Motor unit activity and surface electromyogram power spectrum during increasing force of contraction

Summary Twelve male subjects were tested to determine the relationship between motor unit (MU) activities and surface electromyogram (EMG) power spectral parameters with contractions increasing linearly from zero to 80% of maximal voluntary contraction (MVC). Intramuscular spike and surface EMG signals recorded simultaneously from biceps brachii were analyzed by means of a computer-aided intramuscular MU spike amplitude-frequency (ISAF) histogram and an EMG frequency power spectral analysis. All measurements were made in triplicate and averaged. Results indicate that there were highly significant increases in surface EMG amplitude (71±31.3 to 505±188 V,p<0.01) and mean power frequency (89±13.3 to 123±23.5 Hz,p<0.01) with increasing force. These changes were accompanied by progressive increases in the firing frequency of MU's initially recruited, and of newly recruited MU's with relatively larger spike ampltitudes. The group data in the ISAF histograms revealed significant increases in mean spike amplitude (412±79 to 972±117 V,p<0.01) and mean firing frequency (17.8±5.4 to 24.7±4.1 Hz,p<0.01). These data suggest that surface EMG spectral analysis can provide a sensitive measure of the relative changes in MU activity during increasing force output.     

Oxygen availability and motor unit activity in humans

Summary Six men were studied to determine the interrelationships among blood supply, motor unit (MU) activity and lactate concentrations during intermittent isometric contractions of the hand grip muscles. The subjects performed repeated contractions at 20% of maximal voluntary contraction (MVC) for 2 s followed by 2-s rest for 4 min with either unhindered blood circulation or arterial occlusion given between the 1st and 2nd min. The simultaneously recorded intramuscular MU spikes and surface electromyogram (EMG) data indicated that mean MU spike amplitude, firing frequency and the parameters of surface EMG power spectra (mean power frequency and root mean square amplitude) remained constant during the experiment with unhindered circulation, providing no electrophysiological signs of muscle fatigue. Significant increases in mean MU spike amplitude and frequency were, however, evident during the contractions with arterial occlusion. Similar patterns of significant changes in the surface EMG spectra parameters and venous lactate concentration were also observed, while the integrated force-time curves remained constant. These data would suggest that the metabolic state of the active muscles may have played an important role in the regulation of MU recruitment and rate coding patterns during exercise.     

Influence of handedness on motor unit discharge properties and force tremor

Discharge properties of motor units (MUs) in the first dorsal interosseous muscle (FDI) were studied in the dominant and non-dominant hands of six right-handed (RH) and six left-handed (LH) individuals. MU discharge rates and variability were similar in each hand in RH (186 MUs) and LH (160 MUs) subjects. MU synchronization was less prominent in the dominant hand of RH subjects, with 51% (45/88) of cross-correlograms of MU discharge having significant central peaks, compared with 81% (90/111) for the non-dominant hand. The strength of MU synchronization (expressed as the frequency of extra synchronous discharges above chance) was weaker in the dominant hand of right-handers (0.23 ± 0.03 s^-1 vs 0.39 ± 0.03 s^-1), and synchronous peaks from that hand were slightly broader. Four of six RH subjects had significant differences in synchronization between hands (weaker in dominant hand). In contrast, left-handers had similar incidence (80 vs 82%, n = 161) and strength (0.41 ± 0.03 s^-1 vs 0.37 ± 0.03 s^-1) of MU synchrony in dominant and non-dominant hands. No LH subject had a significant difference in synchronization between hands. Force tremor was quantified in each hand in the same subjects during isometric abduction of FDI at 0.5 N and 3.5 N, and directly correlated with the extent of MU synchronization in the muscle. Tremor root mean square amplitude was similar in dominant and non-dominant hands. Power spectral analysis of the tremor force revealed that the peak frequency in the power spectrum was not influenced by handedness, but power at the peak frequency was higher in the non-dominant hand of RH subjects. Correlations between MU discharge variability and synchrony with measures of tremor amplitude were weak. The reduced MU synchronization in the dominant hand of right-handers may reflect a more restricted distribution of direct projections from motor cortical neurons within the FDI motoneuron pool, or reduced excitability of the cortical neurons during the task. These differences in MU synchronization, however, had an insignificant influence on the magnitude of physiological tremor in the FDI.     

