Comparison of a piezoelectric contact sensor and an accelerometer for examining mechanomyographic amplitude and mean power frequency versus torque relationships during isokinetic and isometric muscle actions of the biceps brachii.

The purpose of this study was to compare a piezoelectric contact sensor with an accelerometer for measuring the mechanomyographic (MMG) signal from the biceps brachii during submaximal to maximal isokinetic and isometric forearm flexion muscle actions. Following determination of isokinetic peak torque (PT) and the isometric maximum voluntary contraction (MVC), 10 adults (mean+/-SD age=22.8+/-2.7yrs) performed randomly ordered, submaximal step muscle actions of the dominant forearm flexors in 20% increments from 20% to 80% PT and MVC. Surface MMG signals were recorded simultaneously from a contact sensor and an accelerometer placed over the belly of the biceps brachii muscle. During the isokinetic and isometric muscle actions, the contact sensor and accelerometer resulted in linear increases in normalized MMG amplitude with torque (r(2) range=0.84-0.97) but the linear slope of the normalized MMG amplitude versus isokinetic torque relationship for the accelerometer was less (p<0.10) than that of the contact sensor. There was no significant (p>0.05) relationship for normalized MMG mean power frequency (MPF, %max) versus isokinetic and isometric torque for the contact sensor, but the accelerometer demonstrated a quadratic (R(2)=0.94) or linear (r(2)=0.83) relationship for the isokinetic and isometric muscle actions, respectively. There were also a number of significant (p<0.05) mean differences between the contact sensor and accelerometer for normalized MMG amplitude or MPF values. These findings indicated that in some cases involving dynamic and isometric muscle actions, the contact sensor and accelerometer resulted in different torque-related responses that may affect the interpretation of the motor control strategies involved.     

Mechanomyographic amplitude and mean power frequency versus torque relationships during isokinetic and isometric muscle actions of the biceps brachii.

The purpose of this investigation was to determine the mechanomyographic (MMG) amplitude and mean power frequency (MPF) versus torque (or force) relationships during isokinetic and isometric muscle actions of the biceps brachii. Ten adults (mean +/- SD age = 21.6 +/- 1.7 years) performed submaximal to maximal isokinetic and isometric muscle actions of the dominant forearm flexors. Following determination of isokinetic peak torque (PT) and the isometric maximum voluntary contraction (MVC), the subjects randomly performed submaximal step muscle actions in 10% increments from 10% to 90% PT and MVC. Polynomial regression analyses indicated that MMG amplitude increased linearly with torque during both the isokinetic (r2 = 0.982) and isometric (r2 = 0.956) muscle actions. From 80% to 100% of isometric MVC, however, MMG amplitude appeared to plateau. Cubic models provided the best fit for the MMG MPF versus isokinetic (R2 = 0.786) and isometric (R2 = 0.940) torque relationships, although no significant increase in MMG MPF was found from 10% to 100% of isokinetic PT. For the isometric muscle actions, however, MMG MPF remained relatively stable from 10% to 50% MVC, increased from 50% to 80% MVC, and decreased from 80% to 100% MVC. The results demonstrated differences in the MMG amplitude and MPF versus torque relationships between the isokinetic and isometric muscle actions. These findings suggested that the time and frequency domains of the MMG signal may be useful for describing the unique motor control strategies that modulate dynamic versus isometric torque production.     

