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PubMed1996

[Effect of respiratory muscle fatigue on their function during exercise].

We evaluated the effect of global inspiratory muscle fatigue (GF) on respiratory muscle control during exercise at 30%, 60%, and 90% of maximal power output in normal subjects. Fatigue was induced by breathing against a high inspiratory resistance until exhaustion. Respiratory pressures, breathing pattern, and perceived breathlessness were measured. Induction of GF had no effect on the ventilatory parameters during mild and moderate exercise. It altered, however, ventilatory response to heavy exercise by increasing breathing frequency and minute ventilation, with minor changes in tidal volume. This was accompanied by an increase in perceived breathlessness. GF significantly increased both the tonic and phasic activities of abdominal muscles that allowed 1) the diaphragm to maintain its function while developing less pressure, 2) the same tidal volume with lesser shortening of the rib cage inspiratory muscles, and 3) relaxation of the abdominal muscles to contribute to lung inflation. The increased work performed by the abdominal muscles may, however, lead to a reduction in their strength. GF may impair exercise performance in some healthy subjects that is probably not related to excessive breathlessness or other ventilatory factors. The respiratory system is remarkably adaptable in maintaining ventilation during exercise even with impaired inspiratory muscle contractility.

Abdominal Muscles↗

PubMed1998

A method for assessing muscle fatigue during sprint exercise in humans using a friction-loaded cycle ergometer.

This study investigated the mechanical changes induced by muscle fatigue caused by repeated sprints and determined whether a friction-loaded cycle ergometer has any advantages for assessing muscle fatigue. Nine subjects performed 15 sprints, each of 5 s with a 25-s rest, on a friction-loaded cycle ergometer. The averaged force, power and velocity of each push-off were calculated. Maximal power decreased by 17.9%, with a concomitant slowing of muscle contraction, but without any change in the maximal force. These results demonstrated that repeated sprints slow down muscle contraction, leading to a fall in maximal power without any loss of force. This would suggest that fast twitch fibres are selectively fatigued by repeated sprints. However, the ergometer used in the present study made it difficult to evaluate the relative influences of contraction velocity and sprinting time. This was certainly the most important limitation. On the other hand, it showed the advantage of measuring instantaneous power and total work dissipated in the environment simultaneously. It also permitted a force-velocity relationship to be obtained from a single sprint and this relationship is known to be closely related to the muscle fibre composition.

Adult↗

PubMed1998

Events of the excitation-contraction-relaxation (E-C-R) cycle in fast- and slow-twitch mammalian muscle fibres relevant to muscle fatigue.

The excitation-contraction-relaxation cycle (E-C-R) in the mammalian twitch muscle comprises the following major events: (1) initiation and propagation of an action potential along the sarcolemma and transverse (T)-tubular system; (2) detection of the T-system depolarization signal and signal transmission from the T-tubule to the sarcoplasmic reticulum (SR) membrane; (3) Ca2+ release from the SR; (4) transient rise of myoplasmic [Ca2+]; (5) transient activation of the Ca2+-regulatory system and of the contractile apparatus; (6) Ca2+ reuptake by the SR Ca2+ pump and Ca2+ binding to myoplasmic sites. There are many steps in the E-C-R cycle which can be seen as potential sites for muscle fatigue and this review explores how structural and functional differences between the fast- and slow-twitch fibres with respect to the E-C-R cycle events can explain to a great extent differences in their fatiguability profiles.

Animals↗

PubMed2004

Muscle fatigue and the mechanisms of task failure.

An alternative approach in the study of muscle fatigue is to address the question, "What causes task failure during a fatiguing contraction?" This approach is described by considering how variation in the type of load supported and contraction intensity influence both the time to task failure and the centrally mediated adjustments in reflex activity and motor unit behavior.

Electromyography↗

PubMed2000

Stretch-shortening cycle: a powerful model to study normal and fatigued muscle.

