EMG activation patterns during force production in precision grip. III. Synchronisation of single motor units

Motor unit (MU) synchronisation during isometric force production in the precision grip was analysed in five subjects performing a visually guided steptracking motor task with three different force levels. With this aim multi-unit electromyographic (EMG) activity of 14 intrinsic and extrinsic finger muscles from 15 experimental sessions was decomposed into the potentials of single MUs. The behaviour of 62 intrinsic and 30 extrinsic MUs in the motor task was quantified. Most MUs displayed a positive correlation between firing rate and grip force. Compared to MUs in extrinsic muscles, intrinsic MUs had steeper regression lines with negative intercepts indicating higher force sensitivity and higher recruitment thresholds. A cross-correlation analysis was performed for 69 intra- and 166 intermuscular MU pairs while steady grip force was exerted at the three force levels. Synchronisation, for at least one force level, was found in 78% of the intra- and 45% of the intermuscular pairs. The occurrence of synchronisation was not stable over the force range tested. Factors influencing the fluctuations in occurrence and strength of synchronisation were investigated. Force increase was not paralleled by increased synchronisation; in contrast, in most MU pairs, especially intermuscular pairs, synchronisation occurred preferentially at the lower force levels. The recruitment threshold appeared to play a determining role in synchronisation: the more similar the thresholds of two MUs, the greater the probability of them being synchronised at this force level. Synchronised MUs fired on average at a lower frequency than non-synchronised ones. Finally, synchronisation at the multi-unit EMG level does not indicate that all underlying MUs are synchronised, nor does the absence of temporal coupling at the multi-unit level indicate that none of the MUs is synchronised.     

Discharge properties of primate forearm motor units during isometric muscle activity

Activity of single motor units (MUs) was recorded in forelimb muscles of rhesus macaques while they generated isometric ramp-and-hold torques about the wrist. Multiunit electromyographic (EMG) activity was recorded from 10-12 identified flexor and extensor muscles of the wrist and digits with implanted EMG wire electrodes. Single MUs from these muscles were recorded with a remotely controlled tripolar microelectrode array. The parent muscle of each MU was determined by compiling MU-triggered averages of multiunit EMGs. The MU firing patterns during the isometric task were determined from response histograms aligned with change in torque. At moderate torque levels, MUs (n = 86) exhibited four types of discharge patterns during the ramp-and-hold trajectory: phasic-tonic (23%), tonic (33%), decrementing (39%), and phasic (5%). Phasic-tonic MUs exhibited a phasic burst of activity during the torque ramp which exceeded the firing rate during the static hold period. Both phasic-tonic and tonic MUs exhibited a constant mean firing rate during the hold period; the discharge of decrementing MUs gradually decreased during the static hold. Phasic MUs fired only during the change in force. The relation between MU firing rate and torque was investigated as the monkeys generated responses of different levels of static torque during the hold period. Mean firing rate during the hold was found to be proportional to static torque up to moderate torque levels, where it reached a maximum. In the linear range, the mean rate-torque slope was 3.4 +/- 1.9 imp/s per 10(5) dyn . cm (n = 9).     

Motor unit properties from three synergistic muscles during ramp isometric elbow extensions

Many tasks require synergistic activation of muscles that possess different architectural, mechanical, and neural control properties. However, investigations of the motor unit (MU) mechanisms which modulate force are mostly restricted to individual muscles and low forces. To explore the pattern of MU recruitment and discharge behavior among three elbow extensors (lateral and long heads of the triceps brachii, and anconeus) during ramp isometric contractions, recruitment thresholds of 77 MUs in five young men were determined and corresponding MU discharge rates were tracked in 1-s epochs over forces ranging from 0 to 75 % of maximal voluntary isometric force (MVC). Across all forces, MUs in the lateral head discharged at higher rates than those in the anconeus (p < 0.001, Δ = 0.23). When all MUs were considered, recruitment thresholds in the long head of the triceps brachii were higher than the lateral head (p < 0.05, Δ = 0.70) with a trend (p = 0.08, Δ = 0.48) for higher recruitment thresholds in the long head compared with the anconeus. Together, these data indicate a potential mechanical disadvantage of the long head of the triceps brachii at 0° shoulder flexion. However, among low-threshold MUs (<10 % MVC), recruitment thresholds were lower in the anconeus than in both heads of the triceps brachii consistent with the expected twitch contractile and fiber type differences among these muscles. These findings illustrate the importance of considering synergistic relations among muscles used for a coordinated task, and the sensitivity of synergies to muscle architectural, mechanical, and possibly specific synaptic input factors.     