Response Relationships in SRR Conditioning

Relationships among orienting responses (ORs), anticipatory responses (ARs), and both unconditioned responses. (UCRs) and the UCR-latency responses which occur on acquisition test trials were examined in an 8 sec CS/UCS skin resistance response (SRR) conditioning schedule, Response characteristics were scored objectively by means of a Linc-8 program based on the rate of change of amplitude, and included response frequency, amplitude, onset and peak latencies. Substantial interaction were observed between AR and UCR characteristics, Negative correlations were obtained between AR and UCR occurrences on reinforced trials, shown to be associated with a long peak latency of the AR. AR/UCR amplitude relationships were also negative, indicating that high amplitude ARs lend to occur with low amplitude UCRs. Results on reinforced trials, however, showed a positive relationship between AR/UCR-latency frequency.     

Correlation-based decomposition of surface electromyograms at low contraction forces

The paper studies a surface electromyogram (SEMG) decomposition technique suitable for identification of complete motor unit (MU) firing patterns and their motor unit action potentials (MUAPs) during low-level isometric voluntary muscle contractions. The algorithm was based on a correlation matrix of measurements, assumed unsynchronised (uncorrelated) MU firings, exhibited a very low computational complexity and resolved the superimposition of MUAPs. A separation index was defined that identified the time instants of an MU's activation and was eventually used for reconstruction of a complete MU innervation pulse train. In contrast with other decomposition techniques, the proposed approach worked well also when the number of active MUs was slightly underestimated, if the MU firing patterns partly overlapped and if the measurements were noisy. The results on synthetic SEMG show 100% accuracy in the detection of innervation pulses down to a signal-to-noise ratio (SNR) of 10 dB, and 93±4.6% (mean± standard deviation) accuracy with 0 dB additive noise. In the case of real SEMG, recorded with an array of 61 electrodes from biceps brachii of five subjects at 10% maximum voluntary contraction, seven active MUs with a mean firing rate of 14.1 Hz were identified on average.     

Electromyography of lumbar erector spinae muscles — influence of posture,interelectrode distance,strength, and fatigue

Summary The aims of the study were to obtain information (1) on surface electromyograms (SEMG) from the lumbar erector spinae muscles at different interelectrode distances and postures during short isometric contractions with constant force, (2) on the relationships between SEMG and extension force at different postures, and (3) on changes in SEMG during fatiguing isometric contractions at different postures and strengths. Six male subjects developed target forces in prone postures without gravity confounding the measurement of the extension torque. The angles between the constantly horizontal upper trunk and thighs were 90° (P1), 135° (P2), 170° (P3), and 190° (P4). Standard deviations of the distribution of SEMG amplitudes (RMS values), autoregressive (AR) time series models of the 15th order and spectral densities, including mean power frequency (MPF), were computed. Smaller interelectrode distances accompanied smaller RMS values and higher MPF. At a constant extension torque of about 110 Nm, RMS values and MPF increased from P1 to P4. Changes of interelectrode distance were of relatively minor importance, compared with the variation in the posture. With increasing torque, the increase in RMS values was steeper at P3 than at P2. The AR structure and MPF did not exhibit distinct effects of force. During sustained contractions at P2 and P3, only the highest force (mean=140 Nm) at P3 caused a significiant decrease of the MPF at the very beginning of the contraction. Endurance at P2 was greater than at P3. Higher forces and/or shorter muscles (P3) induced more pronounced and earlier relative decreases of the MPF and residual variance of AR models. Up to the “failure point”, RMS values increased slightly, but without significant differences.     

