A comparison of EMG-based muscle fatigue assessments during dynamic contractions

Several EMG-based approaches to muscle fatigue assessment have recently been proposed in the literature. In this work, two multivariate fatigue indices developed by the authors: a generalized mapping index (GMI) and the first component of principal component analysis (PCA) were compared to three univariate indices: Dimitrov’s normalized spectral moments (NSM), Gonzalez-Izal’s waveletbased indices (WI), and Talebinejad’s fractal-based Hurst Exponent (HE). Nine healthy participants completed two repetitions of fatigue tests during isometric, cyclic and random fatiguing contractions of the biceps brachii. The fatigue assessments were evaluated in terms of a modified sensitivity to variability ratio yielding the following scores (mean ± std.dev.): PCA: (12.6 ± 5.6), GMI: (11.5 ± 5.4), NSM: (10.3 ± 5.4), WI: (8.9 ± 4.6), HE: (8.0 ± 3.3). It was shown that PCA statistically outperformed WI and HE (p < 0.01) and that GMI outperformed HE (p < 0.02). There was no statistical difference among NSM, WI and HE (p > 0.2). It was found that taking the natural logarithm of NSM and WI, although reducing the parameters’ sensitivity to fatigue, increased SVR scores by reducing variability.     

Muscle sound and electromyogram spectrum analysis during exhausting contractions in man

The changes in the soundmyogram (SMG) and electromyogram (EMG) frequency content during exhausting contractions at 20%, 40%, 60% and 80% of the maximal voluntary contraction (MVC) were investigated by the spectral analysis of the SMG and EMG detected from the biceps brachii muscles of 13 healthy men. The root mean squares (rms) of the two signals were also calculated. Throughout contraction the EMG rms always increased while this was true only at 20% MVC for the SMG. A marked decrease was detected at 60% and 80% MVC. With fatigue the EMG spectra presented a compression towards the lower frequencies at all exercise intensities. The SMG showed a more complex behaviour with a transient increase in its frequency content, followed by a continuous compression of the spectra, at 60% and 80% MVC, and a nearly stable frequency content at lower contraction intensities. This study suggested that different aspects of the changes in the motor unit's activation strategy at different levels of exhausting contractions can be monitored by SMG and EMG signals.     

Changes in conduction velocity, median frequency, and root mean square-amplitude of the electromyogram during 25% maximal voluntary contraction of the triceps brachii muscle, to limit of endurance

The objective of the present study was to investigate whether isometric contraction of the right triceps brachii muscle, of maximal duration and at 25% of the maximal voluntary contraction (MVC), would reduce mean fibre conduction velocity (CV) for the active motor units (MU). In addition to the cross-correlation of surface electromyograms (EMG) for CV determination, median frequency (fm) and root-mean-square amplitude (rms-amplitude) were calculated. The initial 5 min of the recovery of the three parameters was also investigated. The MVC were performed before and after the sustained contraction. Seven males-six in their twenties and one aged 43-participated in the investigation. Mean CV for the unfatigued muscle was 4.5 m.s-1, SD 0.38. On average, CV decreased less than 10% during the sustained contraction (P less than 0.05). The fm decreased almost linearly (46%) during the endurance time, while three quarters of the 250% increase in rms-amplitude took place during the last 50% of the contraction (P less than 0.001, both parameters). The MVC was reduced by 39% immediately after exhaustion (P less than 0.05). During the 1st min of recovery the rms-amplitude decreased by 50%, and the fm increased from 54% to 82% of the initial value (both P less than 0.05). No measurable simultaneous CV restitution occurred. A parallel 15% increase in fm and CV took place during the last 4 min of recovery (both P less than 0.001), while the amplitude remained constant. Since mean CV was essentially unchanged during the last 50% of the endurance time where large changes in fm and rms-amplitude occurred, factors supplementary to CV probably caused the striking changes in fatigue EMG, notably-MU recruitment, synchronization of MU activity, and lowering of MU firing frequencies. Nevertheless, during the last 4 min of recovery the entire increase in fm could be accounted for by the simultaneous increase in CV.     

Modification of myo-electric power spectrum in fatigue from 15% maximal voluntary contraction of human elbow flexor muscles, to limit of endurance: reflection of conduction velocity variation and/or centrally mediated mechanisms?

