Evaluating skeletal muscle electromechanical delay with intramuscular pressure

IntroductionIntramuscular pressure (IMP) is the fluid pressure generated within skeletal muscle and directly reflects individual muscle tension. The purpose of this study was to assess the development of force, IMP, and electromyography (EMG) in the tibialis anterior (TA) muscle during ramped isometric contractions and evaluate electromechanical delay (EMD).MethodsForce, EMG, and IMP were simultaneously measured during ramped isometric contractions in eight young, healthy human subjects. The EMD between the onset of force and EMG activity (Δt-EMG force) and the onset of IMP and EMG activity (Δt EMG-IMP) were calculated.ResultsA statistically significant difference (p < 0.05) was found between the mean force-EMG EMD (36 ± 31 ms) and the mean IMP-EMG EMD (3 ± 21 ms).ConclusionsIMP reflects changes in muscle tension due to the contractile muscle elements.     

Comparison of electromyographic activity during eccentrically versus concentrically loaded isometric contractions

Electromyographic (EMG) amplitude and mechanical tension are directly related during isometric contraction. Maximal voluntary isometric contractions are typically elicited through two different procedures; resisting a load, which is eccentric in nature, and contracting against an immovable object, which is concentric in nature. A wealth of literature exists indicating that EMG amplitude during concentric contractions is greater than that of eccentric contractions of the same magnitude. However, the effects of different methods to elicit isometric contraction on EMG amplitude have yet to be investigated. The purpose of this study was to compare EMG amplitudes under different loading configurations designed to elicit isometric muscle contraction. Twenty healthy volunteers (10 males and 10 females, age = 23 ± 2 yrs, height = 1.7 ± 0.09 m, mass = 69.9 + 16.8 kg) performed a maximal voluntary plantarflexion effort for which the vertical ground reaction force (GRFv) sampled from a force plate and surface EMG of the soleus were recorded. Participants then performed isometric plantarflexion at 20%, 30%, 40%, and 50% GRFv_max in a seated position, from a neutral ankle position, under two different counterbalanced isometric loading conditions (concentric and eccentric). For concentric loading conditions, the subject contracted against an immovable resistance to the specified %GRFv identified via visual and auditory feedback. For eccentric loading conditions, subjects contracted against an applied load placed on the distal anterior thigh that produced the specified %GRFv. This applied load had the tendency to force the ankle into dorsiflexion. Therefore, plantarflexion force, in an attempt to maintain the ankle in a neutral position, resisted lengthening of the plantarflexor musculature, thus representing eccentric loading during an isometric contraction. Mean EMG amplitude was compared across loading levels and types using a 2 (loading type: concentric, eccentric) × 4 (loading level: 20%, 30%, 40%, 50% GRFv) repeated-measures ANOVA. The main effect for loading level was significant (p = 0.007). However, the main effect for loading type, and the loading type × loading level interaction were non-significant (p > 0.05). The present findings provide evidence that isometric muscle contractions loaded in either concentric or eccentric manners elicit similar EMG amplitudes, and are therefore comparable in research settings.     

Recovery of the human biceps electromyogram after heavy eccentric, concentric or isometric exercise.

Five men performed submaximal isometric, concentric or eccentric contractions until exhaustion with the left arm elbow flexors at respectively 50%, 40% and 40% of the prefatigued maximal voluntary contraction force (MVC). Subsequently, and at regular intervals, the surface electromyogram (EMG) during 30-s isometric test contractions at 40% of the prefatigued MVC and the muscle performance parameters (MVC and the endurance time of an isometric endurance test at 40% prefatigued MVC) were recorded. Large differences in the surface EMG response were found after isometric or concentric exercise on the one hand and eccentric exercise on the other. Eccentric exercise evoked in two of the three EMG parameters [the EMG amplitude (root mean square) and the rate of shift of the EMG mean power frequency (MPF)] the greatest (P less than 0.001) and longest lasting (up to 7 days) response. The EMG response after isometric or concentric exercise was smaller and of shorter duration (1-2 days). The third EMG parameter, the initial MPF, had already returned to its prefatigued value at the time of the first measurement, 0.75 h after exercise. The responses of EMG amplitude and of rate of MPF shift were similar to the responses observed in the muscle performance parameters (MVC and the endurance time). Complaints of muscle soreness were most frequent and severe after the eccentric contractions. Thus, eccentric exercise evoked the greatest and longest lasting response both in the surface EMG signal and in the muscle performance parameters.     