Motor unit discharge and force tremor in skill- and strength-trained individuals

We examined motor unit (MU) discharge properties (mean interspike interval, ISI, discharge variability, short-term synchronization, common drive) and force tremor in the first dorsal interosseous (FDI) muscle of five musicians (skill-trained), five weight-lifters (strength-trained) and six untrained subjects during low-force isometric abduction of the index finger. Mean MU ISI was slightly shorter in skill-trained subjects than in untrained subjects. Discharge variability of FDI MUs did not differ significantly between groups. The mean strength of MU synchronization (expressed as the frequency of extra synchronous discharges above chance) was different in skill-trained (0.22±0.02 s^–1, 162 MU pairs), untrained (0.32±0.02 s^–1, 199 MU pairs) and strength-trained subjects (0.44±0.03 s^–1, 183 MU pairs). FDI MU synchrony was weak and of equivalent strength in both hands of skill-trained subjects and the dominant (skilled) hand of untrained subjects. The stronger FDI MU synchrony in the non-dominant hand of untrained subjects was equivalent to that found in both hands of strength-trained subjects. The extent of common modulation of firing rates (common drive) was assessed for a subset of MU pairs and was weaker in skill-trained subjects (0.30±0.04, n=14) than untrained (0.43±0.3, n=14) and strength-trained (0.48±0.03, n=21) subjects. Force tremor was quantified for each hand in the same subjects during isometric index finger abduction at target forces of 0.5 N and 3.5 N. Tremor rms amplitude and peak power in the force frequency spectrum were significantly lower in skill-trained subjects than strength-trained subjects with the 3.5-N target force. The peak tremor frequency was similar in the three groups. The relatively more independent discharge of pairs of FDI MUs in skill-trained subjects was not responsible for the reduced tremor amplitudes in these subjects. Correlations between the overall extent of MU synchrony and common drive in FDI muscles and tremor measures obtained during the same experimental session were all non-significant. Differences in the central descending command signals are the most likely explanation for the more independent discharge of FDI MUs in skill-trained hands, while neural or peripheral muscular factors may be responsible for the weaker tremor. Received: 15 May 1997 / Accepted: 15 August 1997     

Evaluation of postural tremor of finger for neuromuscular diseases and its application to the classification

The purpose of this study is to verify the features of the power spectrum of postural tremors for neuromuscular disease patients and to classify the postural tremors. The subjects were 88 neuromuscular disease patients (30 Parkinson disease (PD), 25 cerebellar disease (CER), 7 multiple sclerosis (MS), 7 neuropathy (NEU), 10 motor neuron disease (MND), 9 myopathy (MYO)). The control subjects were 12 normal young persons and 10 normal aged persons. Postural tremor was detected by accelerator sensor. Postural tremor was recorded under the two postural conditions: The subjects maintained the index finger without or with a weight load of 50 g in a horizontal position while looking at a visual target in front of the tip of the index finger. The power spectrum was calculated by an auto-regressive model (AR model). The peak frequency and the peak power were evaluated under the two conditions. Two frequency components of 8-12 Hz and 20-25 Hz appeared in the postural tremor of both normal subjects and neuromuscular disease patients. The difference of the postural tremor between the subjects mainly appeared in the 8-12 Hz component during the postural tremor with a weight load. MYO patients belonged to one group (called as group P1) due to lower peak power, CER patients belonged to one group (called as group P2) due to higher peak power, and PD and MS patients belonged to one group (called as group P3) due to lower peak frequency and higher peak power. NER and MND patients belonged to one group (called as group N which meant normal group). These results suggested that the peak frequency and the peak power of the 8-12 Hz component were changed by the conditions of both spinal reflex system and central nervous system. An oscillator within the central nervous system produced the underlying frequency of 8-12 Hz component, while the amplitude of 8-12 Hz component was governed by both spinal reflex system and central nervous system. In conclusion, the classification of postural tremor for neuromuscular disease patients was a useful index to elucidate the mechanism of tremor oscillation and to assist in clinical diagnosis of neuromuscular disease.     

