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PubMed1998

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.

Adult↗
PubMed1990

Calculating relaxation allowances for construction operatives - part 2: local muscle fatigue.

Local muscle fatigue usually occurs at low levels of energy output where loads are applied to a localised group of muscles. Local muscle fatigue can be either static or dynamic, depending upon the frequency of loading. Several ergonomics theories relating to local muscle fatigue are reviewed. An alternative method for determining relaxation allowances for construction work is presented.

Journal Article↗
PubMed1997

Measurement of human muscle fatigue.

Human muscle fatigue has been studied using a wide variety of exercise models, protocols and assessment methods. Based on the definition of fatigue as 'any reduction in the maximal capacity to generate force or power output', the different methods to measure fatigue are discussed. It is argued that reliable and valid measures must include either assessment of maximal voluntary contraction force or power, or the force generated by electrical stimulation. By comparing tetanic stimulation and maximal voluntary contraction force one may reveal whether fatigue is of central origin, or whether peripheral mechanisms are involved. Adequate use of twitch interpolation provides an even more sensitive measure for central fatigue. Indirect methods as endurance times and electromyography show variable responses during exercise and no close relationship to fatigue. Hence these methods are of limited value in measurement of human muscle fatigue.

Electromyography↗
PubMed1998

Alterations in information transmission in ensembles of primary muscle spindle afferents after muscle fatigue in heteronymous muscle.

This study showed that fatigue of the ipsilateral medial gastrocnemius muscle caused a clear-cut reduction in the ability of ensembles of primary muscle spindle afferents from the lateral gastrocnemius muscle to discriminate between muscle stretches of varying amplitude. The results were probably caused by reflex-mediated effects from chemosensitive group III and IV afferents onto the gamma-motoneurons projecting to lateral gastrocnemius muscle spindles. The experiments were conducted on seven cats anaesthetized with alpha-chloralose and a total of 41 primary muscle spindle afferents from the lateral gastrocnemius were registered. Afferents were simultaneously recorded in ensembles of three to 10 afferents. A method based on principal component analysis and algorithms for quantification of stimulus discrimination in ensembles of muscle afferents was used prior to, immediately following and five or more minutes after muscle fatigue had been induced to the ipsilateral medial gastrocnemius muscle. It is well established that the primary muscle spindle afferents play an important role in proprioception and kinaesthesia. Therefore the decrease in the accuracy of the information transmitted by ensembles of primary muscle spindle afferents caused by fatigue in an ipsilateral muscle implies concomitant effects on proprioception and kinaesthesia.

Animals↗
PubMed1991

Role of small diameter afferents in reflex inhibition during human muscle fatigue.

1. Previous work has shown that the H reflex excitability of the human soleus motoneurones is reduced during fatigue and is accompanied by a corresponding decrease in electromyographic (EMG) activity during maximal voluntary contractions. These findings were consistent with the existence of a reflex whereby alpha-motoneurones are inhibited by sensory input from the fatigued muscle. 2. To elucidate the contribution of different-sized afferents in such reflex inhibition, compression of the sciatic nerve was used in an attempt to block large myelinated afferents prior to fatigue. 3. Fatigue of the soleus muscle was induced under ischaemic conditions by intermittent electrical stimulation at 15 Hz in ten healthy subjects. These subjects also participated in a control test in which the compression block was followed by ischaemia without fatigue. 4. Following nerve compression alone, both the mean maximal plantarflexion torque and the associated EMG for all ten subjects declined by 18.8 +/- 16.2% (S.D.) and 13.4 +/- 17.2%, respectively. 5. Following fatigue, there were five subjects in whom the large afferents remained blocked and the experimental findings were consistent with the existence of reflex inhibition during fatigue. The mean maximal plantarflexion torque decreased further by 36.2 +/- 7.6% from the value following the compression block compared to a decrease of 5.0 +/- 9.9% in the ischaemia control. The mean EMG associated with these contractions also decreased from post-block values by 56.8 +/- 19.6% following fatigue and by only 6.4 +/- 8.0% following ischaemia alone. 6. The peripheral excitability of the neuromuscular junction and muscle fibre membrane was adequate following fatigue as evidenced by only modest changes in the M wave (muscle compound action potential). The descending motor drive was deemed sufficient because of the absence of any large interpolated twitches superimposed upon the maximal voluntary contraction in all but two subjects. 7. The declines in maximal plantarflexion torque and the associated EMG activity were very similar to those found in a previous study in which the sensory input was unaltered. The findings demonstrated that any reflex inhibition of the alpha-motoneurone pool during fatigue was probably not mediated by large diameter afferents. Rather, it is suggested that the reflex is mediated by smaller diameter afferents originating from the fatigued muscle.

