Recruitment threshold and muscle fiber conduction velocity of single motor units

The average muscle fiber conduction velocity (CV) measured with multicontact surface electrodes has been reported to increase with the contraction force. To understand this behavior better, we studied the relationship between the recruitment threshold and the muscle fiber CV of single motor units (MUs). Myoelectric signals were recorded simultaneously with a linear surface electrode array and a selective needle electrode. From the signals detected by the meedle electrode, the discharges of single MUs were isolated by the decomposition technique. By using the firing of single MUs as the trigger point, we averaged the surface myoelectric signals and extracted the single MU action potentials from the interference surface signals. The CV of single MUs, calculated by a cross-correlation analysis, was higher for MUs recruited at higher contraction force. This result indicates that the larger MUs with higher muscle fiber CV contribute to increase the average CV during varying force contractions.

Influence of proprioceptive feedback on the firing rate and recruitment of motoneurons

We investigated the relationships of the firing rate and maximal recruitment threshold of motoneurons recorded during isometric contraction with the number of spindles in individual muscles. At force levels above 10% of maximal voluntary contraction, the firing rate was inversely related to the number of spindles in a muscle, with the slope of the relationship increasing with force. The maximal recruitment threshold of motor units increased linearly with the number of spindles in the muscle. Thus, muscles with a greater number of spindles had lower firing rates and a greater maximal recruitment threshold. These findings may be explained by a mechanical interaction between muscle fibres and adjacent spindles. During low-level (0% to 10%) voluntary contractions, muscle fibres of recruited motor units produce force twitches that activate nearby spindles to respond with an immediate excitatory feedback that reaches maximal level. As the force increases further, the twitches overlap and tend towards tetanization, the muscle fibres shorten, the spindles slacken, their excitatory firings decrease, and the net excitation to the homonymous motoneurons decreases. Motoneurons of muscles with greater number of spindles receive a greater decrease in excitation which reduces their firing rates, increases their maximal recruitment threshold, and changes the motoneuron recruitment distribution.

A model of motoneuron behavior and muscle-force generation for sustained isometric contractions

A model for the simulation of motoneuron firing behavior and muscle force during sustained constant-force isometric contractions was developed. It provides a non-linear relationship between the excitation to the motoneuron pool of a muscle and the firing behavior of motor units; it implements muscle mechanical changes induced by fatigue and it comprises a feedback loop to maintain the muscle force at a given target level. We simulated a series of repeated force contractions sustained at 20% MVC with the first dorsal interosseous muscle of the hand and the vastus lateralis muscle of the thigh. The model generates force and firing behaviors which are consistent with experimental findings and underscores the influence of muscle mechanical changes on the control behavior of motor units during sustained contractions. The model predicts the increase of force fluctuation with fatigue in both muscles, likely due to recruitment of high-threshold high-amplitude twitch motor units. Force variability is greater in the first dorsal interosseous muscle than in the vastus lateralis muscle at any time during the contraction series, due to the different electrical and mechanical properties of the muscles.

Electro-mechanical stability of surface EMG sensors

This study compared the performance of surface electromyographic (sEMG) sensors for different detection conditions affecting the electro-mechanical stability between the sensor and its contact with the skin. These comparisons were made to gain a better understanding of how specific characteristics of sensor design and use may alter the ability of sEMG sensors to detect signals with high fidelity under conditions of vigorous activity. The first part of the study investigated the effect of different detection surface contours and adhesive tapes on the ability of the sensor to remain in electrical contact with the skin. The second part of the study investigated the effects of different skin preparations and hydrophilic gels on the production of movement artifact resulting from sinusoidal and impact mechanical perturbations. Both parts of the study evaluated sensor performance under dry skin and wet skin (from perspiration) conditions. We found that contouring the detection surface and adding a more adhesive double-sided tape were effective in increasing the forces needed to disrupt the electrical contact between the electrodes and the skin for both dry skin and wet skin conditions. The mechanical perturbation tests demonstrated that hydrophilic gel applied to the detection surface of the sensor produced greater movement artifacts compared to sensors without gel, particularly when the sensors were tested under conditions in which perspiration was present on the skin. The use of a surfactant skin preparation did not influence the amount of movement artifacts that resulted from either the sinusoidal or impact perturbations. The importance of these findings is discussed in terms of their implications for improving sEMG signal fidelity through sensor design modifications and procedures for interfacing them with the skin.

A sEMG-based method for assessing the design of computer mice

Computer users who experience repetitive wrist movements and awkward hand positions are prone to developing upper extremity disorders. Manufacturers have designed various ergonomic mice in response to complaints of pain and discomfort related to computer mouse use. The objective of this work was to validate the use of surface electromyography (sEMG) in assessing the design of nonkeyboard input devices (computer mice). While holding the computer mouse in different grasp positions sEMG of the forearm and hand were recorded during a set of static tasks. The sEMG signal provided information regarding the level of muscle activity and the varied combinations of muscular effort needed to position the hand in a specified posture. A significant decrease in the level of sEMG activity was observed for the pronator muscles when subjects were tested using ergonomic computer mice. The sEMG-based method was validated to be sensitive to the impact of subtle differences in shape/design on the amplitude of the surface EMG data. We also proved a significant effect of hand size and grasp position on the level of muscle activity associated with different mice.

Motor unit recruitment and derecruitment induced by brief increase in contraction amplitude of the human trapezius muscle

The activity pattern of low-threshold human trapezius motor units was examined in response to brief, voluntary increases in contraction amplitude ('EMG pulse') superimposed on a constant contraction at 4-7 % of the surface electromyographic (EMG) response at maximal voluntary contraction (4-7 % EMGmax). EMG pulses at 15-20 % EMGmax were superimposed every minute on contractions of 5, 10, or 30 min duration. A quadrifilar fine-wire electrode recorded single motor unit activity and a surface electrode recorded simultaneously the surface EMG signal. Low-threshold motor units recruited at the start of the contraction were observed to stop firing while motor units of higher recruitment threshold stayed active. Derecruitment of a motor unit coincided with the end of an EMG pulse. The lowest-threshold motor units showed only brief silent periods. Some motor units with recruitment threshold up to 5 % EMGmax higher than the constant contraction level were recruited during an EMG pulse and kept firing throughout the contraction. Following an EMG pulse, there was a marked reduction in motor unit firing rates upon return of the surface EMG signal to the constant contraction level, outlasting the EMG pulse by 4 s on average. The reduction in firing rates may serve as a trigger to induce derecruitment. We speculate that the silent periods following derecruitment may be due to deactivation of non-inactivating inward current ('plateau potentials'). The firing behaviour of trapezius motor units in these experiments may thus illustrate a mechanism and a control strategy to reduce fatigue of motor units with sustained activity patterns.

