Modulation of human cutaneous reflexes during rhythmic cyclical arm movement

The organization and pattern of cutaneous reflex modulation is unknown during rhythmic cyclical movements of the human upper limbs. On the assumption that these cyclic arm movements are central pattern generator (CPG) driven as has been suggested for leg movements such as walking, we hypothesized that cutaneous reflex amplitude would be independent of electromyographic (EMG) muscle activation level during rhythmic arm movement (phase-dependent modulation, as is often the case in the lower limb during locomotion). EMG was recorded from eight muscles crossing the human shoulder, elbow, and wrist joints while whole arm rhythmic cyclical movements were performed. Cutaneous reflexes were evoked with trains of electrical stimulation delivered at non-noxious intensities (approximately 2 x threshold for radiating paresthesia) to the superficial radial nerve innervating the lateral portion of the back of the hand. Phasic bursts of rhythmic muscle activity occurred throughout the movement cycle. Rhythmic EMG and kinematic patterns were similar to what has been seen in the human lower limb during locomotor activities such as cycling or walking: there were extensive periods of reciprocal activation of antagonist muscles. For most muscles, cutaneous reflexes were modulated with the movement cycle and were strongly correlated with the movement-related background EMG amplitude. It is concluded that cutaneous reflexes are primarily modulated by the background muscle activity during rhythmic human upper limb movements, with only some muscles showing phase-dependent modulation.     

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.     

Advanced biofeedback from surface electromyography signals using fuzzy system

The aims of this study were to develop a fuzzy inference-based biofeedback system and investigate its effects when inducing active (shoulder elevation) and passive (relax) pauses on the trapezius muscle electromyographic (EMG) activity during computer work. Surface EMG signals were recorded from clavicular, descending (bilateral) and ascending parts of the trapezius muscles during computer work. The fuzzy system readjusted itself based on the history of previous inputs. The effect of feedback was assessed in terms of muscle activation regularity and amplitude. Active pause resulted in non-uniform muscle activity changes in the trapezius muscle depicted by increase and decrease of permuted sample entropy in ascending and clavicular parts of trapezius, respectively (P < 0.05) compared with no pause. Concomitantly, the normalized root mean square of EMG increased approximately 5% in descending part of trapezius bilaterally (P < 0.01). These findings confirm that advanced feedback can change the pattern of muscle activation.     

Surface EMG amplitude and frequency dependence on exerted force for the upper trapezius muscle: a comparison between right and left sides

Summary Surface electromyographic (EMG) amplitude and mean power frequency (MPF) were used to study the isometric muscular activity of the right versus the left upper trapezius muscles in 14 healthy right-handed women. The EMG activity was recorded simultaneously with force signals during a 10–15 s gradually increasing exertion of force, up to maximal force. Only one side at a time was tested. On both sides there was a significant increase in EMG amplitude (V) during the gradually increasing force from 0% to 100% maximal voluntary contraction (MVC). The right trapezius muscle showed significantly less steep slopes for regression of EMG amplitude versus force at low force levels (0%–40% MVC) compared intra-individually with high force levels (60%–100% MVC). This was not found for the left trapezius muscle. At 40% MVC a significantly lower MPF value was found for the right trapezius muscle intra-individually compared with the left. An increase in MPF between 5% and 40% MVC was statistically significant when both sides were included in the test. The differences in EMG activity between the two sides at low force levels could be due to more slow-twitch (type I fibres) motor unit activity in the right trapezius muscles. It is suggested that this is related to right-handed activity.     

Evidence for repetitive load in the trapezius muscle during a tapping task

Many studies describe the trapezius muscle activation pattern during repetitive key-tapping focusing on continuous activation. The objectives of this study were to determine whether the upper trapezius is phasically active during supported key tapping, whether this activity is cross-correlated with forearm muscle activity, and whether trapezius activity depends on key characteristic. Thirteen subjects (29.7 ± 11.4 years) were tested. Surface EMG of the finger’s extensor and flexor and of the trapezius muscles, as well as the key on–off signal was recorded while the subject performed a 2-min session of key tapping at 4 Hz. The linear envelopes obtained were cut into single tapping cycles extending from one onset to the next onset signal and subsequently time-normalized. Effect size between mean range and maximal standard deviation was calculated to determine as to whether a burst of trapezius muscle activation was present. Cross-correlation was used to determine the time-lag of the activity bursts between forearm and trapezius muscles. For each person the mean and standard deviation of the cross-correlations coefficient between forearm muscles and trapezius were determined. Results showed a burst of activation in the trapezius muscle during most of the tapping cycles. The calculated effect size was ≥0.5 in 67% of the cases. Cross-correlation factors between forearm and trapezius muscle activity were between 0.75 and 0.98 for both extensor and flexor muscles. The cross-correlated phasic trapezius activity did not depend on key characteristics. Trapezius muscle was dynamically active during key tapping; its activity was clearly correlated with forearm muscles’ activity.     

