Phonomyogram and electromyogram relationships with isometric force reinvestigated in man

Contracting muscle generates sounds which can be recorded easily by means of a microphone. To determine if a phomomyogram (PMG) can be used to monitor muscle force, a comparison was made between simultaneous recordings of PMG and monopolar electromyogram (EMG) from the isometrically contracting biceps brachii muscle and the external flexion force. Locations of the monopolar electrode and microphone were identified in relation to the motor point. Whatever the recording site, PMG amplitude was proportional to EMG amplitude and both showed a quadratic relationship to muscle force. Changes in the PMG spectrum with force were similar to those in EMG, i.e. the mean power frequency increased up to about 30% maximal voluntary contraction and then reached a plateau. Despite a slightly higher variability, PMG was shown to be a valid index of muscular isometric force. At the same force, the amplitude of both PMG and EMG was lower in the prone than in the supine position of the hand. This result indicated a selective recording of biceps brachii muscle activity.     

Effects of mechanical assistance on muscle activity and motor performance during isometric elbow flexion

Mechanical assistance on joint movement is generally beneficial; however, its effects on cooperative performance and muscle activity needs to be further explored. This study examined how motor performance and muscle activity are altered when mechanical assistance is provided during isometric force control of ramp-down and hold phases. Thirteen right-handed participants (age: 24.7 ± 1.8 years) performed trajectory tracking tasks. Participants were asked to maintain the reference magnitude of 47 N (REF) during isometric elbow flexion. The force was released to a step-down magnitude of either 75% REF or 50% REF and maintained, with and without mechanical assistance. The ramp-down durations of force release were set to 0.5, 2.5, or 5.0 s. Throughout the experiment, we measured the following: (1) the force output using load cells to compute force variability and overshoot ratio; (2) peak perturbation on the elbow movement using an accelerometer; (3) the surface electromyography (sEMG) from biceps brachii and triceps brachii muscles; and (4) EMG oscillation from the biceps brachii muscle in the bandwidth of 15–45 Hz. Our results indicated that mechanical assistance, which involved greater peak perturbation, demonstrated lower force variability than non-assistance (p < 0.01), while EMG oscillation in the biceps brachii muscle from 15 to 45 Hz was increased (p < 0.05). These findings imply that if assistive force is provided during isometric force control, the central nervous system actively tries to stabilize motor performance by controlling specific motor unit activity in the agonist muscle.     

Force generation performance and motor unit recruitment strategy in muscles of contralateral limbs.

The purpose of the present study was to determine whether the motor unit (MU) recruitment strategy of the agonist and antagonist muscles in the dominant arm differs from that in the non-dominant arm. The median frequency (MF) of the power density spectrum (PDS) of the electromyogram (EMG) was used as a tracking parameter to describe the MU recruitment. In 8 subjects the EMG was recorded from the biceps brachii and triceps brachii of each limb during isometric elbow flexion performed in a ramp fashion. Force was increased from 0 to 100% of the maximum voluntary contraction (MVC) in 3 s following a track displayed on an oscilloscope. When comparing the dominant versus non-dominant arm we found no statistical difference in the MU recruitment pattern of the biceps brachii and the triceps. Because the dominant arm was not always the better performing arm, we grouped the data according to the ability of the subjects to track the ramp signal. In this case we found a statistically significant difference between the better and worse performing arm in the full MU recruitment of the biceps. A more precise and accurate control of the increase in force was obtained when the central nervous system selected a slower and prolonged recruitment of MUs in the agonist muscle.     

EMG and MMG of agonist and antagonist muscles as a function of age and joint angle.

The aim of this study was to determine the effect of elbow joint position on electromyographic (EMG) and mechanomyographic (MMG) activities of agonist and antagonist muscles in young and old women. Surface EMG and MMG were recorded from the triceps and biceps brachii, and brachioradialis muscles during isometric elbow extensions in young and old women. The measurements were carried out at an optimal joint angle (A(o)), as well as at smaller (A(s) = A(o) - 30 degrees ) and larger (A(l) = A(o) + 30 degrees ) angles. The normalized to force EMG amplitude (RMS-EMG/F) was smaller in old women compared to young in all muscles. The RMS-EMG/F of the triceps brachii muscle was not affected by muscle length while that of the biceps brachii and brachioradialis muscles increased at shortest muscle length in both groups. The normalized to force MMG amplitude (RMS-MMG/F) was smaller in old than in young in the triceps brachii muscle only. There was an increase in RMS-MMG/F with triceps brachii and biceps brachii muscle shortening in both groups, and in the brachioradialis muscle -- in young only. Compared to young, older women exhibited a bigger force fluctuation during maximum voluntary contraction, but these did not contribute significantly to the RMS-MMG. Skinfold thickness accounted for the RMS-EMG/F and RMS-MMG/F differences seen between old and young women in the biceps brachii muscle only. It is concluded that, the EMG and MMG response to muscles length change in agonist and antagonist muscles is generally similar in old and young women but the optimal angle shifts toward a bigger value in older women.     