Activation of human quadriceps femoris during isometric, concentric, and eccentric contractions.

Maximal and submaximal activation level of the right knee-extensor muscle group were studied during isometric and slow isokinetic muscular contractions in eight male subjects. The activation level was quantified by means of the twitch interpolation technique. A single electrical impulse was delivered, whatever the contraction mode, on the femoral nerve at a constant 50 degrees knee flexion (0 degrees = full extension). Concentric, eccentric (both at 20 degrees /s velocity), and isometric voluntary activation levels were then calculated. The mean activation levels during maximal eccentric and maximal concentric contractions were 88.3 and 89.7%, respectively, and were significantly lower (P < 0.05) with respect to maximal isometric contractions (95.2%). The relationship between voluntary activation levels and submaximal torques was linearly fitted (P < 0.01): comparison of slopes indicated lower activation levels during submaximal eccentric compared with isometric or concentric contractions. It is concluded that reduced neural drive is present during 20 degrees /s maximal concentric and both maximal and submaximal eccentric contractions. These results indicate a voluntary activation dependency on both tension levels and type of muscular actions in the human knee-extensor muscle group.     

EMG signal amplitude normalization technique in stretch-shortening cycle movements

Analysis of functional movements using surface electromyography (EMG) often involves recording both eccentric and concentric muscle activity during a stretch-shorten cycle (SSC). The techniques used for amplitude normalization are varied and are independent of the type of muscle activity involved. The purpose of this study was: (i) to determine the effect of 11 amplitude normalization techniques on the coefficient of variation (CV) during the eccentric and concentric phases of the SSC; and (ii) to establish the effect of the normalization techniques on the EMG signal under variable load and velocity. The EMG signal of the biceps brachii of eight normal subjects was recorded under four SSC conditions and three levels of isometric contraction. The 11 derived normalization values were total rms, mean rms and peak rms (100 ms time constant) for the isometric contractions and the mean rms and peak rms values of the ensemble values for each set of isotonic contractions. Normalization using maximal voluntary isometric contractions (MVIC), irrespective of rms processing (total, mean or peak), demonstrated greater CV above the raw data for both muscle actions. Mean ensemble values and submaximal isometric recordings reduced the CV of concentric data. No amplitude normalization technique reduced the CV for eccentric data under loaded conditions. An ANOVA demonstrated significant (P < 0.01) main effects for load and velocity on concentric raw data and an interaction (P < 0.05) for raw eccentric data. No significant effects were demonstrated for changes in velocity when the data were normalized using mean rms values. The reduction of the CV should not be at the expense of true biological variance and current normalization techniques poorly serve the analysis of eccentric muscle activity during the SSC.     

Obtaining maximum muscle excitation for normalizing shoulder electromyography in dynamic contractions

Muscle specific maximal voluntary isometric contractions (MVIC) are commonly used to elicit reference amplitudes to normalize electromyographic signals (EMG). It has been questioned whether this is appropriate for normalizing EMG from dynamic contractions. This study compares EMG amplitude when shoulder muscle activity from dynamic contractions is normalized to isometric and isokinetic maximal excitation as well as a hybrid approach currently used in our laboratory. Anterior, middle and posterior deltoid, upper and lower trapezius, pectoralis major, latissimus dorsi and infraspinatus were monitored during (1) manually resisted MVICs, and (2) maximum voluntary dynamic concentric contractions (MVDC) on an isokinetic dynamometer. Dynamic contractions were performed (a) at 30°/s about the longitudinal, frontal and sagittal axes of the shoulder, and (b) during manual bi-rotation of a tilted wheel at 120°/s. EMG from the wheel task was normalized to the maximum excitation from (i) the muscle specific MVIC, (ii) from any MVIC (MVIC_ALL), (iii) for any MVDC, (iv) from any exertion (maximum experimental excitation, MEE). Mean EMG from the wheel task was up to 45% greater when normalized to muscle specific isometric contractions (method i) than when normalized to MEE (method iv). Seventy-five percent of MEE’s occurred during MVDCs. This study presents an 20 useful and effective process for obtaining the greatest excitation from the shoulder muscles when normalizing dynamic efforts.     

