Low-level Isometric Contractions Research Articles

Effect of optimal filtering parameters for autoregressive model AR(p) on motor unit action potential signal

Error is one element of the autoregressive (AR) model, which is supposed to be white noise. Correspondingly assumption that white noise error is a normal distribution in electromyography (EMG) estimation is one of the common causes for error maximization. This paper presents the effect of a suitable choice of filtering function based on the non-invasive analysis properties of motor unit action potential signal, extracted from a non-invasive method-the high spatial resolution (HSR) electromyography (EMG), recorded during low-level isometric muscle contractions. The final prediction error procedure is used to find the number of parameters in the model. The error signal parameter, the simulated deviation from the actual signals, is suitably filtered to obtain optimally appropriate estimates of the parameters of the automatic regression model. It is filtered to acquire optimally appropriate estimates of the parameters of the automatic regression model. Then appropriate estimates of spectral power shapes are obtained with a high degree of efficiency compared with the robust method under investigation. Extensive experiment results for the proposed technique have shown that it provides a robust and reliable calculation of model parameters. Moreover, estimates of power spectral profiles were evaluated efficiently.

Motor unit filter prelearning strategies for decomposition of compound muscle action potentials in high-density surface electromyograms.

We analyze the efficiency of motor unit (MU) filter prelearning from high-density surface electromyographic (HDEMG) recordings of voluntary muscle contractions in the identification of the motor unit firing patterns during elicited muscle contractions. Motor unit filters are assessed from 10 s long low level isometric voluntary contractions by gradient-based optimization of three different cost functions and then applied to synthetic HDEMG recordings of elicited muscle contractions with dispersion of motor unit firings ranging from 13 ms to 1 ms. We demonstrate that the number of identified MUs and the precision of MU identification depend significantly on the selected cost function. Regardless the selected cost function and MU synchronization level, the median precision of motor unit identification in elicited contraction is ≥ 95 % and is comparable to the precision in voluntary contractions. On the other hand, median miss rate increases significantly from < 1 % to ~ 3 % with the tested level of MU synchronization.Clinical Relevance-The identification of MU firings from HDEMG in elicited muscle contractions provides a new tool for in vivo investigation of motor excitability in humans.

Does vibration superimposed on low-level isometric contraction alter motor unit recruitment strategy?

Objective. Beneficial effects, including improved muscle strength and power performance, have been observed during vibration exercise (VE) and partially ascribed to a specific reflex mechanism referred to as Tonic vibration reflex (TVR). TVR involves motor unit (MU) activation synchronized and un-synchronized with the vibration cycle; this suggests VE to alter the temporal MU recruitment strategy. However, the effects of VE on MU recruitment remain poorly understood. This study aims to elucidate the influence of VE on MU recruitment indirectly, by investigating the effects of low-intensity VE on muscle activation. Approach. Twenty volunteers performed isometric contractions on the biceps brachii of the right arm at a baseline (low) force equal to 30% of the maximum voluntary contraction without vibration (control) and with vibration at 20, 30, 40, and 55 Hz. Three vibration amplitudes were employed at 12.5%, 25%, and 50% of the baseline. Mean muscle-fiber conduction velocity (mCV), mean frequency (MF), and root mean square (RMS) value were estimated from surface electromyography as indicators of the alteration in MU recruitment strategies. Main results. The mCV estimates during VE were significantly (p < 0.05) higher compared to the control condition. Furthermore, six VE conditions produced significantly larger RMS values compared to control condition. The estimated MF did not show any consistent trend. Significance. These results suggest that vibration superimposed on low-level isometric contraction alters the MU recruitment strategy, activating larger and faster MUs.

Later stages of diabetic neuropathy affect the complexity of the neuromuscular system at the knee during low-level isometric contractions.

This study evaluates the complexity of force and surface electromyography (sEMG) during knee extension and flexion at low-level isometric contractions in individuals with different degrees of diabetic peripheral neuropathy (DPN). Ten control and 38 diabetic participants performed isometric contractions at 10%, 20%, and 30% of maximal voluntary contraction. Knee force and multichannel sEMG from vastus lateralis (VL) and biceps femoris were acquired. The SD of force and sample entropy (SaEn) of both force and sEMG were computed. Participants with moderate DPN demonstrated high force-SD and low force-SaEn. Severely affected participants showed low SaEn in VL at all force levels. DPN affects the complexity of the neuromuscular system at the knee for the extension task during low-level isometric contractions, with participants in the later stages of the disease (moderate and severe) demonstrating most of the changes. Muscle Nerve 57: 112-121, 2018.