Stretch-shortening cycle (SSC) in human skeletal muscle gives unique possibilities to study normal and fatigued muscle function. The in vivo force measurement systems, buckle transducer technique and optic fiber technique, have revealed that, as compared to a pure concentric action, a non-fatiguing SSC exercise demonstrates considerable performance enhancement with increased force at a given shortening velocity. Characteristic to this phenomenon is very low EMG-activity in the concentric phase of the cycle, but a very pronounced contribution of the short-latency stretch-reflex component. This reflex contributes significantly to force generation during the transition (stretch-shortening) phase in SSC action such as hopping and running. The amplitude of the stretch reflex component - and the subsequent force enhancement - may vary according to the increased stretch-load but also to the level of fatigue. While moderate SSC fatigue may result in slight potentiation, the exhaustive SSC fatigue can dramatically reduce the same reflex contribution. SSC fatigue is a useful model to study the processes of reversible muscle damage and how they interact with muscle mechanics, joint and muscle stiffness. All these parameters and their reduction during SSC fatigue changes stiffness regulation through direct influences on muscle spindle (disfacilitation), and by activating III and IV afferent nerve endings (proprioseptic inhibition). The resulting reduced stretch reflex sensitivity and muscle stiffness deteriorate the force potentiation mechanisms. Recovery of these processes is long lasting and follows the bimodal trend of recovery. Direct mechanical disturbances in the sarcomere structural proteins, such as titin, may also occur as a result of an exhaustive SSC exercise bout.

Humans↗

PubMed1991

Role of exercise-induced potassium fluxes underlying muscle fatigue: a brief review.

The site of exercise-induced muscle fatigue is suggested to be the muscle membrane, which includes the sarcolemma and T-tubule membrane; the excitability of the membrane is dependent on the membrane potential. Significant potassium flux from the intracellular space of contracting muscle may decrease the membrane potential to half its resting value. This is true for isolated muscle preparations as well as for the whole body exercise in humans. Specific K+ channels have been identified, that may account for the intracellular K+ loss. Calcium-sensitive K+ channels open when intracellular Ca2+ concentrations increase, as during excitation. ATP-sensitive K+ channels may be involved but may open only at ATP concentrations well below those attained at exhaustion. However, ATP may be compartmentalized and only the membrane-bound ATP concentration may be of significance. Ca2+ accumulation and ATP depletion cause cell destruction; these changes induce an increased K+ conductance, which may inactivate the membrane and consequently prevent tension development. It is hypothesized that such a safety mechanism is identical to the fatigue mechanism.

Animals↗

PubMed2001

Effect of localized muscle fatigue on vertical ground reaction forces and ankle joint motion during running.

The purpose of this study was to examine the changes in the vertical ground reaction force (VGRF) and ankle joint motion during the first 50% of the stance phase of running following fatiguing exercise of either the dorsiflexors or the invertors of the foot. VGRFs, sagittal and rearfoot kinematic data were collected from 11 female recreational runners running at 2.9 m/second on a treadmill prior to and following localized muscle fatigue of either the invertors or dorsiflexors of the right foot. Loading rate of the impact peak force significantly increased following fatiguing exercise of the dorsiflexors, while the peak magnitudes of the impact and push-off forces remained unchanged. There were significant decreases in dorsiflexion at heel contact, but no significant difference in any rearfoot motion parameters tested following dorsiflexor fatigue. Following fatiguing exercise of the invertors, impact peak magnitude, push-off peak magnitude and the rate of decline of the impact peak force significantly decreased; there was no change in the loading rate of the impact peak force. Invertor fatigue also resulted in a less inverted foot position at heel contact, but there were no significant differences in any other kinematic parameters tested. The results demonstrate that localized muscle fatigue of either the invertors or dorsiflexors can have a significant effect on the loading rates, peak magnitudes and ankle joint motion seen during running. These changes, due to localized muscle fatigue, may play a role in many common lower extremity running injuries.

Adult↗

PubMed2006

EMG analysis of shoulder muscle fatigue during resisted isometric shoulder elevation.

