Reliability of spectral EMG parameters of healthy back extensors during submaximum isometric fatiguing contractions and recovery

Spectral EMG parameters are being used as an objective evaluation of low back rehabilitation programs. The reliability of these spectral parameters is important in determining the validity of this evaluation tool. Two groups of eight subjects, with no history of back pain, were measured: the first group every day for one week; the second group one day per week for four weeks. During each session, subjects performed a 30-second isometric fatiguing contraction of the back extensors at 60% MVC followed by a 60-second rest and a 10-second repeat contraction. The intraclass correlation coefficient (ICC) did not demonstrate high reliability (ICC<0.6) for the rate of median power frequency (MedPF) change (Hz/s), the magnitude of the recovery (Hz) and differences between the dominant and the non-dominant sides within a subject. However, the initial MedPF (Hz) of the fatigue and repeat contractions demonstrated excellent reliability (ICC>0.8) with five or more repeated measures. The practical implication of this work is that the experimental condition must have sufficiently large changes in MedPF (signal)—at least larger than the variability inherent in the MedPF (noise)—to constitute a valid measure.     

Comparison of algorithms for estimation of EMG variables during voluntary isometric contractions.

Many algorithms have been described in the literature for estimating amplitude, frequency variables and conduction velocity of the surface EMG signal detected during voluntary contractions. They have been used in different application areas for the non invasive assessment of muscle functions. Although many studies have focused on the comparison of different methods for information extraction from surface EMG signals, they have been carried out under different conditions and a complete comparison is not available. It is the purpose of this paper to briefly review the most frequently used algorithms for EMG variable estimation, compare them using computer generated as well as real signals and outline the advantages and drawbacks of each. In particular the paper focuses on the issue of EMG amplitude estimation with and without pre-whitening of the signal, mean and median frequency estimation with periodogram and autoregressive based algorithms both in stationary and non-stationary conditions, delay estimation for the calculation of muscle fiber conduction velocity.     

Surface EMG signal processing during isometric contractions

This paper provides an overview of techniques suitable for the estimation, interpretation and understanding of time variations that affect the surface electromyographic (EMG) signal during sustained voluntary or electrically elicited contractions. These variations concern amplitude variables, spectral variables and muscle fiber conduction velocity, are interdependent and are referred to as the ‘fatigue plot'. The fatigue plot provides information suitable for the classification of muscle behavior. In addition, the information obtainable by means of linear electrode arrays is discussed, and applications of mathematical models for the interpretation of array signals are presented. The model approach provides additional ways for the classification of muscle behavior.     

Effects of muscle kinematics on surface EMG amplitude and frequency during fatiguing dynamic contractions

Potvin, J. R. Effects of muscle kinematics on surfaceEMG amplitude and frequency during fatiguing dynamic contractions. J. Appl. Physiol. 82(1): 144-151, 1997.Fifteen male subjects performed a repetitive elbowflexion/extension task with a 7-kg mass until exhaustion. Average jointangle, angular velocity, and biceps brachii surface electromyographic(EMG) amplitude (aEMG) and mean powerfrequency (MPF) were calculated with each consecutive 250-ms segment ofdata during the entire trial. Data were separated into concentric oreccentric phases and into seven 20°-ranges from 0 to 140° ofelbow flexion. A regression analysis was used to estimate the restedand fatigued aEMG and MPF values. aEMG values were expressed as apercentage of amplitudes from maximum voluntary contractions (MVC).Under rested dynamic conditions, the average concentric aEMG amplitudewas 10% MVC higher than average eccentric values. Rested MPF valueswere similar for concentric and eccentric phases, although valuesincreased ~20 Hz from the most extended to flexed joint angles.Fatigue resulted in an average increase in concentric and eccentricaEMG of 35 and 10% MVC, respectively. The largest concentric aEMGincreases (up to 58% MVC) were observed at higher joint velocities,whereas eccentric increases appeared to be related to decreases invelocity. Fatigue had a similar effect on MPF during both concentricand eccentric phases. Larger MPF decreases were observed at shortermuscle lengths such that values within each angle range were verysimilar by the end of the trial. It was hypothesized that this findingmay reflect a biological minimum in conduction velocity beforepropagation failure occurs.

     

Repeatability of surface EMG variables during voluntary isometric contractions of the biceps brachii muscle.

