The effects of interelectrode distance over the innervation zone and normalization on the electromyographic amplitude and mean power frequency versus concentric, eccentric, and isometric torque relationships for the vastus lateralis muscle

Teaching Essential EMG Theory to Kinesiologists and Physical Therapists Using Analogies Visual Descriptions, and Qualitative Analysis of Biophysical Concepts

Electromyography (EMG) is a multidisciplinary field that brings together allied health (kinesiology and physical therapy) and the engineering sciences (biomedical and electrical). Since the physical sciences are used in the measurement of a biological process, the presentation of the theoretical foundations of EMG is most conveniently conducted using math and physics. However, given the multidisciplinary nature of EMG, a course will most likely include students from diverse backgrounds, with varying levels of math and physics. This is a pedagogical paper that outlines an approach for teaching foundational concepts in EMG to kinesiologists and physical therapists that uses a combination of analogies, visual descriptions, and qualitative analysis of biophysical concepts to develop an intuitive understanding for those who are new to surface EMG. The approach focuses on muscle fiber action potentials (MFAPs), motor unit action potentials (MUAPs), and compound muscle action potentials (CMAPs) because changes in these waveforms are much easier to identify and describe in comparison to the surface EMG interference pattern (IP).

The Evolution of the Physical Work Capacity at the Fatigue Threshold Test: Past, Present, and Future

ABSTRACT

Feldpausch, JE, Blok, AL, Frederick, EL, Coburn, JW, and Malek, MH. The evolution of the physical work capacity at the fatigue threshold test: past, present, and future. J Strength Cond Res 35(12): 3529-3536, 2021-The assessment of neuromuscular fatigue using surface electromyography has evolved over the past 40 years while maintaining some of the original key features. In this mini-review article, the goal will be to briefly present a history and systems of the physical working capacity at the fatigue threshold (PWCFT). In addition, we will discuss studies that have investigated the effect of different interventions such as supplementation, exercise, and cognitive fatigue to examine what stimuli influence the PWCFT. The latter section of this mini-review will discuss future studies that may provide additional information related to the underlying physiological mechanism(s) that influences the PWCFT. We will conclude with the practical application of PWCFT in health and sports settings.

Effects of detection system parameters on cross-correlations between MUAPs generated from parallel and inclined muscle fibres

The aim of this study was to investigate the effects of inter-electrode distance (IED), electrode radius (ER) and electrodes configurations on cross-correlation coefficient (CC) between motor unit action potentials (MUAPs) generated in a motor unit (MU) of parallel fibres and in a MU of inclined fibres with respect to the detection system. The fibres inclination angle (FIA) varied from 0° to 180° by a step of 5°. Six spatial filters (the longitudinal single differential (LSD), longitudinal double differential (LDD), bi-transversal double differential (BiTDD), normal double differential (NDD), an inverse binomial filter of order two (IB2) and maximum kurtosis filter (MKF)), three values of IED and three values of ER were considered.

A cylindrical multilayer volume conductor constituted by bone, muscle, fat and skin layers was used to simulate the MUAPs.

The cross-correlation coefficient analysis showed that with the increase of the FIA, the pairs of MUAPs detected by the IB2 system were more correlated than those detected by the five other systems. For each FIA, the findings also showed that the MUAPs pairs detected by BiTDD, NDD, IB2 and MKF systems were more correlated with smaller IEDs than with larger ones, while inverse results were found with the LSD and LDD systems. In addition, the pairs of MUAPs detected by the LDD, BiTDD, IB2 and MKF systems were more correlated with large ERs than with smaller ones. However, inverse results were found with the LSD and NDD systems.

Performance fatigability and neuromuscular responses for bilateral versus unilateral leg extensions in women

The purpose of this study was to compare isokinetic peak torque and the patterns of responses for electromyographic (EMG) and mechanomyographic (MMG), amplitude (AMP) and mean power frequency (MPF) for bilateral (BL) versus unilateral (UL), maximal, isokinetic leg extensions. Eleven recreationally trained women (Mean ± SD: age 22.9 ± 0.9 yrs; body mass 60.5 ± 10.1 kg; height 167.2 ± 6.4 cm) performed 50 maximal, BL and UL isokinetic leg extensions at 60° s^-1 on separate days. Electromyographic and MMG signals from the vastus lateralis of the nondominant leg were recorded. Five separate 2 (Condition [BL and UL]) × 10 (Repetitions [5-50]) repeated measures ANOVAs were performed to examine normalized EMG AMP, EMG MPF, MMG AMP, MMG MPF, and isokinetic torque. The results indicated no significant interactions or main effects for EMG AMP and MMG AMP. There were significant interactions for normalized isokinetic peak torque (p < 0.001, η²_p = 0.493) and MMG MPF (p = 0.003, η²_p = 0.234). For EMG MPF, there was no significant interaction, but significant main effects for Condition (p = 0.003, η²_p = 0.607) and Repetitions (p < 0.001, η²_p = 0.805). The current findings demonstrated greater performance fatigability for UL than BL leg extensions. Both modalities exhibited similar patterns of neuromuscular responses that were consistent with the Muscular Wisdom hypothesis.