Facilitation of triceps brachii muscle contraction by tendon vibration after chronic cervical spinal cord injury.

One way to improve the weak triceps brachii voluntary forces of people with chronic cervical spinal cord injury may be to excite the paralyzed or submaximally activated fraction of muscle. Here we examined whether elbow extensor force was enhanced by vibration (80 Hz) of the triceps or biceps brachii tendons at rest and during maximum isometric voluntary contractions (MVCs) of the elbow extensors performed by spinal cord-injured subjects. The mean +/- SE elbow extensor MVC force was 22 +/- 17.5 N (range: 0-23% control force, n = 11 muscles). Supramaximal radial nerve stimuli delivered during elbow extensor MVCs evoked force in six muscles that could be stimulated selectively, suggesting potential for force improvement. Biceps vibration at rest always evoked a tonic vibration reflex in biceps, but extension force did not improve with biceps vibration during triceps MVCs. Triceps vibration induced a tonic vibration reflex at rest in one-half of the triceps muscles tested. Elbow extensor MVC force (when >1% of control force) was enhanced by vibration of the triceps tendon in one-half of the muscles. Thus triceps, but not biceps, brachii tendon vibration increases the contraction strength of some partially paralyzed triceps brachii muscles.     

Does quantitative EMG differ myotonic dystrophy type 2 and type 1?

Genetic testing is considered the only reliable diagnostic approach in myotonic dystrophy. However it has recently been reported that a considerable number of patients with genetically proven types of the disease have unusual phenotypic presentation. The aim of our study was to evaluate motor unit reorganization reflected by various electrophysiological abnormalities in myotonic dystrophies and to compare findings between type 1 (DM 1) and type 2 myotonic dystrophy (DM2). Quantitative electromyography (EMG) recordings in 63 patients (33 with DM1 and 30 with DM2) from the biceps brachii (BB), rectus femoris (RF), first dorsal interosseus (FDI), and tibialis anterior (TA) muscles were analyzed. Mean amplitude and size index (SI) of motor unit potentials recorded in TA and RF muscles, mean potential duration in TA, and mean SI and the number of outliers with amplitude above the normal range in BB were significantly increased in DM2 as compared to DM1. Myotonic discharges were recorded more frequently in DM1 than in DM2. EMG findings significantly differ between DM1 and DM2. The presence of high amplitude potentials in lower limb muscles in DM2 patients, atypical for myogenic muscle lesions, could be explained by muscle fiber hypertrophy observed in muscle biopsies.     

M-wave properties during progressive motor unit activation by transcutaneous stimulation.

The aim of this study was to interpret changes in experimentally recorded M waves with progressive motor unit (MU) activation based on simulation of the surface electromyogram. Activation order during transcutaneous electrical stimulation was analyzed by investigating M-wave average rectified value, spectral properties, and conduction velocity (CV) during electrically elicited contractions. M-waves were detected from the biceps brachii muscle of 10 healthy male subjects by a linear adhesive array of eight electrodes. Electrical stimulation was delivered to the motor point at either constant current intensity (40, 60, 80, and 100% of the supramaximal stimulation current) or with linearly increasing current. A model of surface electromyogram generation that varied activation order based on MU size and location was used to interpret the experimental results. From the experimental and model analysis, it was found that 1) MUs tended to be activated from low to high CV and from the superficial to the deep muscle layers with increasing transcutaneous electrical stimulation of the biceps brachii muscle, and 2) characteristic spectral frequencies of the M-wave were affected by many factors other than average CV (such as the activation order by MU location or the spread of the MU innervation zones and CVs), thus decreasing with a concomitant increase in CV during progressive MU activation.     

Inhomogeneities in muscle activation reveal motor unit recruitment.