Mechanomyographic and electromyographic responses of the vastus medialis muscle during isometric and concentric muscle actions

The purpose of this study was to examine the patterns for the mechanomyographic (MMG) and electromyographic (EMG) amplitude and mean power frequency (MPF) vs. torque relationships during submaximal to maximal isometric and isokinetic muscle actions. Seven men (mean +/- SD age, 22.4 +/- 1.3 years) volunteered to perform isometric and concentric isokinetic leg extension muscle actions at 20, 40, 60, 80, and 100% of maximal voluntary contraction (MVC) and peak torque (PT) on a Cybex II dynamometer. A piezoelectric MMG recording sensor was placed between bipolar surface EMG electrodes on the vastus medialis. Polynomial regression and separate 1-way repeated-measures analysis of variance were used to analyze the EMG amplitude, MMG amplitude, EMG MPF, and MMG MPF data for the isometric and isokinetic muscle actions. For the isometric muscle actions, EMG amplitude (R(2) = 0.999) and MMG MPF (R(2) = 0.946) increased to MVC, mean MMG amplitude increased to 60% MVC and then plateaued, and mean EMG MPF did not change (p > 0.05) across torque levels. For the isokinetic muscle actions, EMG amplitude (R(2) = 0.988) and MMG amplitude (R(2) = 0.933) increased to PT, but there were no significant mean changes with torque for EMG MPF or MMG MPF. The different torque-related responses for EMG and MMG amplitude and MPF may reflect differences in the motor control strategies that modulate torque production for isometric vs. dynamic muscle actions. These results support the findings of others and suggest that isometric torque production was modulated by a combination of recruitment and firing rate, whereas dynamic torque production was modulated primarily through recruitment.     

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

The purpose of this study was to examine the influence of interelectrode distance (IED) over the estimated innervation zone (IZ) for the vastus lateralis muscle and normalization on the torque-related patterns of responses for electromyographic (EMG) amplitude and mean power frequency (MPF) during concentric isokinetic, eccentric isokinetic, and isometric muscle actions of the leg extensors. Eight men performed submaximal to maximal concentric isokinetic, eccentric isokinetic, and isometric muscle actions of the dominant leg extensors. Surface EMG signals were recorded simultaneously with two bipolar electrode arrangements in single differential configuration (20 and 40 mm IEDs) placed over the estimated IZ for the vastus lateralis muscle and a third electrode arrangement in single differential configuration (20 mm IED) placed distal to the estimated IZ. The results indicated that there were only a few (six of 90 statistical comparisons) significant (p < 0.05) mean differences among the three electrode arrangements for absolute EMG amplitude. There were no mean differences among the three electrode arrangements for absolute or normalized EMG MPF values or normalized EMG amplitude for the three types of muscle actions. Thus, it may be possible to reduce the potential influence of the IZ on amplitude and spectral parameters of the EMG signal through normalization.     

Mechanomyographic and electromyographic responses during submaximal to maximal eccentric isokinetic muscle actions of the biceps brachii

The purpose of this investigation was to determine the mechanomyography (MMG) and electromyography (EMG) amplitude and mean power frequency (MPF) vs. eccentric isokinetic torque relationships for the biceps brachii muscle. Nine adults (mean +/- SD age = 23.1 +/- 2.9 years) performed submaximal to maximal eccentric isokinetic muscle actions of the dominant forearm flexors. After determination of isokinetic peak torque (PT), the subjects randomly performed submaximal step muscle actions in 10% increments from 10 to 90% PT. Polynomial regression analyses indicated that the MMG amplitude vs. eccentric isokinetic torque relationship was best fit with a quadratic model (R(2) = 0.951), where MMG amplitude increased from 10 to 60% PT and then plateaued from 60 to 100% PT. There were linear increases in MMG MPF (r(2) = 0.751) and EMG amplitude (r(2) = 0.988) with increases in eccentric isokinetic torque, but there was no significant change in EMG MPF from 10 to 100% PT. The results suggested that for the biceps brachii, eccentric isokinetic torque was increased to approximately 60% PT through concurrent modulation of the number of active motor units and their firing rates, whereas additional torque above 60% PT was produced only by increases in firing rates. These findings contribute to current knowledge of motor-control strategies during eccentric isokinetic muscle actions and could be useful in the design of training programs.     