Influence of the intensity of a conditioning contraction on the subsequent twitch torque and maximal voluntary concentric torque

This study aimed to clarify the influence of the intensity of a conditioning contraction on subsequent isometric twitch and maximal voluntary concentric torques. Subjects (n=12men) performed voluntary isometric plantar flexion for six seconds as a conditioning contraction, at intensities of 40%, 60%, 80% and 100% of a maximal voluntary isometric contraction (MVIC). Before and immediately after the conditioning contraction, isometric twitch and maximal voluntary concentric (180°/s) plantar flexion torques were determined. Surface electromyograms were recorded from the triceps surae muscles and M-wave amplitudes and root-mean-square values of the electromyographic signals (RMS(EMG)) were calculated. The isometric twitch torque increased significantly after conditioning contraction at all intensities (P<0.05), whereas maximal voluntary concentric torque increased significantly only at 80% and 100% MVIC conditions (P<0.05). It is concluded that during a six second conditioning contraction, the effect of the intensity of a conditioning contraction on subsequent torque development is different between an isometric twitch and maximal voluntary concentric contractions, with the latter being less affected.     

Fluctuations in acceleration during voluntary contractions lead to greater impairment of movement accuracy in old adults.

The purpose of the study was to assess the effect of movement velocity on the relation between fluctuations in acceleration and the ability to achieve a target velocity during voluntary contractions performed by young (29.5 +/- 4.3 yr) and old (74.9 +/- 6.2 yr) adults. Subjects performed concentric and eccentric contractions with the first dorsal interosseus muscle while lifting a submaximal load (15% of maximum) at six movement velocities (0.03-1.16 rad/s). Fluctuations in acceleration, the accuracy of matching the target velocity, and electromyographic (EMG) activity were determined from three trials for each contraction type and movement velocity. The fluctuations in acceleration increased with movement velocity for both concentric and eccentric contractions, but they were greatest during fast eccentric contractions ( approximately 135%) when there was stronger modulation of acceleration in the 5- to 10-Hz bandwidth. Nonetheless, EMG amplitude for first dorsal interosseus increased with movement velocity only for concentric and not eccentric contractions. Consistent with the minimum variance theory, movement accuracy was related to the fluctuations in acceleration for both types of contractions in all subjects. For a given level of fluctuations in acceleration, however, old subjects were three times less accurate than young subjects. Although the EMG amplitude at each speed was similar for young and old adults, only the young adults modulated the power in the EMG spectrum with speed. Thus the fluctuations in acceleration during voluntary contractions had a more pronounced effect on movement accuracy for old adults compared with young adults, probably due to factors that influenced the frequency-domain characteristics of the EMG.     

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.     

Which type of repetitive muscle contractions induces a greater acute impairment of position sense?

The objective of this study was to determine which type of repetitive muscle contractions induces a greater acute impairment of elbow position sense. Eleven male subjects participating in the study underwent (i) an exercise task (ET) consisting of 9 sets of 10 voluntary isometric, concentric, or eccentric contractions randomly performed on three separate sessions, and (ii) a pre- and post-exercise maximal voluntary isometric contraction (iMVC). Prior to and between sets of ET, a proprioception task (PT) consisting of matching the right arm to the left reference arm was performed at three different target angular positions (70°, 110° and 150°). Each ET was immediately followed by 3 PT and 1 min rest. The statistical analysis revealed that post-exercise iMVCs were significantly decreased compared to pre-exercise iMVC in all conditions with a greater drop following the eccentric task. Despite this greater drop, position sense was significantly affected by the concentric exercise task. In addition, the spectral EMG signals significantly shifted towards lower frequencies from the initial values, regardless of exercise task. The results showed that concentric muscle contractions impaired position sense to a greater extent compared to isometric and eccentric contractions.     

Time and frequency domain responses of the mechanomyogram and electromyogram during isometric ramp contractions: a comparison of the short-time Fourier and continuous wavelet transforms.