Comparative study of wavelet denoising in myoelectric control applications

Here, the wavelet analysis has been investigated to improve the quality of myoelectric signal before use in prosthetic design. Effective Surface Electromyogram (SEMG) signals were estimated by first decomposing the obtained signal using wavelet transform and then analysing the decomposed coefficients by threshold methods. With the appropriate choice of wavelet, it is possible to reduce interference noise effectively in the SEMG signal. However, the most effective wavelet for SEMG denoising is chosen by calculating the root mean square value and signal power values. The combined results of root mean square value and signal power shows that wavelet db4 performs the best denoising among the wavelets. Furthermore, time domain and frequency domain methods were applied for SEMG signal analysis to investigate the effect of muscle-force contraction on the signal. It was found that, during sustained contractions, the mean frequency (MNF) and median frequency (MDF) increase as muscle force levels increase.     

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.     

老年及青年Wistar大鼠心电图QRS波时域及频域分析的比较研究

本文报道了老年及青年Ｗｉｓｔａｒ大鼠Ⅱ导联心电图ＱＲＳ波的时程、波幅及频谱特性，并分析了老龄对其时域值及频谱的影响，结果如下：（１）ｍ青年及老年鼠ＱＲＳ波时程及波幅；老年鼠与青年鼠相比，ＱＲＳ波时程延长（Ｐ〈０．０１）崦波幅减少（Ｐ〈０．０５）；（２）青年及老年鼠ＱＲＳ波频谱特征；青年鼠ＱＲＳ波频产呈双峰型，可分为两型：大部分鼠（１３／２１只）第一峰大，少部分鼠（８／２１只）第二峰大，老年鼠频谱图     

Interindividual variability of surface EMG changes during cycling exercise in healthy humans.

We studied surface electromyogram (SEMG) changes during 1-h endurance cycling in 12 healthy subjects of whom five were involved in mountain bike training programme. The work load was set at 50% of the predicted maximal heart rate. The surface EMG and the compound evoked muscle action potential (M-wave) from the vastus lateralis muscle were recorded at rest, during the 1-h cycling period, and the 20-min recovery period. The root mean square (RMS) and the median frequency (MF) of SEMG power spectrum were computed. In all subjects, there was no shift in the median frequency throughout the cycling period and the increase in RMS remained stable. In subjects untrained to endurance cyclism, the M-wave duration increased at the end of the cycling period and these changes persisted for a consecutive 15-min period during recovery of exercise. By contrast, in trained mountain bikers the M-wave duration decreased after 2 min of exercise--the effect persisting for 2 min during recovery. These data suggest that the interpretation of M-wave changes during cycling must take into consideration the sport practices of the subjects and also that SEMG power spectrum and M-wave explore different electrophysiological events.     

Mechanomyogram and force relationship during voluntary isometric ramp contractions of the biceps brachii muscle

The aim of the present study was to examine the non-stationary mechanomyogram (MMG) during voluntary isometric ramp contractions of the biceps brachii muscles using the short-time Fourier transform, and to obtain more detailed information on the motor unit (MU) activation strategy underlying in the continuous MMG/force relationship. The subjects were asked to exert ramp contractions from 5% to 80% of the maximal voluntary contraction (MVC) at a constant rate of 10% MVC/s. The root mean squared (RMS) amplitude of the MMG began to increase slowly at low levels of force, then there was a slight reduction between 12% and 20% MVC. After that, a progressive increase was followed by a decrease beyond 60% MVC. As to the mean power frequency (MPF), a relatively rapid increase up to 30% MVC was followed by a period of slow increment between 30% and 50% MVC. Then temporary reduction at around 50% MVC and a further rapid increase above 60% MVC was observed. The interaction between amplitude and MPF of the MMG in relation to the MU activation strategy is discussed for five force regions defined on the basis of their inflection points in the RMS-amplitude/force and MPF/force relationships. It was found that the MMG during ramp contractions enables deeper insights into the MU activation strategy than those determined during traditional separate contractions. In addition, this contraction protocol is useful not only to ensure higher force resolution in the MMG/force relationship, but also to markedly shorten the time taken for data acquisition and to reduce the risk of fatigue. Accepted: 31 August 2000     

Respiratory and stress-induced activation of low-threshold motor units in the human trapezius muscle