Reduced complexity of force and muscle activity during low level isometric contractions of the ankle in diabetic individuals

BackgroundThis study evaluated the structure and amount of variability of surface electromyography (sEMG) patterns and ankle force data during low-level isometric contractions in diabetic subjects with different degrees of neuropathy. MethodsWe assessed 10 control subjects and 38 diabetic patients, classified as absent, mild, moderate, or severe neuropathy, by a fuzzy system based on clinical variables. Multichannel sEMG (64-electrode matrix) of tibialis anterior and gastrocnemius medialis muscles were acquired during isometric contractions at 10%, 20%, and 30% of the maximum voluntary contraction, and force levels during dorsi- and plantarflexion were recorded. Standard deviation and sample entropy of force signals were calculated and root mean square and sample entropy were calculated from sEMG signals. Differences among groups of force and sEMG variables were verified using a multivariate analysis of variance. FindingsOverall, during dorsiflexion contractions, moderate and severe subjects had higher force standard deviation and moderate subjects had lower force sample entropy. During plantarflexion, moderate subjects had higher force standard deviation and all diabetic subjects had lower entropy. Tibialis anterior presented higher root mean square in absent group and lower entropy in mild subjects. For gastrocnemius medialis, entropy was higher in severe and lower in moderate subjects. InterpretationDiabetic neuropathy affects the complexity of the neuromuscular system during low-level isometric contractions, reducing the system's capacity to adapt to challenging mechanical demands. The observed patterns of neuromuscular complexity were not associated with disease severity, with the majority of alterations recorded in moderate subject.

Detection of Muscle Fatigue during Low-Level Isometric Contraction by Recurrence Quantification Analysis and Monopolar Configuration

Detection of Muscle Fatigue during Low-Level Isometric Contraction by Recurrence Quantification Analysis and Monopolar Configuration

低強度等尺性運動時の筋疲労評価における単極導出法の優位性

現在の筋疲労研究では, 筋の疲労性変化の指標として, 双極導出法により記録した表面筋電図が一般的に用いられている. しかしながら, 双極導出法により記録した表面筋電図では, 低強度運動時の疲労に伴う変化が不明確であるとされている. また, 上腕基部を圧迫し強い筋疲労を擬似的に再現した研究においても, 圧迫条件間に有意差は生じなかったと報告されている. この強い筋疲労は, 等尺性運動時に血管系の末梢抵抗の上昇に伴い発生する可能性があるものである. 低強度運動および等尺性運動は, 生活習慣病に対する運動療法として広く実践されており, 疲労に伴う表面筋電図変化の不明確性は, 解決すべき問題であると考えられる. 本研究では, 双極導出法に比べ導出範囲が広い単極導出法を用いて, 疲労に伴う表面筋電図変化を明確に捉えることを目的とした. 実験では, 血流制限および非血流制限条件で低強度等尺性運動を行い, 単極および双極導出法で表面筋電図を測定した. 血流制限条件は上腕基部の圧迫で実現した. さらに, 近赤外分光法を用いて筋酸素動態, Borg CR-10を用いて自覚的運動強度を測定した. 筋酸素動態および自覚的運動強度は, 筋血流制限と非筋血流制限条件のいずれにおいても疲労性変化を示し, また, 筋血流制限条件の方が有意に大きかった. この筋血流制限の影響は単極導出法で検出されたが, 双極導出法では検出されなかった. 非筋血流制限条件では, 単極導出法により記録した表面筋電図の方が, 双極導出法と比べて疲労性変化が有意に大きかった. 筋血流制限条件においても, 同様な傾向が見られた. これらの結果から, 単極導出法で表面筋電図を記録することにより, 疲労に伴う表面筋電図変化を明確に捉えられる可能性が示唆された.