The repeatability of initial value and rate of change of mean spectral frequency (MNF), average rectified values (ARV) and muscle fiber conduction velocity (CV) was investigated in the dominant biceps brachii of ten normal subjects during sustained isometric voluntary contractions. Four levels of contraction were studied: 10%, 30%, 50% and 70% of the maximal voluntary contraction level (MVC). Each contraction was repeated three times in each of three different days for a total of nine contractions/level/subject and 90 contractions per level across the ten subjects. Repeatability was investigated using the Intraclass Correlation Coefficient (ICC) and the standard error of the mean (SEM) of the estimates for each subject. Contrary to observations in other muscles, CV estimates appeared to be very repeatable both within and between subjects. CV showed a small but significant increase when contraction force increased from 10% to 50% MVC but no change for further increase of force. As force increased, MNF showed a slight decrease possibly related to a wider spreading of the CV values. The rate of time decrement of MNF and CV increased with the level of contraction. The normalized decrement (% of initial value per second) was in general higher for MNF than for CV and was more repeatable between subjects at 10% MVC than at 70% MVC. A final observation is that a resting time of 5 minutes may not be sufficient after a contraction at 50% or 70% MVC.     

Changes in surface EMG parameters during static and dynamic fatiguing contractions.

The effect of contraction types on muscle fiber conduction velocity (MFCV), median frequency (MDF) and mean amplitude (AMP) of surface electromyography was examined in the vastus lateralis of 19 healthy male adults. The subjects performed knee extension both statically and dynamically until they were exhausted. The static contraction was a sustained isometric extension of the knee at a joint angle of 90 degrees with 50% of the maximum voluntary contraction (MVC) load. The dynamic contraction was a repetitive isotonic extension of the knee between the angles of 90 degrees and 180 degrees with the same 50% MVC load at a frequency of 10 times per minute. MFVC during the static contraction significantly decreased during the exercise (p < 0.01). On the other hand, MFVC during the dynamic contraction did not significantly change throughout the exercise. MDF decreased and AMP increased during both types of contractions (p < 0.01). Because the blood flow within the muscle is maintained during the dynamic contraction by enhanced venous return from the contracting muscle, these results suggested that MFVC is affected by the metabolic state in the muscle and the changes in MDF cannot be explained only by that of MFVC.     

Effect of vibration during fatiguing resistance exercise on subsequent muscle activity during maximal voluntary isometric contractions

This investigation was designed to determine if vibration during fatiguing resistance exercise would alter associated patterns of muscle activity. A cross-over design was employed with 8 subjects completing a resistance exercise bout once with a vibrating dumbbell (V) (44 Hz, 3 mm displacement) and once without vibration (NV). For both exercise bouts, 10 sets were performed with a load that induced concentric muscle failure during the 10th repetition. The appropriate load for each set was determined during a pretest. Each testing session was separated by 1 week. Electromyography (EMG) was obtained from the biceps brachii muscle at 12 different time points during a maximum voluntary contraction (MVC) at a 170 degrees elbow angle after each set of the dumbbell exercise. The time points were as follows: pre (5 minutes before the resistance exercise bout), T1-T10 (immediately following each set of resistance exercise), and post (15 minutes after the resistance exercise bout). EMG was analyzed for median power frequency (MPF) and maximum (mEMG). NV resulted in a significant decrease in MPF at T1-T4 (p < or 0.05) and a significant increase in mEMG at T2 during the MVC. V had an overall trend of lower mEMG in comparison to NV. The mEMG and MPF values associated with NV were similar to previously reported investigations. The lower mEMG values and the higher MPF of V in comparison to NV are undocumented. The EMG patterns observed with vibration may indicate a more efficient and effective recruitment of high threshold motor units during fatiguing contractions. This may indicate the usage of vibration with resistance exercise as an effective tool for strength training athletes.     

Reproducibility of surface EMG variables in isometric sub-maximal contractions of jaw elevator muscles.