The study aimed to characterize trapezius motor unit firing pattern in low-amplitude contractions, with emphasis on respiratory modulated activity. Constant-amplitude contractions with shoulder elevation, controlled by feedback of the root mean square detected surface electromyographic (SEMG) signal, typing with arm movement and tasks with mental stress were performed. Single motor unit activity was recorded by a quadrifilar fine-wire electrode. A surface electrode simultaneously recorded SEMG activity. Contraction amplitudes ranged from 1 to 10% of the SEMG signal at maximum voluntary contraction (1–10% EMG_max). The majority (∼80%) of motor units recorded during constant-amplitude contractions showed firing rate modulation at the respiratory frequency. Respiratory firing rate modulation was clear for low amplitude contractions (< 3% EMG_max), but was reduced at higher amplitudes (3–5.9% EMG_max). Most motor units had peak firing rate at the transition from inspiration to expiration, but peak firing rate at the transition from expiration to inspiration or at the first harmonic frequency was also observed. The SEMG signal showed little or no respiratory modulation, possibly because respiratory phase varied between motor units. Respiratory modulation of firing rates was significantly reduced in experiments with mental stress and was rarely observed in typing experiments. Both central respiratory drive and peripheral afferent input may contribute to respiratory modulation of firing rates; however, animal studies indicate a central source of the respiratory modulated input. We speculate that the reduction in respiratory modulation of motor activity with mental stress is due to activation of alternative pathways providing excitatory input to trapezius motoneurons.     

Coactivation patterns of the medial and lateral hamstrings based on joint position and movement velocity during isokinetic movements

The purpose of this investigation was to determine the effect of movement velocity (100 degrees x s(-1), 200 degrees x s(-1), 300 degees x s(-1), and 400 degrees x s(-1)) and joint position (0 degrees - 20 degrees [L0] 30 degrees - 50 degrees [L30], and 70 degrees - 90 degrees [L70] knee flexion) on reciprocal coactivation patterns of the medial and lateral hamstrings as determined by the amplitude and frequency spectrum of surface electromyography (SEMG). Thirteen female subjects (age = 22.7 +/- 2.1 years, mean height = 161.1 +/- 6.6 cm, mean weight = 63.5 +/- 5.8 kg) participated in the study. Bipolar surface electrodes were placed over the biceps femoris (BF) and medial hamstrings (MH) for determination of the root mean square (SEMGrms) and median frequency (SEMGmf) of the SEMG. Normalized SEMGrms values for the MH and BF were determined as a percentage of agonist SEMGrms activity for the same muscle during its agonist phase. Data were analyzed using separate 2 x 3 x 4 (muscle x position x angular velocity) repeated measures analysis of variance (ANOVA). For SEMGrms, there were significant muscle (p < 0.01) and position (p < or = 0.0001) main effects. Post-hoc analyses indicated the BF displayed greater muscle amplitude than the MH and that there was greater muscle amplitude at the L0 position (as the knee approached terminal extension). No velocity effect was noted (p > 0.05). For SEMGmf there were muscle x position (p < or = 0.05) and muscle x position x velocity (p < or = 0.01) interaction effects. Post-hoc analyses indicated the BF displayed a higher frequency spectrum than the MH at the L0 position. Secondly, velocity affected the BF and MH frequency spectrum such that values for both the MH and BF were lowest at 200 degrees x s(-1) and highest at 300 degrees x s(-1) (BF) and 400 degrees x s(-1) (MH). Velocity had little impact on the frequency spectrum in the midrange of the ROM (L30 position). Higher SEMGrms and SEMGmf values for the BF could be explained by the locking or screw home mechanism of the knee, and a way in which the human motor control system provides the limb with a dynamic braking system to control both extension and lateral rotational forces during the final stage of knee extension. It would appear that the way in which the body performs this function is not only to increase the amplitude of BF muscle firing but also to shift toward the recruitment of more fast-twitch motor units.     

