Electromyogram changes during sustained contraction after resistance training in women in their 3rd and 8th decades

The present study aimed at investigating the neuromuscular adaptations to 6 weeks of resistance training in women in their third (6 experimental, 8 controls) and eighth decades (8 experimental, 8 controls). The surface electromyogram (sEMG) was measured from the biceps brachii muscle during constant-force isometric contractions lasting 12 s at 80% of maximal voluntary contraction (MVC). All the signals were analysed adopting in the time domain the root mean square (RMS) as a measure of amplitude and in the frequency domain the median frequency (MDF) of the power spectrum. Quantitative analysis was performed from the 3rd to the 6th second, to describe the early phase of the contraction (“Early”), starting from point at which 80% of the MVC was reached, and from the 9th to the 12th second, to describe the last part of the constant-force sustained contraction (“Late”). After training, the MVC increased by 22.4% in the young (P < 0.0001) and by 13.4% in the older (P < 0.05) women. The “Early” RMS increased by 60.4% with respect to the pre-training condition in the young (P < 0.01) but not in the older women. In contrast, the “Late” RMS increased by 46.7% in the older (P < 0.05) but not in the young women. The MDF remained unchanged in both groups. These results indicate that young and older women showed different training-induced adaptation of the motor unit (MU) activation pattern, in order to keep a constant level of force during a sustained isometric contraction at 80% of MVC. Accepted: 11 March 2000     

Vibrations and sounds produced during sustained voluntary muscle contraction

In 20 normal subjects, sounds or vibrational signals were recorded from the biceps brachii muscle during voluntary isometric contraction. Fourier analysis showed presence of predominantly low frequency components with little contributions beyond 60 or 70 Hz. Relatively high amplitude peaks occurring below 20 Hz were seen in the frequency spectrum with the most prominent of these peaks occurring at a mean frequency of 11.3 Hz. A majority of subjects had one or two additional peaks, each appearing on either side of the major peak with mean frequencies of 7.3 and 16.2 Hz. In general, subjects who could not sustain a very steady contraction had more peaks compared to those who were able to maintain a smoother contraction. Between 20 and 50 Hz, several well-defined but much smaller peaks were also seen. The frequencies of some of these small peaks can be expressed as exact harmonics of the previously described larger peaks in 85% of subjects.     

Relationship between mechanomyogram signals and changes in force of human forefinger flexor muscles during voluntary contraction

In previous studies on mechanomyogram (MMG) signals no analysis of these signals accompanying force generation has been performed. Therefore, we have recorded MMG signals (previously referred to as muscle sound or acoustomyographic signals) during voluntary contractions of forefinger flexor muscles in 31 young subjects. These subjects made contractions to produce force records of triangular or trapeziform shape. The peak target force amounted to 10, 20 or 40 N which represented less than 40% of maximal voluntary contraction. The MMG signals during the transient phases of force generation at three different rates were analysed. The MMG intensity level calculated for MMG records and the peak-to-peak amplitude of MMG signals correlated with both the velocity of force increase and the contraction force. The occurrence of the strongest MMG signals corresponded to changes in contractile force. Therefore, it is suggested that measurements of these parameters could be a useful tool in studies of changes in contractile force.     

Lactate oxidation by skeletal muscle during sustained contraction in vivo

Summary Estimation of lactate oxidation in vivo was carried out by a tracer technique in the muscles of the lower hindleg of the dog during tetani of different duration. The fractional (%) rate of lactate oxidation increased markedly, compared with that of the resting muscle, after the first 2 min of stimulation. Lactate oxidation afforded a large contribution to the total oxygen consumption. Fatigue appeared to play a role in limiting lactate oxidation in the later phase of sustained contraction.     