Motor unit potential morphology differences in individuals with non-specific arm pain and lateral epicondylitis

BackgroundThe pathophysiology of non-specific arm pain (NSAP) is unclear and the diagnosis is made by excluding other specific upper limb pathologies, such as lateral epicondylitis or cervical radiculopathy. The purpose of this study was to determine: (i) if the quantitative parameters related to motor unit potential morphology and/or motor unit firing patterns derived from electromyographic (EMG) signals detected from an affected muscle of patients with NSAP are different from those detected in the same muscle of individuals with lateral epicondylitis (LE) and/or control subjects and (ii) if the quantitative EMG parameters suggest that the underlying pathophysiology in NSAP is either myopathic or neuropathic in nature.MethodsSixteen subjects with NSAP, 11 subjects with LE, eight subjects deemed to be at-risk for developing a repetitive strain injury, and 37 control subjects participated. A quantitative electromyography evaluation was completed using decomposition-based quantitative electromyography (DQEMG). Needle- and surface-detected EMG signals were collected during low-level isometric contractions of the extensor carpi radialis brevis (ECRB) muscle. DQEMG was used to extract needle-detected motor unit potential trains (MUPTs), and needle-detected motor unit potential (MUP) and surface detected motor unit potential (SMUP) morphology and motor unit (MU) firing rates were compared among the four groups using one-way analysis of variance (ANOVA). Post hoc analyses were performed using Tukey's pairwise comparisons.ResultsSignificant group differences were found for all MUP variables and for MU firing rate (p < 0.006). The post-hoc analyses revealed that patients with NSAP had smaller MUP amplitude and SMUP amplitude and area compared to the control and LE groups (p < 0.006). MUP duration and AAR values were significantly larger in the NSAP, LE and at-risk groups compared to the control group (p < 0.006); while MUP amplitude, duration and AAR values were smaller in the NSAP compared to the LE group. SMUP duration was significantly shorter in the NSAP group compared to the control group (p < 0.006). NSAP, LE and at-risk subjects had lower mean MU firing rates than the control subjects (p < 0.006).ConclusionThe size-related parameters suggest that the NSAP group had significantly smaller MUPs and SMUPs than the control and LE subjects. Smaller MUPs and SMUPs may be indicative of muscle fiber atrophy and/or loss. A prospective study is needed to confirm any causal relationship between smaller MUPs and SMUPs and NSAP as found in this work.

Reliability of quantitative EMG analysis of the extensor carpi radialis muscle

This study investigated the within-subject, intra- and inter-operator reliability of quantitative electromyographic (EMG) analysis using decomposition-based quantitative electromyography (DQEMG). Needle and surface-detected EMG signals were collected during low-level isometric contractions of the extensor carpi radialis (ECR) muscle. DQEMG was used to extract needle-detected motor unit potential (MUP) trains and surface-detected MUPs (SMUPs) associated with each train. Two independent experienced operators re-decomposed and processed the MUP data on two separate occasions. One-way analyses of variance (ANOVA) were performed to identify within-subject differences (test–retest, n = 6), and operator and trial differences (intra- and inter-operator, n = 20) for the MUP morphological variables. The within-subject reliability, as well as the intra- and inter-operator reliability were estimated using intraclass correlation coefficients (ICCs). The 95% limits of agreement were calculated to measure within-subject and between operator agreements. MUP durations were found to be significantly different between days ( p < 0.05). For intra- and inter-operator reliability, a significant difference was found within and between the operators for MUP duration, and between the operators for MUP number of turns ( p < 0.05). SMUP morphological parameters yielded higher ICC values for both intra (0.96–0.99) and inter (0.96–0.99) operator scores when compared to MUP morphological parameters (intra-operator: 0.81–0.99; and inter-operator: 0.67–0.98). Mean motor unit (MU) firing rate was found to be a highly reliable measure for both intra- and inter-operators (0.99–0.97, respectively). The between-operator agreement was above 95% for all morphological parameters. These results concerning within-subject, intra- and inter-operator reliability and levels of agreement for quantitative motor unit analysis suggest that DQEMG provides sufficiently consistent results to allow it to be effectively used for QEMG analysis. Needle-detected MUP morphology although clinically useful in the diagnosis and monitoring of neuropathies, was not as reliable as surface MUP QEMG measures.