Disturbances of motor output in a cat hindlimb muscle after acute dorsal spinal hemisection

Summary Force and electromyogram (EMG) responses of the medial gastrocnemius muscle were assessed during isometric contractions in 8 decerebrate cat preparations, before and after acute dorsal hemisection of the spinal cord at the T12 level. The measures derived included the relation between static force and mean rectified EMG, the EMG amplitude distribution, EMG power spectral density, and force power spectral density. Our findings were that the spinal lesion induced modifications in the shape of the EMG amplitude distribution, a substantial increase in mean rectified EMG per unit force, and increases in EMG spectral power and force spectral power over a broad band of frequencies. In 7/8 preparations, there was disproportionate enhancement of EMG spectral power below 40 Hz, with a commensurate reduction in the EMG mean power frequency (MPF) in 6 of these 7 cases. Recordings of motoneuron discharge from 9 decerebrate preparations taken before and after the spinal hemisection revealed that the lesion-induced changes in EMG and force power spectra were accompanied by lower mean discharge rates, and by a compression of the range of recruitment force. These changes in motoneuron rate and recruitment were probably responsible for the changes in EMG and force measures, especially for the relative increase in low-frequency EMG power. If these acute disturbances of motoneuron rate and recruitment persist in chronic human neurological disorders, they represent an important and largely unrecognized source of muscular weakness and increased fatigability.     

Localization and contractile properties of intrinsic longitudinal motor units of the rat tongue

Tongue dysfunction is a hallmark of many human clinical disorders, yet we lack even a rudimentary understanding of tongue neural control. Here, the location and contractile properties of intrinsic longitudinal motor units (MUs) of the rat tongue body are described to provide a foundation for developing and testing theories of tongue motor control. One hundred and sixty-five MUs were studied by microelectrode penetration and stimulation of individual motor axons coursing in the terminal portion of the lateral (retrusor) branch of the hypoglossal nerve in the rat. Uniaxial MU force was recorded by a transducer attached to the protruded tongue tip, and MU location was estimated by electromyographic (EMG) electrodes implanted into the anterior, middle, and posterior portions of the tongue body. All MUs produced retrusive force. MU twitch force ranged from 2-129 mg (mean = 35 mg) and tetanic force ranged from 9-394 mg (mean = 95 mg). MUs reached maximal twitch force in 8-33 ms (mean = 15 ms) and were resistant to fatigue; following 2 min of stimulation, MUs (n = 11) produced 78-131% of initial force. EMG data were collected for 105 MUs. For 65 of these MUs, the EMG response was confined to a single electrode location: for 26 MUs to the anterior, 21 MUs to the middle, and 18 MUs to the posterior portion of the tongue. Of the remaining MUs, EMG responses were observed in two (38/40) or all three (2/40) tongue regions. These data provide the first contractile measures of identified intrinsic tongue body MUs and the first evidence that intrinsic longitudinal MUs are restricted to a portion of tongue length. Localization of MU territory suggests a role for intrinsic MU in the regional control of the mammalian tongue observed during feeding and speech.     

Evaluation of motor unit firing rates by standard concentric needle electromyography.