This paper presents a novel method to quantify spatial changes in muscle activation pattern by multi-channel surface electromyography (MCSEMG) in order to investigate motor unit recruitment variation. The method is based on non-uniform distributions of motor units that cause spatial inhomogeneous muscle activation. To evaluate the method, 15 subjects performed three isometric elbow flexion contractions consisting of slow sinusoidal changes in force ranging from 0% to 80% of the maximal voluntary contraction. MCSEMG electrodes were placed in a 10 x 13 grid over the biceps brachii muscle. From all channels, root mean square (RMS) values of bipolar leadings were computed over 0.5 s epochs over the whole recording. Thereafter, correlation coefficients were calculated between the RMS values at one epoch, with the RMS values at another epoch. Results showed consistent spatial changes in the distribution of RMS at different contraction levels up to 80% of maximal voluntary contraction and when comparing increasing and decreasing contractions at the same force level. These findings are reproducible within and between subjects, and in agreement with physiological phenomena and therefore indicate that the spatial inhomogeneities of motor unit properties in the biceps brachii muscle can be used to study changes in motor unit recruitment.     

Changes in motor unit discharge rate are not associated with the amount of twitch potentiation in old men.

This study examined, in nine old men (82 +/- 4 yr), whether there is an association between the magnitude of change in motor unit discharge rate and the amount of twitch potentiation after a conditioning contraction (CC). The evoked twitch force and motor unit discharge rate during isometric ramp-and-hold contractions (10-18 s) of the triceps brachii muscle at 10, 20, and 30% of the maximal voluntary contraction were determined before and 10 s, 2 min, 6 min, and 11 min after a 5-s CC at 75% maximal voluntary contraction. After the CC, there was a potentiation of twitch force (approximately twofold), and the discharge rate of the 47 sampled motor units declined (P < 0.05) an average of 1 Hz 10 s after the CC, compared with the control condition. The CC had no effect on the variability (coefficient of variation) of both force and discharge rate, as well as the electromyographic activity recorded over the triceps brachii and biceps brachii muscles. In contrast to our earlier study of young men (Klein CS, Ivanova TD, Rice CL, and Garland SJ, Neurosci Lett 316: 153-156, 2001), the magnitude of the reduction in discharge rate after the CC was not associated (r = 0.06) with the amount of twitch potentiation. These findings suggest an age-related alteration in the neural strategy for adjusting motor output to a muscle after a CC.     

Regenerative potential of human skeletal muscle during aging

In this study, we have investigated the consequences of aging on the regenerative capacity of human skeletal muscle by evaluating two parameters: (i) variation in telomere length which was used to evaluate the in vivo turn-over and (ii) the proportion of satellite cells calculated as compared to the total number of nuclei in a muscle fibre. Two skeletal muscles which have different types of innervation were analysed: the biceps brachii, a limb muscle, and the masseter, a masticatory muscle. The biopsies were obtained from two groups: young adults (23 +/- 1.15 years old) and aged adults (74 +/- 4.25 years old). Our results showed that during adult life, minimum telomere lengths and mean telomere lengths remained stable in the two muscles. The mean number of myonuclei per fibre was lower in the biceps brachii than in the masseter but no significant change was observed in either muscle with increasing age. However, the number of satellite cells, expressed as a proportion of myonuclei, decreased with age in both muscles. Therefore, normal aging of skeletal muscle in vivo is reflected by the number of satellite cells available for regeneration, but not by the mean number of myonuclei per fibre or by telomere lengths. We conclude that a decrease in regenerative capacity with age may be partially explained by a reduced availability of satellite cells.     

Impaired neuromuscular function during isometric, shortening, and lengthening contractions after exercise-induced damage to elbow flexor muscles

The purpose of this study was to examine the effect of exercise-induced damage of the elbow flexor muscles on steady motor performance during isometric, shortening, and lengthening contractions. Ten healthy individuals (age 22+/-4 yr) performed four tasks with the elbow flexor muscles: a maximum voluntary contraction, a one repetition maximum (1 RM), an isometric task at three joint angles (short, intermediate, and long muscle lengths), and a constant-load task during slow (approximately 7 degrees/s) shortening and lengthening contractions. Task performance was quantified as the fluctuations in wrist acceleration (steadiness), and electromyography was obtained from the biceps and triceps brachii muscles at loads of 10, 20, and 40% of 1 RM. Tasks were performed before, immediately after, and 24 h after eccentric exercise that resulted in indicators of muscle damage. Maximum voluntary contraction force and 1-RM load declined by approximately 45% immediately after exercise and remained lower at 24 h ( approximately 30% decrease). Eccentric exercise resulted in reduced steadiness and increased biceps and triceps brachii electromyography for all tasks. For the isometric task, steadiness was impaired at the short compared with the long muscle length immediately after exercise (P<0.01). Furthermore, despite no differences before exercise, there was reduced steadiness for the shortening compared with the lengthening contractions after exercise (P=0.01), and steadiness remained impaired for shortening contractions 24 h later (P=0.01). These findings suggest that there are profound effects for the performance of these types of fine motor tasks when recovering from a bout of eccentric exercise.     

