Analysis of various conditions in order to measure electromyography of isometric contractions in water and on air

The aim of this study was to verify if there are differences in the amplitude of signals from surface electromyography (EMG) during maximal and submaximal voluntary isometric contraction (MVC and 50% MVC, respectively) under different conditions, in our case, water and air, with and without extra protection (water-resistant tape) on the electrode. The isometric force and muscle activation of the MVC and 50% MVC of the biceps brachial muscle of nine healthy trained men were measured simultaneously, performed in water and on air, with and without protection of the EMG electrode. The multivariate analysis of variance with a post hoc Tukey test was applied to detect significant differences between the levels of muscular force. For the amplitude values of the EMG signal, the Wilcoxon signed rank test was applied to compare all experimental conditions in order to detect a significance of p < 0.05. The values of isometric force were not significantly different among conditions (MVC and 50% MVC). The results showed a significant difference among conditions in the water without extra protection compared to the conditions on air with and without extra protection and in water with extra protection. Reduced EMG amplitude was seen in water without extra protection from 37.04% to 55.81% regarding the other conditions. However, no significant difference was seen among conditions in water with extra protection in relation the conditions on air (with and without extra protection). This study suggest that it is necessary to use a water-resistant tape as an extra protection on the electrode when using EMG underwater, to avoid having a significant decrease in the EMG amplitude underwater and not to suffer interference from the water. There was no significant difference among the recordings of EMG with and without the use of protection on air; therefore, the protection does not influence the recording of EMG amplitude and isometric force on air.     

Human isometric force production and electromyogram activity of knee extensor muscles in water and on dry land

This study was designed to determine trial-to-trial and day-to-day reproducibility of isometric force and electromyogram activity (EMG) of the knee extensor muscles in water and on dry land as well as to make comparisons between the two training conditions in muscle activity and force production. A group of 20 healthy subjects (12 women and 8 men) were tested three times over 2 weeks. A measurement session consisted of recordings of maximal and submaximal isometric knee extension force with simultaneous recording of surface EMG from the vastus medialis, vastus lateralis and biceps femoris muscles. To ensure identical measurement conditions the same patient elevator chair was used in both the dry and the wet environment. Intraclass correlation coefficients (ICC) and coefficients of variation (CV) showed high trial-to-trial (ICC = 0.95-0.99, CV = 3.5%-11%) and day-to-day reproducibility (ICC=0.85-0.98, CV=11%-19%) for underwater and dry land measurements of force and EMG in each muscle during maximal contractions. The day-to-day reproducibility for submaximal contractions was similar. The interesting finding was that underwater EMG amplitude decreased significantly in each muscle during maximal (P < 0.01-P < 0.001) and submaximal contractions (P < 0.05-P < 0.001). However, the isometric force measurements showed similar values in both wet and dry conditions. The water had no disturbing effect on the electrodes as shown by slightly lowered interelectrode resistance values, the absence of artefacts and low noise levels of the EMG signals. It was concluded that underwater force and EMG measurements are highly reproducible. The significant decrease of underwater EMG could have electromechanical and/or neurophysiological explanations.     

Rectus femoris surface myoelectric signal cross-talk during static contractions

The clinical application of EMG requires that the recorded signal is representative of the muscle of interest and is not contaminated with signals from adjacent muscles. Some authors report that surface EMG is not suitable for obtaining information on a single muscle but rather reflects muscle group function [J. Perry, C.S. Easterday, D.J. Antonelli, Surface versus intramuscular electrodes for electromyography of superficial and deep muscles. Physical Therapy 61 (1981) 7–15]. Other authors report however, that surface EMG is adequate to determine individual muscle function, once guidelines pertaining to data acquisition are followed [D.A. Winter, A.J. Fuglevand, S.E. Archer. Cross-talk in surface electromyography: theoretical and practical estimates. Journal of Electromyography and Kinesiology 4 (1994) 15–26]. The aim of this study was to determine whether surface EMG was suitable for monitoring rectus femoris (RF) activity during static contractions. Five healthy subjects, having given written informed consent, participated in this trial. Surface and fine wire EMG from the rectus femoris and the vastus lateralis (VL) muscles were recorded simultaneously during a protocol of static contractions consisting of knee extensions and hip flexions. Ratios were used to quantify the relationship between the surface EMG amplitude value and the fine wire EMG amplitude value for the same contraction. The results showed that hip flexion contractions elicited RF activation only and that knee extension contractions elicited fine wire activity in VL only. When the relationship between RF surface and RF fine wire electrodes was compared for hip flexion and knee extension contractions, it was observed that for all subjects, there was a tendency for increased RF surface activity in the absence of RF fine wire activity during knee extensions. It was concluded that the activity recorded by the RF surface electrode arrangement during knee extension consisted of EMG from the vastii, i.e., cross-talk and that vastus intermedius was the most likely origin of the erroneous signal. Therefore it is concluded that for accurate EMG information from RF, fine wire electrodes are necessary during a range of static contractions.     