Motor unit firing rates at slight voluntary contraction were studied by standard concentric needle electromyography. Employing digital signal analysis techniques firing rates of motor units (MU) could be evaluated as long as four or less different MUs were activated in the vicinity of the concentric needle electrode. The extension of the recording area is defined by the recording properties of the electrode and the upper limit of rise-time for all MUPs being evaluated. Distant MUs, generating volume conducted potentials with rise-times greater than 0.8 ms, were excluded. In biceps muscles of 15 healthy controls the firing rate of the MU activated first was evaluated at that moment, when a second MU was recruited and was found to be 12.1 +/- 2.1 Hz (mean +/- S.D., n = 40). The firing rate of the fastest MU out of 2, 3, or 4 simultaneously active MUs was 10.7 +/- 2.5 Hz, 10.9 +/- 2.5 Hz and 10.6 +/- 2.4 Hz respectively. Hence at low innervation level there is no increase of firing rate with rising number of activated MUs. The upper normal limit of MU firing rate (3 sigma interval) is calculated as 17 Hz, irrespective whether 1, 2, 3 or 4 MUs are active within the recording area. Fifteen patients with partially denervated biceps muscles were investigated. Maximal firing rates were increased in 10 patients, all showing moderate or severe paresis (grade 1-3). In 10 patients suffering from myopathies firing rates always were normal. The presented data may serve as an additional criterion in evaluating MU firing rates during standard clinical EMG.     

Effects of aging on force variability,single motor unit discharge patterns,and the structure of 10, 20, and 40 Hz EMG activity

The purpose of this investigation was to examine the discharge properties of single motor units and the structure of the rectified 10, 20, and 40 Hz electromyographic (EMG) activity to determine a physiological correlate for the greater force variability with aging. Young (n=10; mean: 22+/-1 years), old (n=10; mean: 67+/-2 years), and older-old (n=10; mean: 82+/-5 years) adult humans produced isometric second finger abduction force in both constant and sine-wave tasks at 5, 10, 20, and 40% of their maximal voluntary contraction. Force and fine-wire intramuscular electromyography were recorded from the first dorsal interosseous muscle. The amount and time-dependent structure of the discharge rate variability of single motor units and Fourier analysis of the rectified intramuscular EMG was performed. Force output variability increased across the young, old, and older-old groups. The amount and time-dependent structure of the discharge rate variability of single motor units did not differ between the young and aging groups. There was a progressive decrease in the relative power of approximately 40 Hz EMG activity from the young>old>older-old subjects across the 5, 10, 20, and 40% maximum voluntary contraction (MVC) force levels. There was also a progressive increase in the relative power of the approximately 10 Hz EMG activity from young相似文献   

The role of synchrony and oscillations in the motor output

 There is currently much interest in the synchronisation of neural discharge and the potential role it may play in information coding within the nervous system. We describe some recent results from investigations of synchronisation within the motor system. Local field potentials (LFPs) and identified pyramidal tract neurones (PTNs) were recorded from the primary motor cortex of monkeys trained to perform a precision grip task. The LFPs showed bursts of oscillatory activity at 20–30 Hz, which were coherent with the rectified electromyographs (EMG) of contralateral hand and forearm muscles. This oscillatory synchronisation showed a highly specific task dependence, being present only during the part of the task when the animal maintained a steady grip and not during the movement phases before or after it. PTNs were phase-locked to LFP oscillations, implying that at least part of the coherence between cortical activity and EMG was mediated by corticospinal fibres. The phase locking of the PTNs to LFP oscillations produced task-dependent oscillatory synchronisation between PTN pairs, as assessed by the single-unit cross-correlation histogram. Recordings were also made from normal human subjects performing a precision grip similar to that used in the monkey recordings. Pairs of EMGs recorded from intrinsic hand and forearm muscles showed 20–30 Hz coherence, which modulated during task performance, being present only during periods of steady contraction. We suggest that these changes in EMG-EMG synchronisation reflect changing levels of synchronous drive from the corticospinal system. The generation of oscillations in the cortex is discussed in the light of results from a model of local cortical circuits. Other modelling work has shown that synchrony in the corticospinal inputs could act to recruit motoneurones more efficiently, producing more output force from a muscle than asynchronous inputs firing at the same mean rate. A speculative hypothesis is presented on the role of synchronous oscillations in the motor system, which is consistent with experimental observations to date. Received: 17 July 1998 / Accepted: 10 December 1998     