The effect of glycerol on torque, electromyography, and mechanomyography

The purpose of this investigation was to determine the effect of hyperhydration on the electromyographic (EMG) and mechanomyographic (MMG) responses during isometric and isokinetic muscle actions of the biceps brachii. Eight (22.1 +/- 1.8 years, 79.5 +/- 22.8 kg) subjects were tested for maximal isometric, submaximal isometric, and maximal concentric isokinetic muscle strength in either a control (C) or hyperhydrated (H) state induced by glycerol ingestion while the EMG and MMG signals were recorded. Although fluid retention was significantly greater during the H protocol, the analyses indicated no change in torque, EMG amplitude, EMG mean power frequency (MPF), MMG amplitude, or MMG MPF with hyperhydration. These results indicated that glycerol-induced fluid retention does not affect the torque-producing capabilities of a muscle, the impulses (EMG) going to a muscle, or muscular vibrations (MMG). It has been suggested that EMG and MMG can be used as direct electrical/mechanical monitoring, which could be presented to trainers and athletes; however, before determining the utility of these signals, the MMG and EMG responses should be examined under a variety of conditions such as in the present study.     

Gender comparisons of mechanomyographic amplitude and mean power frequency versus isometric torque relationships

This study compared the patterns of mechanomyographic (MMG) amplitude and mean power frequency vs. torque relationships in men and women during isometric muscle actions of the biceps brachii. Seven men (mean age 23.9 +/- 3.5 yrs) and 8 women (mean 21.0 +/- 1.3 yrs) performed submaximal to maximal isometric muscle actions of the dominant forearm flexors. Following determination of the isometric maximum voluntary contraction (MVC), they randomly performed submaximal step muscle actions in 10% increments from 10% to 90% MVC. Polynomial regression analyses indicated that the MMG amplitude vs. isometric torque relationship for the men was best fit with a cubic model (R(2) = 0.983),,where MMG amplitude increased slightly from 10% to 20% MVC, increased rapidly from 20% to 80% MVC, and plateaued from 80% to 100% MVC. For the women, MMG amplitude increased linearly (r(2) = 0.949) from 10% to 100% MVC. Linear models also provided the best fit for the MMG mean power frequency vs. isometric torque relationship in both the men (r(2) = 0.813) and women (r(2) = 0.578). The results demonstrated gender differences in the MMG amplitude vs. isometric torque relationship, but similar torque-related patterns for MMG mean power frequency. These findings suggested that the plateau in MMG amplitude at high levels of isometric torque production for the biceps brachii in the men, but not the women, may have been due to greater isometric torque, muscle stiffness, and/or intramuscular fluid pressure in the men, rather than to differences in motor unit activation strategies for modulating isometric torque production.     

Electromyographic instantaneous amplitude and instantaneous mean power frequency patterns across a range of motion during a concentric isokinetic muscle action of the biceps brachii.

The purpose of this study was to examine the electromyographic (EMG) instantaneous amplitude (IA) and instantaneous mean power frequency (IMPF) patterns for the biceps brachii muscle across a range of motion during maximal and submaximal concentric isokinetic muscle actions of the forearm flexors. Ten adults (mean +/- SD age = 22.0 +/- 3.4 years) performed a maximal and a submaximal [20% peak torque (PT)] concentric isokinetic forearm flexion muscle action at a velocity of 30 degrees s(-1). The surface EMG signal was detected from the biceps brachii muscle with a bipolar electrode arrangement, and the EMG IA and IMPF versus time relationships were examined for each subject using first- and second-order polynomial regression models. The results indicated that there were no consistent patterns between subjects for EMG IA or IMPF with increases in torque across the range of motion. Some of the potential nonphysiological factors that could influence the amplitude and/or frequency contents of the surface EMG signal during a dynamic muscle action include movement of the muscle fibers and innervation zone beneath the skin surface, as well as changes in muscle fiber length and the thickness of the tissue layer between the muscle and the recording electrodes. These factors may affect the EMG IA and IMPF patterns differently for each subject, thereby increasing the difficulty of drawing any general conclusions regarding the motor control strategies that increase torque across a range of motion.     