Triceps surae stretch and voluntary contraction alters maximal M-wave magnitude.

Reliability of the motor response (M-wave) is fundamental in many reflex studies; however it has recently been shown to change during some investigations. The aim of this investigation was to determine if triceps surae stretch and voluntary contraction, or recording and analysis techniques, affect the maximal M-wave magnitude. The maximal M-wave was investigated in human gastrocnemius and soleus during different foot positions and during triceps surae contraction. Both bipolar and monopolar-recoding methods, and area and peak-to-peak (PTP) amplitude analysis methods were used. RESULTS: Maximal M-wave magnitude changed significantly between test muscle conditions, and is largest during dorsiflexion, probably due to changes in muscle bulk and recording electrode relationship. The maximal M-wave was up to 88% smaller when recorded by bipolar electrodes compared to monopolar electrodes, which is discussed in relation to signal cancellation. Area analysis provided more significant differences in M-wave magnitude between test muscle conditions than did PTP amplitude analysis, and the maximal M-wave shape changed significantly between test muscle conditions. This study suggests that maximal M-wave magnitude can vary depending on muscle condition, it highlights the importance of using correct recording and analysis techniques, and questions the reliability of using M-wave magnitude to monitor the relationship between the nerves and stimulating electrodes.     

Consensus for experimental design in electromyography (CEDE) project: Amplitude normalization matrix

The general purpose of normalization of EMG amplitude is to enable comparisons between participants, muscles, measurement sessions or electrode positions. Normalization is necessary to reduce the impact of differences in physiological and anatomical characteristics of muscles and surrounding tissues. Normalization of the EMG amplitude provides information about the magnitude of muscle activation relative to a reference value. It is essential to select an appropriate method for normalization with specific reference to how the EMG signal will be interpreted, and to consider how the normalized EMG amplitude may change when interpreting it under specific conditions. This matrix, developed by the Consensus for Experimental Design in Electromyography (CEDE) project, presents six approaches to EMG normalization: (1) Maximal voluntary contraction (MVC) in same task/context as the task of interest, (2) Standardized isometric MVC (which is not necessarily matched to the contraction type in the task of interest), (3) Standardized submaximal task (isometric/dynamic) that can be task-specific, (4) Peak/mean EMG amplitude in task, (5) Non-normalized, and (6) Maximal M-wave. General considerations for normalization, features that should be reported, definitions, and “pros and cons” of each normalization approach are presented first. This information is followed by recommendations for specific experimental contexts, along with an explanation of the factors that determine the suitability of a method, and frequently asked questions. This matrix is intended to help researchers when selecting, reporting and interpreting EMG amplitude data.     

The nature of facilitation of leg muscle motor evoked potentials by knee flexion

Knee flexion is a movement that initiates rising from a sitting position, which is a common therapeutic exercise for patients unable to ambulate. We investigated how voluntary isometric biceps femoris contraction affects motor evoked potential (MEP) amplitude following transcranial magnetic stimulation, background electromyographic (EMG) amplitude, and H-reflex amplitude in ipsilateral leg muscles. Subjects were seated on the edge of a bed with their hips and knees flexed at 90°, and the soles of their feet on the floor. MEP and background EMG were recorded from the tibialis anterior (TA) and soleus (SOL), and H reflexes from SOL of 30 volunteers. Background EMG and MEP also were recorded while voluntarily contracting tested muscles. Biceps femoris contraction increased MEP and background EMG for TA and SOL ( p < 0.01). Maximal background EMG and MEP increased with increasing voluntary contraction of tested muscles ( p < 0.005). Regression slope differed little between TA and SOL. Biceps femoris contraction facilitated MEP comparably for TA and SOL, while SOL background EMG exceeded that of TA ( p < 0.02). The relationship between MEP facilitation and background EMG changed to favor more efficient facilitation in TA ( p < 0.05), but not SOL ( p > 0.1). MEP recorded from TA and SOL with subthreshold stimuli using needle electrodes were more frequent with biceps femoris contraction ( p < 0.04). H-reflex amplitude of SOL decreased during biceps femoris contraction ( p < 0.001). We concluded that biceps femoris contraction affects leg muscle MEP, background EMG, and H reflexes differently.     