The purpose of this study was to determine if a difference existed in the rate of fatigue of select shoulder muscles during isometric shoulder elevation and if the measured rate of fatigue was consistent from day to day. Shoulder muscle fatigue has been associated with alterations in joint mechanics and possibly contributes to shoulder dysfunction. While research exists, there is limited information on an objective and reliable measure of shoulder fatigue. Sixteen asymptomatic subjects were evaluated. The subjects held a weight equivalent to 60% of his/her Maximum Voluntary Isometric Contraction (MVIC) while elevating in the scapular plane. Surface electrodes were applied to collect electromyographic activity from the upper trapezius, middle deltoid, serratus anterior, and lower trapezius muscles while the arm was held at 90 degrees elevation. Data collection ceased when the subject was no longer able to maintain 90 degrees of elevation. The subject then rested and a second trial performed. One week later, the two-trial procedure was repeated. A significant interaction of trial x day x muscle was found for the rate of fatigue. Post hoc analysis revealed that the rate of fatigue of the middle deltoid was significantly greater than the other muscles tested. The intraday reliability was good for all muscles but interday reliability was poor except for the middle deltoid. This study suggests that the middle deltoid appears to fatigue faster than the other shoulder muscles tested at the selected level of shoulder elevation. This should be considered in designing a rehabilitation program to develop a sequence that does not overly fatigue the middle deltoid.

Adult↗

PubMed1992

[An analysis of respiratory muscle fatigue under a resistive load with the breathing of gas mixtures with different oxygen levels].

In anesthetized cats, the respiration muscles' fatigue under normoxia occurred in heavy mechanical load equal to 80% of the maximal inspiratory effort. Under hypoxia the force of the diaphragm contraction did never decrease whereas when breathing with hypoxic gas mixture weakening of the contractile ability occurred yet under moderate load. In breathing with additional mechanical load under hypoxia, mainly the peripheral part of the respiratory system's motor apparatus suffers, as the weakening of diaphragm's electrical activity and reduction of its contractions' force occurred during unfalling activity of the diaphragmatic nerve. The oxygen content in arterial blood affects the contractile ability of respiratory muscles. However, the increase in the energy supply level does not necessarily exclude the possibility of respiratory muscles' fatigue, but just decelerates its development.

Air↗

PubMed1997

Trunk muscle fatigue and associated EMG changes during a dynamic iso-inertial test.

This study was designed to investigate the relationship between trunk muscle fatigue and associated changes in the electromyographic (EMG) signals during a dynamic iso-inertial test. Eleven subjects performed dynamic trunk flexion/extension movements against 40% maximum voluntary contraction (MVC) torque until exhaustion in a tri-axial trunk dynamometer. EMG parameters in the time and frequency domain were studied by analysing changes of the signal amplitudes and the spectral density (using the zero-crossing-rate and the median frequency). The kinematics of the movement were analysed according to the movement velocities and the deviations from the required movement plane. The flexion and extension velocities decreased from the beginning to the end of the test. Movement deviations from the sagittal plane into the frontal and transverse plane increased with increasing test duration, as did the EMG amplitude. The median frequency during periods with maximum muscle activity decreased, as did the zero-crossing-rate. The increase in amplitude and decrease in median frequency were more pronounced in the trunk flexors than in the trunk extensors. The parameters of median frequency, zero-crossing-rate and amplitude seem to be sensitive identifiers of muscle fatigue during well-controlled dynamic contractions. While the kinematic data did not yield any information on the mechanisms of the fatigue, changes in the EMG parameters demonstrated that the duration of the test was limited by the fatigue of the trunk flexors.

Adult↗

PubMed2002

Muscle fatigue: the role of metabolism.

This paper examined the role of metabolites in causing muscle fatigue. Previous studies have shown that Pi (H2PO4-, HPO4-2) and H+ may be important factors in causing fatigue. A key question is the potential interaction between metabolic end-products and calcium related excitation-contraction coupling fatigue (ECC). An in vivo rat muscle model was used to measure tension development and metabolic end-products in response to electrical stimulation. Two stimulation protocols were used, high intensity stimulation followed by a medium intensity stimulation (High Group), and low intensity stimulation followed by a medium intensity stimulation (Low Group). Metabolic fatigue was based on concentrations of (H2PO4-measured with phosphorus magnetic resonance spectroscopy. ECC fatigue was measured as the fatigue in excess of metabolic fatigue, and as the relative decline of force at low compared to high stimulation frequencies. During the initial stimulation period, the High Group had greater metabolic fatigue (p < 0.001) and greater ECC fatigue (p = 0.007). During the second stimulation period and recovery, the High Group had no difference in metabolic fatigue (p = 0.07) and greater ECC fatigue (p = 0.015). These results present a method for determining the relative amounts of metabolic and ECC fatigue, and suggest that metabolites can increase the amount of ECC fatigue.

