in the botulinum toxin injection for the treatment of spasticity.     

Distribution of sensory receptors in the flexor carpi radialis muscle of the cat

The structures and distribution of encapsulated muscle receptors were examined in serial transverse sections of flexor carpi radialis in the adult cat. Four types of receptors (muscle spindles, Golgi tendon organs, paciniform, and Pacinian corpuscles) were identified. Their structures resembled those encountered in other limb muscles. Pacinian corpuscles were rare and occurred only in the external fascial coat of the muscle near its origin. The other three receptor types were distributed in an uneven but consistent pattern throughout the muscle. As noted previously (Gonyea and Ericson, '77), spindles were largely confined to a deep muscle region comprising less than 20% of the muscle volume, located directly between the long tendon of origin and the tendon of insertion. This region contains the largest proportion of type SO muscle fibers (Gonyea and Ericson, '77). Tendon organs and paciniform corpuscles were concentrated along the tendons that lined the spindle-rich muscle region. This region appeared to be composed of extrafusal fibers that were shorter and of more oblique pinnation than those in other regions. The localization of muscle receptors to the "oxidative" core of the muscle in its direct line of pull may have functional implications for afferent input to the spinal cord which are discussed. In addition, the possibility is raised that there are more paciniform corpuscles in flexor carpi radialis (and possibly other muscles) than previously thought.     

Myoelectric manifestations of fatigue in vastus lateralis,medialis obliquus and medialis longus muscles

The purpose of this study was to determine whether surface electromyography (EMG) assessment of myoelectric manifestations of muscle fatigue is capable of detecting differences between the vastus lateralis and medialis muscles which are consistent with the results of previous biopsy studies. Surface EMG signals were recorded from the vastus medialis longus (VML), vastus medialis obliquus (VMO) and vastus lateralis (VL) muscles during isometric knee extension contractions at 60% and 80% of the maximum voluntary contraction (MVC) for 10 s and 60 s, respectively. Initial values and rate of change of mean frequency (MNF), average rectified value (ARV) and conduction velocity (CV) of the EMG signal were calculated. Comparisons between the two force levels revealed that the initial values of MNF for the VL muscle were greater at 80% MVC compared to 60% MVC (P < 0.01). Comparisons between the vasti muscles demonstrated lower initial values of CV for VMO compared to VL at 60% MVC (P < 0.01) and lower than VML and VL at 80% MVC (P < 0.01). In addition, initial values of MNF were higher for VL with respect to both VML and VMO at 80% MVC (P < 0.01) and initial estimates of ARV were higher for VMO compared to VML at both force levels (P < 0.01 at 60% MVC and P < 0.05 at 80% MVC). For the sustained contraction at 80% MVC, VL demonstrated a greater decrease in CV over time compared to VMO (P < 0.05).These findings suggest that surface EMG signals and their time course during sustained isometric contractions may be useful to non-invasively describe functional differences between the vasti muscles.     

Effect of muscle length on surface EMG wave forms in isometric contractions

To elucidate the influence of muscle length on surface EMG wave form, comparisons were made of surface EMGs of the biceps and triceps brachii muscles during isometric contractions at different muscle lengths. Muscle lengths were altered by setting the elbow joint angle at several intervals between the limits of extension and flexion. The intensity of the isometric contractions was 25% of maximum voluntary contraction at the individual joint angles. Slowing was obvious in the EMG wave forms of biceps as muscle length increased. The so-called 'Piper rhythm' appeared when the muscle was more than moderately lengthened. The slowing trend with muscle lengthening, though less marked, was also seen in triceps. Zero-cross analysis revealed quasi-linear relationships between muscle length and slowing. Frequency analysis confirmed the development of 'Piper rhythm'. An attempt was made to interpret the slowing associated with muscle lengthening in terms of the propagation of myoelectric signals in muscle fibers. given the effect of muscle length on EMG wave forms, a careful control of joint angle may be required in assessing local making fatigue when using EMG spectral indices.     

Responses of the anterolateral abdominal muscles during cough and expiratory threshold loading in the cat.

The present study was conducted to determine the pattern of activation of the anterolateral abdominal muscles during the cough reflex. Electromyograms (EMGs) of the rectus abdominis, external oblique, internal oblique, transversus abdominis, and parasternal muscles were recorded along with gastric pressure in anesthetized cats. Cough was produced by mechanical stimulation of the lumen of the intrathoracic trachea or larynx. The pattern of EMG activation of these muscles during cough was compared with that during graded expiratory threshold loading (ETL; 1-30 cmH(2)O). ETL elicited differential recruitment of abdominal muscle EMG activity (transversus abdominis > internal oblique > rectus abdominis congruent with external oblique). In contrast, both laryngeal and tracheobronchial cough resulted in simultaneous activation of all four anterolateral abdominal muscles with peak EMG amplitudes 3- to 10-fold greater than those observed during the largest ETL. Gastric pressures during laryngeal and tracheobronchial cough were at least eightfold greater than those produced by the largest ETL. These results suggest that, unlike their behavior during expiratory loading, the anterolateral abdominal muscles act as a unit during cough.     