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% EMG_max). EMG pulses at 15–20% EMG_max 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% EMG_max 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.     

Functional subdivision of the upper trapezius muscle during low-level activation

The electromyographic (EMG) amplitude was recorded using bipolar surface electrodes placed at different positions above the upper trapezius muscle of 16 healthy subjects. One of the aims of this study was to investigate the variation in EMG activity between electrode positions. For this purpose three tasks were performed: a mental activation test, a dynamic movement test and 90° arm abduction. The EMG signals were full-wave rectified and averaged within windows that were 0.2?s in length. Normalized EMG activity showed significantly different EMG amplitudes at different electrode positions for two of the three tasks. The second aim of this study was to investigate whether the upper trapezius muscle may be functionally subdivided. For this purpose the normalized EMG amplitudes of each task were compared with the EMG amplitude recorded during submaximal shoulder elevation. While the EMG level was similar at one electrode position, significant differences were found at some of the other electrode positions, indicating a functional subdivision of the muscle. The present results indicate that for comparisons of upper trapezius EMG activity levels between some tasks or between subjects, it is worthwhile to make EMG recordings at several electrode positions.     

Effects of sleep on the tonic drive to respiratory muscle and the threshold for rhythm generation in the dog.

1. The present study was designed to determine the effect of sleep on the tonic output to respiratory muscle and on the level of chemical respiratory stimulation required to produce rhythmic respiratory output. 2. Chronically implanted electrodes recorded expiratory (triangularis sterni) and inspiratory (diaphragm and parasternal intercostal) electromyographic (EMG) activities in three trained dogs during wakefulness and sleep. The dogs were mechanically hyperventilated via an endotracheal tube inserted into a permanent tracheostomy. During the studies, a cold block of the cervical vagus nerves was maintained to avoid the complicating effects of vagal inputs on respiratory drive and rhythm. 3. During wakefulness, steady-state hypocapnia (partial pressure of CO2, PCO2 = 30 mmHg) abolished inspiratory EMG activity, resulting in apnoea, but the expiratory muscle became tonically active. Compared to wakefulness, the level of the tonic expiratory EMG activity was decreased in non-REM (non-rapid eye movement) sleep (median decrease = 34%, P = 0.005) and was further decreased in REM sleep (median decrease = 78%, P < 0.0001). During REM sleep, the tonic expiratory EMG activity was highly variable (mean coefficient of variation = 39% compared to 7% awake, P < 0.0001) and in some periods of REM, bursts of inspiratory EMG activity and active breathing movements were observed despite the presence of hypocapnia. 4. During constant mechanical hyperventilation, progressive increases in arterial PCO2 (in hyperoxia) were produced by rebreathing. Measurement of the CO2 threshold for the onset of spontaneous breathing showed that this threshold was not different between wakefulness and non-REM sleep (mean difference = 0.1 mmHg from paired observations, 95% confidence interval for the difference = -1.0 to +1.1 mmHg, P = 0.898). 5. The results show that sleep reduces the tonic output to respiratory muscles but does not increase the CO2 threshold for the generation of rhythmic respiratory output. These observations suggest that changes in the tonic drives to the respiratory motoneurones may be a principal mechanism by which changes in sleep-wake states produce changes in respiratory output.     