Neuronal mechanisms underlying physiological tremor

1. Tremor force was recorded during stationary isometric contractions of intrinsic hand muscles of normal subjects. Subjects maintained a steady force level between their thumb and forefinger for 30 s. The force level varied from weak (0.2 kg) to strong contractions (7 kg). These experimental conditions were the same as those in two preceding studies, where single motor-unit activity (14) and the correlation between the discharges of two simultaneously recorded motor units and physiological tremor (11) have been investigated. 2. Two alterations of the power spectra were observed at successively stronger contractions: increase of tremor amplitude and changes in the shape of the power spectrum. At all force levels, the power spectra of tremor force show the well-known decay of tremor amplitude from the lower to the higher frequencies with a local peak at 6--10 Hz. This peak does not show a significant change with respect to frequency when the force level is varied. It is shifted toward lower frequencies in a pathological condition (Parkinsonism) where the recruitment firing rates of the motor units are significantly lower than in the normal. 3. Higher frequencies (greater than 20 Hz) are barely present in the power spectrum during the very weak contractions. They become significant as the contractions become stronger. 4. The steep decay of the power spectrum toward higher frequencies has a similar slope (--43 dB/decade) as the reduction in amplitude of the unfused part of the muscle contractions with increasing stimulus rates (--38 dB/decade). The cutoff of the power spectrum above 25 Hz parallels the achievement of total fusion of muscle twitches above this rate. 5. The results are consistent with the hypothesis that the power spectrum over the range of 6--25 Hz is mainly caused by the unfused parts of the twitch contractions of motor units firing between recruitment (6--8/s) and total fusion of the twitches (25--30/s). The decline of the power spectrum toward higher frequencies can be explained by mechanical damping, which results from increasing fusion of the twitch contractions. The low-frequency part of the power spectrum is assumed to be the result of the slow force deviations produced by changes in the net output of the motoneuron pool. 6. These assumptions were supported by additional animal experiments where the number and rate of force-producing elements could be controlled. Bundles of ventral root filaments innervating cat soleus and gastrocnemius muscles were stimulated synchronously and asynchronously at a number of different rates. The force output of the strain gauge was recorded, filtered, and analyzed in the same way as the human force records. 7. Stimualtion of one nerve bundle at one fixed frequency led to a sharp peak in the power spectrum at that frequency plus peaks of decreasing height representing the harmonics of the stimulation frequency. The height of the peaks decreased at --37 dB/decade. 8...     

Contribution of motor unit activity enhanced by acute fatigue to physiological tremor of finger.

The contribution of motor unit activity to a physiological tremor (hereafter called as tremor) in a middle finger is studied by both a power spectrum and a correlation analysis in which the correlation coefficient and the coherence spectrum are obtained when five kinds of loads, 0, 50, 100, 150, and 200 g, are added to the middle finger for two minutes in a loading experiment on twelve male subjects. A weight of 200 g is applied to the subjects for ten minutes in a fatigue experiment. Throughout both experiments, the middle finger remains stretched from the load of the weight. The tremor is measured by an accelerometer (MT-3T, Nihon Kohden, Japan) attached to the middle finger, and the surface electromyogram (EMG) is measured by bipolar electrodes placed on m. extensor digitorum communis. A power spectrum analysis is carried out on the tremor and EMG, and a correlation analysis is performed on the relationship between the tremor and the demodulated EMG. It is found in the loading experiment that when the weight on the finger increases, the amplitude of the tremor oscillation increases since the activity of the motor units of the muscle is enhanced by the phenomenon of recruitment. Two frequency components of the tremor spectra at 10 Hz and 25 Hz under a no load condition reflect the components of the activity of the motor units of the muscle because the tremor shows a significant correlation in the frequency zone of 10 Hz and 25 Hz with the demodulated EMG. The lower frequency component of the tremor spectrum at 10 Hz results in synchronized activity of the motor units, while the higher frequency at 25 Hz occurs from the stretch reflex loop via the motoneurons of the spinal cord. The shift of the higher frequency component to the lower frequency domain due to the load of the weight originates from the prolongation of the response time of the finger mechanical system because the lag time at the peak of the correlation coefficient increases with the load of the weight. It is found in the fatigue experiment that the amplitude of the tremor oscillation increases with the progress of fatigue. The increase is caused by the recruitment of the motor unit activity of the muscle holding the finger as well as by the synchronization of the firings of the motoneurons. The progress of the synchronization is verified by the fact that the mean power frequency (MPF) of the EMG spectrum decreases and the correlation between the tremor and the demodulated EMG increases with the progress of fatigue. The mechanisms of the increase of the amplitude of the tremor oscillation under the load of the weight to the finger and under the state of fatigue of the finger are elucidated by the analysis of the tremor and EMG.     