Task-related behaviour of motor units in the human temporalis muscle

Muscle activity patterns in some complex human jaw muscles appear to be task sensitive. However, it is presently uncertain how changes in motor task affect motor unit (MU) behaviour in the human temporalis muscle. In this study, activity was recorded from 40 MUs in the anterior region of the muscle. The lowest sustainable firing frequency (LSFF) was reached by slow increases and decreases in firing rate, then firing was maintained at the lowest possible rate without significant pauses. An array of consecutive interspike intervals (ISI) were sampled digitally and used to measure the LSFF for each task associated with the MU. In a controlled paradigm, MU reflex inhibition was measured during the performance of different tasks. Single electrical pulses of non-noxious intensity were delivered to the gingiva near the maxillary canine tooth. During continuous MU firing at a controlled firing frequency of 10 Hz, series of pulses were delivered with increasing delays, after preselected spikes. The MUs fired continuously during the performance of 1–4 postural and tooth-contact tasks. There were significant differences in LSFFs between tasks in those MUs associated with multiple tasks. In the reflex study, all MUs were inhibited, but the magnitude of the inhibition was highly task dependent. Thus, both LSFF and reflex inhibition of temporalis MUs appear to vary with the motor task and are sensitive to the position of the jaw and the direction and location of tooth contact along the tooth row. This behaviour most likely reflects task-related changes in output from orofacial and muscle afferents.     

Sleep/wake firing patterns of human genioglossus motor units

Although studies of the principal tongue protrudor muscle genioglossus (GG) suggest that whole muscle GG electromyographic (EMG) activities are preserved in nonrapid eye movement (NREM) sleep, it is unclear what influence sleep exerts on individual GG motor unit (MU) activities. We characterized the firing patterns of human GG MUs in wakefulness and NREM sleep with the aim of determining 1) whether the range of MU discharge patterns evident in wakefulness is preserved in sleep and 2) what effect the removal of the "wakefulness" input has on the magnitude of the respiratory modulation of MU activities. Microelectrodes inserted into the extrinsic tongue protrudor muscle, the genioglossus, were used to follow the discharge of single MUs. We categorized MU activities on the basis of the temporal relationship between the spike train and the respiration cycle and quantified the magnitude of the respiratory modulation of each MU using the eta (eta(2)) index, in wakefulness and sleep. The majority of MUs exhibited subtle increases or decreases in respiratory modulation but were otherwise unaffected by NREM sleep. In contrast, 30% of MUs exhibited marked sleep-associated changes in discharge frequency and respiratory modulation. We suggest that GG MUs should not be considered exclusively tonic or phasic; rather, the discharge pattern appears to be a flexible feature of GG activities in healthy young adults. Whether such flexibility is important in the response to changes in the chemical and/or mechanical environment and whether it is preserved as a function of aging or in individuals with obstructive sleep apnea are critical questions for future research.     

Motor unit recruitment and rate coding in response to fatiguing shoulder abductions and subsequent recovery

The purpose of the present study was to investigate motor unit (MU) recruitment and firing rate, and the MU action potential (MUAP) characteristics of the human supraspinatus muscle during prolonged static contraction and subsequent recovery. Eight female subjects sustained a 30° shoulder abduction, requiring 11–12% of maximal voluntary contraction (MVC), for 30 min. At 10 and 30 min into the recovery period, the shoulder abduction was repeated for 1 min. The rating of perceived exertion for the shoulder region increased to “close to exhaustion” during the prolonged contraction, and the surface electromyography (EMG) recorded from the deltoid and trapezius muscles showed signs of local muscle fatigue. From the supraspinatus muscle, a total of 23,830 MU firings from 265 MUs were identified using needle electrodes. Of the identified MUs, 95% were continuously active during the 8-s recordings, indicating a low degree of MU rotation. The mean (range) MU firing rate was 11.2 (5.7–14.5) Hz, indicating the relative force contribution of individual MUs to be larger than the overall mean shoulder muscle load. The average MU firing rate remained stable throughout the prolonged abduction, although firing rate variability increased in response to fatigue. The average concentric MUAP amplitude increased by 38% from the beginning (0–6 min) to the end (24–29 min) of the contraction period, indicating recruitment of larger MUs in response to fatigue. In contrast, after 10 min of recovery the average MU amplitude was smaller than seen initially in the prolonged contraction, but not different after 30 min, while the MU firing rate was higher during both tests. In conclusion, MU recruitment plays a significant role during fatigue, whereas rate coding has a major priority during recovery. Furthermore, a low degree of MU rotation in combination with a high relative load at the MU level may imply a risk of overloading certain MUs during prolonged contractions. Accepted: 6 June 2000     