Adult↗
PubMed1986

Water and electrolyte fluxes during exercise and their relation to muscle fatigue.

UNLABELLED: Muscle fatigue - defined as impaired contractility - coincided in rat skeletal muscle during in vitro experiments with a decrease in intracellular potassium concentration, [K], and a decrease in resting membrane potential, RMP. In man changes in intra- and extracellular electrolyte concentrations and RMP were calculated from muscle biopsies and blood samples during voluntary contractions. At exhaustion in maximal exercise the intracellular [K] decreased by 20-40 mM from a resting value of 165 mM. Simultaneously extracellular [K] increased by 1-2 mM, and also the intra- as well as extracellular [Na] tended to increase. The calculated RMP therefore decreased from a resting value of -89 mV to -75 mV. During prolonged submaximal exercise similar changes occurred although the rate of these changes was slower. The K lost from the exercising muscles was being taken up by other tissues, and during recovery the fatigued muscles regain the previously lost K. IN CONCLUSION: the K-gradient across the muscle membrane decreased significantly during maximal as well as submaximal exercise. At exhaustion this change is of an order of magnitude, which may well impair the excitability of the muscle membrane and thereby the contractility of the muscle fibres.

Animals↗
PubMed2006

Evidence for a supraspinal contribution to human muscle fatigue.

1. Muscle fatigue can be defined as any exercise-induced loss of ability to produce force with a muscle or muscle group. It involves processes at all levels of the motor pathway between the brain and the muscle. Central fatigue represents the failure of the nervous system to drive the muscle maximally. It is defined as a progressive exercise-induced reduction in voluntary activation or neural drive to the muscle. Supraspinal fatigue is a component of central fatigue. It can be defined as an exercise-induced decline in force caused by suboptimal output from the motor cortex. 2. When stimulus intensity is set appropriately, transcranial magnetic stimulation (TMS) over the motor cortex during an isometric maximal voluntary contraction (MVC) of the elbow flexors commonly evokes a small twitch-like increment in flexion force. This increment indicates that, despite the subject's maximal effort, motor cortical output at the moment of stimulation was not maximal and was not sufficient to drive the motoneurons to produce maximal force from the muscle. An exercise-induced increase in this increment demonstrates supraspinal fatigue. 3. Supraspinal fatigue has been demonstrated during fatiguing sustained and intermittent maximal and submaximal contractions of the elbow flexors where it accounts for about one-quarter of the loss of force of fatigue. It is linked to activity and the development of fatigue in the tested muscles and is little influenced by exercise performed by other muscles. 4. The mechanisms of supraspinal fatigue are unclear. Although changes in the behaviour of cortical neurons and spinal motoneurons occur during fatigue, they can be dissociated from supraspinal fatigue. One factor that may contribute to supraspinal fatigue is the firing of fatigue-sensitive muscle afferents that may act to impair voluntary descending drive.

Brain↗
PubMed1998

Early development of EMG localized muscle fatigue in hand muscles of patients with chronic heart failure.