Fatigue-Mediated Loss of Complexity is Contraction-Type Dependent in Vastus Lateralis Electromyographic Signals

The purpose of this study was to investigate the effect of fatigue status and contraction type on complexity of the surface electromyographic (sEMG) signal. Twelve females (mean age ± SD = 21.1 ± 1.4 years) performed three fatigue-inducing protocols that involved maximal concentric, eccentric, or isometric knee-extensor contractions over three non-consecutive sessions. Pre- and post-fatigue assessments were also completed each session and consisted of three maximal efforts for each type of contraction. Complexity of sEMG signals from the vastus lateralis was assessed using Sample Entropy (SampEn) and Detrended Fluctuation Analysis (DFA) as expressed using the scaling exponent α. The results showed that fatigue decreased (p < 0.05) sEMG complexity as indicated by decreased SampEn (non-fatigued: 1.57 ± 0.22 > fatigued: 1.46 ± 0.25) and increased DFA α (non-fatigued: 1.27 ± 0.26 < fatigued: 1.32 ± 0.23). In addition, sEMG complexity was different among contraction types as indicated by SampEn (concentric: 1.58 ± 0.22 > eccentric: 1.47 ± 0.27 and isometric: 1.50 ± 0.21) and DFA α (concentric: 1.27 ± 0.18 < isometric: 1.32 ± 0.18). Thus, these findings suggested sEMG complexity is affected by fatigue status and contraction type, with the degree of fatigue-mediated loss of complexity dependent on the type of contraction used to elicit fatigue.

The influence of electromyographic recording methods and the innervation zone on the mean power frequency-torque relationships

This study examined the effects of electromyographic (EMG) recording methods and innervation zone (IZ) on the mean power frequency (MPF)-torque relationships. Nine subjects performed isometric ramp muscle actions of the leg extensors from 5% to 100% of maximal voluntary contraction with an eight channel linear electrode array over the IZ of the vastus lateralis. The slopes were calculated from the log-transformed monopolar and bipolar EMG MPF-torque relationships for each channel and subject and 95% confidence intervals (CI) were constructed around the slopes for each relationship and the composite of the slopes. Twenty-two to 55% of the subjects exhibited 95% CIs that did not include a slope of zero for the monopolar EMG MPF-torque relationships while 25-75% of the subjects exhibited 95% CIs that did not include a slope of zero for the bipolar EMG MPF-torque relationships. The composite of the slopes from the EMG MPF-torque relationships were not significantly different from zero for any method or channel, however, the method and IZ location slightly influenced the number of significant slopes on a subject-by-subject basis. The log-transform model indicated that EMG MPF-torque patterns were nonlinear regardless of recording method or distance from the IZ.

Quantifying the effects of electrode distance from the innervation zone on the electromyographic amplitude versus torque relationships

The present study applied a log-transformation model to compare the electromyographic (EMG) amplitude versus torque relationships from monopolar EMG signals up to 35 mm proximal and distal from the innervation zone (IZ). Seven men (age = 23 ± 2 year; mass = 82 ± 10 kg) and two women (age = 21 ± 1 year; mass = 62 ± 8 kg) performed isometric ramp contractions of the right leg extensors with an eight-channel linear electrode array positioned over the vastus lateralis with the IZ located between channels 4 and 5. Linear regression models were fit to the log-transformed monopolar EMG(RMS)-torque relationships with the b terms (slope) and the a terms (Y-intercept) calculated for each channel and subject. The b terms for channels 4, 5, and 6 were higher (P ≤ 0.05) than the more distal channels 7 and 8 (P < 0.05). In contrast, there were no differences (P > 0.05) among the a terms of the eight channels. Thus, the shapes of the monopolar EMG(RMS)-torque relationships were altered as a function of distance between the IZ and recording area, which may be helpful for clinicians and researchers who infer changes in motor control strategies based on the shapes of the EMG(RMS)-torque relationships.