The monkey vertical vestibuloocular response: a frequency domain study

We studied the vertical vestibuloocular response (VVOR) in seven cynomolgus monkeys. Eye movements were measured by the search coil method. We tested the monkeys by rotating them about their interaural axis, which was colinear with gravity. Each monkey was tested by using a standard rotational paradigm that consisted of discrete sinusoidal oscillations at three frequencies (0.01, 0.1, and 1.0 Hz) and six peak velocities (5, 10, 30, 60, 100, and 150 degrees/S). The standard rotational paradigm was applied twice for each of two conditions. The first condition (EOD) consisted of rotations with the animal's vision occluded; the second condition (EOL) consisted of rotations during which the animal was allowed to view a well-lighted room. Using various statistics, we tested the linearity of the sinusoidal slow-phase velocity component of the VVOR. The largest nonlinearity found was a skewness of approximately 14% in the waveform of f = 0.01 Hz. We did not find an amplitude asymmetry between slow-phase eye velocity upward (SPVU) and slow-phase eye velocity downward (SPVD) greater than 6% for any oscillation. Nonlinearities present in the VVOR during testing with vision occluded (EOD condition) disappeared with the addition of vision (EOL condition). Intensity function plots [peak slow-phase eye velocity vs. peak rotator (head) velocity] revealed that at f = 0.01, 0.1, and 1.0 Hz over the intensity range from +/-30 degrees/s to +/-150 degrees/s, the VVOR is highly linear. The lowest correlation coefficient associated with linear regressions of the intensity function data at each frequency was 0.99. Analyses of frequency response functions for the bandwidth f = 0.01 to 1.0 Hz, revealed the following: 1) mean amplitude ratio (AR) and phase overlap for four different stimulus intensities (30, 60, 100, and 150 degrees/s); 2) no significant differences (Mann-Whitney U test, P greater than 0.05) between any AR or phase value for mean peak SPVU and mean peak SPVD re appropriately directed head velocity; 3) no significant differences (Mann-Whitney U test, P greater than 0.05) between AR and phase values for animals tested and then retested 1 mo later with five intervening standard rotational paradigms; 4) a large effect of vision in producing a VVOR with near-unity gain and near-perfect phase compensation.(ABSTRACT TRUNCATED AT 400 WORDS)     

Reliability of SEMG spike parameters during concentric contractions

This study examined the reliability of four surface electromyographic (SEMG) spike parameters during concentric (isotonic) contractions: mean spike amplitude, mean spike frequency, mean spike slope, and the mean number of peaks per spike. Eighteen subjects performed rapid elbow flexion on a horizontal angular displacement device that was used to measure joint torque. The SEMG activity of the biceps brachii was monitored with Beckman Ag/AgCl electrodes. The testing schedule consisted of four hundred trials distributed equally over four sessions. The stability of the means across sessions and the consistency of scores within subjects was determined for the first five (1-5) and last five (96-100) trials of each session to examine the possible influence of a "warm up" effect. All measures exhibited a significant (p < 0.01) increase across test days. However, the intraclass correlation coefficients for the first five (1-5) trials ranged from 0.76 to 0.83, which was quite good. The stability and consistency for most of the criterion measures increased for the last five (96-100) trials of each session. This resulted in a higher range of coefficients from 0.85 to 0.93. Subjects became more homogeneous with respect to the mean number of peaks per spike and the R decreased to 0.65. It was concluded that the four SEMG spike parameters could be reliably measured to assess changes in muscle activity patterns. The adaptations in SEMG spike activity suggest that repetitive dynamic contractions enhanced the ability to recruit more fast-twitch motor units across test days.     

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.