Electromyography of pronators and supinators in great apes.

We obtained electromyographic recordings from the supinator, biceps brachii, pronator quadratus, and pronator teres muscles of a chimpanzee and a gorilla and from the supinator, pronator quadratus, and biceps brachii muscles of an orangutan as they stood and walked quadrupedally on horizontal and inclined surfaces, engaged in suspensory behavior, reached overhead, and manipulated a variety of foods and artifacts. In Pan troglodytes and Pan gorilla, as in Homo sapiens, the supinator muscle is the prime supinator, with the biceps brachii muscle serving to augment speed or force of supination. Primary of the pronator quadratus muscle over the pronator teres muscle during pronation is less clear in the African apes than in humans. Possibly, pongid radial curvature or forelimb elongation or both factors are related to the somewhat different patterns of activity that we observed in the pronator muscles of Pan versus those reported for Homo sapiens. In Pongo pygmaeus, as in P. troglodytes and P. gorilla, the pronator quadratus muscle acts as a pronator and the supinator muscle acts to supinate the hand at the radioulnar joints. The biceps brachii muscle is active at low levels as the orangutan supinates its hand with the elbow flexed.     

A Wavelet-Based Method to Predict Muscle Forces From Surface Electromyography Signals in Weightlifting

The purpose of this study was to develop a wavelet-based method to predict muscle forces from surface electromyography (EMG) signals in vivo.The weightlifting motor task was implemented as the case study.EMG signals of biceps brachii,triceps brachii and deltoid muscles were recorded when the subject carried out a standard weightlifting motor task.The wavelet-based algorithm was used to process raw EMG signals and extract features which could be input to the Hill-type muscle force models to predict muscle forces.At the same time,the musculoskeletal model of subject's weightlifting motor task was built and simulated using the Computed Muscle Control (CMC) method via a motion capture experiment.The results of CMC were compared with the muscle force predictions by the proposed method.The correlation coefficient between two results was 0.99(p＜0.01).However,the proposed method was easier and more efficiency than the CMC method.It has potential to be used clinically to predict muscle forces in vivo.     

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.     

Less is more: high pass filtering, to remove up to 99% of the surface EMG signal power, improves EMG-based biceps brachii muscle force estimates.

It is generally assumed that raw surface EMG (sEMG) should be high pass filtered with cutoffs of 10-30 Hz to remove motion artifact before subsequent processing to estimate muscle force. The purpose of the current study was to explore the benefits of filtering out much of the raw sEMG signal when attempting to estimate accurate muscle forces. Twenty-five subjects were studied as they performed rapid static, anisotonic contractions of the biceps brachii. Biceps force was estimated (as a percentage of maximum) based on forces recorded at the wrist. An iterative approach was used to process the sEMG from the biceps brachii, using progressively greater high pass cutoff frequencies (20-440 Hz in steps of 30 Hz) with first and sixth order filters, as well as signal whitening, to determine the effects on the accuracy of EMG-based biceps force estimates. The results indicate that removing up to 99% of the raw sEMG signal power resulted in significant and substantial improvements in biceps force estimates. These findings challenge previous assumptions that the raw sEMG signal power between about 20 and 500 Hz should used when estimating muscle force. For the purposes of force prediction, it appears that a much smaller, high band of sEMG frequencies may be associated with force and the remainder of the spectrum has little relevance for force estimation.     

Muscle-specific enzyme activity patterns of the capillary bed of human oro-facial,masticatory and limb muscles

Enzyme-histochemical methods were used to analyse the activities of alkaline phosphatase (AP), dipeptidylpeptidase IV (DPP IV) and adenosine triphosphatase (ATPase) in capillaries of four different human oro-facial muscles, the major and minor zygomatic, the orbicularis oris and buccinator, one masticatory, the masseter and two limb muscles, the biceps brachii and first dorsal interosseus muscles. In all muscles, except for the orbicularis oris, the majority of the capillaries lacked enzyme activity. Therefore, none of these enzymes seems to be reliable as a general marker for human muscle capillaries. In general, the capillaries of the limb muscles and the major and minor zygomatic and the buccinator, were similar in their staining pattern for AP and ATPase, but differed in DPP IV staining. The orbicularis oris muscle differed from the other muscles by showing the largest proportion of capillaries with AP and ATPase activity. The masseter muscle had the largest proportion of capillaries stained for DPP IV. The muscle specific differences in enzyme activity of the capillaries are in agreement with our previous findings of specific differences between limb, oro-facial and masticatory muscles with respect to capillary supply and composition of fibre types and myosins. The results reflect functional specialization of the capillary bed of human muscles.     