Electromyographic and mechanomyographic estimation of motor unit activation strategy in voluntary force production

Electromyographic and mechanomyographic estimation of motor unit activation strategy in voluntary force production. In order to determine whether electromyogram (EMG) and mechanomyogram (MMG) are suitable for the noninvasive estimation of the motor unit (MU) activation strategy, the EMG/force and MMG/force relationships were examined simultaneously during isometric ramp contractions in biceps brachii muscle. The highest mean power frequency (MPF) of the EMG, which reflects the full MU recruitment, was determined at 51% MVC. Two obvious inflection points were identified on the MMG-amplitude/force relationship that showed an initial slow increase followed by a rapid increase and a progressive decrease at higher force levels. Our results suggest that the MMG amplitude allows the estimation of the beginning of recruitment of MUs that innervate the first-twitch fibers in addition to identification of the full MU recruitment. The rate coding strategy was qualitatively reflected by the MMG-MPF/force relationship. We conclude that the MU activation strategy is estimated in more detail by the MMG than by the EMG.     

Physiological properties of the motor units of the wrist extensor muscles in man

Summary The physiological properties of 355 motor units (MUs) recorded in the extensor carpi radialis muscles were studied in 34 healthy human subjects during isometric contractions. MU selective twitches were educed from the whole muscle force using the spike-triggered averaging method. The twitch contraction times and twitch forces were measured. From these data it was attempted to estimate the distribution of fast and slow MUs in the muscles studied. MU recruitment thresholds were systematically measured during stereotyped slow ramp contractions (force increase=0.25 N·s^-1). Degrees of correlation between contraction times, twitch forces and recruitment thresholds were pair analysed by computing simple regression curves and correlation coefficients. The degrees of correlation were compared between 245 MUs recorded in 34 subjects and 66 MUs recorded in a single subject. Analysis of the instantaneous discharge frequency of 132 MUs showed the existence of a remarkable degree of correlation (correlation coefficient, r=-0.75) between the frequency rise times (discharge onset to maximal frequency) and the MU twitch contraction times; i.e., the frequency rise times increase when the twitch contraction times decrease. The possibility that muscle contraction may be differentially modulated on the basis of this discharge property of the MUs is discussed. The results are compared to previous data and the limitations of the spike-triggered averaging method applied to long muscles in man are extensively discussed.     

10 Hz periodic component influences lower frequency component of the physiological tremor at low force levels

A positive correlation has been reported between the amplitudes of the 10 Hz and lower frequency components of the physiological tremor (PT) at low force levels, though the generation mechanisms based on motor unit (MU) firing properties are different. This study aimed to investigate the causal relation between these fluctuations. A computer simulation was performed to alter the fluctuation intensity, which enabled manipulation of MU firing properties. Two types of MU contributions to synchronization activity were considered to influence the intensity of the 10 Hz PT: (1) number of MUs involved in synchronization and (2) synchrony between MUs. The impact of oscillatory excitatory input from the central nervous system on the generation of the 10 Hz PT was also evaluated. The results showed that the lower frequency fluctuation (LF fluctuation) was influenced by the number of MUs contributing to the 10 Hz PT amplitude. The synchrony between MUs and the oscillatory excitatory input had no influence on the LF fluctuation. In conclusion, MU synchronization in a certain frequency range increased the fluctuations not only at the synchronizing frequency but also at lower frequencies, and the number of MUs involved in synchronization was a plausible factor to explain the correlation between the 10 Hz and LF fluctuations.     