Time-frequency parameters of the surface myoelectric signal for assessing muscle fatigue during cyclic dynamic contractions

The time-dependent shift in the spectral content of the surface myoelectric signal to lower frequencies has proven to be a useful tool for assessing localized muscle fatigue. Unfortunately, the technique has been restricted to constant-force, isometric contractions because of limitations in the processing methods used to obtain spectral estimates. A novel approach is proposed for calculating spectral parameters from the surface myoelectric signal during cyclic dynamic contractions. The procedure was developed using Cohen class time-frequency transforms to define the instantaneous median and mean frequency during cyclic dynamic contractions. Changes in muscle length, force, and electrode position contribute to the nonstationarity of the surface myoelectric signal. These factors, unrelated to localized fatigue, can be constrained and isolated for cyclic dynamic contractions, where they are assumed to be constant for identical phases of each cycle. Estimation errors for the instantaneous median and mean frequency are calculated from synthesized signals. It is shown that the instantaneous median frequency is affected by an error slightly lower than that related to the instantaneous mean frequency. In addition, we present a sample application to surface myoelectric signals recorded from the first dorsal interosseous muscle during repetitive abduction/adduction of the index finger against resistance. Results indicate that the variability of the instantaneous median frequency is related to the repeatability of the biomechanics of the exercise.

Motor control of low-threshold motor units in the human trapezius muscle

The firing pattern of low-threshold motor units was examined in the human trapezius and first dorsal interosseous (FDI) muscles during slowly augmenting, low-amplitude contractions that were intended to mimic contractile activity in postural muscles. The motor unit activity was detected with a special needle electrode and was analyzed with the assistance of computer algorithms. The surface electromyographic (EMG) signal was recorded. Its root-mean-square (RMS) value was calculated and presented to the subject who used it to regulate the muscle force level. In the trapezius, there was minimal, if any, firing rate modulation of early recruited motor units during slow contractions (< or =1% EMG(max)/s), and later recruited motor units consistently presented higher peak firing rates. As the force rate of the contraction increased (3% EMG(max)/s), the firing rates of the motor units in the trapezius approached an orderly hierarchical pattern with the earliest recruited motor units having the greatest firing rate. In contrast, and as reported previously, the firing rates of all motor units in the FDI always presented the previously reported hierarchical "onion-skin" pattern. We conclude that the low-threshold motor units in the postural trapezius muscle, that is the motor units that are most often called on to activate the muscle in postural activities, have different control features in slow and fast contractions. More detailed analysis revealed that, in the low force-rate contractions of the trapezius, recruitment of new motor units inhibited the firing rate of active motor units, providing an explanation for the depressed firing rate of the low-threshold motor units. We speculate that Renshaw cell inhibition contributes to the observed deviation of the low-threshold motor units from the hierarchical onion-skin pattern.

Electromyographic analysis of grand-plié in ballet and modern dancers

PURPOSE

The purpose of this report is to describe lower extremity muscle activity in grand-plié, as determined by EMG analysis; to compare and contrast muscle function in grand-plié and demi-plié to support the hypothesis that grand-plié is not simply a deeper demi-plié, but rather a fundamentally different movement in terms of muscle use; and to present further evidence in support of the hypothesis that ballet dancers use muscles differently than modern dancers in dance movement.

METHODS

Surface electromyography was used to analyze lower extremity muscle activity during grand-plié in first position with lower extremities turned out in five ballet and seven modern female professional dancers.

RESULTS

Electromyographic (EMG) activity of tibialis anterior included continuous activity from heel-off during the lowering phase, through midcycle, and ending at heel-on during the rising phase in all grand-pliés; the majority of tibialis anterior EMG tracings in ballet dancers had additional activity at the end of the rising phase. All EMG tracings for vastus lateralis and medialis included a peak of activity during the lowering phase, a decrease (valley) at midcycle, followed by another peak during the rising phase; increased activity at the end of the rising phase was observed in most grand-plié in ballet, and not modern, dancers. Adductor EMG activity was also observed in all tracings with a peak during the lowering phase from heel-off to midcycle, a valley at midcycle, followed by a peak of activity in early rising phase; the midcycle valley was of lower, and the rising phase peak of higher, activity in ballet compared with modern dancers. Variation of EMG patterns was observed for lateral and medial gastrocnemius, gluteus maximus, and hamstrings.

CONCLUSIONS

The data support the concept that lower extremity muscle activity in dance movement is comprised of three major types: (a) unique, characteristic activity required for the execution of the movement; (b) varied activity which is characteristic of dancers of different dance idioms; and (c) varied activity which may depend on factors such as balance, personal habit, and individual training background. Furthermore, EMG activity of vastus lateralis and medialis at the midcycle valley in grand-plié was significantly less in ballet dancers than in modern dancers despite similar degree of knee flexion, suggesting that ballet dancers may have lower patellofemoral joint reaction force at midcycle than modern dancers.

Hand dominance and motor unit firing behavior

Daily preferential use was shown to alter physiological and mechanical properties of skeletal muscle. This study was aimed at revealing differences in the control strategy of muscle pairs in humans who show a clear preference for one hand. We compared the motor unit (MU) recruitment and firing behavior in the first dorsal interosseous (FDI) muscle of both hands in eight male volunteers whose hand preference was evaluated with the use of a standard questionnaire. Myoelectric signals were recorded while subjects isometrically abducted the index finger at 30% of the maximal voluntary contraction (MVC) force. A myoelectric signal decomposition technique was used to accurately identify MU firing times from the myoelectric signal. In MUs of the dominant hand, mean values for recruitment threshold, initial firing rate, average firing rate at target force, and discharge variability were lower when compared with the nondominant hand. Analysis of the cross-correlation between mean firing rate and muscle force revealed cross-correlation peaks of longer latency in the dominant hand than in the nondominant side. This lag of the force output with respect to fluctuations in the firing behavior of MUs is indicative of a greater mechanical delay in the dominant FDI muscle. MVC force was not significantly different across muscle pairs, but the variability of force at the submaximal target level was higher in the nondominant side. The presence of lower average firing rates, lower recruitment thresholds, and greater firing rate/force delay in the dominant hand is consistent with the notion of an increased percentage of slow twitch fibers in the preferentially used muscle, allowing twitch fusion and force buildup to occur at lower firing rates. It is suggested that a lifetime of preferred use may cause adaptations in the fiber composition of the dominant muscle such that the mechanical effectiveness of its MUs increased.

Classification of back muscle impairment based on the surface electromyographic signal

A surface electromyographic (EMG) procedure for classifying muscle impairments in persons with low back pain (LBP) is described. The procedure was studied using a device, the Back Analysis System (BAS), to acquire and process EMG signals from six bilateral muscle sites during sustained isometric contractions designed to progressively fatigue the lower back. Back muscle impairment was determined on the basis of the different ways in which the EMG median frequency parameters change as a function of contraction duration and muscle site. The article describes a series of studies that have been useful in developing an automated procedure for identifying back muscle impairment by comparing individual test results to a normative database. To date, the research results have produced multivariate discriminant functions that have identified two muscle impairment categories associated with deconditioning and imbalances secondary to LBP. We have found that the functions can distinguish individuals with and without LBP with an accuracy of approximately 90%. Other studies are described in which the technique is applied to monitoring changes in muscle performance capability that occur following rehabilitation for LBP. Many of our findings here are also compared to the results of independent studies by others using similar procedures. The need for further research and development of the technique to improve its clinical applicability is also described.