Oscillatory firing of single human sphincteric alpha 2 and alpha 3-motoneurons reflexly activated for the continence of urinary bladder and rectum. Restoration of bladder function in paraplegia

1. By recording with 2 pairs of wire electrodes from human sacral nerve roots (S3-S5) rhythmic as well as occasional firing was observed in alpha 2 and alpha 3-motoneurons in response to physiologic stimulation of the urinary bladder and the anal canal. The rhythmic firing consisted of periodically occurring impulse trains, most likely produced by true spinal oscillators which drove the motoneurons. 2. Alpha 2-motoneurons, innervating fast fatigue-resistant muscle fibres, were observed to fire with impulse trains of about 2 to 4 action potentials (Ap's). These impulse trains occurred every 110 to 170 msec (5-9 Hz). Alpha 3-motoneurons, innervating slow fatigue-resistant muscle fibres, fired about every 1400 msec (approximately 0.7 Hz) with impulse trains of about 11 to 60 Ap's. Alpha 1-motoneurons, innervating fast fatigue muscle fibres, and gamma-motoneurons were not observed in the continuous oscillatory firing mode. 3. Sphincteric motoneurons were observed most likely in the oscillatory firing mode in response to the sustained stretch (reflex) of the external and sphincter or to retrograde filling of the bladder (urethro-sphincteric guarding reflex), in order to preserve continence. A urethral sphincteric alpha 2-motoneuron increased its mean activity from 0.5 to 18 Ap's/sec during retrograde filling by changing its firing pattern from the occasional spike mode via the transient oscillatory firing mode to the continuous oscillatory mode. Up to a filling of the bladder of 500 ml the mean activity of the stretch receptors, measuring probably mural tension, increased roughly proportionally and the sphincteric motoneuron increased its activity to about 1 Ap/sec in the occasional spike mode. Up to 600 ml, the motoneuron responded in the transient oscillatory mode with mean activities of up to 5 Ap's/sec. With higher bladder fillings, the flow receptors afferents fired additionally, probably according to pressure symptoms, and the motoneuron switched into the continuous oscillatory firing mode and increased its activity up to 18 Ap's/sec at 700 ml. When the bladder was about 800 ml full, the stretch afferent activity decreased, the flow receptor activity increased strongly and the alpha 2-motoneuron activity decreased; the overflow incontinence had probably started. Micturition was not observed, probably because of brain death. 4. It is suggested that one adequate stimulus for an alpha 2-motoneuron of the external anal sphincter to jump into the oscillatory firing mode, was the activity from secondary spindle afferent (SP2) fibres from external anal sphincter muscle spindles.(ABSTRACT TRUNCATED AT 400 WORDS)     

Mechanical performance and electromyography during repeated maximal isokinetic shoulder forward flexions in female cleaners with and without myalgia of the trapezius muscle and in healthy controls

This cross-sectional study aimed at investigating the influence of occupational exposure to static and highly repetitive work involving the neck and shoulder muscles, myalgia of and tender point in the trapezius muscle on biomechanical output, and electromyogram (EMG) variables (mean frequency MNF, signal amplitude and ability to relax) during maximal forward flexions of the shoulder muscles. Groups of 25 cleaners suffering from chronic myalgia of the trapezius muscle, 25 cleaners free from myalgia of the trapezius muscle and 21 teachers performed 150 forward flexions using an isokinetic dynamometer. Perception of fatigue was reported and surface EMG was recorded from four muscles during the endurance test. The cleaners were stronger than the teachers. Myalgia was associated with lower levels of endurance and a high degree of perceived fatigue. The ability to relax the trapezius muscle decreased with age and was even lower in cleaners with and without myalgia. Higher MNF of the deltoid muscle but not of the trapezius muscle was found in the group suffering from myalgia compared to the groups free from myalgia. This cross-sectional study indicated that myalgia of the trapezius muscle did not influence the strength but did influence the endurance of the forward flexor muscles of cleaners. The observed decrease in the ability to relax the trapezius muscle in cleaners compared to healthy teachers might be indicative of a future insufficiency in the muscle. Prospective studies are needed to define the significance of the results presented here.     

Mechanical performance and electromyography during repeated maximal isokinetic shoulder forward flexions in female cleaners with and without myalgia of the trapezius muscle and in healthy controls

This cross-sectional study aimed at investigating the influence of occupational exposure to static and highly repetitive work involving the neck and shoulder muscles, myalgia of and tender point in the trapezius muscle on biomechanical output, and electromyogram (EMG) variables (mean frequency MNF, signal amplitude and ability to relax) during maximal forward flexions of the shoulder muscles. Groups of 25 cleaners suffering from chronic myalgia of the trapezius muscle, 25 cleaners free from myalgia of the trapezius muscle and 21 teachers performed 150 forward flexions using an isokinetic dynamometer. Perception of fatigue was reported and surface EMG was recorded from four muscles during the endurance test. The cleaners were stronger than the teachers. Myalgia was associated with lower levels of endurance and a high degree of perceived fatigue. The ability to relax the trapezius muscle decreased with age and was even lower in cleaners with and without myalgia. Higher MNF of the deltoid muscle but not of the trapezius muscle was found in the group suffering from myalgia compared to the groups free from myalgia. This cross-sectional study indicated that myalgia of the trapezius muscle did not influence the strength but did influence the endurance of the forward flexor muscles of cleaners. The observed decrease in the ability to relax the trapezius muscle in cleaners compared to healthy teachers might be indicative of a future insufficiency in the muscle. Prospective studies are needed to define the significance of the results presented here. Accepted: 26 June 2000     