Electromyogram and perceived fatigue changes in the trapezius muscle during typewriting and recovery

The purpose of the present study was to investigate the development and recovery of muscle fatigue in the upper trapezius muscle by analyzing electromyographic signals. Six male subjects performed a simulated typewriting task for four 25-min sessions. During fatigue and the following rest periods, subjective fatigue and surface electromyography (EMG) from the trapezius muscle during isometric contraction at 30% maximum voluntary contraction (MVC) were periodically measured in the interval. We detected a significant decrease in muscle fiber conduction velocity (MFCV) (P = 0.008) and median frequency (MDF) (P = 0.026) as well as an increase in root mean square (RMS) (P = 0.039) and subjective fatigue (P = 0.0004) during the fatigue period. During the recovery period, subjective fatigue decreased drastically and significantly (P = 0.0004), however, the EMG parameters did not recover completely. Thus, physiological muscle fatigue in the trapezius developed in accordance with subjective muscle fatigue during typewriting. On the other hand, differences between the physiological and subjective parameters were found during recovery. Further studies should be necessary to reveal the discrepancy could be a major factor of a transition from temporal phenomena to serious chronic muscle fatigue and to identify the necessity of some guidelines to prevent VDT work-related chronic muscle fatigue in the trapezius.     

Conduction velocity of quiescent muscle fibers decreases during sustained contraction

We tested the hypothesis that conduction velocity of quiescent muscle fibers decreases during sustained contraction due to the activity of the active motor units in the muscle. Ten subjects trained for the identification of a target motor unit in the abductor pollicis brevis with feedback on surface EMG signals detected with a two-dimensional array of 61 electrodes. The subjects activated the target motor unit in two 10-s long contractions, before (contraction C1) and after (C3) a 3-min contraction (C2), all in ischemic condition. The target motor unit was not activated during C2. Eight of the 10 subjects (control group) performed a second experimental session identical to the first but with a resting period of 3 min instead of the contraction C2. Exerted force and target motor unit discharge rate were not different between the two subject groups and between C1 and C3 (mean +/- SD, over C1 and C3; C2 group: 15.8 +/- 10.4% maximal voluntary contractions and 13.1 +/- 1.9 pps; control group: 15.6 +/- 22.1% maximal voluntary contractions and 14.5 +/- 1.9 pps, respectively). Muscle fiber conduction velocity of the target motor unit decreased in C3 with respect to C1 in the C2 group (3.59 +/- 0.57 and 3.34 +/- 0.47 m/s for C1 and C3, respectively; P < 0.05) but not in the control group (3.47 +/- 0.68 and 3.46 +/- 0.73 m/s). In the C2 group, the percent decrease in conduction velocity of the target motor unit between C1 and C3 (6.4 +/- 7.1%) was not significantly different from the percent decrease in the average conduction velocity of the motor units active during C2 (9.6 +/- 5.4%). In conclusion, the contraction-induced modifications in electrophysiological membrane properties of muscle fibers are partly independent on fiber activation.     

System identification of evoked mechanomyogram from abductor pollicis brevis muscle in isometric contraction

The purpose of this study is to verify the applicability of a sixth-order model to the mechanomyogram (MMG) system of the parallel-fibered muscle, which was identified from the MMG of the pennation muscle. The median nerve was stimulated, and an MMG and torque of the abductor pollicis brevis muscle were measured. The MMGs were detected with either a capacitor microphone or an acceleration sensor. The transfer functions between stimulation and the MMG and between stimulation and torque were identified by the singular value decomposition method. The torque and the MMG, which were detected with a capacitor microphone, DMMG, were approximated with a second- and a third-order model, respectively. The natural frequency of the torque, reflecting longitudinal mechanical characteristics, did not show a significant difference from that of the DMMG. The MMG detected with an acceleration sensor was approximated with a fourth-order model. The natural frequencies of the AMMG reflecting the muscle and subcutaneous tissue in the transverse direction were obtained. Both DMMG and AMMG have to be measured to investigate the model of the MMG system for parallel-fibered muscle. The MMG system of parallel-fibered muscle was also modeled with a sixth-order model.     