Development of new protocols and analysis procedures for the assessment of LBP by surface EMG techniques

Spectral parameters of the surface electromyographic (EMG) signal from lumbar back muscles assessed during a fatiguing isometric contraction can be used to classify different categories of low back pain (LBP) subjects and control subjects without LBP. In the test protocol currently used at the NeuroMuscular Research Center at Boston University, subjects contract their back muscles at 80% of their maximal voluntary contraction (MVC) force. This fatigue-based protocol has been successfully applied to persons with subacute or chronic LBP; those in acute pain, however, have not been included because of their inability to perform a maximal exertion. In this paper we will examine the force sensitivity of the currently used EMG parameters and also give an overview of some of our efforts to develop new test procedures. Our goal is to develop force-insensitive surface EMG parameters that can be used for classification purposes in populations of subjects who develop low trunk extension forces. In addition, the development of a model to predict MVC from anthropometrical measurements will be presented.

Rank-ordered regulation of motor units

Myoelectric signals were detected from the tibialis anterior muscle of 5 subjects with a quadrifilar needle electrode while the subjects generated isometric forces that increased linearly with time (10% of maximal voluntary contraction/s) up to maximal voluntary level. Motor unit firing rates were studied as a function of force throughout the full range of muscle force output. The relationship between force and firing rate was found to contain three distinct regions. At recruitment and near maximal force levels, firing rates increased more rapidly with force than in the intermediate region. Furthermore, in the regions with rapid increases, the rate of change of firing rate was correlated to the recruitment threshold, with higher recruitment threshold motor units displaying greater rates of change. In the intermediate region, all motor units had similar rates of change of firing rate. A weak positive correlation was found between initial firing rate and recruitment threshold. Firing rates of motor units at any instant were found to be ordered according to the recruitment order: at any given time in the contraction motor units with lower recruitment thresholds had higher firing rates than units with higher recruitment thresholds. Firing rates of all motor units were observed to converge to the same value at maximal forces. Mechanisms underlying motor unit recruitment and firing rate modulation are discussed in the context of a conceptual model.

The effects of spaceflight on open-loop and closed-loop postural control mechanisms: human neurovestibular studies on SLS-2

Stabilogram-diffusion analysis was used to examine how prolonged periods in microgravity affect the open-loop and closed-loop postural control mechanisms. It was hypothesized that following spaceflight: (1) the effective stochastic activity of the open-loop postural control schemes in astronauts is increased; (2) the effective stochastic activity and uncorrelated behavior, respectively, of the closed-loop postural control mechanisms in astronauts are increased; and (3) astronauts utilized open-loop postural controls schemes for shorter time intervals and smaller displacements. Four crew members and two alternates from the 14-day Spacelab Life Sciences 2 Mission were included in the study. Each subject was tested under eyes-open, quiet-standing conditions on multiple preflight and postflight days. The subjects' center-of-pressure trajectories were measured with a force platform and analyzed according to stabilogram-diffusion analysis. It was found that the effective stochastic activity of the open-loop postural control schemes in three of the four crew members was increased following spaceflight. This result is interpreted as an indication that there may be in-flight adaptations to higher-level descending postural control pathways, e.g., a postflight increase in the tonic activation of postural muscles. This change may also be the consequence of a compensatory (e.g., "stiffening") postural control strategy that is adopted by astronauts to account for general feeling of postflight unsteadiness. The crew members, as a group, did not exhibit any consistent preflight/postflight differences in the steady-state behavior of their closed-loop postural control mechanisms or in the functional interaction of their open-loop and closed-loop postural control mechanisms. These results are interpreted as indications that although there may be in-flight adaptations to the vestibular system and/or proprioceptive system, input from the visual system can compensate for such changes during undisturbed stance.

Open-loop and closed-loop postural control mechanisms in Parkinson's disease: increased mediolateral activity during quiet standing

Stabilogram-diffusion analysis was used to gain insights into how idiopathic Parkinson's disease (IPD) affects the postural control mechanisms involved in maintaining erect stance. Twenty-two subjects with IPD and twenty-four healthy elderly subjects were studied under eyes-open, quiet-standing conditions. The postural control mechanisms in the parkinsonian subjects, compared to the healthy elderly, were characterized by an increase in the effective stochastic activity in the mediolateral direction. Mediolateral posturographic measures were also associated with a history of falls and poor performance on clinical measures of balance. It is hypothesized that the increase in mediolateral activity in subjects with IPD may reflect an attempt to maintain potentially stabilizing movements during quiet standing in the face of impaired movement in the anteroposterior direction. This study supports the notion that mediolateral instability is an important posturographic marker of functional balance impairment in the elderly.

Effects of muscle fiber type and size on EMG median frequency and conduction velocity

This paper describes an in vitro method for comparing surface-detected electromyographic median frequency (MF) and conduction velocity (CV) parameters with histochemical measurements of muscle fiber type composition and cross-sectional area (CSA). Electromyographic signals were recorded during electrically elicited tetanic contractions from rat soleus, extensor digitorum longus, and diaphragm muscles placed in an oxygenated Krebs bath. Fibers were typed as slow oxidative, fast oxidative glycolytic, and fast glycolytic based on histochemical enzyme stains. Muscles with a greater percentage of fast glycolytic and fast oxidative glycolytic fibers exhibited greater initial values of MF and CV as well as a greater reduction in these variables over the course of the contraction. Regression indicated that fiber type composition could be predicted based on two MF parameters. A weighted measure of muscle fiber CSA was found to be linearly related to both initial MF and CV. The results of this study suggest that MF and CV parameters recorded during a muscular contraction are related to muscle fiber type composition and muscle fiber CSA.

Age-related changes in open-loop and closed-loop postural control mechanisms

In an earlier posturographic investigation (Collins and De Luca 1993) it was proposed that open-loop and closed-loop control mechanisms are involved in the regulation of undisturbed, upright stance. In this study, stabilogram-diffusion analysis was used to examine how the natural aging process affects the operational characteristics of these control mechanisms. Stabilogram-diffusion analysis leads to the extraction of repeatable center-of-pressure (COP) parameters that can be directly related to the steady-state behavior and functional interaction of the neuromuscular mechanisms underlying the maintenance of erect posture. Twenty-five healthy young males (aged 19-30 years) and twenty-five elderly males (aged 71-80 years) who were free of major gait and postural disorders were included in the study. An instrumented force platform was used to measure the time-varying displacements of the COP under each subject's feet during quiet standing. The COP trajectories were analyzed as one-dimensional and two-dimensional random walks, according to stabilogram-diffusion analysis. Using this technique, it was demonstrated cross-sectionally that healthy aging is associated with significant changes in the 'quasi-static' dynamics of the postural control system. (It was also shown that more traditional posturographic analyses, i.e., summary statistics, were not sensitive enough to detect these age-related differences.) It was found that the steady-state behavior of the open-loop postural control mechanisms in the elderly is more positively correlated and therefore perhaps more unstable, i.e., the output of the overall system has a greater tendency to move or drift away from a relative equilibrium point over the short term. In contrast with this result, it was also found that the steady-state behavior of the closed-loop postural control mechanisms in the elderly is more negatively correlated and therefore perhaps more stable, i.e., over the longer term, there is an increased probability that movements away from a relative equilibrium point will be offset by corrective adjustments back towards the equilibrium position. In addition, it was demonstrated that the elderly utilize open-loop control schemes for longer time intervals and correspondingly larger COP displacements during periods of undisturbed stance. This result suggests that in the elderly there is a greater delay, on average, before closed-loop feedback mechanisms are called into play. Finally, it was shown that there is an increased heterogeneity of postural control abilities in healthy older adults.