The association between nocturnal trapezius muscle activity and shoulder and neck pain

The study investigated the possible association between nocturnal trapezius muscle activity and shoulder and neck pain. Sixty female subjects participated in the study, 33 were classified as pain-afflicted on the basis of shoulder and neck pain reports for the previous 6 months. Electromyographic (EMG) activity was monitored bilaterally from the trapezius in all recordings. EMG recording of the deltoid, biceps, and hand flexors was included for 26 subjects (17 with pain) to provide a comparative basis for evaluation of the trapezius recordings. There was considerable variation in the amount of muscle activity between subjects; however, some subjects presented a continuous, low-level activity pattern throughout the presumed sleep period. Subjects classified as pain-afflicted had significantly higher activity level and longer duration of trapezius EMG activity than the pain-free subjects. The deltoid had significantly more activity than the trapezius, while the biceps and the hand flexors presented similar activity level and duration as the trapezius. Nocturnal trapezius activity was not associated with pain exacerbation the same night. We suggest nocturnal trapezius muscle activity is a pointer to physiological mechanisms that contribute to some forms of shoulder and neck pain, such as trapezius myalgia, but nocturnal muscle activity may not be casually implicated in the pain-induction process.     

Neural mechanisms of intermuscular coherence: implications for the rectification of surface electromyography

Oscillatory activity plays a crucial role in corticospinal control of muscle synergies and is widely investigated using corticospinal and intermuscular synchronization. However, the neurophysiological mechanisms that translate these rhythmic patterns into surface electromyography (EMG) are not well understood. This is underscored by the ongoing debate on the rectification of surface EMG before spectral analysis. Whereas empirical studies commonly rectify surface EMG, computational approaches have argued against it. In the present study, we employ a computational model to investigate the role of the motor unit action potential (MAUP) on the translation of oscillatory activity. That is, diverse MUAP shapes may distort the transfer of common input into surface EMG. We test this in a computational model consisting of two motor unit pools receiving common input and compare it to empirical results of intermuscular coherence between bilateral leg muscles. The shape of the MUAP was parametrically varied, and power and coherence spectra were investigated with and without rectification. The model shows that the effect of EMG rectification depends on the uniformity of MUAP shapes. When output spikes of different motor units are convolved with identical MUAPs, oscillatory input is evident in both rectified and nonrectified EMG. In contrast, a heterogeneous MAUP distribution distorts common input and oscillatory components are only manifest as periodic amplitude modulations, i.e., in rectified EMG. The experimental data showed that intermuscular coherence was mainly discernable in rectified EMG, hence providing empirical support for a heterogeneous distribution of MUAPs. These findings implicate that the shape of MUAPs is an essential parameter to reconcile experimental and computational approaches.     

Integrative re-organization mechanism for reducing tremor in Parkinson's disease patients

A special movement therapy, called coordination dynamics therapy, has been reported to have the potential to improve central nervous system (CNS) functioning in Parkinson's disease patients. Electromyography using surface electrodes (sEMG) showed that the rhythmic muscle activity leading to Parkinsonian tremor was generated in the patients by the impairment of two kinds of inhibition. First, some premotor spinal oscillators organized themselves in the CNS neuronal networks without strong adequate input and second, the oscillators synchronized their firing to give rise to rhythmic muscle activity and tremor. In this paper it will be shown that highly coordinated arm and leg movements, generated when exercising on a special coordination dynamics therapy device, can reduce Parkinsonian tremor in amplitude and frequency and improve CNS functioning in the short-term memory. sEMG measurements showed upon exercising on the special coordination dynamics therapy device that the motor program improved in the short-term memory and tremor muscle activity became coordinated with the volitional motor program and reduced in size and frequency. Higher load exercising seemed to better reduce tremor muscle activity, probably because the physiologic CNS organization was more integrative then and could 'bind' stronger simultaneous pathologic tremor activity. Moreover, the rhythmic synchronized motor unit firing in different arm and leg muscles was synchronized or coordinated and changed in frequency and amplitude. It is concluded that the integrative re-organization mechanism to reduce Parkinsonian tremor is the phase and frequency coordination between neuron firing of the physiologic neuronal network state, generated by the highly coordinated arm and leg movements, and the simultaneous pathologic tremor network state, generated by the uninhibited neurons, firing synchronized oscillatory.     