The aims of this study were: (1) to develop and assess reproducibility of a new method for measuring masticatory force in the intercuspal position; (2) to test the reproducibility of surface EMG signal amplitude and spectral variables in constant force contractions of jaw elevator muscles and its dependency on inter-electrode distance. The study was performed on the masseter and temporalis anterior muscles of both sides of nine healthy volunteers. An intraoral compressive-force sensor was used to measure maximal voluntary contraction forces in the intercuspal position and to provide a visual feedback on sub-maximal forces to the subject. Three experimental sessions were performed in three days. In each session, three isometric contractions at 80% of the maximal force were sustained by the subjects for 30s. The intra-class correlation coefficient (ICC) of the maximal force measure was 71.9%. ICC of average rectified value and mean power spectral frequency of the EMG signal increased with inter-electrode distance, with values larger than 70% with 30 mm inter-electrode distance. It was concluded that surface EMG variables measured in isometric contractions of the jaw elevator muscles with the proposed force recording system show good reproducibility for clinical applications when a 30 mm inter-electrode distance is considered.     

EMG power spectra of elbow extensors during ramp and step isometric contractions

The goal of the present study was to compare electromyogram (EMG) power spectra obtained from step (constant force level) and ramp (progressive increase in the force level) isometric contractions. Data windows of different durations were also analysed for the step contractions, in order to evaluate the stability of EMG power spectrum statistics. Fourteen normal subjects performed (1) five ramp elbow extensions ranging from 0 to 100% of the maximum voluntary contraction (MVC) and (2) three stepwise elbow extensions maintained at five different levels of MVC. Spectral analysis of surface EMG signals obtained from triceps brachii and anconeus was performed. The mean power frequency (MPF) and the median frequency (MF) of each power spectrum were obtained from 256-ms windows taken at 10, 20, 40, 60 and 80% MVC for each type of contraction and in addition on 512-, 1024- and 2048-ms windows for the step contractions. No significant differences (P greater than 0.05) were found in the values of both spectral statistics between the different window lengths. Even though no significant differences (P greater than 0.05) were found between the ramp and the step contractions, significant interactions (P less than 0.05) between these two types of contraction and the force level were found for both the MPF and the MF data. These interactions point out the existence of different behaviours for both the MPF and the MF across force levels between the two types of contraction.     

Identification of isometric contractions based on High Density EMG maps

Identification of motion intention and muscle activation strategy is necessary to control human–machine interfaces like prostheses or orthoses, as well as other rehabilitation devices, games and computer-based training programs. Pattern recognition from sEMG signals has been extensively investigated in the last decades, however, most of the studies did not take into account different strengths and EMG distributions associated to the intended task. The identification of such quantities could be beneficial for the training of the subject or the control of assistive devices. Recent studies have shown the need to improve pattern-recognition classification by reducing sensitivity to changes in the exerted strength, muscle-electrode shifts and bad contacts. Surface High Density EMG (HD-EMG) obtained from 2-dimensional arrays can provide much more information than electrode pairs for inferring not only motion intention but also the strategy adopted to distribute the load between muscles as well as changes in the spatial distribution of motor unit action potentials within a single muscle because of it.The objectives of this study were: (a) the automatic identification of four isometric motor tasks associated with the degrees of freedom of the forearm: flexion–extension and supination–pronation and (b) the differentiation among levels of voluntary contraction at low-medium efforts. For this purpose, monopolar HD-EMG maps were obtained from five muscles of the upper-limb in healthy subjects. An original classifier is proposed, based on: (1) Two steps linear discriminant analysis of the EMG information for each type of contraction, and (2) features extracted from HD-EMG maps and related to its intensity and distribution in the 2D space. The classifier was trained and tested with different effort levels. Spatial distribution-based features by themselves are not sufficient to classify the type of task or the effort level with an acceptable accuracy; however, when calculated with the “isolated masses” method proposed in this study and combined with intensity-base features, the performance of the classifier is improved. The classifier is capable of identifying the tasks even at 10% of Maximum Voluntary Contraction, in the range of effort level developed by patients with neuromuscular disorders, showing that intention end effort of motion can be estimated from HD-EMG maps and applied in rehabilitation.     

Knee extensor torque and quadriceps femoris EMG during perceptually-guided isometric contractions.