Physiological characteristics of motor units in the brachioradialis muscle across fatiguing low-level isometric contractions

The purpose of this study was to determine (i) if decomposition-based quantitative electromyography (DQEMG) could detect changes in motor unit potential (MUP) morphology and motor unit (MU) firing pattern statistics associated with muscle fatigue, (ii) if any detected changes are correlated with surface electromyographic (SEMG) signs of fatigue, and (iii) if significant fatigue-dependent changes are repeatable within individuals. Mean MU firing rates and the morphology of MUPs detected using needle and surface electrodes during constant-torque isometric contractions held until exhaustion were investigated in the brachioradialis (BR) muscle in 10 healthy volunteers (mean age = 28.6 yr, SD ± 3.9). Time dependant changes were investigated using an analysis of variance with normalized time as a main effect. Partial correlation coefficients were computed using a repeated measures analysis of covariance to determine if changes in MU firing rates, needle-detected MUPs and surface-detected MUPs (SMUPs) were related to changes in SEMG signal amplitude and frequency parameters. Intraclass correlation coefficients (ICCs) were used to determine the within-subject repeatability of changes in MU firing rates, and MUP and SMUP parameters. Significant decreases in mean MU firing rates were found along with significant increases in various duration and area related parameters in both MUPs and SMUPs across the fatiguing contraction. The SEMG signal demonstrated the expected changes with fatigue: an increase in amplitude and a decrease in frequency content. SEMG amplitude was significantly positively correlated with SMUP peak-to-peak voltage ( r = 0.85, p < 0.05), and SMUP area ( r = 0.86, p < 0.05). Mean power frequency was significantly negatively correlated with SMUP negative peak duration ( r = −0.74, p < 0.05). The significant time-dependent changes were reliably observed (ICCs were 0.94 for MUP peak to peak amplitude, 0.97 for MUP area and 0.95 for MUP area to amplitude ratio, 0.95 for SMUP peak-to-peak voltage, 0.83 for SMUP area, 0.99 for SMUP negative peak amplitude and 0.88 for SMUP negative peak area). The decreases in mean MU firing rates measured along with the increases in amplitude, duration and area parameters of MUPs and SMUPs and their partial correlation with SEMG amplitude during submaximal fatiguing contractions of the BR, suggest that recruitment is a main cause of increased SEMG amplitude parameters with fatigue. We conclude that DQEMG can be effectively and reliably used to detect changes in physiological characteristics of MUs that accompany fatigue.

Firing Characteristics of Motor Units during the First 90 Seconds of a Fatiguing Low-Level Contraction

The mechanisms responsible for the changes in surface electromyographic (EMG) signal amplitude and frequency content during prolonged fatiguing low-level contractions are not well understood. In addition, the normal behaviour of motor units (MU) during these contractions also remains elusive. PURPOSE: This study was designed (i) to determine decomposition-based quantitative electromyography (DQEMG), could detect changes in motor unit firing statistics that accompany fatigue in the brachioradialis (BR) and extensor carpi radialis (ECR) muscles over the first 90 seconds of sustained, low-level isometric contractions, (ii) to determine if time-dependent changes in surface EMG parameters are correlated with changes in MU firing rates, and (iii) to determine if time-dependant changes seen in the MU firing characteristics are repeatable within individuals. METHODS: Changes in mean MU firing rates were investigated in the BR (n=1 0) and ECR (n=10) muscles in healthy volunteers (mean age = 28.6 yr, SD ± 3.9) during constant-torque isometric contractions held until exhaustion. Three BR contractions were sustained at 25% of the maximum voluntary contraction (MVC) and two ECR contractions were sustained at 20% MVC. Mean MU firing rates were estimated following the decomposition of 30-second EMG data windows. Time dependant changes in firing rates were investigated using an analysis of variance with time (first, second and third 30 second epochs) included as a main effect. Pearson product-moment correlation coefficients were computed to determine if changes in MU firing rates were related to changes in surface EMG amplitude and frequency parameters (α=0.05). Intra-class correlation coefficients (ICC) were used to determine the within-subject repeatability of the MU firing rate changes, and SEMG parameters. RESULTS: Mean MU firing rate decreased within the first 90 seconds in both muscle groups, and was repeatable across trials using the DQEMG method. Significant correlations between changes in mean firing rates and surface EMG amplitude and frequency content were found in the ECR muscle but not the BR muscle. The reliability of firing rate changes within subjects was moderate in both the ECR (ICC=0.72) and BR muscles (ICC=0.59). The SEMG mean power frequency significantly decreased over contraction time in the ECR muscle, and was found to be a highly reliable measure in both muscles (ICC=0.99 and 0.98, respectively). CONCLUSIONS: Changes in MU characteristics were observed as early as 90 seconds into a low level contraction. Differences found between the two muscles may be due to contraction level or fiber type distributions.