The purposes of this study were to examine the mechanomyographic (MMG) and electromyographic (EMG) time and frequency domain responses of the vastus lateralis (VL) and rectus femoris (RF) muscles during isometric ramp contractions and compare the time-frequency of the MMG and EMG signals generated by the short-time Fourier transform (STFT) and continuous wavelet transform (CWT). Nineteen healthy subjects (mean+/-SD age=24+/-4 years) performed two isometric maximal voluntary contractions (MVCs) before and after completing 2-3, 6-s isometric ramp contractions from 5% to 100% MVC with the right leg extensors. MMG and surface EMG signals were recorded from the VL and RF muscles. Time domains were represented as root mean squared amplitude values, and time-frequency representations were generated using the STFT and CWT. Polynomial regression analyses indicated cubic increases in MMG amplitude, MMG frequency, and EMG frequency, whereas EMG amplitude increased quadratically. From 5% to 24-28% MVC, MMG amplitude remained stable while MMG frequency increased. From 24-28% to 76-78% MVC, MMG amplitude increased rapidly while MMG frequency plateaued. From 76-78% to 100% MVC, MMG amplitude plateaued (VL) or decreased (RF) while MMG frequency increased. EMG amplitude increased while EMG frequency changed only marginally across the force spectrum with no clear deflection points. Overall, these findings suggested that MMG may offer more unique information regarding the interactions between motor unit recruitment and firing rate that control muscle force production during ramp contractions than traditional surface EMG. In addition, although the STFT frequency patterns were more pronounced than the CWT, both algorithms produced similar time-frequency representations for tracking changes in MMG or EMG frequency.     

Neuromuscular fatigue during dynamic maximal strength and hypertrophic resistance loadings

The purpose of this study was to compare the acute neuromuscular fatigue during dynamic maximal strength and hypertrophic loadings, known to cause different adaptations underlying strength gain during training. Thirteen healthy, untrained males performed two leg press loadings, one week apart, consisting of 15 sets of 1 repetition maximum (MAX) and 5 sets of 10 repetition maximums (HYP). Concentric load and muscle activity, electromyography (EMG) amplitude and median frequency, was assessed throughout each set. Additionally, maximal bilateral isometric force and muscle activity was assessed pre-, mid-, and up to 30 min post-loading. Concentric load during MAX was decreased after set 10 (P<0.05), while the load was maintained throughout HYP. Both loadings caused large reductions in maximal isometric force (MAX=-30±6.4% vs. HYP=-48±9.7%, P<0.001). The decreased concentric and isometric strength during MAX loading was accompanied by reduced EMG amplitude (P<0.05). Conversely, hypertrophic loading caused decreased median frequency only during isometric contractions (P<0.01). During concentric contractions, EMG amplitude increased and median frequency decreased in HYP (P<0.01). Our results indicate reduced neural drive during MAX loading and more complex changes in muscle activity during HYP loading.     

A comparison between mechanomyographic condenser microphone and accelerometer measurements during submaximal isometric, concentric and eccentric contractions.

The aim of this study was to compare mechanomyogram (MMG) recorded by a condenser microphone (MIC) and an accelerometer (ACC) during submaximal isometric, concentric and eccentric contractions in 14 males. The maximal voluntary force (MVC) of the biceps brachii was measured. The subjects were asked to do short duration isometric, concentric and eccentric contraction at 10%, 30%, 50%, 70% MVC twice. For the concentric and eccentric contraction, the subject bent his arm for 3s (concentric) then held it for 3s and extended (eccentric) during 3s. The normalized root mean square (RMS) and mean power frequency (MPF) increased linearly with increased force for both transducers. There was a correlation between MIC MPF and ACC MPF at 10%, 30%, 50% MVC, and between MIC RMS and ACC RMS at 30% MVC during isometric contractions. There was significantly higher MPF for the ACC than for the MIC in concentric and eccentric modes, while the RMS did not differ among transducers in the three contraction modes. The RMS and MPF values coefficient of variations were significantly larger during anisometric contractions compared with isometric contractions and were lower for the accelerometer than for the microphone. The present results obtained during isometric, concentric and eccentric contractions of increased intensity showed that the information contained in microphone- and accelerometer-based MMG signals is different despite similar trends. It can be concluded that at low-moderate movement velocity, concentric contractions can be investigated by means of accelerometer and microphone.     