BACKGROUND: Patients with chronic heart failure (CHF) frequently complain of fatigue and exercise intolerance that are not directly related to the severity of cardiac failure. A not well-defined muscle function impairment is generally considered the cause of such symptoms. The frequency compression of electromyographic (EMG) signal power spectrum during isometric contractions is commonly accepted as an index of the fatigue occurring in the muscle (localized muscle fatigue). PURPOSE AND METHODS: The purpose of the study was to evaluate muscle fatigue development in a selected group of CHF patients by studying the compression of the EMG signal power spectrum. The first dorsal interosseus of the right, dominant hand was investigated at two levels of contraction: 40% and 80% of the maximal voluntary contraction (MVC). RESULTS: In CHF patients there was early development of localized muscle fatigue during the high level of contraction (80% of MVC). CONCLUSION: This study demonstrates the presence of an early development of localized muscle fatigue in CHF patients and confirms the possibility of an increased glycolytic metabolism. Moreover, the changes seem to show that muscle impairment is not limited to large muscles, but also occurs in small muscles of the hands, frequently used during daily activities. Finally, this study confirms the validity of EMG spectral analysis techniques in evaluating muscle fatigue of CHF patients, suggesting a possible use in the rehabilitation of such patients when the technique is correctly used.

Adult↗
PubMed1998

Cervical magnetic stimulation as a method to discriminate between diaphragm and rib cage muscle fatigue.

Inspiratory muscle fatigue can probably determine hypercapnic respiratory failure. Diaphragm fatigue is detected by electrical phrenic stimulation (ELS), but there is no simple tool to assess rib cage muscle (RCM) fatigue. Cervical magnetic stimulation (CMS) costimulates the phrenic nerves and RCM. We reasoned that changes in transdiaphragmatic pressure twitch (Pdi,tw) with CMS and ELS should be different after selective diaphragm vs. RCM fatigue. Five volunteers performed inspiratory resistive tasks while voluntarily uncoupling diaphragm and RCM. Baseline Pdi,twELS and Pdi,twCMS were 28.57 +/- 1.68 and 32.83 +/- 2.92 cmH2O. After selective diaphragm loading, Pdi,twELS and Pdi,twCMS were reduced by 39 and 26%, with comparable decreases in gastric pressure twitch (Pga,tw). Esophageal pressure twitch (Pes,tw) was better preserved with CMS. Therefore Pes,tw/Pga,tw was lower with ELS than CMS (-1.24 +/- 0.16 vs. -1.73 +/- 0.11, P = 0.05). After selective RCM loading, there was no diaphragm fatigue, but Pes,twCMS was significantly reduced (-30%). These findings support the role of rib cage stiffening by CMS-related RCM contraction in the ELS-CMS differences and suggest that CMS can be used to assess RCM fatigue.

Adult↗
PubMed1988

Respiratory muscle fatigue.

Respiratory muscle fatigue is caused by excessive effort relative to the strength and endurance of the respiratory muscles. It can be manifested by reductions in respiratory drive (central fatigue), by impaired neuromuscular transmission (transmission fatigue), by decreased contractility (contractile fatigue), or by a combination of these factors. Respiratory muscle fatigue probably contributes to the difficulties some patients have with weaning from mechanical ventilation, the symptoms of exercise intolerance and dyspnea in chronic lung disease, and CO2 retention. Therapy depends on a reduction in the required level of respiratory effort and/or an improvement in respiratory muscle strength and endurance.

Dyspnea↗
PubMed2005

Changes in motor cortex excitability during muscle fatigue in amyotrophic lateral sclerosis.

To further investigate the pathophysiology of amyotrophic lateral sclerosis (ALS), the silent period (SP) evoked by transcranial magnetic stimulation during a fatiguing muscle contraction was evaluated in 15 patients and in 15 healthy subjects. Physiological lengthening of the SP duration was not observed in patients with disease duration of > or = 2 years. Decreased intracortical inhibition, probably secondary to dysfunction of the inhibitory interneurons that modulate the corticomotoneuronal firing, appears in later stages of disease. Normal motor cortex adaptation is impaired and cortical hyperexcitability might be unmasked during fatigue in ALS patients with longer disease duration.

Adult↗
PubMed1983

Invariance of ankle dynamic stiffness during fatiguing muscle contractions.