Analysis of Variance↗

PubMed1998

Skeletal muscle fatigue in normal subjects and heart failure patients. Is there a common mechanism?

Skeletal muscle fatigue develops gradually during all forms of exercise, and develops more rapidly in heart failure patients. The fatigue mechanism is still not known, but is most likely localized to the muscle cells themselves. During high intensity exercise the perturbations of the Na+ and K+ balance in the exercising muscle favour depolarization, smaller action potentials and inexcitability. The Na+, K+ pump becomes strongly activated and limits, but does not prevent the rise in extracellular Na+, K+ pump concentration and intracellular Na+ concentration. However, by virtue of its electrogenic property the pump may contribute in maintaining excitability and contractility by keeping the cells more polarized than the ion gradients predict. With prolonged exercise perturbations of Na+ and K+ are smaller and fatigue may be associated with altered cellular handling of Ca2+ and Mg2+. Release of Ca2+ from the sarcoplasmic reticulum (SR) is reduced in the absence of changes of the cellular content of Ca2+ and Mg2+. In heart failure several clinical reports indicate severe electrolyte perturbations in skeletal muscle. However, in well controlled studies small or insignificant changes are found. We conclude that with high intensity exercise perturbations of Na+ and K+ in muscle cells may contribute to fatigue, whereas with endurance type of exercise and in heart failure patients the skeletal muscle fatigue is more likely to reside in the intracellular control of Ca2+ release and reuptake.

Heart Failure↗

PubMed1997

The validity of stimulus-evoked EMG for studying muscle fatigue characteristics of paraplegic subjects during dynamic cycling movement.

The fatigue characteristics of paralyzed muscles were investigated during dynamic cycling movement induced by functional electrical stimulation (FES). The peak-to-peak (PTP) amplitude of stimulus-evoked electromyogram (EMG), after suppression of stimulus artifact, was adopted as fatigue indicator. Compared to static contraction, the effects of dynamic movement factors on the stimulus-evoked EMG, such as the intermittent stimulation, joint angle, and contraction speed, were first evaluated in separate experiments. The results of isolated tests laid the foundation for interpreting the data obtained in two FES-cycling experiments, performed under maximum stimulation or in controlled cycling speeds. The effects of intermittent stimulation and joint angle caused periodic changes in PTP amplitude which can be alleviated by averaging the PTP amplitude of one cycle. Under the same stimulation intensity, our results indicated that slower muscle contraction speed would have larger PTP amplitude and vice versa. For the limited number of subjects with paraplegia studied, our results showed that the use of EMG PTP as reliable muscle fatigue indicator during dynamic movement is only valid at the same cycling speed or corresponding contraction speed. The decline of the PTP amplitude decreased with the decay of muscle force can be observed during cycling movement; however, reduction of cycling speed had the opposite effect on PTP amplitude. Observations from the hyperbolic modeling of fatigue process demonstrated that the EMG PTP of a fatigued muscle under dynamic movement decreased at a slower rate.

Adult↗

PubMed1988

Relationship between numbers and frequencies of stimuli in human muscle fatigue.

The effect of stimulus frequency on the rate of muscle fatigue has been studied on dorsiflexor muscles of the human ankle. It was found that significantly fewer stimuli were required to abolish twitch and tetanic torque when the stimuli were delivered at 15 Hz rather than 30 Hz. At both stimulus frequencies twitch torque disappeared before tetanic torque. The difference in numbers of stimuli required for fatigue was not due to impaired excitation of muscle fibers at either of the two frequencies. At both stimulating frequencies, twitch fatigue appeared to be due to a defect in excitation-contraction coupling and/or the contractile machinery.