Influence of amplitude cancellation on the simulated surface electromyogram.

The purpose of the study was to quantify the influence of selected motor unit properties and patterns of activity on amplitude cancellation in the simulated surface electromyogram (EMG). The study involved computer simulations of a motor unit population with physiologically defined recruitment and rate coding characteristics that activated muscle fibers whose potentials were recorded on the skin over the muscle. Amplitude cancellation was quantified as the percent difference in signal amplitude when motor unit potentials were summed before and after rectification. The simulations involved varying the level of activation for the motor unit population, the recording configuration, the upper limit of motor unit recruitment, peak discharge rates, the amount of motor unit synchronization, muscle fiber length, the thickness of the subcutaneous tissue, and the motor unit properties that change with advancing age. The results confirmed a previous experimental report (Day SJ and Hulliger M, J Neurophysiol 86: 2144-2158, 2001) that amplitude cancellation in the surface EMG can reach 62% at maximal activation. A decrease in the range of amplitudes of the motor unit potentials, as can occur during fatiguing contractions, increased amplitude cancellation up to approximately 85%. Differences in the amount of amplitude cancellation were observed across all simulated conditions, and resulted in substantial changes in the absolute magnitude of the EMG signal. The most profound factors influencing amplitude cancellation were the number of active motor units and the duration of the action potentials. The effects of amplitude cancellation were minimal (<5%) when the EMG amplitude was normalized to maximal values, with the exception of variations in peak discharge rate and recruitment range, which resulted in differences up to 17% in the normalized EMG signal across conditions. These results indicate the amount of amplitude cancellation that can occur in various experimental conditions and its influence on absolute and relative measures of EMG amplitude.     

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.     

A differential suction electrode for recording electromyographic activity from the pelvic floor muscles: Crosstalk evaluation

The purpose of this study was to determine whether a differential suction electrode (DSE) probe is less prone to recording crosstalk from the hip adductors or external rotators than the Femiscan? probe when recording electromyography (EMG) data from the pelvic floor muscles (PFMs). Twenty nulliparous, continent women performed hip adduction and hip external rotation contractions at 25%, 50% and 100% of maximal voluntary effort both while keeping their PFMs relaxed and while contracting their PFMs as strongly as possible. All tasks were performed while DSEs were adhered to the vaginal wall at the level of the pubococcygeus group bilaterally, and also with the Femiscan? probe in situ. The order of the probes was randomized. For each task, the peak smoothed PFM EMG amplitude was compared between hip forces and probes using a two-way repeated measures analysis of variance (ANOVA) including the interaction between contraction level and probe (α = 0.05). There was a significant contraction level by probe interaction for each task. In most cases the Femiscan? probe recorded significantly higher PFM EMG activity during hip adduction and external rotation tasks at 25% and 50% MVC compared to what it recorded when the hip musculature remained relaxed, whereas the DSE probe did not. As such, the DSE probe appears to be less susceptible to crosstalk from the hip adductors and external rotators than the Femiscan? probe at these hip muscle contraction levels. Both probes recorded significantly higher EMG activity when maximal contractions of the hip adductors and external rotators were performed therefore, no conclusion can be made regarding whether the activity recorded from the PFMs during maximal hip adduction and external rotation is the result of crosstalk or co-activation.     

Estimation of monopolar signals from sphincter muscles and removal of common mode interference

The detection of surface electromyogram (EMG) by multi-electrode systems is applied in many research studies. The signal is usually recorded by means of spatial filters (linear combination of the potential under at least two electrodes) with vanishing sum of weights. Nevertheless, more information could be extracted from monopolar signals measured with respect to a reference electrode away from the muscle. Under certain conditions, surface EMG signal along a curve parallel to the fibre path has zero mean (property approximately satisfied when EMG is sampled by an array of electrodes that covers the entire support of the signal in space). This property allows estimating monopolar from single differential (SD) signals by pseudoinversion of the matrix relating monopolar to SD signals. The method applies to EMG signals from the external anal sphincter muscle, recorded using a specific cylindrical probe with an array of electrodes located along the circular path of the fibres. The performance of the algorithm for the estimation of monopolar from SD signals is tested on simulated signals. The estimation error of monopolar signals decreases by increasing the number of channels. Using at least 12 electrodes, the estimation error is negligible. The method applies to single fibre action potentials, single motor unit action potentials, and interference signals.The same method can also be applied to reduce common mode interference from SD signals from muscles with rectilinear fibres. In this case, the last SD channel defined as the difference between the potentials of the last and the first electrodes must be recorded, so that the sum of all the SD signals vanishes. The SD signals estimated from the double differential signals by pseudoinvertion are free of common mode.     