The influence of circadian rhythm during a sustained submaximal exercise and on recovery process.

The purpose of this study was to examine the time-of-day effects on muscle fatigue and recovery process following an isometric fatiguing contraction. Sixteen male subjects were tested at two times (06:00h and 18:00h) and were requested to perform a sustained submaximal contraction of the elbow flexors, consisting in maintaining 40% of their absolute strength as long as they could. Isometric maximal voluntary contractions (MVC) were performed before (Pre), immediately after (Post), and up to 10min after the endurance task. Endurance time, peak torque (PT) and electromyographic (EMG) activities of the biceps brachii and triceps brachii were recorded and analysed. Results showed that under Pre-test conditions, PT developed at 18:00h was higher than at 06:00h. No time-of-day effect appears for the endurance time and EMG activities during the test. No time-of-day effect was observed on either MVC or EMG recovery. From the results of this study, it seems that both muscle fatigue and recovery process are not time-of-day dependent. We conclude that circadian rhythm of the force do not influence the evaluation of muscle capacities during a submaximal exercise corresponding at 40% of MVC.     

The effects of electrode placement and innervation zone location on the electromyographic amplitude and mean power frequency versus isometric torque relationships for the vastus lateralis muscle.

The purposes of this investigation were to examine the effects of electrode placement and innervation zone (IZ) location on: (a) the torque-related patterns of responses for absolute and normalized electromyographic (EMG) amplitude and mean power frequency (MPF) and (b) the mean absolute and normalized EMG amplitude and MPF values. In addition, the present study examined the variability between subjects for the location of the IZ for the vastus lateralis (VL). Eight men (mean+/-SD age=23.0+/-4.3yr) performed submaximal to maximal isometric muscle actions of the dominant leg extensors. During each muscle action, fifteen channels of bipolar surface EMG signals were detected from the vastus lateralis using a linear electrode array aligned with the long axis of the muscle fibers. The results indicated that there were differences among channels 1-15 for the patterns of responses and mean values for absolute and normalized EMG amplitude and MPF versus isometric torque. Thus, normalized EMG amplitude and MPF values from different individuals cannot be compared if the EMG signals were detected from different locations over the muscle. In addition, absolute and relative (to femur length) estimates of IZ location for the VL resulted in similar inter-subject variability.     

Changes in the surface electromyogram during increasing isometric shoulder forward flexions

When using electromyographic techniques in the evaluation of muscular load it is necessary to determine the mathematical relationship between the torque and the amplitude of the electromyographic signal. Isometric gradually increasing contractions up to 100% MVC can then be used. Often more than linear increases for the amplitude (RMS)--force regression have been reported. The present study was designed to test whether changes in power spectral density function take place during a gradually increasing isometric contraction (duration 10 s). Twenty-two clinically healthy females performed an increasing isometric shoulder forward flexion for 10 s using an isokinetic dynamometer. Electromyographic activity was measured in trapezius, deltoid, infraspinatus and biceps brachii using surface electrodes. Mean torque values were determined together with mean power frequency (MPF) and root mean square values (RMS) from the EMG signals for each 256 ms period. The RMS-torque regressions showed higher regression coefficients during the 6th to 9th sec than during the first 5 s. No significant correlation existed between MPF for the four muscles and the torque. A gradual decrease in MPF was generally found from the 6th s. It is concluded that this decrease in power spectral density function might have contributed to the significantly higher regression coefficient for the RMS torque regression at the high output part of the gradually increasing isometric contraction.     

Variations in the relationship between the frequency content of EMG signals and the rate of torque development in voluntary and elicited contractions.