Adult↗

PubMed1990

Muscle fatigue in acute respiratory failure.

Fatigue of the respiratory muscles is now well documented but still remains a process that cannot be assessed easily. Several methods are available to detect fatigue; they represent windows allowing different viewpoints on the same phenomenon. The definition and determinants of respiratory muscle fatigue and some of the methods used to detect it are reviewed. On the basis of the available evidence provided by each of these methods, the role played by muscle fatigue in acute respiratory failure (ARF) is discussed.

Acute Disease↗

PubMed2002

Development of muscle fatigue as assessed by electromyography and mechanomyography during continuous and intermittent low-force contractions: effects of the feedback mode.

The goal of the present study was to investigate the significance of low-force continuous or intermittent static contraction and feedback mode (visual or proprioceptive) on the development of muscle fatigue as assessed by electromyography (EMG) and mechanomyography (MMG). Visual (force control) and proprioceptive (displacement control) feedback was investigated during intermittent (6 s contraction, 4 s rest) and continuous static contractions at 10% and 30% of the maximum voluntary contraction (MVC). Mean force, force fluctuation, rating of perceived exertion and root mean square (RMS) and mean power frequency (MPF) of the EMG and MMG signals were analysed. The general pattern for MMG RMS and EMG RMS values and the rating of perceived exertion was an increase with contraction time, while the EMG MPF values decreased ( P<0.05). The increase in RMS values was generally more pronounced for the MMG compared with the EMG, while the decrease in MPF values was more consistent for the EMG compared with the MMG signal. During the intermittent contractions, the main effect was on MPF for both EMG and MMG. Lower force fluctuation and larger rating of perceived exertion ( P<0.05), greater slopes of EMG and MMG RMS and MPF values versus time were observed with proprioceptive feedback compared with visual feedback. The findings suggest that (1) the EMG and MMG signals give complementary information about localised muscle fatigue at low-level contraction: they responded differently in terms of changes in the time and frequency domain during continuous contraction, while they responded in concert in the frequency domain during intermittent contractions, and (2) the different centrally mediated motor control strategies used during fatiguing contraction may be dependent upon the feedback modality.

Adult↗

PubMed1991

A real-time FFT analyser for monitoring muscle fatigue.

The TMS 32010 system for a real-time muscle fatigue monitor is described; it is based on mean frequency shift in the electromyographic signal (EMG). The mean frequency of the EMG is obtained, in real time, from its power spectrum, with a 2 Hz resolution. This is made possible by combining novel hardware and software.

Computer Systems↗

PubMed1987

Does rib cage-abdominal paradox signify respiratory muscle fatigue?

Studies suggesting that abnormal motion of the rib cage (RC) and abdomen (Ab) may indicate respiratory muscle fatigue have not separated the influence of respiratory load from that of fatigue in its pathogenesis. We hypothesized that abnormalities on RC-Ab motion are primarily related to increased load rather than fatigue. We tested this hypothesis in subjects breathing against resistive loads while maintaining 30 and 60% of maximum mouth pressure (Pmmax). RC-Ab asynchrony and paradox and the degree of variation in compartmental contribution to tidal volume were measured by inductive plethysmography and quantitated by the Konno-Mead method of analysis. Comparing measurements of base line and 30 and 60% of Pmmax indicated that the degree of asynchrony, paradox, and variation in compartmental contribution were significantly related to the level of the load; significant abnormalities were observed at even 30% of Pmmax, a target pressure that can be sustained indefinitely. In another group of subjects, fatigue was induced by sustaining 60% of Pmmax to the limits of tolerance. Indexes of abnormal RC-Ab motion increased from base line during the 1st min of loaded breathing but displayed no progression from the beginning to the end of the fatigue run. Immediately on discontinuation of the load, the indexes returned to levels similar to base line despite persistence of the fatigue state. These results in healthy subjects breathing against severe resistances indicate that RC-Ab asynchrony and paradox and variation in compartmental contribution to tidal volume are predominantly due to increases in respiratory load rather than muscle fatigue.

Abdomen↗