Non-invasive assessment of single motor unit mechanomyographic response and twitch force by spike-triggered averaging

A method for non-invasive assessment of single motor unit (MU) properties from electromyographic (EMG), mechanomyographic (MMG) and force signals is proposed. The method is based on the detection and classification of single MU action potentials from interference multichannel surface EMG signals and on the spike-triggered average of the MMG (detected by an accelerometer) and force signals. The first dorsal interosseous (FDI) and abductor digiti minimi (ADM) muscles were investigated at contraction levels of 2% and 5% of the maximum voluntary contraction (MVC) force. A third contraction was performed by selective activation of a single MU with surface MU action potential visual feedback provided to the subject. At 5% MVC, the mean (±standard error) single MU MMG peak-to-peak value was 11.0±1.8 mm s⁻² (N=17) and 32.3±6.5 mm s⁻² (N=20) for the FDI and AMD muscles, respectively. The peak of the twitch force was, at the same contraction livel, 7.41±1.34 mN and 14.42±2.92 mN, for the FDI and ADM muscles, respectively. The peak-to-peak value of the MMG was significantly different for the same MU at different contraction levels, indicating a non-linear summation of the single MU contributions. For the FDI muscle, the MMG peak-to-peak value of individual MUs was 21.5±7.8 mm s⁻², when such MUs were activated with visual feedback provided to the subject, whereas, for the same MUs, it was 11.8±3.8 mm s⁻², when the subject maintained a constant force level of 2% MVC. The method proposed allows the non-invasive assessment of single MU membrane and contractile properties during voluntary contractions.     

Electromyogram and mechanomyogram changes in fresh and fatigued muscle during sustained contraction in men

In surface electromyogram (EMG) and mechanomyogram (MMG) the electrical and mechanical activities of recruited motor units (MU) are summated. Muscle fatigue influences the electrical and mechanical properties of the active MU. The aim of this study was to evaluate fatigue-induced changes in the electrical and mechanical properties of MU after a short recovery period, using an analysis of force, surface EMG and MMG. In seven subjects the EMG and MMG were recorded from the biceps brachii muscle during sustained isometric effort at 80% of the maximal voluntary contraction (MVC), before (test 1) and 10 min after (test 2) a fatiguing exercise. From the time and frequency domain analysis of the signals, the root mean square (rms) and the mean frequency (f¯) of the power spectrum were calculated. The results were that the mean MVC was 412 (SEM 90) N and 304 (SEM 85) N in fresh and fatigued muscle, respectively; during tests 1 and 2 the mean EMG rms increased from 0.403 (SEM 0.07) mV to 0.566 (SEM 0.09) mV and from 0.476 (SEM 0.07) mV to 0.63 (SEM 0.09) mV, respectively; during test 1 the mean MMG rms decreased from 9.4 (SEM 0.8) mV to 5.7 (SEM 0.9) mV; in contrast, during test 2 constantly lower values were observed throughout contraction; during tests 1 and 2 the EMG f¯ declined from 122 (SEM 7) Hz to 74 (SEM 7) Hz and from 106 (SEM 8) Hz to 60 (SEM 7) Hz, respectively; during test 1 the MMG f¯ increased in the first 6 s from 19.3 (SEM 1.4) Hz to 23.9 (SEM 2.9) Hz, falling to 13.9 (SEM 1.3) Hz at the end of contraction; in contrast, during test 2 the MMG f¯ declined continuously from 18.7 (SEM 1) Hz to 12.4 (SEM 0.8) Hz. The lower MVC after the fatiguing exercise and the changes in the EMG parameters confirmed that 10 min after the fatiguing exercise, the mechanical and electrical activities of MU were altered. In addition, the MMG results suggested that after a 10-min recovery, some highly fatigable MU might not be recruitable.     

Hand dominance during constant force isometric contractions: evidence of different cortical drive commands

The purpose of this study was to investigate force variability and sensoriomotor strategies of dominant and nondominant hands of right and left-handed subjects during a submaximal isometric force production task. Twelve right-handed adults (9 men and 3 women; 23?±?3?year) and twelve left-handed adults (4 men and 8 women; 24?±?3?year) performed an isometric constant force contraction sustained at 30 and 50% of maximal force for 10?s. Surface EMG signals were obtained from forearm flexors and extensors. Force signals were analyzed in the time (CV of force) and frequency (0-10?Hz) domain. The neural activation of the involved muscles was investigated from the EMG structure using the cross-wavelet spectra of the interference EMG signals of six different frequency bands of the EMG signals were quantified (5-13, 13-30, 30-60, 60-100, 100-150 and 150-200?Hz). The major findings were: (1) dominant and nondominant hands of right- and left-handed subjects exhibited similar CV of force; (2) the power spectrum of force is influenced by handedness, with greater 1-3?Hz oscillations for left-handed subjects when compared to right-handed subjects; (3) right-handed subjects have greater 30-60?Hz neuromuscular activation when compared to left-handed subjects. Our results indicate that right-handed individuals may rely preferentially in visual feedback to carry out a task with visual and proprioceptive feedback because of the left hemisphere specialization on the visuomotor control.     