Our purpose was to characterize the relationship between EMG mean power frequency (MPF) or median frequency (MF) and rate of torque development in voluntary ballistic and electrically elicited isometric contractions. Twenty-three healthy adults participated in two sets of experiments performed on elbow flexor muscles. For Experiment 1, subjects were asked to generate voluntary ballistic contractions by reaching four different target torque levels (20, 40, 60 and 100% of the maximal voluntary contraction (MVC)) as fast as they could. For Experiment 2, electrical (M-waves) and mechanical (twitches) responses to electrical stimulation of the nerves supplying the biceps brachii and brachioradialis muscles were recorded with the subjects at rest and with a background isometric contraction of 15% MVC. MPF, MF and rate of torque development (% MVC/s) were calculated for both voluntary and elicited contractions. Significant positive correlations were observed between MPF and rate of torque development for the voluntary contractions, whereas significant negative correlations were observed between the two variables for elicited contractions. This suggests that factors other than muscle fiber composition influence the frequency content of EMG signals and/or the rate of torque development, and that the effect of these factors will vary between voluntary and elicited contractions.     

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.     

Neural contributions to concentric vs. eccentric exercise-induced strength loss

The purpose of this study was to examine the strength, electromyographic (EMG), and mechanomyographic (MMG) responses after workouts designed to elicit fatigue and muscle damage vs. only fatigue. Thirteen men (mean ± SD age = 23.7 ± 2.2 years) performed 6 sets of 10 maximal concentric isokinetic (CONexercise) or eccentric isokinetic (ECCexercise) muscle actions of the dominant forearm flexors on 2 separate days. Before (PRE) and after (POST) these workouts, peak torque (PT), surface EMG, and MMG signals were measured during maximal concentric isokinetic, eccentric isokinetic, and isometric muscle actions of the forearm flexors. The subjects also visited the laboratory for a control (CTL) visit with quiet resting between the PRE and POST measurements, rather than performing the CONexercise or ECCexercise. The results showed that there were significant 26 and 25% decreases in PT after the CONexercise and ECCexercise, respectively, and these decreases were statistically equivalent for the concentric, eccentric, and isometric muscle actions. There were also 19 and 23% reductions in normalized EMG amplitude after the CONexercise and ECCexercise, respectively, but no changes in EMG mean frequency (MNF), MMG amplitude, or MMG MNF. These findings demonstrated a neural component(s) to the strength decrement after CONexercise and ECCexercise. It is possible that after these 2 types of exercise, activation of free nerve endings that are sensitive to muscle damage and pH changes resulted in inhibition of alpha motor neurons, causing decreased muscle activation and torque. These findings suggest that training programs designed to minimize strength loss during competition should consider the fact that at least some of this loss is because of neural factors.     

Eccentric exercise increases EMG amplitude and force fluctuations during submaximal contractions of elbow flexor muscles.

The purpose of this study was to determine the effect of eccentric exercise on the ability to exert steady submaximal forces with muscles that cross the elbow joint. Eight subjects performed two tasks requiring isometric contraction of the right elbow flexors: a maximum voluntary contraction (MVC) and a constant-force task at four submaximal target forces (5, 20, 35, 50% MVC) while electromyography (EMG) was recorded from elbow flexor and extensor muscles. These tasks were performed before, after, and 24 h after a period of eccentric (fatigue and muscle damage) or concentric exercise (fatigue only). MVC force declined after eccentric exercise (45% decline) and remained depressed 24 h later (24%), whereas the reduced force after concentric exercise (22%) fully recovered the following day. EMG amplitude during the submaximal contractions increased in all elbow flexor muscles after eccentric exercise, with the greatest change in the biceps brachii at low forces (3-4 times larger at 5 and 20% MVC) and in the brachialis muscle at moderate forces (2 times larger at 35 and 50% MVC). Eccentric exercise resulted in a twofold increase in coactivation of the triceps brachii muscle during all submaximal contractions. Force fluctuations were larger after eccentric exercise, particularly at low forces (3-4 times larger at 5% MVC, 2 times larger at 50% MVC), with a twofold increase in physiological tremor at 8-12 Hz. These data indicate that eccentric exercise results in impaired motor control and altered neural drive to elbow flexor muscles, particularly at low forces, suggesting altered motor unit activation after eccentric exercise.     