Upright, correlated random walks: A statistical-biomechanics approach to the human postural control system

The task of maintaining erect stance involves a complex sensorimotor control system, the output of which can be highly irregular. Even when a healthy individual attempts to stand still, the center of gravity of his or her body and the center of pressure (COP) under his or her feet continually move about in an erratic fashion. In this study, we approach the problem of characterizing postural sway from the perspective of random-walk theory. Specifically, we analyze COP trajectories as one-dimensional and two-dimensional random walks. These analyses reveal that over short-term intervals of time during undisturbed stance the COP behaves as a positively correlated random walk, whereas over long-term intervals of time it resembles a negatively correlated random walk. We interpret this novel finding as an indication that during quiet standing the postural control system utilizes open-loop and closed-loop control schemes over short-term and long-term intervals, respectively. From this perspective, our approach, known as stabilogram-diffusion analysis, has the advantage that it leads to the extraction of COP parameters which can be directly related to the steady-state behavior and functional interaction of the neuromuscular mechanisms underlying the maintenance of erect stance. (c) 1995 American Institute of Physics.

The effects of visual input on open-loop and closed-loop postural control mechanisms

In an earlier posturographic investigation (Collins and De Luca 1993) it was proposed that open-loop and closed-loop control mechanisms are involved in the regulation of undisturbed, upright stance. In this study, stabilogram-diffusion analysis was used to examine how visual input affects the operational characteristics of these control mechanisms. Stabilogram-diffusion analysis leads to the extraction of repeatable center-of-pressure (COP) parameters that can be directly related to the resultant steady-state behavior and functional interaction of the neuromuscular mechanisms underlying the maintenance of erect posture. Twenty-five healthy male subjects (aged 19-30 years) were included in the study. An instrumented force platform was used to measure the time-varying displacements of the COP under each subject's feet during quiet standing. The subjects were tested under eyes-open and eyes-closed conditions. The COP trajectories were analyzed as one-dimensional and two-dimensional random walks, according to stabilogram-diffusion analysis. Using this technique, it was found that visual input affects the performance of the postural control system in one of two different ways--either it significantly modifies the steady-state behavior of the open-loop postural control mechanisms, or it significantly alters the characteristics of the other closed-loop feedback mechanisms that are involved in balance control. This result is interpreted as an indication that the visual system is integrated into the postural control system in one of two different ways. The experimental population was roughly evenly divided between these two schemes. For the first group (13 of 25 subjects), visual input principally caused a decrease in the "effective" stochastic activity of the open-loop control mechanisms in both the mediolateral and anteroposterior directions. For the second group (12 of 25 subjects), visual input caused an increase in the effective stochastic activity and uncorrelated behavior of the closed-loop control mechanisms in the anteroposterior direction only. On the basis of these results, it is hypothesized that visual input, in both schemes, serves to decrease the stiffness of the musculoskeletal system. In the former case, this may be accomplished by decreasing the level of muscular activity across the joints of the lower limb, whereas, in the latter case, reduced stiffness may be achieved by reducing the gain(s) of the other postural feedback mechanisms, i.e., the proprioceptive and/or vestibular systems. Using stabilogram-diffusion analysis, it was also found that the two groups of subjects behaved similarly under eyes-closed conditions.(ABSTRACT TRUNCATED AT 400 WORDS)

Spectral electromyographic assessment of back muscles in patients with low back pain undergoing rehabilitation

STUDY DESIGN

A surface electromyographic procedure for evaluating back muscle impairment was studied in patients undergoing rehabilitation for low back pain.

OBJECTIVES

The results were analyzed to determine whether the electromyographic procedure was able to: 1) distinguish muscle impairment between patients with low back pain and normal subjects, and 2) monitor changes in muscle function after low back pain rehabilitation.

METHODS

Patients with chronic low back pain (n = 85) were tested to measure the median frequency of the electromyographic signals from six lumbar electrode sites during sustained trunk extensions. A subset (n = 28) of these patients was re-tested after low back pain rehabilitation. A discriminant function for classifying subjects into "low back pain" and "normal" groups was formulated using the electromyographic data from a subset of the patients with low back pain (n = 28) and a normative sample (n = 42). Results for this "learning" sample were compared with results using the same function on the remaining "holdout" sample of patients (n = 57) and an additional normative sample (n = 6). Differences in electromyographic parameters before and after rehabilitation also were analyzed.

RESULTS

The discriminant function classified subjects into low back pain and normal groups, with 86% and 89% correct classification for the "learning" and "holdout" samples, respectively. These classification results were independent of trunk extensor strength. Changes in median frequency after the rehabilitation program were consistent with improvements in back muscle fatigability.

CONCLUSION

These findings demonstrate how electromyographic spectral measurements may be used to identify and monitor back muscle impairment in patients undergoing rehabilitation for low back pain.

Electrically evoked myoelectric signals in back muscles: effect of side dominance

This work had two goals, to study the effect of hand dominance on myoelectric signal variables and fatigue indexes in back muscles and to assess the repeatability of the estimates of such variables. Myoelectric manifestations of muscle fatigue were studied in the right and left longissimus dorsi muscles of five right-handed and five left-handed normal male subjects. Myoelectric signals (M waves), evoked by stimulation applied to a muscle's motor point, were detected with surface electrodes. Each test consisted of eliciting a tetanic contraction of 30 s duration with supramaximal stimulation at 25 Hz and was repeated five times on 5 different days for each subject. The mean and median frequencies of the resulting power spectra of the M waves were plotted vs. time, and fatigue indexes were obtained from the time course of these variables. Only two-thirds of the elicited contractions provided signals of sufficiently good quality to obtain reliable estimates of the mean and median frequencies. Criteria for acceptability are described. Analysis of variance and paired comparisons showed a statistically significant effect of side dominance on fatigue indexes in the right-handed subjects but not in the left-handed subjects. Normalized initial slope and other fatigue indexes based on spectral variables demonstrated myoelectric manifestations of fatigue that were greater on the dominant side. We surmise that the differences are related to the fiber type modifications associated with the unilateral usage of the upper limbs and the consequent activation of the nondominant side of the back.