Our objective was to determine the effect of muscle fatigue on the dynamic stiffness of the human ankle. Four subjects were required to maintain constant-force contractions of tibialis anterior until the required force could no longer be maintained. Repeated pseudo-random displacements of ankle angular position were applied throughout each contraction. The dynamic relation between ankle angular position and ankle torque was identified by determining non-parametric compliance impulse response functions (CIRFs). The CIRFs were redetermined every 2.55S throughout the sustained contractions to provide a quantitative measure of changes in ankle stiffness dynamics. Inspection of these CIRFS revealed little change in shape or magnitude throughout the contractions, despite large increases in tibialis anterior EMG. The dynamics were further quantified by estimating the equivalent joint inertia, viscosity and elasticity associated with each CIRF. As each contraction progressed, the inertial and elastic terms remained constant whereas the viscous term decreased slightly. These findings demonstrate that fatigue of tibialis anterior during sustained constant mean force contractions results in little change in the mechanical dynamics of the human ankle.

Adult↗
PubMed1985

Dynamic and static stretch responses in muscle spindle receptors in fatigued muscle.

Dynamic and static stretch responses in muscle spindles were investigated in fatigued muscle to determine if acute adaptations do occur in receptor discharge as has been shown after contractions of short duration. Fatigue to 60-50% maximum tetanic tension was induced in the isolated gastrocnemius muscle in 16 cats by sustained, 7 X threshold electrical stimulation (100 Hz) of the cut L7 ventral root and S1 ventral root. Group Ia and II afferent fiber responses to slow ramp stretches (5 mm X s-1) and vibration (100 Hz) applied to the Achilles tendon were monitored before and immediately after muscle tetany to fatigue. Changes in firing characteristics were similar when results from faster (25-30 mm X s-1) ramp stretches were contrasted. During muscle fatigue, decreases in response latency to displacement and increases in resting discharge, mean frequency during stretch, and frequency of firing to vibration were predominant in both afferent fiber types. Static responses were generally lower, indicating a decrease in position sensitivity. Resting muscle force and passive peak muscle stiffness were consistently higher following contraction. The sum effects of these proprioceptive afferent and mechanical muscle responses would be to increase muscle stiffness and thus resist yield in muscle length to perturbations at lower muscle forces. The magnitude of these adaptations in proprioceptive discharge appears dependent on intrafusal muscle fiber activation.

Adaptation, Physiological↗
PubMed1998

Neural control in human muscle fatigue: changes in muscle afferents, motoneurones and motor cortical drive [corrected].

To understand the neural factors which contribute to fatigue, it is not satisfactory to regard fatigue as occurring only when a task can no longer be performed. Changes in muscle afferent feedback, motoneuronal discharge, motor cortical output, and perceived effort develop well before an endurance limit in limb muscles. During sustained maximal contractions the discharge of motoneurones declines, commonly to below the level required to produce maximal force from the muscle whose contractile speed is usually slowed. Thus, some 'central' fatigue develops. Recent findings using transcranial stimulation have revealed that the motor cortex is one site at which suboptimal output develops during human muscle fatigue. There is a need to study the reflex effects on motoneurones and the excitability of the motor cortex in experimental animals, as well as to apply rigorous methods to assess these processes in voluntary exercise in human subjects [corrected].

Humans↗
PubMed2002

Glucose infusion attenuates muscle fatigue in rat plantaris muscle during prolonged indirect stimulation in situ.