The repeated-bout effect: influence on biceps brachii oxygenation and myoelectrical activity

This study investigated biceps brachii oxygenation and myoelectrical activity during and following maximal eccentric exercise to better understand the repeated-bout effect. Ten men performed two bouts of eccentric exercise (ECC1, ECC2), consisting of 10 sets of 6 maximal lengthening contractions of the elbow flexors separated by 4 wk. Tissue oxygenation index minimum amplitude (TOI(min)), mean and maximum total hemoglobin volume by near-infrared spectroscopy, torque, and surface electromyography root mean square (EMG(RMS)) during exercise were compared between ECC1 and ECC2. Changes in maximal voluntary isometric contraction (MVC) torque, range of motion, plasma creatine kinase activity, muscle soreness, TOI(min), and EMG(RMS) during sustained (10-s) and 30-repeated isometric contraction tasks at 30% (same absolute force) and 100% MVC (same relative force) for 4 days postexercise were compared between ECC1 and ECC2. No significant differences between ECC1 and ECC2 were evident for changes in torque, TOI(min), mean total hemoglobin volume, maximum total hemoglobin volume, and EMG(RMS) during exercise. Smaller (P < 0.05) changes and faster recovery of muscle damage markers were evident following ECC2 than ECC1. During 30% MVC tasks, TOI(min) did not change, but EMG(RMS) increased 1-4 days following ECC1 and ECC2. During 100% MVC tasks, EMG(RMS) did not change, but torque and TOI(min) decreased 1-4 days following ECC1 and ECC2. TOI(min) during 100% MVC tasks and EMG(RMS) during 30% MVC tasks recovered faster (P < 0.05) following ECC2 than ECC1. We conclude that the repeated-bout effect cannot be explained by altered muscle activation or metabolic/hemodynamic changes, and the faster recovery in muscle oxygenation and activation was mainly due to faster recovery of force.     

Fatigue and muscle activation during submaximal elbow flexion in children with cerebral palsy

The purpose of this study was to investigate whether children with cerebral palsy (CP), like typically developing peers, would compensate for muscle fatigue by recruiting additional motor units during a sustained low force contraction until task failure.Twelve children with CP and 17 typically developing peers performed one submaximal isometric elbow flexion contraction until the task could no longer be sustained at on average 25% (range 10–35%) of their maximal voluntary torque. Meanwhile surface electromyography (EMG) was measured from the biceps brachii and triceps brachii, and acceleration variations of the forearm were detected by an accelerometer. Slopes of the change in EMG amplitude and median frequency and accelerometer variation during time normalised to their initial values were calculated.Strength and time to task failure were similar in both groups. Children with CP exhibited a lower increase in EMG amplitude of the biceps brachii and triceps brachii during the course of the sustained elbow flexion task, while there were no significant group differences in median frequency decrease or acceleration variation increase. This indicates that children with CP do not compensate muscle fatigue with recruitment of additional motor units during sustained low force contractions.     

The mechanomyography of persons after stroke during isometric voluntary contractions.