Firing properties of motor units during fatigue in subjects after stroke.

The purpose of this work was to investigate the electromyographic (EMG) fatigue representations in muscles of subjects after stroke at the level of motor unit, based on the analysis of mean power frequency (MPF) in the power density spectrum (PDS) for intramuscular EMG and our previous modeling and experiment studies on the neuromuscular transmission failure (NTF). NTF due to the local muscular fatigue had been captured in motor unit signals from healthy subjects during a submaximal fatigue contraction previously. In this study, the EMG signals for the biceps brachii muscles were collected by needle electrodes from the affected and unaffected arms of six hemiplegic subjects after stroke, and from the dominated arm of six healthy subjects during a full maximum voluntary contraction (MVC) and a subsequent 20% MVC. The MPF of EMG trials detected intramuscularly during the full and 20% MVCs, and the parameters of motor unit action potential trains (MUAPTs) during 20% MVC were analyzed in three groups: the normal (from healthy subjects), unaffected (from subjects after stroke), and affected (from subjects after stroke). It was found that during the full MVC the MPFs of the normal and unaffected groups decreased more than the affected when monitored by a moving time window of 2 s. The comparison on the overall MPF during the full MVC for these three groups over the whole time course of the EMG signal (18 s) were: the affected overall MPF was higher than the unaffected (P < 0.05); and the unaffected overall MPF was larger than the normal (P < 0.05). However, no significant decrease in MPF was found for these three groups during 20% MVC. The NTF was captured in most MUAPTs in the groups of the normal and unaffected rather than in the affected group, symbolized by the lowered rates of change (RCs) of firing rate (FR) (P < 0.05), more MUAPTs with positive RCs of maximum oscillation (MO) in MUAPT power density spectra (P < 0.05), and the significant higher RCs of minimum inter-pulse interval (MINI) (P < 0.05) in the normal and unaffected compared to the affected group. Enhanced neural drives to the motor units of the unaffected and affected groups were observed during 20% MVC, which possibly came from the bilateral neural inputs due to the disinhibition of the ipsilateral projections in subjects after stroke. For identifying the fatigue associated with NTF, the motor unit firing parameters, FR, MINI, and MO, were more sensitive than the MPF. The results obtained in this work provided a further understanding on the EMG of the fatigue processes in paretic and non-paretic muscles during voluntary contractions.     

The reliability of histochemical fibre typing of human necropsy muscles

The reliability of muscle fibre typing of post mortem specimens was investigated with special reference to the influence of time and temperature. In specimens stored at +4 degrees C, muscle fibre typing could be reliably performed up to at least ten and fifteen days post mortem for the masseter and biceps brachii muscles respectively. The corresponding figures for storage at room temperature were three and six days. The difference in the preservation of enzyme activity between masticatory and limb muscles might be related to the demonstrated difference in the fibre type composition and thus the enzyme content and energy sources.     

The reliability of histochemical fibre typing of human necropsy muscles

Summary The reliability of muscle fibre typing of post mortem specimens was investigated with special reference to the influence of time and temperature. In specimens stored at +4° C, muscle fibre typing could be reliably performed up to at least ten and fifteen days post mortem for the masseter and biceps brachii muscles respectively. The corresponding figures for storage at room temperature were three and six days. The difference in the preservation of enzyme activity between masticatory and limb muscles might be related to the demonstrated difference in the fibre type composition and thus the enzyme content and energy sources.     

Surface EMG-force modelling for the biceps brachii and its experimental evaluation during isometric isotonic contractions

The aim of this study was to evaluate a surface electromyography (sEMG) signal and force model for the biceps brachii muscle during isotonic isometric contractions for an experimental set-up as well as for a simulation. The proposed model includes a new rate coding scheme and a new analytical formulation of the muscle force generation. The proposed rate coding scheme supposes varying minimum and peak firing frequencies according to motor unit (MU) type (I or II). Practically, the proposed analytical mechanogram allows us to tune the force contribution of each active MU according to its type and instantaneous firing rate. A subsequent sensitivity analysis using a Monte Carlo simulation allows deducing optimised input parameter ranges that guarantee a realistic behaviour of the proposed model according to two existing criteria and an additional one. In fact, this proposed new criterion evaluates the force generation efficiency according to neural intent. Experiments and simulations, at varying force levels and using the optimised parameter ranges, were performed to evaluate the proposed model. As a result, our study showed that the proposed sEMG–force modelling can emulate the biceps brachii behaviour during isotonic isometric contractions.