AR modeling of myoelectric interference signals during a ramp contraction

We investigated the time-varying behaviour of the autoregressive (AR) parameters in a myoelectric (ME) signal detected during a linear force increasing contraction. The AR parameters of interest were the reflection coefficients, the AR model spectrum, and the prediction errors. We used well-conditioned ME signals for which the complete time record of the motor units firings was available. In addition, the influence of the recruitment of a new motor unit, the conduction velocity of action potentials, and additive broad-band noise were investigated using simulated ME signals. The simulated ME signals were constructed from a selected group of the available motor unit action potential trains. The results revealed that, as the contraction progressed, the AR parameters displayed a time-varying behavior which coincided with the recruitment of newly recruited motor units whose spectrum of the waveform differed from that of the rest of the ME signal. This property of the AR parameters was obscured by the presence of broad-band noise and low-amplitude motor unit action potentials, both of which are more pronounced during low-level force contractions.

Common drive of motor units in regulation of muscle force

The neuromuscular system is responsible for all our interactions with our environment. Although recent decades have witnessed numerous discoveries that have shed light into various properties of this system, the basic principles underlying its overall operation still remain poorly understood. In this article, Carlo J. De Luca and Zeynep Erim discuss the concept of common drive of motor units that provides a possible scheme for the control of motor units, unifying various seemingly isolated findings that have been reported. According to this concept, a pool of motor units that makes up a muscle is controlled collectively during a contraction of that muscle. The unique firing patterns of individual motor units are effected, not by separate command signals sent to these units, but by one common drive to which motor units respond differently. The specific architecture of the system and the orderly gradation in the inherent properties of individual elements enable a single source to control the activities of all the motor units in a given pool. Such an arrangement relieves the CNS from the burden of monitoring and regulating each motor unit separately.

Electromyographic analysis of standing posture and demi-plié in ballet and modern dancers

Surface electromyography was used to analyze lower extremity muscle activity during standing posture and demi-plié in first position with lower extremities turned out, in five ballet and seven modern female professional dancers. In standing posture, increased electromyographic (EMG) activity above baseline was detected most frequently at the medial gastrocnemius (54% standing repetitions) and tibialis anterior (29%) electrodes (all dancers); in ballet dancers, increased EMG activity during standing was significantly less frequent at the medial gastrocnemius, but more frequent at the tibialis anterior, than in modern dancers. In demi-plié, the tibialis anterior had a discrete peak of EMG activity at midcycle in all dancers (97% demi-pliés). All dancers also had midcycle EMG activity in both vastus lateralis and medialis (100% demi-pliés). At the end of rising phase of demi-plié, ballet dancers had greater EMG activity than at midcycle in vastus lateralis (100% demi-pliés) and medialis (92%); in modern dancers, end-rising phase voltage was lower than at midcycle for vastus lateralis (71% demi-pliés) and medialis (83%). Genu recurvatum > or = 10 degrees was observed at the beginning and end of demi-plié in all ballet dancers, but not in modern dancers. There was marked variation of EMG activity during demi-plié in the lateral gastrocnemius, medial gastrocnemius, gluteus maximus, hamstrings, and adductors. The results support the hypothesis that ballet and modern dancers have different patterns of muscle use in standing posture and demi-plié, which in part may be a result of differences in genu recurvatum and turnout between the two groups.

Synchronization of motor-unit firings in several human muscles

1. Synchronization of concurrently active motor-unit firings was studied in six human muscles performing isometric constant-force contractions at 30% of the maximal level. The myoelectric signal was detected with a quadrifilar needle electrode and was decomposed into its constituent motor-unit action-potential trains with the Precision Decomposition technique, whose accuracy has been proven previously. 2. Synchronization was considered as the tendency of two motor units to fire at fixed time intervals with respect to each other more often than would be expected if the motor units fired independently. A rigorous statistical technique was used to measure the presence of peaks in the cross-interval histogram of pairs of motor-unit action-potential trains. The location of the center of peak as well as their width and amplitude were measured. A synch index was developed to measure the percentage of firings that were synchronized. The percentage of concurrently active motor-unit pairs that contained synchronized firings was measured. 3. Synchronization of motor-unit firings was observed to occur in two modalities. The short-term modality was seen as a peak in the cross-interval histogram centered about zero-time delay (0.5 +/- 2.9 ms, mean +/- SD) and with an average width of 4.5 +/- 2.5 ms. The long-term modality was seen as a peak centered at latencies ranging from 8 to 76 ms. On the average, the peaks of the long-term synchronization were 36% lower but had approximately the same width as the peaks for the short-term synchronization. Short-term synchronization was seen in 60% of the motor-unit paris, whereas long-term synchronization was seen in 10% of the pairs. 4. Short-term synchronization occurred in bursts of consecutive firings, ranging in number from 1 to 10, with 91% of all synchronized firing occurring in groups of 1 or 2; and the bursts of discharges appeared at sporadic times during the contraction. 5. The amount of synchronization in motor-unit pairs was found to be low. In the six muscles that were tested, an average of 8.0% of all the firings were short-term synchronized, and an average of 1.0% were long-term synchronized. The synch index was statistically indistinguishable (P = 0.07-0.89) among the different muscles and among 9 of the 11 subjects tested. 6. Sixty percent of concurrently active motor-unit pairs displayed short-term synchronization, 10% of the pairs displayed long-term synchronization, and 8% displayed both modalities.(ABSTRACT TRUNCATED AT 400 WORDS)

Open-loop and closed-loop control of posture: a random-walk analysis of center-of-pressure trajectories

A new conceptual and theoretical framework for studying the human postural control system is introduced. Mathematical techniques from statistical mechanics are developed and applied to the analysis and interpretation of stabilograms. This work was based on the assumption that the act of maintaining an erect posture could be viewed, in part, as a stochastic process. Twenty-five healthy young subjects were studied under quite-standing conditions. Center-of-pressure (COP) trajectories were analyzed as one-dimensional and two-dimensional random walks. This novel approach led to the extraction of repeatable, physiologically meaningful parameters from stabilograms. It is shown that although individual stabilograms for a single subject were highly variable and random in appearance, a consistent, subject-specific pattern emerged with the generation of averaged stabilogram-diffusion plots (mean square COP displacement vs time interval). In addition, significant inter-subject differences were found in the calculated results. This suggests that the steady-state behavior of the control mechanisms involved in maintaining erect posture can be quite variable even amongst a population of age-matched, anthropometrically similar, healthy individuals. These posturographic analyses also demonstrated that COP trajectories could be modelled as fractional Brownian motion and that at least two control systems-a short-term mechanism and a long-term mechanism-were operating during quit standing. More specifically, the present results suggest that over short-term intervals open-loop control schemes are utilized by the postural control system, whereas over long-term intervals closed-loop control mechanisms are called into play. This work strongly supports the position that much can be learned about the functional organization of the postural control system by studying the steady-state behavior of the human body during periods of undisturbed stance.