Carbohydrate ingestion increases both endurance time to exhaustion during prolonged exercise, and the ability to perform resistance exercise. The mechanism(s) underlying the increased performance following glucose ingestion remain(s) unclear. The purpose of the present study was to verify the hypothesis that glucose infusion could attenuate peripheral muscle fatigue in the anaesthetized rat during prolonged indirect electrical stimulation in situ. For this purpose the plantaris muscle was electrically stimulated (50 Hz for 200 ms every 2.7 s; 5 V; pulse width, 0.05 ms) in situ through the sciatic nerve to perform concentric contractions for 60 min while infusing intravenously either saline alone (7.25 ml kg(-1) h(-1)), or saline and glucose (1 g kg(-1) h(-1): plasma glucose 11 +/- 1.1, vs. 4.9 +/- 0.2 mm with infusion of saline) (8 rats per group). Glucose infusion attenuated the reduction in submaximal peak dynamic force (55% decrease vs. 70% decrease in rats infused with saline alone, P < 0.05). In a third group of rats (n = 8), infusion of glucose 30 min after the start of stimulation partially restored submaximal peak dynamic force (P < 0.05). Maximum dynamic and isometric forces at the end of the period of stimulation were also higher (P < 0.05) in rats infused with glucose (4.0 +/- 0.2 and 4.3 +/- 0.2 N, respectively) than saline alone (3.0 +/- 0.2 and 3.5 +/- 0.2 N, respectively). The beneficial effect of glucose infusion on peripheral muscle force during prolonged stimulation was not associated with a reduction in muscle glycogen utilisation, nor with a reduction of fatigue at the neuromuscular junction, as assessed through maximal direct muscle stimulation (200 Hz for 200 ms; 150 V; pulse width, 0.05 ms). However, changes in M-wave peak-to-peak amplitude, duration and total area suggest that glucose infusion, and/or the associated increase in plasma insulin concentration, may prevent the deterioration of electrical properties of the muscle fibre membrane.

Animals↗
PubMed1980

Mechanical and electrical correlates of isometric muscle fatigue in skeletal muscle in the cat.

The amplitude and duration of motor unit action potentials, the rise time, peak tension, and half relaxation time of an isometric twitch, the force-velocity relationship, and tetanic tension were measured at the beginning (fresh muscle) and at the end of a fatiguing isometric contraction at a tension of either 40 or 70% of the initial strength in the soleus (a slow twitch muscle) and the medial gastrocnemius (a fast twitch muscle) of the cat. These same parameters were also measured at set intervals following these contractions to assess their rate of recovery to pre-exercise values. At The end of a fatiguing contraction at either tension examined, Vmx, twitch tension and tetanic tension, were all reduced while there was a prolonged twitch duration and duration of the motor unit action potential for both types of muscle. The height of the motor unit action potential was only marginally effected by muscle fatigue. Following the fatiguing contraction, the endurance required several hours to recover in the medial gastrocnemius muscle but recovered fully within 15 min after either tension in the soleus muscle. Tetanic tension and twitch tension both required less than 10 min for full recovery in the medial gastrocnemius muscle but recovered fully to the pre-exercise values within 3 min following fatiguing isometric contractions in the soleus muscles. Vmx, and the height and duration of the motor unit action potential both recovered within 1 min following the end of the exercise.

Animals↗
PubMed2002

Nonlinear cortical modulation of muscle fatigue: a functional MRI study.

Muscle fatigue has been studied for over a century, but almost no data are available to indicate how the brain perceives fatigue and modulates its signals to the fatiguing muscle. In this study, brain activation was measured by functional magnetic resonance imaging (fMRI) during a sustained (2-min) maximal-effort handgrip contraction while handgrip force and finger muscle electromyographic (EMG) data were recorded simultaneously by a magnetic resonance environment-adapted force-EMG measurement system. The results showed decoupled progresses in brain and muscle activities when muscle was fatigued and correlated behaviors among the cortical areas being analyzed. While handgrip force and EMG signals declined in parallel during the course of muscle fatigue, fMRI-measured brain activities first substantially increased and then decreased. This similar signal modulation occurred not only in the primary sensorimotor areas but also in the secondary and association cortices (supplementary motor, prefrontal, and cingulate areas). The nonlinear changes of brain signal may reflect an early adjustment to strengthen the descending command for force-loss compensation and subsequent inhibition by sensory feedback as fatigue became more severe. The close association in the activation pattern in many cortical regions may reflect integrated processing of information in the brain.

Afferent Pathways↗