Patterns of leg muscle recruitment vary between novice and highly trained cyclists

This study compared patterns of leg muscle recruitment and coactivation, and the relationship between muscle recruitment, coactivation and cadence, in novice and highly trained cyclists. Electromyographic (EMG) activity of tibialis anterior (TA), tibialis posterior (TP), peroneus longus (PL), gastrocnemius lateralis (GL) and soleus (SOL) was recorded using intramuscular fine-wire electrodes. Four experimental conditions of varying cadence were investigated. Differences were evident between novice and highly trained cyclists in the recruitment of all muscles. Novice cyclists were characterized by greater individual variance, greater population variance, more extensive and more variable muscle coactivation, and greater EMG amplitude in periods between primary EMG bursts. Peak EMG amplitude increased linearly with cadence and was not different at individual preferred cadence in either novice or highly trained cyclists. However, EMG amplitude in periods between primary EMG bursts, as well as the duration of primary EMG bursts, increased with increasing cadence in novice cyclists but were not influenced by cadence in highly trained cyclists. Our findings suggest that muscle recruitment is highly skilled in highly trained cyclists and less refined in novice cyclists. More skilled muscle recruitment in highly trained cyclists is likely a result of neuromuscular adaptations due to repeated performance of the cycling movement in training and competition.     

Recovery following exhaustive dynamic exercise in the human biceps muscle

The aim of the study was to investigate the recovery of the maximum voluntary contraction force (MVC), the endurance time and electromyographical (EMG) parameters following exhaustive dynamic exercise of the m. biceps brachii. EMG recordings were made in ten healthy subjects using bipolar surface electrodes placed over the common belly of the left arm biceps muscle. Up to 25 h post-exercise, the maximum contraction force and the EMG signal were recorded alternately at regular intervals. The EMG signal was recorded during 30-s contractions at 40% of the pre-fatigued MVC. Four hours and 25 h post-exercise, the endurance time of a 40% pre-fatigued MVC was recorded. Up to 25 h after the exercise the maximum contraction force, the endurance time and the EMG parameters were significantly different from the pre-exercise values. Nine out of ten subjects complained that muscle soreness had developed. Thus, long-lasting changes are found after exhaustive dynamic exercise, not only in the MVC and the muscle's endurance capacity, but also in the EMG signal.     

Influence of amplitude cancellation on the accuracy of determining the onset of muscle activity from the surface electromyogram

The purpose of the study was to quantify the influence of amplitude cancellation on the accuracy of detecting the onset of muscle activity based on an analysis of simulated surface electromyographic (EMG) signals. EMG activity of a generic lower limb muscle was simulated during the stance phase of human gait. Surface EMG signals were generated with and without amplitude cancellation by summing simulated motor unit potentials either before (cancellation EMG) or after (no-cancellation EMG) the potentials had been rectified. The two sets of EMG signals were compared at forces of 30% and 80% of maximum voluntary contraction (MVC) and with various low-pass filter cut-off frequencies. Onset time was determined both visually and by an algorithm that identified when the mean amplitude of the signal within a sliding window exceeded a specified standard deviation (SD) above the baseline mean. Onset error was greater for the no-cancellation conditions when determined automatically and by visual inspection. However, the differences in onset error between the two cancellation conditions appear to be clinically insignificant. Therefore, amplitude cancellation does not appear to limit the ability to detect the onset of muscle activity from the surface EMG.     