The aim of this study was to examine superficial quadriceps femoris (QF) EMG and torque at perceived voluntary contraction efforts. Thirty subjects (15 males, 15 females) performed 9, 5 s, sub-maximal contractions at prescribed levels of perceived voluntary effort at points 1-9 on an 11-point scale (0-10), in a random order. Surface electromyograms (EMG) of the vastus medialis (VM), vastus lateralis (VL), and rectus femoris (RF) muscles, as well as QF peak torque (PT), average torque (AT), and torque coefficient of variation (C.V.), were sampled. The raw EMG signals were full-wave rectified and integrated over the middle three s of each contraction. The sampled EMG signals, and PT and AT at each perceived exertion level were normalized to the average of three maximal voluntary contractions. The normalized EMG and torque values at each perceived exertion level were then compared to equivalent percent values (i.e., 10% at a perceived level of 1). The results demonstrated that at all perceived exertion levels, with the exception of the RF at a level of 2 which was equivalent to 20%, and the VL and RF muscles at a level 1 in which activation was greater than 10%, activation was significantly less than the equivalent percent value at each point on the scale. VM EMG was found to be less than the VL and RF from contraction levels 3-9. PT was shown to be less than the equivalent percent values at contraction levels 6-9. The AT was found to be lower than the expected percent value at perceived effort levels 2-9. Torque C.V. was not found to be different across the range of perceived effort. The major findings of this study suggested that humans over-estimate voluntary QF muscle torque when guided by perceptual sensations. It is also suggested that the produced EMG signals revealed a reliance on the VL muscle for knee extensor torque generation at sub-maximal levels.     

Firing rates of motor units in human vastus lateralis muscle during fatiguing isometric contractions.

We investigated the firing rate of motor units in the vastus lateralis muscle in five healthy young men (mean = 21.4 yr, SD = 0.9) during a sequence of isometric constant-torque contractions repeated to exhaustion. The contractions were sustained at 20% of the maximal voluntary level, measured at the beginning of the test sequence. Electromyographic (EMG) signals were recorded via quadrifilar fine-wire electrodes and subsequently decomposed into their constituent motor unit action potentials to obtain the motor unit firing times. In addition, we measured the whole muscle mechanical properties during the fatigue task using electrical stimulation. The firing rate of motor units first decreased within the first 10-20% of the endurance time of the contractions and then increased. The firing rate increase was accompanied by recruitment of additional motor units as the force output remained constant. The elicited twitch and tetanic torque responses first increased and then decreased. The two processes modulated in a complementary fashion at the same time. Our data suggest that, when the vastus lateralis muscle is activated to maintain a constant torque output, its motoneuron pool receives a net excitatory drive that first decreases to compensate for the short-lived potentiation of the muscle force twitch and then increases to compensate for the diminution of the force twitch. The underlying inverse relationship between the firing rate and the recruitment threshold that has been reported for nonfatigued contractions is maintained. We, therefore, conclude that the central nervous system control of vastus lateralis motor units remains invariant during fatigue in submaximal isometric isotonic contractions.     

Linear vs. non-linear mapping of peak power using surface EMG features during dynamic fatiguing contractions

This study compares a non-linear (neural network) and a linear (linear regression) power mapping using a set of features of the surface electromyogram recorded from the vastus medialis and lateralis muscles. Fifteen healthy participants performed 5 sets of 10 repetitions leg press using the individual maximum load corresponding what they could perform 10 times (10RM) with 120 s of rest between them. The following sEMG variables were computed from each extension contraction and used as inputs to both approaches: mean average value (MAV), median frequency (Fmed), the spectral parameter proposed by Dimitrov (FInsm5), average (over the observation interval) of the instantaneous mean frequency obtained from a Choi–Williams distribution (MFM), and wavelet indices ratio between moments at different scales (WIRM1551, WIRM1M51, WIRM1522, WIRE51, and WIRW51). The non-linear mapping (neural network) provided higher correlation coefficients and signal-to-noise ratios values (although not significantly different) between the actual and the estimated changes of power compared to linear mapping (linear regression) using the sEMG variables alone and a combination of WIRW51 and MFM (obtained by a stepwise multiple linear regression). In conclusion, non-linear mapping of force loss during dynamic knee extension exercise showed higher signal-to-noise ratio and correlation coefficients between the actual and estimated power output compared to linear mapping. However, since no significant differences were observed between linear and non-linear approaches, both were equally valid to estimate changes in peak power during fatiguing repetitive leg extension exercise.     