Use of the surface EMG signal for performance evaluation of back muscles

For well over a decade my associates and I have been developing an objective, noninvasive technique to evaluate the performance of low-back muscles, with emphasis on being able to distinguish between healthy and dysfunctioned backs. Our approach is based on the well-known fact that the EMG signal undergoes a compression in the frequency domain during a sustained muscle contraction. In particular we track the median frequency of EMG signals detected from six muscles in the lower back during an isometric extension of the trunk. The measurements are taken with the Back Analysis System which consists of a postural restraining device, special electrodes for detecting the EMG signals, a muscle fatigue monitor which calculates the median frequencies, and the appropriate software. We have found that the pattern of fatigue exhibited by the six median frequency curves can be used to distinguish individuals who have low-back pain from those who do not with an accuracy of at least 84%. An even more relevant and timely application of our technique is for quantifying the progression of the performance of low-back muscles during a rehabilitation program. Although more work is required to explore the intricacies of the technique, present results provide a convincing indication that it is reliable and that it is ready to be placed into practice.

Firing rate interactions among human orbicularis oris motor units

Groups of human motor units from the same muscle exhibit joint fluctuations in firing rate during voluntary muscle contraction. In an effort to determine whether similar behavior would be observed in a muscle lacking muscle spindles, motor unit firing behavior was examined in the human orbicularis oris inferior (OOI) during mild voluntary effort. Motor unit activity was recorded with a quadrifilar needle inserted in the OOI. Firing occurrences were identified using a motor unit decomposition procedure. Cross-correlation of motor unit firing rates revealed a tendency for motor unit firing rates to covary, although the effect was somewhat more variable than that observed previously in other skeletal muscles. There was also a statistically significant tendency for pairs of motor units to fire at simultaneous or near-simultaneous (+/- 5 ms) intervals (synchronization). Firing rate variability in OOI motor units was not significantly different (p > .05) from that observed in the FDI. Thus, the present results suggest that the common drive of human motor unit activity may not depend on the presence of muscle spindles.

Lateral dominance and motor unit firing behavior

Twelve subjects were classified as left-handed (LH) or right-handed (RH) using Annett's hand dominance classification. Motor unit recordings were obtained from the first dorsal interosseous (FDI) muscle of each hand using a quadrifilar needle electrode. Firing occurrences of individual motor units were then identified and the firing rates of all motor units recorded during the contraction were cross-correlated. The results demonstrated significantly greater firing rate cross-correlation scores in the dominant hand than in the non-dominant hand for both LH and RH subjects. This association between hand dominance and the common drive of motor unit firing rates lends credence to the idea that one or more CNS sites may influence conjoint motor unit firing behavior.

pH-induced effects on median frequency and conduction velocity of the myoelectric signal

H+ accumulation at the sarcolemma is believed to play a key role in determining the electrophysiological correlates of fatigue. This paper describes an in vitro method to externally manipulate muscle pH while measuring the resultant effect on surface-detected median frequency (MDF) and conduction velocity (CV) parameters. Hamster muscle diaphragm strips (n = 8) were isolated with the phrenic nerve intact and placed in an oxygenated Krebs bath (26 degrees C). The muscle was clamped to a noncompliant load cell to measure isometric contractile tension. Tetanic contraction was developed via 40-Hz supermaximal stimulation of the phrenic nerve. Differential signals were recorded from three electromyogram (EMG) detection surfaces for computation of CV (via the phase shift in the EMG signals) and MDF. Repeated trials were conducted at bath pHs of 7.4, 7.0, and 6.6. Bath pH was altered by aerating predetermined concentrations of O2 and CO2 into the bath. Decreases in bath pH resulted in decreases in both initial MDF and initial CV. The differences in initial MDF and initial CV were significant (P less than 0.001) for each of the bath pH conditions. In general, the change in bath pH resulted in an equal percent change in initial MDF and initial CV. This suggests that the change in bath pH caused a decrease in CV without significantly altering the fundamental shape of the M wave. In contrast, the EMG was altered differently during stimulated contractions. During stimulation, the rate of decay of CV was 65% of the rate of decay of MDF.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of percutaneous stimulation on motor unit firing behavior in man

Motor unit firing behavior in human first dorsal interosseous (FDI) muscle was studied during controlled constant force isometric contractions. The threshold at which motor units were recruited and the mean firing rate at 50% of maximal voluntary contraction (MVC) were evaluated following stimulation of the skin area over the second digit. Stimulation of cutaneous receptors tended to increase the recruitment threshold of most of the motor units recruited under 20% MVC, while high-threshold motor units (those recruited over 30% MVC) generally exhibited a decrease in recruitment threshold. Less dramatic changes in motor unit firing rates were observed, but those motor units recruited over 30% MVC exhibited some increase in firing rate. The relationship between the change in recruitment threshold and change in motor unit firing rate is not rigid and seems to be susceptible to considerable synaptic noise.

The effects of external bending moments on lumbar muscle force distribution

A detailed biomechanical model of the low-back musculature that predicts muscle-force distribution in response to external loading is presented. The paper shows that the class of loading tasks that involve an erect posture and an arbitrary load placed on the upper limbs can be described as a loading plane whose axes are the flexion and lateral bending moments. Under these conditions, the individual muscle forces are described as a three-dimensional surface defined by the loading plane and termed the muscle activity surface (MAS). The MAS and the loading plane intersect along the switching curve which separates the load combinations that activate the muscle from those that do not. The paper suggests the existence of a recruitment order of low back muscles in response to external loads and presents a comprehensive framework for examining earlier studies that used EMG measurements to validate physiological and mechanical predictions.

Myoelectric manifestations of fatigue in voluntary and electrically elicited contractions

The time course of muscle fiber conduction velocity and surface myoelectric signal spectral (mean and median frequency of the power spectrum) and amplitude (average rectified and root-mean-square value) parameters was studied in 20 experiments on the tibialis anterior muscle of 10 healthy human subjects during sustained isometric voluntary or electrically elicited contractions. Voluntary contractions at 20% maximal voluntary contraction (MVC) and at 80% MVC with duration of 20 s were performed at the beginning of each experiment. Tetanic electrical stimulation was then applied to the main muscle motor point for 20 s with surface electrodes at five stimulation frequencies (20, 25, 30, 35, and 40 Hz). All subjects showed myoelectric manifestations of muscle fatigue consisting of negative trends of spectral variables and conduction velocity and positive trends of amplitude variables. The main findings of this work are 1) myoelectric signal variables obtained from electrically elicited contractions show fluctuations smaller than those observed in voluntary contractions, 2) spectral variables are more sensitive to fatigue than conduction velocity and the average rectified value is more sensitive to fatigue than the root-mean-square value, 3) conduction velocity is not the only physiological factor affecting spectral variables, and 4) contractions elicited at supramaximal stimulation and frequencies greater than 30 Hz demonstrate myoelectric manifestations of muscle fatigue greater than those observed at 80% MVC sustained for the same time.