Electromyogram spectrum changes during sustained contraction related to proton and diprotonated inorganic phosphate accumulation: a 31P nuclear magnetic resonance study on human calf muscles

Summary The calf muscles of five clinically healthy men were submitted to isometric exercise and examined by ³¹P nuclear magnetic resonance (NMR) spectroscopy and electromyography (EMG) to evaluate the influence of proton (H⁺) and diprotonated forms of inorganic phosphate (H₂PO ₄ ^– ) accumulation on EMG spectrum changes. The experiments were performed in a supraconducting magnet (2.35 Tesla, 35-cm effective diameter) using a surface coil (7-cm diameter) positioned against the calf muscles. The EMG surface electrodes were applied on the gastrocnemius medialis muscle and acquisition of both NMR and EMG signals was synchronized. The exercise consisted of a sustained isometric contraction at 70% of the maximal voluntary contraction until exhaustion. A continuous decrease in phosphocreatine content and a large concomitant increase in H₂PO ₄ ^– was observed in the calf muscles of each subject. A significant increase in H⁺ concentration was also found when considering the whole population but intracellular acidosis was low for two subjects. Moreover, a quasilinear decrease in mean power frequency (MPF) was found during the test. Changes in MPF were correlated with variations in H⁺ and H₂PO ₄ ^– concentration but a more significant relationship was found when MPF changes were correlated with H₂PO ₄ ^– concentration. An interpretation of EMG spectrum changes in terms of an accumulation of by-products of anaerobic metabolism and an increase in the relative number of activated slow fibres is proposed.     

Regional tissue fluid pressure in rat calf muscle during sustained contraction or stretch

The tissue fluid equilibration pressure in central and peripheral regions of rat calf muscles has been measured by needles with large smoothed sideholes. Short equilibration periods were normally achieved although saline was not infused. Tissue fluid pressure during rest remained constant at -2 to 0 mmHg. During sustained isometric contraction the average tissue fluid pressure rose almost linearly with increasing force of contraction. The average pressure in the central inner zone of the muscle reached 220±80 mmHg during maximal force, whereas the average pressure in the outer peripheral zone increased to 85±56 mmHg, markedly less than in the central zone. Thus, this difference in regional tissue pressure may possibly explain the greater impediment to blood flow in central than in peripheral regions during contraction shown previously by Wisnes & Kirkebø (1976). Although a corresponding regional pressure difference was observed during passive stretch of the muscle, the absolute tissue pressures were much smaller. However, the heterogeneous pattern of muscle fiber directions and relative displacement of various muscle elements during work, may induce shear forces causing focal vessel obstructions that are different during contraction and stretch.     

Motor unit conduction velocity during sustained contraction of the vastus medialis muscle

The aim of the study was to analyze motor unit conduction velocity at varying force of the vastus medialis muscle during sustained contraction. Surface (8-electrode array) and intramuscular (two wire electrodes) EMG signals were recorded from the distal part of the dominant vastus medialis muscle of ten healthy male subjects. The subjects sat on a chair with the knee 90° flexed and performed seven 180-s long contractions at forces in the range 2.5–30% of the maximal voluntary contraction force. For each force level, the discharge patterns of the newly recruited motor units with respect to the previous force level were identified from the intramuscular recordings and used as trigger for averaging the surface EMG signals. Motor unit conduction velocity was estimated from the averaged surface EMG. Average discharge rate at which motor units were analyzed was the same for each force level (mean ± SD, 8.3 ± 0.8 pulses per second). Motor unit conduction velocity at the beginning of the contraction and its rate of change over time increased with force (P < 0.05). Conduction velocity at the beginning of the contraction estimated from the interference surface EMG (4.44 ± 0.66 m/s) and from single motor units (4.75 ± 0.56 m/s) were positively correlated (R ² = 0.46; P < 0.0001) but significantly different (P < 0.05). The results indicate that single motor unit conduction velocity and its rate of change during sustained contraction, assessed at a fixed discharge rate, depend on force level.     