Forearm blood flow responses to fatiguing isometric contractions in women and men

Previous studies suggest that women experience less vascular occlusion than men when generating the same relative contractile force. This study examined forearm blood flow (FBF) in women and men during isometric handgrip exercise requiring the same relative force. Thirty-eight subjects [20 women and 18 men, 22.8 +/- 0.6 yrs old (means +/- SE)] performed low- and moderate-force handgrip exercise on two occasions. Subjects performed five maximum voluntary contractions (MVC) before exercise to determine 20% and 50% MVC target forces. Time to task failure (TTF) was determined when the subject could not maintain force within 5% of the target force. Mean blood velocity was measured in the brachial artery with the use of Doppler ultrasonography. Arterial diameter was measured at rest and used to calculate absolute FBF (FBFa; ml/min) and relative FBF (FBFr; ml.min(-1).100 ml(-1)). Women generated less (P < 0.05) absolute maximal force (208 +/- 10 N) than men (357 +/- 17 N). The TTF was longer (P < 0.05) at 20% MVC for women (349 +/- 32 s) than for men (230 +/- 23 s), but no difference between the sexes was observed at 50% MVC (women: 69 +/- 5 s; men: 71 +/- 8 s). FBFa and FBFr increased (P < 0.05) from rest to TTF in both women and men during 20% and 50% MVC trials. FBFr was greater in women than in men at > or =30% TTF during 50% MVC. At exercise durations > or =60% of TTF, FBFa was lower (P < 0.05) in women than in men during handgrip at 20% MVC. Despite the longer exercise duration for women at the lower contraction intensity, FBFr was similar between the sexes, suggesting that muscle perfusion is matched to the exercising muscle mass independent of sex.     

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.     

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.     

Simulation analysis of interference EMG during fatiguing voluntary contractions. Part II--changes in amplitude and spectral characteristics.

Capabilities of amplitude and spectral methods for information extraction from interference EMG signals were assessed through simulation and preliminary experiment. Muscle was composed of 4 types of motor units (MUs). Different hypotheses on changes in firing frequency of individual MUs, intracellular action potential (IAP) and muscle fibre propagation velocity (MFPV) during fatigue were analyzed. It was found that changes in amplitude characteristics of interference signals (root mean square, RMS, or integrated rectified value, IEMG) detected by intramuscular and surface electrodes differed. RMS and IEMG of surface detected interference signals could increase even under MU firing rate reduction and without MU synchronisation. IAP profile lengthening can affect amplitude characteristics more significantly than MU firing frequency. Thus, an increase of interference EMG amplitude is unreliable to reflect changes in the neural drive. The ratio between EMG amplitude and contraction response can hardly characterise the so-called 'neuromuscular efficiency'. The recently proposed spectral fatigue indices can be used for quantification of interference EMG signals. The indices are practically insensitive to MU firing frequency. IAP profile lengthening and decrease in MFPV enhanced the index value, while recruitment of fast fatigable MUs reduced it. Sensitivity of the indices was higher than that of indices traditionally used.     

Effects of lung volume on diaphragm EMG signal strength during voluntary contractions

The use ofesophageal recordings of the diaphragm electromyogram (EMG) signalstrength to evaluate diaphragm activation during voluntary contractionsin humans has recently been criticized because of a possible artifactcreated by changes in lung volume. Therefore, the first aim of thisstudy was to evaluate whether there is an artifactual influence of lungvolume on the strength of the diaphragm EMG during voluntarycontractions. The second aim was to measure the required changes inactivation for changes in lung volume at a given tension, i.e., thevolume-activation relationship of the diaphragm. Healthy subjects(n = 6) performed contractions of thediaphragm at different transdiaphragmatic pressure (Pdi) targets (range20-160 cmH₂O) whilemaintaining chest wall configuration constant at different lungvolumes. The diaphragm EMG was recorded with a multiple-arrayesophageal electrode, with control of signal contamination andelectrode positioning. The effects of lung volume on the EMG werestudied by comparing the crural diaphragm EMG root mean square (RMS),an index of crural diaphragm activation, with an index of globaldiaphragm activation obtained by normalizing Pdi to the maximum Pdi atthe given muscle length(Pdi/Pdi_max@L) at thedifferent lung volumes. We observed a direct relationship between RMSand Pdi/Pdi_max@L independent of diaphragm length. The volume-activation relationship ofthe diaphragm was equally affected by changes in lung volume as thevolume-Pdi relationship (60% change from functional residual capacityto total lung capacity). We conclude that the RMS of the diaphragm EMGis not artifactually influenced by lung volume and can be used as areliable index of diaphragm activation. The volume-activationrelationship can be used to infer changes in the length-tensionrelationship of the diaphragm at submaximal activation/contractionlevels.