Isometric flexion of the right elbow at 15% of the maximal voluntary contraction (MVC) was maintained to the limit of endurance (elbow angle 135°). The surface electromyogram (EMG) of the brachioradialis (BR) and biceps brachii (BB) muscles was recorded for calculation of conduction velocity (CV) by the cross-correlation method, and determination of median frequency (fm) and root mean square (rms) amplitude. Perceived exertion was rated for both muscles, and heart rate and blood pressure were measured. The EMG of ten brief 15% MVC contractions distributed over a 30-min recovery period was also recorded. Eleven males in their twenties volunteered for the investigation. The average endurance time was 906 (SD 419) s. Mean CV for the unfatigued muscles was 4.2 (SD 0.41) m·s^–1 (BR), and 4.3 (SD 0.29) m·s^–1 (BB). The contraction caused a significant decrease in CV of BR (12%,P<0.001) whereas CV variation of BB remained insignificant. Concurrently the meanf _m of both muscles dropped to approximately 66% of their initial values and their average rms amplitudes grew by approximately 380% (BR and BB:P<0.001, both parameters). The 1st min of recovery lowered the rms amplitudes by approximately 60% (BR and BB:P<0.01), while thef _m increased to approximately 88% of the initial recording (BR,P<0.01; BB,P<0.05). The accompanying small increases in CV were beyond the level of significance. Over the next 29 min a significant parallel restitution inf _m and CV took place; changes inf _m evidenced a simple one to one reflection of relative CV variation. A similar uncomplicated linear causality between relative changes in CV andf _m was hypothesized for the endurance contraction. Consequently, the 12% CV decrease of the BR accounted for only one-third of the fatigue inducedf _m reduction of 33%, while two-thirds were assumed to be attributable to centrally mediated regulatory interventions in motor unit (MU) performance. Independent of contributions from the virtually unchanged CV, thef _m of the BB muscle decreased by 35%; from one subject exhibiting a remarkably manifest burst-type pattern of MU activity it is argued that synchronization/grouping of MU firing predominantly determined the power redistribution in the BB spectrum.     

Myo-electric fatigue manifestations revisited: power spectrum, conduction velocity, and amplitude of human elbow flexor muscles during isolated and repetitive endurance contractions at 30% maximal voluntary contraction

A brief survey of the literature on manifestations of myo-electric fatigue has disclosed a surprisingly sharp conflict between early studies, focusing on neuromotor regulatory mechanisms, and more recent studies which stress the determinant influence of local metabolism and skewed homeostasis. Favoured explanations concerning changes in the electromyographic (EMG) spectrum were synchronization/grouping of motor unit (MU) firing and conduction velocity (CV) decreases of the action potential propagation. The notion of mutual exclusivity interwoven with these theories prompted us to reinvestigate the EMG of moderate level, static endurance contraction. Ten men in their twenties performed isometric elbow flexion (elbow angle 135°) at 30%6 maximal voluntary contraction (MVC), and the surface EMG of the brachioradialis (BR) and biceps brachii (BB) muscles was recorded. Initially the CV — determined by cross-correlation — was 4.3 m · s^–1 (BR) and 4.6 m · s^–1 (BB). At exhaustion the CV of the BR muscle had declined by 33%, roughly twice the decrease of the BB CV. Substantially larger relative median frequency (f_m) reductions of 50% (BR) and 43% (BB) were found. Simultaneously, the root-mean-square amplitudes grew by 150% (BR) and 120% (BB). All changes during contraction reached the same level of significance (P<0.001, both muscles). From the largely uniform relative increases inf_m and CV during the last 4 min of a 5-min recovery period, variations in CV were suggested to produce equivalent shifts inf_m. The gradually increasing discrepancies between relative decreases inf_m and CV during contraction presumably reflected centrally mediated regulation of MU firing patterns (notably synchronization). After the 5-min recovery another 11 endurance contractions at 30% MVC were executed, separated by 5-min intervals. The series of contractions reduced the endurance time to one-third of the 153 s initially sustained, while the terminal CV recordings increased by 1.0 (BR) and 0.6 (BB) m · s^–1, and the terminalf_m increased by 24 (BR) and 14 (BB) Hz. The relative CV decreased in direct proportion to the endurance time and thef_m decreases varied with the CV; the findings did not support a causal link between CV decrease (signifying impaired fibre excitability) and the force failure of exhaustion.     

Test–Retest Reliability of Static EMG Scan Configural Profiling

Measurement techniques or instruments are typically evaluated along the dimensions of reliability and validity. The focus of this investigation was to assess the test–retest reliability of a static EMG scan profile (sESP) method using 64 chronic pain participants. The test–retest interval was 30–33 days. Reliability coefficients were expressed using the Profile Similarity Coefficient (r _p) in place of the more traditional Pearson Product Moment Correlation. sESP reliabilities were calculated for posture laterality for the head and neck, back, and overall profiles (head, neck, and back combined). The reliability coefficients ranged from .57 to .80. The back profile was the least reliable with a range of .55–.59 whereas the overall profiles were the most reliable, .78–.80. The analysis method was judged to be very conservative with its use of r _p, a protracted intertest interval period, and weighting the data by their variances. These results can be viewed as setting the lower reliability limit for sESP.     