Reflex changes in muscle spindle discharge during a voluntary contraction

1. This study was undertaken to determine whether low-threshold cutaneous and muscle afferents from mechanoreceptors in the foot reflexly affect fusimotor neurons innervating the plantar and dorsiflexors of the ankle during voluntary contractions. 2. Recordings were made from 29 identified muscle spindle afferents innervating triceps surae and the pretibial flexors. Trains of electrical stimuli (5 stimuli, 300 impulses per second) were delivered to the sural nerve at the ankle (intensity: 2-4 times sensory threshold) and to the posterior tibial nerve at the ankle (intensity: 1.5-3 times motor threshold for the small muscles of the foot). The stimuli were delivered while the subject maintained an isometric voluntary contraction of the receptor-bearing muscle, sufficient to accelerate the discharge of each spindle ending. This ensured that the fusimotor neurons directed to the ending were active and influencing the spindle discharge. The effects of these stimuli on muscle spindle discharge were assessed using raster displays, frequencygrams, poststimulus time histograms (PSTHs) and cumulative sums ("CUSUMs") of the PSTHs. Reflex effects onto alpha-motoneurons were determined from poststimulus changes in the averaged rectified electromyogram (EMG). Reflex effects of these stimuli onto single-motor units were assessed in separate experiments using PSTHs and CUSUMs. 3. Electrical stimulation of the sural or posterior tibial nerves at nonnoxious levels had no significant effect on the discharge of the 14 spindle endings in the pretibial flexor muscles. The electrical stimuli also produced no significant change in discharge of 11 of 15 spindle endings in triceps surae. With the remaining four endings in triceps surae, the overall change in discharge appeared to be an increase for two endings (at latencies of 60 and 68 ms) and a decrease for two endings (at latencies of 110 and 150 ms). The difference in the incidence of the responses of spindle endings in tibialis anterior and in triceps surae was significant (P less than 0.05, chi 2 test). 4. For both triceps surae and pretibial flexor muscles the electrical stimuli to sural or posterior tibial nerves had clear effects on the alpha-motoneuron pool, whether assessed using surface EMG or the discharge of single-motor units. Based on EMG recordings using intramuscular wire electrodes, the reflex effects differed for the gastrocnemii and soleus. 5. In this study, reflex changes in the discharge of human spindle endings were more difficult to demonstrate than comparable changes in the discharge of alpha-motoneurons.(ABSTRACT TRUNCATED AT 400 WORDS)     

Surface mechanomyogram reflects the changes in the mechanical properties of muscle at fatigue

The contractile properties of muscle are usually investigated by analysing the force signal recorded during electrically elicited contractions. The electrically stimulated muscle shows surface oscillations that can be detected by an accelerometer; the acceleration signal is termed the surface mechanomyogram (MMG). In the study described here we compared, in the human tibialis anterior muscle, changes in the MMG and force signal characteristics before, and immediately after fatigue, as well as during 6?min of recovery, when changes in the contractile properties of muscle occur. Fatigue was induced by sustained electrical stimulation. The final aim was to evaluate the reliability of the MMG as a tool to follow the changes in the mechanical properties of muscle caused by fatigue. Because of fatigue, the parameters of the force peak, the peak rate of force production and the peak of the acceleration of force production (d² F/dt ²) decreased, while the contraction time and the half-relaxation time (½-RT) increased. The MMG peak-to-peak (p-p) also decreased. The attenuation rate of the force oscillation amplitude and MMG p-p at increasing stimulation frequency was greater after fatigue. With the exception of ½-RT, all of the force and MMG parameters were restored within 2?min of recovery. A high correlation was found between MMG and d² F/dt ² in un-fatigued muscle and during recovery. In conclusion, the MMG reflects specific aspects of muscle mechanics and can be used to follow the changes in the contractile properties of muscle caused by localised muscle fatigue.     