Periodontal anaesthesia reduces common 8 Hz input to masseters during isometric biting

During isometric contractions of the jaw muscles, oscillations in the rectified masseteric EMG record that are coherent with the mandibular force output are evident at ~8 Hz. We have investigated the load dependence of these oscillations under both force and EMG feedback conditions and the extent to which these oscillations are coupled bilaterally in the jaw muscles. We further investigated the extent to which afferent information arising from the periodontium during biting influenced the extent of ~8 Hz EMG tremor and the bilateral coupling between masseters at this frequency. Using coherence analysis we have shown that a significant load-independent coupling of EMG between the closing muscles of the jaw occurs at ~8 Hz as a result of common ~8 Hz input to the masseters. This common input is significantly reduced when afferent information from the periodontium is blocked. These results suggest that afferent information arising from the periodontium enhances the expression of peripheral tremulous activity, which may be important for optimising the response of the jaw to changes in forces occurring between the teeth.     

Influence of motor unit synchronization on amplitude characteristics of surface and intramuscularly recorded EMG signals

The increase in muscle strength without noticeable hypertrophic adaptations is very important in some sports. Motor unit (MU) synchronisation and higher rate of MU activation are proposed as possible mechanisms for such a strength and electromyogram (EMG) increase in the early phase of a training regimen. Root mean square and/or integrated EMG are amplitude measures commonly used to estimate the adaptive changes in efferent neural drive. EMG amplitude characteristics could change also because of alteration in intracellular action potential (IAP) spatial profile. We simulated MUs synchronization under different length of the IAP profile. Different synchronization was simulated by variation of the percent of discharges in a referent MU, to which a variable percent of remaining MUs was synchronized. Population synchrony index estimated the degree of MU synchronization in EMG signals. We demonstrate that the increase in amplitude characteristics due to MU synchronization is stronger in surface than in intramuscularly detected EMG signals. However, the effect of IAP profile lengthening on surface detected EMG signals could be much stronger than that of MU synchronization. Thus, changes in amplitude characteristics of surface detected EMG signals with progressive strength training could hardly be used as an indicator of changes in neural drive without testing possible changes in IAPs.     

The influence of experimental muscle pain on motor unit activity during low-level contraction

In the present study we compared motor unit (MU) activity in a painful extensor carpi ulnaris (ECU) muscle to that of a pain-free control. According to the pain adaptation model the activity of the painful ECU muscle may be inhibited and its antagonist activity increased during wrist extension performed as a pre-defined low-force ramp. The pre-defined low force may then be maintained by increased activity in the pain-free synergist muscles such as the extensor carpi radialis (ECR) muscle. Nine females (31–47 years old) participated in the study. Maximal voluntary contraction (MVC) of the wrist extensors was performed. A catheter was inserted into the ECU muscle to allow the injection of hypertonic saline to evoke muscle pain, and a concentric needle was inserted for the recording of MU activity. Surface electromyograms were recorded from a synergist and an antagonist (ECR and flexor carpi radialis) to the painful ECU muscle. A force ramp of isometric wrist extensions up to 10% MVC, with a force increase of 1% MVC · s⁻¹, were performed followed by 60 s of sustained contraction at 10% MVC. The number of MUs recruited was almost identical for baseline and with pain, and no effect of experimental muscle pain was found on the properties of the MUs (amplitude, area) or their firing characteristics (mean firing rate, firing variability) during low-force ramp contraction. During the sustained 10% MVC, no effect of pain was found for concentric or surface EMG of the forearm muscles. At low force levels no pain-induced modulations were found in MU activity, when the mechanical condition was similar to that of a control situation. Accepted: 30 May 2000