The effect of foot orthotics on myoelectric fatigue in the vastus lateralis during a simulated skier’s squat

Fatigue in the legs is a problem experienced by skiers. It has been suggested that optimal orthotics may reduce muscle fatigue for a given movement task by minimising muscle activity (Med. Sci. Sports Exerc. 31 (1999) S421). The aims were to determine whether EMG would provide an independent method of analysing myoelectric fatigue in the vastus lateralis (VL) during a skier’s squat and whether orthotics could affect this fatigue response. Six skiers performed skier’s squats for as long as possible with no orthotic, low volume orthotics and high volume orthotics in their ski boots. Bipolar, active surface electrodes recorded EMG activity in the VL throughout each squat. Results for the EMG median frequency showed a significant shift in the power density spectrum towards the lower frequencies (P<0.05) at the end of the contraction, suggesting that myoelectric fatigue was occurring and was measurable using EMG. All conditions displayed a significant decrease in median frequency at the end of the contraction (P=0.001). The high volume orthotic showed a significant reduction in myoelectric fatigue, however, there was no difference in the duration of squats across the three conditions (P>0.05). Subjective and objective findings support the use of the high volume foot orthotic for skiers.     

Changes in intra-abdominal pressure, trunk muscle activation and force during isokinetic lifting and lowering

Intra-abdominal pressure (IAP), force and electromyographic (EMG) activity from the abdominal (intra-muscular) and trunk extensor (surface) muscles were measured in seven male subjects during maximal and sub-maximal sagittal lifting and lowering with straight arms and legs. An isokinetic dynamometer was used to provide five constant velocities (0.12–0.96 m·s^–1) of lifting (pulling against the resistance of the motor) and lowering (resisting the downward pull of the motor). For the maximal efforts, position-specific lowering force was greater than lifting force at each respective velocity. In contrast, corresponding IAPs during lowering were less than those during lifting. Highest mean force occurred during slow lowering (1547 N at 0.24 m·s^–1) while highest IAP occurred during the fastest lifts (17.8 kPa at 0.48–0.96 m·s^–1). Among the abdominal muscles, the highest level of activity and the best correlation to variations in IAP (r=0.970 over velocities) was demonstrated by the transversus abdominis muscle. At each velocity the EMG activity of the primary trunk and hip extensors was less during lowering (eccentric muscle action) than lifting (concentric muscle action) despite higher levels of force (r between –0.896 and –0.851). Sub-maximal efforts resulted in IAP increasing linearly with increasing lifting or lowering force (r=0.918 and 0.882, respectively). However, at any given force IAP was less during lowering than lifting. This difference was negated if force and IAP were expressed relative to their respective lifting and lowering maxima. It appears that the IAP increase primarily accomplished by the activation of the transversus abdominis muscle can have the dual function of stabilising the trunk and reducing compression forces in the lumbar spine via its extensor moment. The neural mechanisms involved in sensing and regulating both IAP and trunk extensor activity in relation to the type of muscle action, velocity and effort during the maximal and sub-maximal loading tasks are unknown.     

增加刺激频率不影响大鼠萎缩比目鱼肌强直收缩疲劳性

为观察模拟失重对大鼠比目鱼肌（soleus,SOL）与趾长伸肌（extensor digitorum longus,EDL）间断强直收缩功能的影响,以及对刺激频率的调节作用,采用离体骨骼肌条灌流技术,观测其产生强直收缩最大张力的最适刺激频率、疲劳性与疲劳后恢复过程。结果表明：对照组大鼠SOL强直收缩的最适刺激频率为60Hz,尾部悬吊1周大鼠SOL的最适刺激频率亦为60Hz,尾部悬吊2周后,其最适刺激频率增高为80Hz,4周后则为100Hz;在最适刺激频率作用下,悬吊大鼠SOL间断强直收缩的最大张力（Po）在悬吊1与2周未见改变,第4周才呈现显著性降低（P〈0．01）。间断强直收缩5min后,对照组大鼠SOL张力降低到22．8％Po：悬吊1、2与4周组疲劳性均增加,与其同步对照组相比均有显著性差异（P〈0．01）。疲劳性强直收缩后,在20min内对照大鼠SOL张力基本恢复到疲劳前水平,而悬吊1、2与4周组则不能完全恢复（P〈0．05）。对照组大鼠EDL的最适刺激频率为120Hz,悬吊1、2与4周组EDL的最适刺激频率、疲劳性以及疲劳后恢复过程均未发生改变。以上结果提示,增加刺激频率可对悬吊1与2周大鼠SOL强直收缩最大张力的降低有代偿作用,但不能代偿悬吊4周大鼠SOL最大收缩张力的降低,亦不影响悬吊大鼠SOL间断强直收缩疲劳性的增加与疲劳后恢复的减缓。     