Effect of arm-shoulder fatigue on carpenters at work

Summary The purpose of the study was to analyse the effect of arm-shoulder fatigue on manual performance. Ten experienced carpenters performed three standardized tasks (nailing, sawing and screwing). Electromyographic activity was recorded from six arm-shoulder muscles and the performances were video-filmed. After 45 min of standardized arm-cranking (arm-shoulder-fatiguing exercise of approximately 70%–80% maximal oxygen consumption), the tasks were repeated. The number of work movements and the time taken for each task were recorded and the quality of the work performed was compared. After the fatiguing exercise, only nailing was perceived as being harder and more mistakes were made during nailing and sawing. Movement performance was not influenced during nailing but was slightly slower during sawing and faster during screwing. However, there were increased mean EMG amplitudes in the upper trapezius and biceps muscles during nailing, in the upper trapezius, anterior deltoid and infraspinatus muscles during sawing and in the anterior deltoid muscle during screwing. Of the muscles studied the upper trapezius and anterior deltoid muscles increased their activity most after the arm-shoulder-fatiguing exercise.     

Fibre type proportion and fibre size in trapezius muscle biopsies from cleaners with and without myalgia and its correlation with ragged red fibres, cytochrome-c-oxidase-negative fibres, biomechanical output, perception of fatigue, and surface electromyography during repetitive forward flexions

In the literature enlarged/increased cross-sectional area (CSA) of type I muscle fibres has been reported as a morphological mark of work-related localised myalgia in the descending part of the trapezius muscle of women. These studies did not use enough subjects or lacked an adequate control group. The recording of surface electromyograms (EMG) is central to the research field of work-related myalgia. However, the influence of intrinsic muscle properties – such as the effect of muscle fibre distribution upon surface EMG – has to be better understood in order properly to evaluate this method as a possible diagnostic and preventive tool. This study had two aims. Firstly, it investigated the muscle fibre distribution and CSA in work-related myalgia in trapezius muscles. Secondly, the multivariate relationships among muscle morphology and histochemistry [ragged-red (RR) fibres, and cytochrome-c-oxidase-negative-fibre changes] EMG, perceived fatigue, and biomechanical output of shoulder flexions were analysed. The raw data have been presented in an earlier study. The participants in this study were 25 female cleaners with work-related myalgia of the trapezius muscle and 25 female cleaners not experiencing work-related myalgia of this muscle. The control group comprised 21 healthy female teachers who had not been exposed to highly repetitive work or static muscle work. Smaller CSA of type II fibres were found in cleaners compared to teachers. In this study the CSA of type I fibres of the trapezius muscles associated with myalgia were no greater than in muscles without myalgia. The prevalence of RR fibres together with age, fibre type proportions, CSA and working as a cleaner correlated with the ability to relax as recorded electromyographically. The relative mean frequency of the EMG of the trapezius muscle correlated with the prevalence of RR fibres, but it did not correlate with the proportions and CSA of different fibre types. Low biomechanical outputs and low signal amplitude increases of the EMG during the test were associated with high proportions of type IIB fibres. The smaller CSA of type II fibres in cleaners might have reflected a different muscle activation pattern due to different occupational demands in cleaners than in teachers. Morphological or histopathological variables can influence the three EMG variables investigated. Electronic Publication     

Neural regulation of rhythmic arm and leg movement is conserved across human locomotor tasks