Localized muscular fatigue duration, EMG parameters and accuracy of rapid limb movements

While much is known about the physiological basis of local muscular fatigue, little is known about the kinematic and electromyographic (EMG) consequences of brief fatiguing isometric contractions. Five male subjects performed a horizontal elbow flexion-extension reversal movement over 90° in 250 ms to reversal before and after one of five single maximal isometric elbow flexions ranging in duration from 15–120 s. Surface EMG signals were recorded from the biceps brachii, the long head of the triceps, the clavicular portion of the pectoralis major, and the posterior deltoid. Spatial and temporal errors were computed from potentiometer output. During the fatiguing bouts, maximum voluntary force dropped linearly an average of 4% in the 15 s condition and 58% in the 120 s condition relative to maximum force. The associated biceps rectified-integrated EMG signal increased from the onset of each fatigue bout for 15–30 s, then decreased over the remainder of the longer bouts. Following the fatigue bout, subjects undershot the target distance on the first movement trial in all conditions. Following short fatigue durations (i.e. 15–30 s), the peak biceps EMG amplitude was disrupted and movement velocity decreased, but both measures recovered within seconds. As fatigue duration increased, progressive decreases in peak velocity occurred with increased time to reversal, reduced EMG amplitude, and longer recovery times. However, the relative timing of the EMG pattern was maintained suggesting the temporal structure was not altered by fatigue. The findings suggest that even short single isometric contractions can disrupt certain elements of the motor control system.     

Force depression in human quadriceps femoris following voluntary shortening contractions.

The purpose of this study was to investigate whether the isometric muscle force, redeveloped following maximal-effort voluntary shortening contractions in human skeletal muscle, is smaller than the purely isometric muscle force at the corresponding length. Isometric knee extensor moments, surface electromyographic (EMG) signals of quadriceps femoris, and interpolated twitch moments (ITMs) were measured while 10 subjects performed purely isometric knee extensor contractions at a 60 degrees knee angle and isometric knee extensor contractions at a 60 degrees knee angle preceded by maximal-effort voluntary shortening of the quadriceps muscles. It was found that the knee extensor moments were significantly decreased for the isometric-shortening-isometric contractions compared with the isometric contractions for the group as a whole, whereas the corresponding EMG and ITM values were the same. This study is the first to demonstrate force depression following muscle shortening for voluntary contractions. We concluded that force depression following muscle shortening is an actual property of skeletal muscle rather than a stimulation artifact and that force depression during voluntary contraction is not accompanied by systematic changes in muscle activation as evaluated by EMG and ITM.     

Effects of electromyostimulation versus voluntary isometric training on elbow flexor muscle strength

The purpose of this study was to determine whether 7 weeks of standardized (same number and duration of repetitions, sets and rest strictly identical) electromyostimulation training of the elbow flexor muscles would induce strength gains equivalent to those of voluntary isometric training in isometric, eccentric and concentric contractions. Twenty-five males were randomly assigned to an electromyostimulated group (EMS, n = 9), a voluntary isometric group (VOL, n = 8), or a control group (CON, n = 8). Maximal voluntary isometric, eccentric and concentric strength, electromyographic (EMG) activity of the biceps and triceps brachii muscles, elbow flexor muscle activation (twitch interpolation technique) and contractile properties were assessed before and after the training period. The main findings were that the isometric torque gains of EMS were greater than those of VOL after the training period (P < 0.01) and that the eccentric and concentric torque gains were equivalent. In both groups, we observed that the mechanical twitch (Pt) was increased (P < 0.05) and that torque improvements were not mediated by neural adaptations. Considering the respective intensities of the training programs (i.e., submaximal contractions for EMS versus maximal for VOL), it can be concluded that electromyostimulation training would be more efficient than voluntary isometric training to improve both isometric and dynamic strength.     

Muscle soreness and intramuscular fluid pressure: comparison between eccentric and concentric load

This study investigates the dynamic and resting intramuscular pressures associated with eccentric and concentric exercise of muscles in a low-compliance compartment. The left and righ leg anterior compartments of eight healthy males (ages 22-32 yr) were exercised by either concentric or eccentric contractions of the same load (400 submaximal contractions at constant rate, 20/min for 20 min at a load corresponding to 15% of individual maximal dorsiflexion torque). Tissue fluid pressures were measured with the slit-catheter technique before, during, and after the exercise. Average peak intramuscular pressure generated during eccentric exercise (236 mmHg) was significantly greater than during concentric exercise (157 mmHg, P less than 0.001). Peak isometric contraction pressure in the eccentrically exercised compartment was significantly higher both within 20 min postexercise and on the second postexercise day (P less than 0.001). Resting pressure 2 days postexercise was significantly higher on the eccentrically exercised side (10.5 mmHg) compared with the concentrically exercised (4.4 mmHg, P less than 0.05). The ability to sustain tension during postexercise isometric contractions was impaired on the "eccentric" side. Soreness was exclusively experienced in the eccentrically exercised muscles. We conclude that eccentric exercise causes significant intramuscular pressure elevation in the anterior compartment, not seen following concentric exercise, and that this may be one of the factors associated with development of delayed muscle soreness in a tight compartment.     