EMG muscle scanning: Comparison to attached surface electrodes

Electromyographic (EMG) muscle scanning measures brief samples of integrated muscle action potentials from individual muscles using a hand-held scanner with post-style electrodes. This scanning technique is widely used by biofeedback practitioners to quickly assess muscle activity in the diagnosis of musculoskeletal disorders. In an effort to compare muscle scanning with the established technique using attached surface electrodes, ten healthy subjects (25–35 years old) were scanned using 2-second sampling at five bilateral muscle sites while simultaneously monitoring the same sites with surface electrodes. This was repeated using 10-second scanning samplings. Pearson's product-moment correlations between scanning for 2 seconds and prolonged surface recording at all sites were 0.54–0.89. Scanning for 10 seconds improved the correlations to 0.68–0.91. EMG scanning for 2 seconds compares favorably with attached surface electrode recording. Comparisons are further improved by 10-second scans.     

Do toads have a jump on how far they hop? Pre-landing activity timing and intensity in forelimb muscles of hopping Bufo marinus

During jumping or falling in humans and various other mammals, limb muscles are activated before landing, and the intensity and timing of this pre-landing activity are scaled to the expected impact. In this study, we test whether similarly tuned anticipatory muscle activity is present in hopping cane toads. Toads use their forelimbs for landing, and we analysed pre-landing electromyographic (EMG) timing and intensity in relation to hop distance for the m. coracoradialis and m. anconeus, which act antagonistically at the elbow, and are presumably important in stabilizing the forelimb during landing. In most cases, a significant, positive relationship between hop distance and pre-landing EMG intensity was found. Moreover, pre-landing activation timing of m. anconeus was tightly linked to when the forelimbs touched down at landing. Thus, like mammals, toads appear to gauge the timing and magnitude of their impending impact and activate elbow muscles accordingly. To our knowledge these data represent the first demonstration of tuned pre-landing muscle recruitment in anurans and raise questions about how important the visual, vestibular and/or proprioceptive systems are in mediating this response.     

A fuzzy-genetic model for estimating forces from electromyographical activity of antagonistic muscles due to planar lower arm movements: the effect of nonlinear muscle properties

The aim of this paper is to create a model for mapping the surface electromyogram (EMG) signals to the force that generated by human arm muscles. Because the parameters of each person's muscle are individual, the model of the muscle must have two characteristics: (1) The model must be adjustable for each subject. (2) The relationship between the input and output of model must be affected by the force-length and the force-velocity behaviors are proven through Hill's experiments. Hill's model is a kinematic mechanistic model with three elements, i.e. one contractile component and two nonlinear spring elements.In this research, fuzzy systems are applied to improve the muscle model. The advantages of using fuzzy system are as follows: they are robust to noise, they prove an adjustable nonlinear mapping, and are able to model the uncertainties of the muscle.Three fuzzy coefficients have been added to the relationships of force-length (active and passive) and force-velocity existing in Hill's model. Then, a genetic algorithm (GA) has been used as a biological search method that can adjust the parameters of the model in order to achieve the optimal possible fit.Finally, the accuracy of the fuzzy genetic implementation Hill-based muscle model (FGIHM) is invested as following: the FGIHM results have 12.4% RMS error (in worse case) in comparison to the experimental data recorded from three healthy male subjects. Moreover, the FGIHM active force-length relationship which is the key characteristics of muscles has been compared to virtual muscle (VM) and Zajac muscle model. The sensitivity of the FGIHM has been evaluated by adding a white noise with zero mean to the input and FGIHM has proved to have lower sensitivity to input noise than the traditional Hill's muscle model.     

Instrumental measurement of the gypsy moth pre-flight behavioural response to pheromone

ABSTRACT. Vibrations of the thorax and electrical activity (EMG) of gypsy moth flight muscles were recorded during wing fanning following pheromone stimulation. The percentage of positive responses and durations of bursts of flight muscle activity increased with the logarithm of pheromone dose, whereas latency decreased. The results correlated well with wing fanning responses of freely moving gypsy moths exposed to nearly identical stimulus conditions. Typical dose-response curves in the range of 0.04-400ng disparlure were obtained in both types of experiments. These methods provide an electrical analogue of wing fanning behaviour.