The influence of the type of contraction on the masseter muscle EMG power spectrum

Different behaviours of the EMG power spectrum across increasing force levels have been reported for the masseter muscle. A factor that could explain these different behaviours may be the type of contraction used, as was recently shown for certain upper limb muscles⁵. The purpose of this study was to compare, between two types of isometric contractions, the behaviour of EMG power spectrum statistics (median frequency (MF) and mean power frequency (MPF)) obtained across increasing force levels. Ten women exerted, while biting in the intercuspal position, three 5 s ramp contractions that increased linearly from 0 to 100% of the maximal voluntary contraction (MVC). They also completed three step contractions (constant EMG amplitude) at each of the following levels: 20, 40, 60 and 80% MVC. EMG signals from the masseter muscle were recorded with miniature surface electrodes. The RMS, as well as the MPF and MF of the power spectrum were calculated at 20, 40, 60 and 80% MVC for each type of contraction. As expected, the RMS values showed similar increases with increasing levels of effort for both types of contractions. Different behaviours for both MPF (contraction^*force interaction, ANOVA, P<0.05) and MF (contraction^*force interaction, ANOVA, P>0.05) across increasing levels of effort were found between the two types of contraction. The use of step contractions gave rise to a decrease of both MPF and MF with increasing force, while the use of ramp contractions gave rise to an increase in both statistics up to at least 40% MVC followed by a decrease at higher force levels. These findings suggest that the type of contraction used does influence the behaviour of the spectral statistics across increasing force levels and that this could explain the differences obtained in previous studies for the masseter muscle.     

Effect of muscle length on electromyogram in a canine diaphragm strip preparation

The relationship between diaphragm electromyogram (EMG), isometric force, and length was studied in the canine diaphragm strip with intact blood supply and innervation under three conditions: supramaximal tetanic (100 Hz) phrenic nerve stimulation (STPS; n = 12), supramaximal phrenic stimulation at 25 Hz (n = 15), and submaximal phrenic stimulation at 25 Hz (n = 5). In the same preparation, the EMG-length relationship was also examined with direct muscle stimulation when the neuromuscular junction was blocked. EMG from three different sites and via two types of electrodes (direct or sewn-in and surface) were recorded during isometric contraction at different lengths. Direct EMGs were recorded from two bipolar electrodes sutured into the strip, one near its central end and the other near its costal end. A third EMG electrode configuration summed potentials from the whole strip by recording potentials between central and costal sites. Surface EMGs were recorded by a bipolar spring clip electrode that made contact with upper and lower surfaces of the muscle strip with light pressure. In all conditions of stimulation with different types of electrodes, all EMGs decreased significantly (P less than 0.05) when muscle length was changed from 50 to 120% of resting length (L0). Minimal and maximal force outputs were observed at 50 and 120% of L0, respectively, in all experiments. The results of this study indicated that the muscle length is a significant variable that affects the EMG recording and that the diaphragmatic EMG may not be an accurate reflection of phrenic nerve activity.(ABSTRACT TRUNCATED AT 250 WORDS)     

Comparison of force and EMG measures in normal and reinnervated tibialis anterior muscles of the rat.

The relationship between motor unit force and the recorded voltage produced by activated muscle unit fibres (electromyogram, EMG) was examined in normal and reinnervated rat tibialis anterior muscles. The number, cross-sectional area, and radial distance from the recording electrode of muscle fibres in a given unit, obtained directly from a sample of glycogen-depleted motor units, were analysed in relation to the magnitude of the EMG signal produced by that unit. EMG peak to peak amplitude and area varied as approximately the square root of twitch force in both normal and reinnervated units. Furthermore, the EMG amplitude increased approximately as the total cross-sectional area of the motor unit (number of muscle fibres x the average cross-sectional area of the fibres) and inversely with approximately the square root of the distance of fibres from the recording electrodes on the surface of the muscle.     

Electromyographic measures of muscle activation and changes in muscle architecture of human elbow flexors during fatiguing contractions.

The study compared changes in intramuscular and surface recordings of EMG amplitude with ultrasound measures of muscle architecture of the elbow flexors during a submaximal isometric contraction. Ten subjects performed a fatiguing contraction to task failure at 20% of maximal voluntary contraction force. EMG activity was recorded in biceps brachii, brachialis, and brachioradialis muscles using intramuscular and surface electrodes. The rates of increase in the amplitude of the surface EMG for the long and short heads of biceps brachii and brachioradialis were greater than those for the intramuscular recordings measured at different depths. The amplitude of the intramuscular recordings from three muscles increased at a similar rate (P = 0.13), as did the amplitude of the three surface recordings from two muscles (P = 0.83). The increases in brachialis thickness (27.7 +/- 5.7 to 30.9 +/- 3.5 mm; P < 0.05) and pennation angle (10.9 +/- 3.5 to 16.5 +/- 4.8 degrees ; P = 0.003) were not associated with the increase in intramuscular EMG amplitude (P > 0.58). The increase in brachioradialis thickness (22.8 +/- 4.8 to 25.5 +/- 3.4 mm; P = 0.0075) was associated with the increase in the amplitude for one of two intramuscular EMG signals (P = 0.007, r = 0.79). The time to failure was more strongly associated with the rate of increase in the amplitude of the surface EMG than that for the intramuscular EMG, which suggests that the surface measurement provides a more appropriate measure of the change in muscle activation during a fatiguing contraction.     