Effects of postural muscle fatigue on the relation between segmental posture and movement

The purpose of this study was to examine whether fatigue of postural muscles might influence the coordination between segmental posture and movement. Seven healthy adults performed series of fifteen fast wrist flexions and extensions while being instructed to keep a dominant upper limb posture as constant as possible. These series of voluntary movements were performed before and after a fatiguing submaximal isometric elbow flexion, and also with or without the help of an elbow support. Surface EMG from muscles Deltoı̈deus anterior, Biceps brachii, Triceps brachii, Flexor carpi ulnaris, Extensor carpi radialis were recorded simultaneously with wrist, elbow and shoulder accelerations and wrist and elbow displacements. Fatigue was evidenced by a shift of the elbow and shoulder muscles EMG spectra towards low frequencies. Kinematics of wrist movements and corresponding activations of wrist prime-movers, as well as the background of postural muscle activation before wrist movement were not modified. There were only slight changes in timing of postural muscle activations. These data indicate that postural fatigue induced by a low-level isometric contraction has no effect on voluntary movement and requires no dramatic adaptation in postural control.

Upper trapezius muscle mechanomyographic and electromyographic activity in humans during low force fatiguing and non-fatiguing contractions

The purposes of this study were firstly to compare and investigate localised fatigue in the upper trapezius muscle in various arm positions as assessed by mechanomyographic (MMG) and surface electromyographic (EMG) signals and secondly to study the effects of different normalisation methods on MMG and EMG signals during non-fatiguing and fatiguing low level isometric contractions. The MMG, EMG and rate of perceived exertion were recorded from 11 subjects in five arm positions (0 degrees abduction and 0 degrees flexion, 45 degrees and 90 degrees flexion, 45 degrees and 90 degrees abduction) with different bilateral arm loads during 3 s for non-fatiguing (0-0.5-1 kg hand-load) and 3 min for fatiguing contractions (1 kg hand-load). The root mean square (RMS), average rectified value (ARV), mean power frequency (MNF), and median power frequency (MDF) of the MMG and EMG signals were computed and normalised with respect to the initial values obtained in the current arm position or in the reference position (0 degrees abduction and 0 degrees flexion) corresponding to the normal postural activity of the trapezius muscle. For fatiguing contractions, differences in magnitude of the increase in the RMS or ARV and decrease in the MNF or MDF were observed for EMG and MMG. The MMG amplitude and spectral changes followed the subjective sensation of fatigue and were not correlated to their EMG counterparts, suggesting that they may reflect different phenomena. For non-fatiguing contractions, normalisation to the current arm position entailed the loss of dynamic amplitude changes suggesting that a single reference contraction in the middle part of the range of movement is enough for proper normalisation of EMG and MMG signals. For fatiguing contractions, normalisation of the EMG and MMG to some extent can lead to a misleading interpretation. Assessment of the upper trapezius muscle by means of MMG may be valuable in ergonomics.

Influence of posture and training on the endurance time of a low-level isometric contraction

Classically, the critical force of a muscle (the relative force below which an isometric contraction can be maintained for a very long time without fatigue) is comprised of between 15 and 20% of its maximum voluntary contraction (MVC). However, some authors believe that the value is below 10% MVC. If such is the case, signs that accompany the establishment of muscle fatigue (EMG changes, continuous increase in systolic blood pressure [SBP] and heart rate [HR]) would have to appear more rapidly and with a higher intensity if the muscle is already partially fatigued at the start of maintaining a contraction at 10% MVC. Twelve healthy untrained participants carried out two isometric contractions with the digit flexors: one (test A) began with a maximum contraction sustained for 4 min followed without interruption by a contraction at 10% MVC for 61 min; the other (test B) was a contraction maintained at 10% MVC for 65 min. For test B, after an initial increase of 4 bpm with respect to at rest, HR remained stable until the end of contraction, SBP progressively increased by 24 mm Hg in 28 min, then remained unchanged until the end, and there were no significant changes in EMG (absence of spectral deviation towards low frequencies). For test A, in spite of the initial maximum contraction, changes in the parameters being studied (total maintenance time, HR, SBP, EMG) during maintenance at 10% MVC were identical to those for test B. The results show that (1) when the number and intensity of the co-contractions are minimized by applying an appropriate posture, it is possible to sustain an isometric contraction at 10% MVC for at least 65 min without the appearance of signs of muscle fatigue; (2) the critical force of the digit flexors is higher than 10% MVC.