Reduced susceptibility to eccentric exercise-induced muscle damage in resistance-trained men is not linked to resistance training-related neural adaptations

The purpose of this study was to examine the acute effects of maximal concentric vs. eccentric exercise on the isometric strength of the elbow flexor, as well as the biceps brachii muscle electromyographic (EMG) responses in resistance-trained (RT) vs. untrained (UT) men. Thirteen RT men (age: 24 ± 4 years; height: 180.2 ± 7.7 cm; body weight: 92.2 ± 16.9 kg) and twelve UT men (age: 23 ± 4 years; height: 179.2 ± 5.0 cm; body weight: 81.5 ± 8.6 kg) performed six sets of ten maximal concentric isokinetic (CON) or eccentric isokinetic (ECC) elbow flexion exercise in two separate visits. Before and after the exercise interventions, maximal voluntary contractions (MVCs) were performed for testing isometric strength. In addition, bipolar surface EMG signals were detected from the biceps brachii muscle during the strength testing. Both CON and ECC caused isometric strength to decrease, regardless of the training status. However, ECC caused greater isometric strength decline than CON did for the UT group (p = 0.006), but not for the RT group. Both EMG amplitude and mean frequency significantly decreased and increased, respectively, regardless of the training status and exercise intervention. Resistance-trained men are less susceptible to eccentric exercise-induced muscle damage, but this advantage is not likely linked to the chronic resistance training-induced neural adaptations.     

Neuromuscular adjustments that constrain submaximal EMG amplitude at task failure of sustained isometric contractions

The amplitude of the surface EMG does not reach the level achieved during a maximal voluntary contraction force at the end of a sustained, submaximal contraction, despite near-maximal levels of voluntary effort. The depression of EMG amplitude may be explained by several neural and muscular adjustments during fatiguing contractions, including decreased net neural drive to the muscle, changes in the shape of the motor unit action potentials, and EMG amplitude cancellation. The changes in these parameters for the entire motor unit pool, however, cannot be measured experimentally. The present study used a computational model to simulate the adjustments during sustained isometric contractions and thereby determine the relative importance of these factors in explaining the submaximal levels of EMG amplitude at task failure. The simulation results indicated that the amount of amplitude cancellation in the simulated EMG (～ 40%) exhibited a negligible change during the fatiguing contractions. Instead, the main determinant of the submaximal EMG amplitude at task failure was a decrease in muscle activation (number of muscle fiber action potentials), due to a reduction in the net synaptic input to motor neurons, with a lesser contribution from changes in the shape of the motor unit action potentials. Despite the association between the submaximal EMG amplitude and reduced muscle activation, the deficit in EMG amplitude at task failure was not consistently associated with the decrease in neural drive (number of motor unit action potentials) to the muscle. This indicates that the EMG amplitude cannot be used as an index of neural drive.     

Validation of a portable EMG device to assess muscle activity during free-living situations

Portable amplifiers that record electromyograms (EMGs) for longer than four hours are commonly priced over $20,000 USD. This cost, and the technical challenges associated with recording EMGs during free-living situations, typically restrict EMG use to laboratory settings. A low-cost system (μEMG; OT Bioelecttronica, 100€), using specialized concentric bipolar electrodes, has been developed specifically for free-living situations. The purpose of this study was to validate the μEMG system by comparing EMGs from μEMG with a laboratory-based alternative (Telemyo 900; Noraxon USA, Inc.). Surface EMGs from biceps brachii (BB) and tibialis anterior (TA) of ten subjects were recorded simultaneously with both systems as subjects performed maximal voluntary contractions (MVCs), submaximal contractions at 25%, 50%, and 75% MVC, seven simulated activities of daily living (ADLs), and >60 min of simulated free-living inside the laboratory. In general, EMG parameters (e.g., average full-wave rectified EMG amplitude) derived from both systems were not significantly different for all outcome variables, except there were small differences across systems in baseline noise and absolute EMG amplitudes during MVCs. These results suggest that μEMG is a valid approach to the long-term recording of EMG.