Fatigue, recovery, and low back pain in varsity rowers

The purpose of this study was to determine whether surface electromyography (EMG) from the erector spinae muscles could correctly identify individuals with low back pain without a population of elite athletes. A similar technique had previously been successful in identifying low back pain patients within a non-athletic population. A Back Analysis System was used to compute the median frequency of the EMG power density spectrum to monitor metabolic changes in back muscles associated with muscle fatigue. Twenty-three members of a men's collegiate varsity crew team consisting of port (N = 13) and starboard (N = 10) rowers were tested in a laboratory during a fatigue-inducing isometric contraction sustained at a relatively high, constant force. Six of the rowers tested were further classified as having low back pain. A brief test contraction was repeated at a fixed interval following the fatiguing contraction to monitor recovery. A two-group discriminant analysis procedure correctly classified 100% of the rowers with low back pain and 93% of the rowers without back pain on the basis of the median frequency data. The median frequency parameters related to recovery were the best discriminators of back pain. A similar analysis correctly classified 100% of the port rowers and 100% of the starboard rowers on the basis of their spectral parameters. The best discriminating variables in this instance were the median frequency parameters relating to both fatigability and recovery. Results from this study demonstrate that low back pain and asymmetrical muscle function in rowers can be assessed on the basis of EMG spectral analysis.

Inference of motor unit recruitment order in voluntary and electrically elicited contractions

The relationship between surface myoelectric signal parameters and the level of voluntary or electrically elicited contractions was studied in 32 experiments on the tibialis anterior muscle of 22 healthy human subjects. Contractions were performed at 20 and 80% of the maximum voluntary contraction torque. Two levels of stimulation current were used, yielding, respectively, a maximum M wave and an M wave approximately 30% of the maximum. A four-bar electrode probe was used to detect single- and double-differential signals from which mean and median frequency of the power spectrum and average muscle fiber conduction velocity were estimated. Measurements obtained from voluntary contractions showed a positive correlation between contraction levels and both conduction velocity and spectral parameters. Conduction velocity increased by 21.2 +/- 10.9% when voluntary contraction level increased from 20 to 80% of the maximal value. Spectral parameters increased by similar amounts. Tetanic electrical stimulation was applied to a muscle motor point for 20 s via surface electrodes. Rectangular current pulses with 0.1-ms width and frequencies of 20, 25, 30, 35, and 40 Hz were used. Four types of behavior were observed with increasing stimulation level: 1) the two spectral parameters and conduction velocity both increased with stimulation in 15 experiments, 2) the two spectral parameters decreased and conduction velocity increased in 8 experiments, 3) the two spectral parameters and conduction velocity both decreased in 6 experiments, and 4) the two spectral parameters increased and conduction velocity decreased in 3 experiments. Conduction velocity increased with increasing stimulation current in 72% of the experiments, indicating a recruitment order similar to that of voluntary contractions, although it decreased in the other 28% of the cases, indicating a reverse order of recruitment. Contrary to what is observed in direct stimulation of nerves, motor units are not in general recruited in reverse order of size during electrical stimulation of a muscle motor point. This discrepancy may be the result of geometric factors or a lack of correlation between axonal branch diameter and the diameter of the parent motoneuron axon. Changes of conduction velocity and spectral parameters in opposite directions may be the result of the combined effect of the motor unit recruitment order and of the different tissue filtering function associated with the geometric location of the recruited motor units within the muscle.

Lumbar muscle fatigue and chronic lower back pain

There currently is a clinical need for an objective technique to assess muscle dysfunction associated with chronic lower back pain. A Back Analysis System for objectively measuring local fatigue in the back extensor muscles is presented. The reliability and validity of this technique was evaluated by testing chronic low-back pain patients and control subjects without back pain. Concurrent surface electromyograms (EMG) were detected from multiple back muscles during sustained isometric contractions at different force levels of trunk extension. Median frequency parameters of the EMG power density spectrum were monitored to quantify localized muscle fatigue. Results indicated: 1) high reliability estimates for repeated trials; 2) significant differences (P less than 0.05) in median frequency parameters between lower back pain patients and control subjects for specific combinations of contractile force level and muscle site tested; 3) Median Frequency parameters correctly classified lower back pain and control subjects using a two-group discriminant analysis procedure. The applicability of this technique as a treatment outcome measure and diagnostic screening method for lower back pain patients is discussed.

1989 Volvo Award in biomechanics. Mechanical recruitment of low-back muscles. Theoretical predictions and experimental validation

A biomechanical model for studying lumbar muscle load sharing for a class of physical tasks that involve gravitational loading (holding weights) of the upper body in an erect posture is presented. The model assumes that the lumbar muscles balance the externally applied flexion and lateral bending moments. The concept of a 'loading plane' whose axes are the two bending moments is introduced. Any point in the plane can be viewed as a 'loading-point' describing a combination of bending moments that are applied to the body. The study of lumbar-muscle load sharing revealed loading conditions that required activation or deactivation of a particular muscle. The loading plane thus could be divided into regions of activity and inactivity for each muscle, separated by a 'switching curve.' The concept of 'switching curves' proved very useful for examining previously described physiologic assumptions on the loading conditions of particular muscle groups, and for grouping the 22 muscles described in the model into ten functional units. Electromyographic validation studies were conducted and showed a high degree of correlation between the model predictions and actual measurements for the contralateral (with respect to the load) muscles and to a lesser degree of correlation for the ipsilateral muscles.

Unusual motor unit firing behavior in older adults

Motor unit firing behavior was studied in the first dorsal interosseus (FDI) and tibialis anterior (TA) muscles of 10 aged subjects during slow, isometric contractions. Previous study in younger individuals had shown that motor units are recruited and derecruited in an orderly manner whereby the early-recruited units are the last to be derecruited. However, there were several examples in the old subjects in which some high-threshold motor units were derecruited at much lower levels of force. Concurrent antagonist firing in an effort to maintain the required precision is considered a likely candidate for such prolonged motor unit activation.

Surface myoelectric signal cross-talk among muscles of the leg

Surface myoelectric signals were detected from the skin surface above the tibialis anterior muscle, the peroneus brevis muscle, the soleus muscle and the tibial bone during selective maximal electrical stimulation of the tibialis anterior muscle in 12 normal subjects. The double differential technique developed by Broman et al. (1985) was used to determine if the detected signal was due to volume conduction from the tibialis anterior fibers. The peak-to-peak (PP), average rectified (ARV) and root mean square (RMS) amplitudes of the M waves were computed for each detection location. The values detected on the tibial bone, on the peroneus and on the soleus muscles were normalized with respect to those detected on the tibialis anterior and ranged from 4.8% to 33.0% (PP), 4.7% to 36.0% (ARV), and 7.7% to 37.4% (RMS) for the tibial bone area; from 4.0% to 20.0% (PP), 3.5% to 10.0% (ARV), and 3.0% to 10.0% (RMS) for the peroneus brevis muscle area; and from 3.0% to 8.0% (PP), 3.4% to 9.1% (ARV), and 2.0% to 9.8% (RMS) for the soleus muscle area. Neither peak-to-peak values, average rectified values nor root mean square values appeared to be correlated with leg size. It is concluded that a surface myoelectric signal detected on the skin above a leg muscle and having a peak-to-peak amplitude of up to 16.6% of a signal detected above a neighboring muscle may be due to cross-talk rather than to activation of the muscle below the electrode.