Reliability of surface EMG matrix in locating the innervation zone of upper trapezius muscle

The identification of the motor unit (MU) innervation zone (IZ) using surface electromyographic (sEMG) signals detected on the skin with a linear array or a matrix of electrodes has been recently proposed in the literature. However, an analysis of the reliability of this procedure and, therefore, of the suitability of the sEMG signals for this purpose has not been reported. The purpose of this work is to describe the intra and inter-rater reliability and the suitability of surface EMG in locating the innervation zone of the upper trapezius muscle. Two operators were trained on electrode matrix positioning and sEMG signal analysis. Ten healthy subjects, instructed to perform a series of isometric contractions of the upper trapezius muscle participated in the study. The two operators collected sEMG signals and then independently estimated the IZ location through visual analysis. Results showed an almost perfect agreement for intra-rater and inter-rater reliability. The constancy of IZ location could be affected by the factors reflecting the population of active MUs and their IZs, including: the contraction intensity, the acquisition period analyzed, the contraction repetition. In almost all cases the IZ location shift due to these factors did not exceed 4mm. Results generalization to other muscles should be made with caution.

Differences in the log-transformed electromyographic-force relationships of the plantar flexors between high- and moderate-activated subjects

The present study examined the log-transformed electromyographic amplitude (EMG) versus force relationships for the medial gastrocnemius (MG) and soleus (SOL) in high- and moderate-activated subjects. Twenty-five (age=21±2 year; mass=62±12 kg) participants performed six submaximal contractions (30-90% maximal voluntary contraction [MVC]) with the interpolated twitch technique (ITT) performed at 90% MVC to calculate percent voluntary activation (% VA). Sixteen participants with>90% VA at 90% MVC were categorized high-activated group; the remaining nine were the moderate-activated group. Linear regression models were fit to the log-transformed EMG-force relationships. The slope (b value) and the antilog of the Y-intercept (a value) were calculated. The b values from the MG EMG-force relationships were higher (P<0.05) for the high-activated group (1.27±0.13) than the moderate-activated group (0.88±0.06). The a values and p-p M-wave amplitude values (collapsed across twitches [superimposed and potentiated]) were larger (P<0.05) for the MG (1.17±0.40 and 8.98±0.46 mV) than the SOL (0.24±0.07 and 4.48±0.20 mV) when collapsed across groups. The b value from the log-transformed EMG-force relationships is an attractive model to determine if a subject has the ability to achieve high activation of their MG without muscle or nerve stimulation.

Methodology of electromyographic analysis of the trunk muscles during walking in healthy subjects: a literature review

PURPOSE

To review and discuss the literature about the use of trunk muscle electromyography - including the use of surface or fine-wire electrodes, site of application and muscle selection - during gait analysis in healthy subjects.

METHODS

The databases Pubmed, Web of Knowledge and Cochrane Library were searched. Articles were included when EMG activity of at least one trunk muscle was measured in healthy subjects during walking.

RESULTS

In the 33 selected articles 491 healthy subjects walked with different velocities on a treadmill and/or overground. The activity of the M. erector spinae, M. multifidus, M. obliquus externus and internus, M. rectus abdominus, M. trapezius, M. latissimus dorsi, M. transversus abdominus, M. iliopsoas and M. quadrates lumborum was measured. Twenty-nine studies used surface electrodes, one study fine-wire electrodes, and the other three studies used a combination. There is no consensus on the exact placement site of the electrodes.

CONCLUSION

Surface electrodes were used more often than fine-wire electrodes and the descriptions of the electrode locations were mostly vague and not consistent among the different studies. There is need for further research to make specific recommendations about the type of electrodes in combination with the optimal locations of application of these electrodes.

A noninvasive, log-transform method for fiber type discrimination using mechanomyography

This study examined the log-transformed mechanomyographic (MMG(RMS)) and electromyographic (EMG(RMS)) amplitude vs. force relationships for aerobically-trained (AT), resistance-trained (RT), and sedentary (SED) individuals. Subjects performed isometric ramp contractions from 5% to 90% maximal voluntary contraction. Muscle biopsies were collected and thigh skinfolds, MMG and EMG were recorded from the vastus lateralis muscle. Linear regression models were fit to the log-transformed EMG(RMS) and MMG(RMS) vs. force relationships. The slope (b coefficient) and the antilog of the y-intercept (a coefficient) were calculated. The AT group had the highest percentage of type I fiber area, the RT group had the highest percentage of type IIa fiber area, and the SED group had the highest percentage of type IIx fiber area. The a coefficients were higher for the AT group than the RT and SED groups in both the MMG(RMS) and EMG(RMS) vs. force relationships, whereas the b coefficients were lower for the AT group than the RT and SED groups only in the MMG(RMS) vs. force relationship. The group differences among the a coefficients may have reflected subcutaneous fat acting as a filter thereby reducing EMG(RMS) and MMG(RMS). The lower b coefficients for the AT group in the MMG(RMS) patterns may have reflected fiber area-related differences in motor unit activation strategies.