Architecture of the hind limb muscles of cats: Functional significance

Force, velocity, and displacement properties of a muscle are determined in large part by its architectural design. The relative effect of muscle architecture on these physiological variables was studied by determining muscle weight, fiber length, average sarcomere length, and approximate angle of pinnation for 24 cat hind limb muscles. Muscle lengths ranged from 28.3 to 144 mm, whereas fiber lengths ranged from 8.4 to 105.5 mm. Generally, fiber to muscle length ratios were similar throughout a muscle. Estimated angles of pinnation of muscle fibers varied from 0 to 21° with most having an angle of less than 10°. The cross-sectional area of the knee extensors was similar to the knee flexors (16.43 vs. 16.83 cm²) whereas the cross-sectional area of the ankle extensors was more than six times greater than the ankle flexors (18.59 vs. 2.83 cm²). There was a 6.7-fold difference in the maximal force between muscles, when normalized to a constant weight, that could be attributed to architectural features. Rations of wet weight to predicted maximal tetanic tension for each muscle and group were calculated to compare the relative priority of muscle force versus muscle length-velocity for a given mass of muscle. These ratios varied from 0.4 to 4.84. The ratios suggest that velocity and/or displacement is a priority for the hamstrings, whereas force is a priority for the quadriceps and lower leg muscles. As much as a 12.6-fold difference in maximal velocity between muscles can be attributed to differences in fiber lengths. This can be compared to approximately a 2.5-fold difference in maximal velocity reported to occur as a result of biochemical (intrinsic) differences.     

Are the myoelectric manifestations of fatigue distributed regionally in the human medial gastrocnemius muscle?

Myoelectric fatigue typically manifests as variations in the amplitude and spectrum of surface electromyograms (EMGs). Interestingly, these variations seem to be represented locally in different muscles. In this study, we ask whether such a regional distribution of myoelectric fatigue extends to the medial gastrocnemius (MG) muscle. If the MG muscle is activated locally during fatiguing contractions, or if the most fatigable MG fibers are located at distinct muscle regions, then, the myoelectric manifestations of MG fatigue are expected to appear locally in a grid of surface electrodes. With a matrix of surface electrodes (7 × 15 single-differential EMGs) we show that myoelectric fatigue, indeed, manifests regionally in the MG muscle of 12 subjects, who exerted intermittent, fatiguing plantar flections at 50% of their maximal effort. Contrary to the root mean square amplitude, the median frequency of surface EMGs varied consistently across subjects throughout the plantar flections (P = 0.002). On average, changes in EMG spectrum were represented at 78–93 (interquartile interval) out of the 105 channels in the matrix, though with different degrees across channels. For all participants, about 29% of the channels detected significantly greater reductions in median frequency when compared to all channels in the matrix (P < 0.003). Strikingly, these channels were not sparsely distributed; they rather occupied localized skin regions across subjects. Physiologically, our results suggest that, during sub-maximal fatiguing tasks, myoelectric manifestations of MG fatigue are represented in spatially localized muscle regions. Technically, the possibility of studying myoelectric fatigue in the MG muscle appears to depend on the electrode location.     

Muscle fluid shift does not alter EMG global variables during sustained isometric actions

Body fluid redistribution occurs in astronauts traveling in space, potentially altering interstitial water content and hence impedance. This in turn may impact the features of electromyographic (EMG) signals measured to compare in-flight muscle function with pre- and post-flight conditions. Thus, the current study aimed at investigating the influence of similar fluid shifts on EMG spectral variables during muscle contractile activity. Ten men performed sustained isometric actions (120 s) at 20% and 60% of maximum voluntary contraction (MVC) following 1-h rest in the vertical or supine position.From single differential EMG signals, recorded from the soleus (SOL), the medial (MG) and lateral (LG) gastrocnemius muscles, initial value and rate of change over time (slope) of mean power frequency (MNF) and average rectified value (ARV) were assessed. MNF initial value showed dependence on muscle (P < 0.01), but was unaffected by body tilt. MNF rate of change increased (P < 0.001) with increased force and differed across muscles (P < 0.05), but was not influenced (P = 0.85) by altered body position. Thus, fluid shift resulting from vertical to supine tilt had no impact on myoelectrical manifestations of muscle fatigue. Furthermore, since such alteration of body fluid distribution resembles that occurring in microgravity, our findings suggest this may not be a methodological limitation, when comparing EMG fatigue indices on Earth versus in space.     

Mean frequency and signal amplitude of the surface EMG of the quadriceps muscles increase with increasing torque--a study using the continuous wavelet transform.

The continuous wavelet transform (CWT), a time-frequency method, was used when calculating mean frequency of the power spectrum (MNF) and signal amplitude (RMS) of the surface EMG to investigate their relationships to force during a gradually increasing knee extension (ramp). Based upon the CWT, MNF was redefined to include time dependence on the EMG signal frequency contents, the short-time MNF (STMNF). Surface EMG was recorded from vastus lateralis, rectus femoris and vastus medialis in 21 clinically healthy subjects during a brief, gradually increasing contraction up to 100% of a maximum voluntary contraction (MVC), with a duration of approximately 10 s. The relationships between the EMG variables and force using linear regression were determined for each subject. For vastus lateralis, we also investigated if certain aspects of the muscle morphology (i.e., proportions and areas of different fibre types) influenced the EMG-force relationship.For the majority of subjects (17-18 out of 21 subjects) there were significant positive correlations between STMNF and force in the three muscles. No sex differences were found in intercepts or regression coefficients of STMNF. The muscle morphology had a significant influence on the STMNF-force intercept and the regression coefficient. Positive and highly significant linear correlations between RMS and force were found for all subjects and all three muscles.In conclusion, time frequency methods can be applied when investigating EMG during brief contractions associated with non-stationarity. In a great majority of the subjects, and in the three muscles, significant linear force dependencies were found for STMNF. Thus, when evaluating muscle fatigue, e.g., in ergonomic situations, it is important to consider the force level as one factor that can influence the results. Morphological variables (fibre proportions and fibre areas) influenced the STMNF-force relationship in vastus lateralis.