Voluntary control of motor units in human antagonist muscles: coactivation and reciprocal activation

1. Myoelectric (ME) activity of several motor units was detected simultaneously from the human flexor pollicis longus and extensor pollicis longus muscles, the only two muscles that control the interphalangeal joint of the thumb. The ME signals were detected while the subjects produced isometric force outputs to track three different paradigms: triangular trajectories, random-force trajectories requiring both flexion and extension contractions, and net zero force resulting from stiffening the joint by voluntarily coactivating both muscles. 2. The ME signals were decomposed into their constituent motor-unit action potential trains. The firing rate behavior of the concurrently active motor units was studied using cross-correlation techniques. 3. During isometric contractions, the firing rates of motor units within a muscle were greatly cross-correlated with essentially zero time shift with respect to each other. This observation confirms our previous report of this behavior, which has been called common drive. Common drive was also found among the motor units of the agonist and antagonist muscles during voluntary coactivation to stiffen the interphalangeal joint. This observation suggests two interesting points: 1) that the common drive mechanism has a component of central origin, and 2) that the central nervous system may control the motoneuron pools of an agonist-antagonist muscle pair as if they were one pool when both are performing the same task. 4. During force reversals, the firing rates of motor units reverse in an orderly manner: earlier recruited motor units decrease their firing rate before later recruited motor units. This orderly reversal of firing rates is consistent with the concept of orderly recruitment and derecruitment. 5. A control scheme is suggested to explain the behavior of the motor units in both muscles during force reversal. It consists of centrally mediated reciprocally organized flexion and extension commands along with a common coactivation command to both muscles. This control scheme allows for coactivation and reciprocal activation of an agonist-antagonist set. 6. The agonist-antagonist pair was observed to generate a net force in two control modalities: proportional activation and reciprocal activation. In proportional activation, the agonist-antagonist set is coactivated during either of two states: when uncertainty exists in the required task or when a compensatory force contraction is perceived to be required.(ABSTRACT TRUNCATED AT 400 WORDS)

Compression induced damage on in-situ severed and intact nerves

The effect of rapid as well as sustained compressive forces applied to the surface of intact and severed peroneal nerves of rabbits was studied. Considerable effort was taken to ensure a quantitative and consistent experimental paradigm. Stimuli were delivered to the sciatic nerve, and the compound action potential was recorded in the peroneal nerve, with compressive forces applied more proximally on the peroneal nerve. It was found that the conduction of action potentials on the larger nerve fibers was more sensitive to compressive force than that of the smaller nerve fibers, although all nerve fibers stopped conducting when sufficient compression was applied to the nerve. The effect on the conduction of action potentials on the nerve fibers appeared to be determined both by Laplace's law (as previously reported by others) and the viscoelastic properties of the entire nerve. Relatively low compressive forces (20 gm applied over approximately 7 sq mm) were found to decrease the neutral conduction of the larger nerve fibers for at least two hours, whereas stagnation of blood circulation was not found to affect measurably the neural conduction of all the nerve fibers for up to two hours.

New motor control assessment techniques for evaluating individuals with severe handicaps: a case study

Prescription of assistive devices for motor-handicapped individuals requires assessment of their motor capabilities. When patients' motor deficits are particularly severe, wide individual differences in the location and type of abnormalities complicate the assessment process. The precision of assessment has been greatly increased in recent years by the use of quantitative, computer-aided motion analysis, which facilitates statistical examination and comparison with normal individuals. This paper discusses a case study wherein a 24-year-old male nonvocal cerebral palsy patient was assessed for his ability to operate assistive communication devices. Three computer-aided measurement protocols were employed to evaluate the patient and two controls: performance using the patient's existing communication aid was evaluated in terms of rate and accuracy of communication using standardized spelling and response time tasks; volitional myoelectric activity was surveyed to identify possible myoelectric control sites for communication aid operation; a study of head position and its time derivatives was conducted to explore the feasibility of proportional control of a communication aid. Comparison of handicapped and control subject data indicated that, despite several characteristic motor control deficits, the handicapped subject was capable of proportional control of lateral head rotation and binary control of frontalis myoelectric signals. These movements could be used to operate a proportionally-controlled, direct-selection communication aid that could substantially increase the subject's communication rate. Work in progress includes the expansion of the handicapped and unimpaired subject databases and further development of the techniques discussed here to include three-dimensional motion analysis and objective measurement of muscle fatigue.

Effects of electrode location on myoelectric conduction velocity and median frequency estimates

The effect of surface electrode location on the estimates of the median frequency and conduction velocity of the myoelectric signal was investigated. The locations were identified with respect to the innervation zone and the tendonous portion of the tibialis anterior muscle. Considerable modifications in the median frequency and conduction velocity parameters were noted. The highest values of the median frequency occurred at the region of the innervation zone and tendonous insertion of the muscle, and decreased proportionally with distance from these areas. The rate of change of median frequency was not effected by electrode location. Estimates of conduction velocity were most stable in a region between the distal tendon and the adjacent innervation zone. This region also provided the best linear fit when comparing conduction velocity to median frequency estimates. The implications for signal detection procedures are discussed.

Median frequency of the myoelectric signal. Effects of hand dominance

A study was performed to investigate the existence of any distinction in the fatiguability of corresponding contralateral muscles in the hand as a function of hand dominance. The first dorsal interosseous muscle was studied. The median frequency of the myoelectric signal was employed to describe the fatigue behavior of the muscle. It was found that during sustained contractions the median frequency decreased faster in the non-dominant hand of right handed individuals, whereas, no statistically significant distinction could be found in left handed individuals. This distinction was evident in both male and female subjects. This study demonstrates that continued preferential usage of a muscle is associated with altered electrical properties of the myoelectric signal and that the median frequency of the signal provides an appropriate measure of the modifications. It is argued that the findings may provide an indication of modifications in the metabolic properties of muscle fibers induced by a lifetime of preferred functional use.

Motor unit recruitment and firing rates interaction in the control of human muscles

Muscle contractions are modulated by the number of motor units recruited and their respective firing rates. The work described in this report documents an interplay between recruitment and firing rates of motor units. The recruitment of a new motor unit appears to have a disfacilitatory influence on the firing rates of previously activated motor units. It is speculated that this effect is likely to be mediated, at least partially, via the stretch reflex loop and possibly by the recurrent inhibition of the Renshaw circuit. Such a mechanism would be functionally useful in providing smooth control of muscle output via peripheral circuitry (consisting of proprioceptive reflexes and recurrent inhibition), thus lessening the amount of detailed supervision of the alpha-motoneuron pool required by the central nervous system.