Methodological aspects of SEMG recordings for force estimation – A tutorial and review

Insight into the magnitude of muscle forces is important in biomechanics research, for example because muscle forces are the main determinants of joint loading. Unfortunately muscle forces cannot be calculated directly and can only be measured using invasive procedures. Therefore, estimates of muscle force based on surface EMG measurements are frequently used. This review discusses the problems associated with surface EMG in muscle force estimation and the solutions that novel methodological developments provide to this problem. First, some basic aspects of muscle activity and EMG are reviewed and related to EMG amplitude estimation. The main methodological issues in EMG amplitude estimation are precision and representativeness. Lack of precision arises directly from the stochastic nature of the EMG signal as the summation of a series of randomly occurring polyphasic motor unit potentials and the resulting random constructive and destructive (phase cancellation) superimpositions. Representativeness is an issue due the structural and functional heterogeneity of muscles. Novel methods, i.e. multi-channel monopolar EMG and high-pass filtering or whitening of conventional bipolar EMG allow substantially less variable estimates of the EMG amplitude and yield better estimates of muscle force by (1) reducing effects of phase cancellation, and (2) adequate representation of the heterogeneous activity of motor units within a muscle. With such methods, highly accurate predictions of force, even of the minute force fluctuations that occur during an isometric and isotonic contraction have been achieved. For dynamic contractions, EMG-based force estimates are confounded by the effects of muscle length and contraction velocity on force producing capacity. These contractions require EMG amplitude estimates to be combined with modeling of muscle contraction dynamics to achieve valid force predictions.     

Surface EMG alterations induced by underwater recording.

BACKGROUND AND PURPOSE: This study aims to verify if amplitude and spectral characteristics of surface EMG signal are modified due to recording in a wet environment. METHODS: Isometric contractions of the biceps brachii muscle of ten subjects were performed in several different set-up combinations, both in dry (D) and in water from hydrotherapy pools (PW), with (PWM) or without moving the pool water and with (T) or without water-resistant adhesive taping. RESULTS: In PW condition the amplitude of the recorded signal is reduced to 5-10% of the corresponding signal recorded in D condition. In PWM the power spectrum is drastically reduced and altered by the water movement that introduces an increase of spectral power in the frequency range 0-20 Hz. The use of T modality allows to record signals with both amplitude and spectral frequencies comparable with those obtained in the D conditions. DISCUSSION AND CONCLUSION: This work demonstrates the need for water resistant taping when EMG signals are recorded in water. Signals recorded without such a protective film are strongly affected in their amplitude and frequency characteristics by the conductivity and the movement of water.     

Reproducibility of surface EMG variables and peak torque during three sets of ten dynamic contractions.

The interpretation of the electromyogram (EMG) of dynamic contractions might be difficult because the movement per se introduces additional factors that could affect its characteristics. There is a lack of studies concerning the reproducibility of surface EMG registrations during dynamic contractions. The aim was to investigate the during-the-day reproducibility (using intra-class correlation; ICC) of the peak torque (PT) and the EMG variables (without removing the electrodes) of dynamic contractions. Ten healthy subjects performed three sets of 10 dynamic maximum right-knee extensions with a one-hour interval in between, using an isokinetic dynamometer and the PT was determined. EMG signals were recorded from the right vastus lateralis, rectus femoris and vastus medialis muscles using surface electrodes and the mean frequency of the power spectrum (MNF [Hz]) and the signal amplitude (RMS [microV]), were computed. The ability to relax in-between the maximum extensions was calculated as a ratio of the RMS during the passive flexion phase and the RMS during the active extension phase of each contraction cycle: the signal amplitude ratio (SAR). Both PT (ICC = 0.99) and RMS (ICC = 0.83-0.98) had good reproducibility. The reproducibility of MNF was good for all muscles when the mean of contraction nos.: 1-10 was used. Vastus lateralis had the highest ICC among the three muscles. The reproducibility of SAR was generally poor (ICC < 0.60). The present study showed good reproducibility for common EMG variables (MNF and RMS) obtained during maximum isokinetic contractions.