This study was to investigate the properties of mechanomyography (MMG), or muscle sound, of the paretic muscle in the affected side of hemiplegic subjects after stroke during isometric voluntary contractions, in comparison with those from the muscle in the unaffected side of the hemiplegic subjects and from the healthy muscle of unimpaired subjects. MMG and electromyography (EMG) signals were recorded simultaneously from the biceps brachii muscles of the dominant arm of unimpaired subjects (n=5) and the unaffected and affected arms of subjects after stroke (n=8), when performing a fatiguing maximal voluntary contraction (MVC) associated with the decrease in elbow flexion torque, and then submaximal elbow flexions at 20%, 40%, 60% and 80% MVCs. The root mean squared (RMS) values, the mean power frequencies (MPF, in the power density spectrum, PDS) of the EMG and MMG, and the high frequency rate (HF-rate, the ratio of the power above 15Hz in the MMG PDS) were used for the analysis. The MMG RMS decreased more slowly during the MVC in the affected muscle compared to the healthy and unaffected muscles. A transient increase could be observed in the MMG MPFs from the unaffected and healthy muscles during the MVC, associated with the decrease in their simultaneous EMG MPFs due to the muscular fatigue. No significant variation could be seen in the EMG and MMG MPFs in the affected muscles during the MVC. The values in the MPF and HF-rate of MMG from the affected muscles were significantly lower than those from the healthy and unaffected muscles (P<0.05) at the high contraction level (80% MVC). Both the MMG and EMG RMS values in the healthy and unaffected groups were found to be significantly higher than the affected group (P<0.05) at 60% and 80% MVCs. These observations were related to an atrophy of the fast-twitch fibers and a reduction of the neural input in the affected muscles of the hemiplegic subjects. The results in this study suggested MMG could be used as a complementary to EMG for the analysis on muscular characteristics in subjects after stroke.     

Intramuscular and surface electromyogram changes during muscle fatigue

Twelve male subjects were tested to determine the effects of motor unit (MU) recruitment and firing frequency on the surface electromyogram (EMG) frequency power spectra during sustained maximal voluntary contraction (MVC) and 50% MVC of the biceps brachii muscle. Both the intramuscular MU spikes and surface EMG were recorded simultaneously and analyzed by means of a computer-aided intramuscular spike amplitude-frequency histogram and frequency power spectral analysis, respectively. Results indicated that both mean power frequency (MPF) and amplitude (rmsEMG) of the surface EMG fell significantly (P less than 0.001) together with a progressive reduction in MU spike amplitude and firing frequency during sustained MVC. During 50% MVC there was a significant decline in MPF (P less than 0.001), but this decline was accompanied by a significant increase in rmsEMG (P less than 0.001) and a progressive MU recruitment as evidenced by an increased number of MUs with relatively large spike amplitude. Our data suggest that the surface EMG amplitude could better represent the underlying MU activity during muscle fatigue and the frequency powers spectral shift may or may not reflect changes in MU recruitment and rate-coding patterns.     

Effects of two days of isokinetic training on strength and electromyographic amplitude in the agonist and antagonist muscles

The purpose of this study was to examine the effects of 2 days of isokinetic training of the forearm flexors and extensors on strength and electromyographic (EMG) amplitude for the agonist and antagonist muscles. Seventeen men (mean +/- SD age = 21.9 +/- 2.8 years) were randomly assigned to 1 of 2 groups: (a) a training group (TRN; n = 8), or (b) a control group (CTL; n = 9). The subjects in the TRN group were tested for maximal isometric and concentric isokinetic (randomly ordered velocities of 60, 180, and 300 degrees x s(-1)) torque of the dominant forearm flexors and extensors before (pretest) and after (posttest) 2 days of isokinetic strength training. Each training session involved 6 sets of 10 maximal concentric isokinetic muscle actions of the forearm flexors and extensors at a velocity of 180 degrees x s(-1). The subjects in the CTL group were also tested for strength but did not perform any training. Surface EMG signals were detected from the biceps brachii and triceps brachii muscles during the strength testing. The results indicated that there were no significant (p > 0.05) pre- to post-test changes in forearm flexion and extension torque or EMG amplitude for the agonist and antagonist muscles. Thus, unlike previous studies of the quadriceps femoris muscles, these findings for the forearm flexors and extensors suggested that 2 days of isokinetic training may not be sufficient to elicit significant increases in strength. These results may have implications for the number of visits that are required for rehabilitation after injury, surgery, or both.