System identification of the mechanomyogram from single motor units during voluntary isometric contraction

A mechanomyogram (MMG) from single motor units of the anconeus muscle in voluntary isometric contraction was recorded from seven subjects using a spike-triggered averaging technique. The MMG system, in which the input was an ideal impulse and the output was the MMG detected with an acceleration sensor, was identified as the fifth-order model by the subspace-based state-space model identification method. The transfer function of the MMG system was factorized to the second- and the first-order models. The second-order model was compared to the standard form of the second-order model, and its resonance frequency was calculated. The resonance frequencies of the second-order models were 166 ± 61 and 93 ± 27 Hz, which were within the range of the values estimated from mechanical impedance in the literature. The equivalent mechanical model of the MMG system of the single motor unit was proposed on the basis of the fifth-order model. The model might be useful to evaluate the visco-elastic properties of the anconeus muscle.     

The effects of beta-blockade on electrically stimulated contraction in fatigued human triceps surae muscle

The contractile characteristics of the triceps surae muscle group were examined before and after repeated isometric contractions in two groups of eight healthy young males. Single twitches and trains of stimuli at 10, 20, 50 and 100 Hz were delivered to the muscle using supramaximal voltages. Subjects were treated with beta-blockade (2 X 80 mg oral propranolol, beta-b) or matched placebo in a double-blind crossover design. Four different exercise conditions were studied: (I) maximal voluntary contraction (MVC); (II) MVC during circulatory occlusion; (III) electrical stimulation at 20 Hz using 50% of voltage required for maximal torque production; and (IV) electrical stimulation with occlusion. Each contraction was for 5 s with 5 s recovery. Total duration of exercise was 10 min for non-occluded contractions and to a 50% decline in torque output with occlusion. At rest prior to exercise, maximal voluntary contraction was significantly reduced (5.7%) by beta-b during 40 observations in 16 subjects. Following exercise without occlusion (I and III), the reduction in torque output of the muscle at 10 and 20 Hz stimulation was generally greater during beta-b than placebo. This low frequency fatigue was longer-lasting with beta-b. The shorter lasting reduction in torque at 50 and 100 Hz was generally not different between beta-b placebo. After exercise with occlusion (II and IV), the torque output at all stimulation frequencies was reduced to a similar extent in both placebo and beta-b at most comparison points. Twitch responses after exercise with occlusion showed decreases in peak tension and time to peak tension and a lengthening of one-half relaxation time in both placebo and beta-b. It was concluded that the greater reduction in torque output of the triceps surae muscle group at low frequencies during beta-b was probably a consequence of a reduction in blood flow relative to the placebo treatment. This relative low frequency fatigue could be responsible for the increased perception of effort in patients exercising during beta-blocker therapy.     

The effect of sustained isometric muscle contraction on various muscle functions

Summary Nine physiotherapists were included in a training programme whose object was to elucidate the effect of isometric contractions of long duration.The isometric endurance of the left elbow flexors was trained 10 times daily until exhaustion. The load was 60% of 1 RM. The elbow joint was kept flexed 90 °, the forearm being horizontal. The training period was 5 weeks with no training on Sundays.Before and after the experiment the subjects had tests of dynamic strength, isometric strength, dynamic endurance, isometric endurance, and the ability to perform repeated isometric contractions of 5 seconds' duration, interrupted by an interval of 2 seconds.The training increased the isometric endurance by 84% and the ability to perform repeated isometric contractions by 219%. On the other hand, the dynamic endurance was only increased by 92.5%, a slight increase when compared with that which may result from experiments training the dynamic endurance.The effect upon the isometric and dynamic strength was moderate, an increase of 11% and 15.4% respectively — moderate when compared with what is obtained by training with maximum loads.The results are discussed.     

Relationships between muscle fibre conduction velocity and frequency parameters of surface EMG during sustained contraction

Summary A surface electrode array has been used to investigate the relationship between muscle fibre conduction velocity and the frequency spectrum during sustained isometric contractions of the biceps brachii. Measurement of muscle fibre conduction velocity was made directly, using the zero-crossing time delay method with two pairs of bipolar electrodes. It was found that the average conduction velocity during an intense (12 kg) sustained contraction decreased by about 20% at the end of the contracting period. Except for peak frequency, changes in the spectral parameters decreased in a similar manner. These results indicate that, during fatiguing contraction, spectral modifications are partly due to reduction in the action potential conduction velocity along the muscle fibres.