It has been proposed that different forms of rhythmic human limb movement have a common central neural control ('common core hypothesis'), just as in other animals. We compared the modulation patterns of background EMG and cutaneous reflexes during walking, arm and leg cycling, and arm-assisted recumbent stepping. We hypothesized that patterns of EMG and reflex modulation during cycling and stepping (deduced from mathematical principal components analysis) would be comparable to those during walking because they rely on similar neural substrates. Differences between the tasks were assessed by evoking cutaneous reflexes via stimulation of nerves in the foot and hand in separate trials. The EMG was recorded from flexor and extensor muscles of the arms and legs. Angular positions of the hip, knee and elbow joints were also recorded. Factor analysis revealed that across the three tasks, four principal components explained more than 93% of the variance in the background EMG and middle-latency reflex amplitude. Phase modulation of reflex amplitude was observed in most muscles across all tasks, suggesting activity in similar control networks. Significant correlations between EMG level and reflex amplitude were frequently observed only during static voluntary muscle activation and not during rhythmic movement. Results from a control experiment showed that strong correlation between EMG and reflex amplitudes was observed during discrete, voluntary leg extension but not during walking. There were task-dependent differences in reflex modulation between the three tasks which probably arise owing to specific constraints during each task. Overall, the results show strong correlation across tasks and support common neural patterning as the regulator of arm and leg movement during various rhythmic human movements.     

Experimental pain leads to reorganisation of trapezius electromyography during computer work with active and passive pauses

The aim of this laboratory study was to investigate acute effects of experimental muscle pain on spatial electromyographic (EMG) activity of the trapezius muscle during computer work with active and passive pauses. Twelve healthy male subjects performed four sessions of computer work for 2 min in one day, with passive (relax) and active (30% maximum voluntary contraction of shoulder elevation) pauses given every 40 s without and with presence of experimental pain. Surface EMG signals were recorded from four parts of the trapezius. The centroid of exposure variation analysis along the time axis was lower during computer work with active pauses when compared with passive one in all muscle parts (P < 0.05). In presence of experimental pain, EMG amplitude increased in transverse and ascending parts and relative rest time decreased in ascending part. The results of this study showed a more variable trapezius activity pattern and increased activity with active compared with passive pauses, a lowered trapezius rest with presence of experimental pain, and increased activity in the transverse and ascending parts of trapezius due to experimental pain during computer work. Acute pain led to muscle activation pattern during computer work considered to increase the risk of developing work-related musculoskeletal disorders.     

Behavior of jaw muscle spindle afferents during cortically induced rhythmic jaw movements in the anesthetized rabbit

The regulation by muscle spindles of jaw-closing muscle activity during mastication was evaluated in anesthetized rabbits. Simultaneous records were made of the discharges of muscle spindle units in the mesencephalic trigeminal nucleus, masseter and digastric muscle activity (electromyogram [EMG]), and jaw-movement parameters during cortically induced rhythmic jaw movements. One of three test strips of polyurethane foam, each of a different hardness, was inserted between the opposing molars during the jaw movements. The induced rhythmic jaw movements were crescent shaped and were divided into three phases: jaw-opening, jaw-closing, and power. The firing rate of muscle spindle units during each phase increased after strip application, with a tendency for the spindle discharge to be continuous throughout the entire chewing cycle. However, although the firing rate did not change during the jaw-opening and jaw-closing phases when the strip hardness was altered, the firing rate during the power phase increased in a hardness-dependent manner. In addition, the integrated EMG activity, the duration of the masseteric bursts, and the minimum gape increased with strip hardness. Spindle discharge during the power phase correlated with jaw-closing muscle activity, implying that the change in jaw-closing muscle activity associated with strip hardness was caused by increased spindle discharge produced through insertion of a test strip. The increased firing rate during the other two phases may be involved in a long-latency spindle feedback. This could contribute to matching the spatiotemporal pattern of the central pattern generator to that of the moving jaw.     

Motor unit substitution in long-duration contractions of the human trapezius muscle.

We examined the activity pattern of low-threshold motor units in the human trapezius muscle during contractions of 10 min duration. Three procedures were applied in sequence: 1) static contraction controlled by maintaining a constant low level of the surface electromyogram (EMG)-detected root-mean-square signal, 2) a manipulation task with mental concentration, and 3) copying a text on a word processor. A quadrifilar fine-wire electrode was used to record single motor unit activity. Simultaneously, surface electrodes recorded the surface EMG signal. During these contractions, low-threshold motor units showed periods of inactivity and were substituted by motor units of higher recruitment threshold. This phenomenon was not observed during the first few minutes of the contraction. In several cases the substitution process coincided with a short period of inactivity in the surface EMG pattern. Substitution was observed in five of eight experiments. These observations may be explained by a time-variant recruitment threshold of motor units, sensitive to their activation history and temporal variation in the activity patterns. We speculate that the substitution phenomenon protects motor units in postural muscles from excessive fatigue when there is a demand for sustained low-level muscle activity.