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At least 127 records · Page 7Linked to original sources
PubMed2003

Inspiratory muscle fatigue in swimmers after a single 200 m swim.

Inspiratory muscle fatigue may occur in as little as 6 min during high-intensity spontaneously breathing exercise. The aims of this study were to determine whether inspiratory muscle fatigue occurs during swimming exercise and whether inspiratory muscle strength differs between the supine and standing body positions. Seven competitive swimmers were recruited to perform a single 200 m front-crawl swim, corresponding to 90-95% of race pace. Inspiratory muscle strength was measured at residual volume using a hand-held mouth pressure meter that measured maximal inspiratory pressure in the upright and supine positions. At baseline, maximal inspiratory pressure in the supine position was significantly lower than maximal inspiratory pressure in the upright position (112 +/- 20.4 and 133 +/- 16.7 cmH2O, respectively; P < or = 0.01). Post-exercise maximal inspiratory pressure in the supine position (80 +/- 15.7 cmH2O) was significantly lower than baseline maximal inspiratory pressure in the supine position (P < or = 0.01). The results indicate that a single 200 m front-crawl swim corresponding to 90-95% of race pace was sufficient to induce inspiratory muscle fatigue in less than 2.7 min. Furthermore, although diaphragm muscle length is optimized when supine, our results indicate that the force output of the diaphragm and inspiratory accessory muscles is greater when upright than when supine.

Adult↗
PubMed1999

Muscle fatigue from intermittent stimulation with low and high frequency electrical pulses.

OBJECTIVE: To evaluate muscle fatigue resulting from intermittent low frequency and high frequency stimulation for the application of closed-loop control in functional electrical stimulation (FES). DESIGN: Nonrandomized trial. SETTING: General community, a referral center, institutional practice, and ambulatory care. PATIENTS: Twenty healthy nondisabled men volunteered for the normal muscle group. Four paraplegic men with implanted percutaneous intramuscular electrodes for FES volunteered for the paralyzed muscle group. INTERVENTION: The stimulation frequency was set at low (20 Hz) or high (100 Hz). Stimulation was administered in 4-second bursts at the start of 60-second, 120-second, and 240-second periods (duty cycles of 1/15, 1/30, and 1/60, respectively). MAIN OUTCOME MEASUREMENTS: Knee extensor torques were measured during intermittent electrical stimulation. A strength decrement index (SDI) was used to assess muscle fatigue. Actual knee extensor torques in the paraplegic men were also measured with an isokinetic dynamometer. RESULTS: Muscle fatigue was significantly greater at 20 Hz than at 100 Hz for both the nondisabled and the paraplegic subjects (p < .0001). Muscle fatigue at the 1/15 cycle was significantly reduced (p < .01). CONCLUSIONS: Muscle fatigue was greater at the lower frequency (20 Hz) than at the higher frequency (100 Hz) during intermittent electrical stimulation, suggesting that intermittent high frequency stimulation may be valuable in the development of closed-loop control strategies for FES.

Adult↗
PubMed2002

Muscle fatigue in animal models of neuromuscular disease.

This review summarizes the literature regarding the fatigue properties of skeletal muscles in animal models of neuromuscular disease and the effect of exercise training on these properties in diseased muscles. Results of these studies are not consistent. Reasons for the variability may include the use of differing experimental conditions, varying stimulation protocols, different methods of muscle preparation, and a range of temperatures and ages. Despite the variability and small sample sizes, these studies of muscle fatigue have revealed definite trends. Whereas the slow oxidative soleus muscle from the diseased animals fatigued more slowly or at the same rate as controls, the extensor digitorum longus (a fast, glycolytic muscle) from the diseased animals fatigued more rapidly than controls. The diaphragm-a fast, oxidative, glycolytic muscle that is generally fatigue resistant-did not reveal significant changes in fatigability. Exercise training had a minimal impact on the fatigue properties of skeletal muscle from diseased animals. As molecular and biochemical mechanisms are further identified and defined, additional therapeutic interventions may become available for these disorders. It is hoped that this review will serve as a guide for investigators who use an animal model to evaluate interventions for neuromuscular diseases.

Animals↗
PubMed1992

The effect of inspiratory muscle fatigue on breathing pattern and ventilatory response to CO2.

1. The effects of inducing inspiratory muscle fatigue on the subsequent breathing pattern were examined during resting unstimulated breathing and during CO2 rebreathing. In addition, we examined whether induction of inspiratory muscle fatigue alters CO2 responsiveness. 2. Global inspiratory muscle fatigue and diaphragmatic fatigue were achieved by having subjects breathe against an inspiratory resistive load while generating a predetermined fraction of either their maximal mouth pressure or maximal transdiaphragmatic pressure until they were unable to generate the target pressure. 3. Induction of inspiratory muscle fatigue had no effect on the subsequent breathing pattern during either unstimulated breathing or during CO2 rebreathing. 4. Following induction of inspiratory muscle fatigue, the slope of the ventilatory response to CO2 was significantly decreased from 18.8 +/- 3.3 during control to 13.8 +/- 2.1 l min-1 (% end-tidal CO2 concentration)-1 with fatigue (P < 0.02).

Adult↗
PubMed1997

Palpebral ptosis and muscle fatiguability associated with perineurial cell ensheathment of muscle fibers: a new disease of the neuromuscular junction?

Perineurial cell ensheathment of muscle fibers has been reported only in one patient. Here we describe a new case with identical morphologic features and a similar, but milder clinical course characterized by progressive muscle weakness and bilateral palpebral ptosis. EMG examination (including repetitive stimulation) and antibodies against acetylcholine receptors were normal. Muscle biopsy revealed several muscle fibers encircled by stratified rings of homogeneous material in which elongated nuclei were visible; this material was positively stained by antibodies directed at epithelial membrane antigen. On ultrastructural examination these encircling-fiber spirals had the characteristics of perineurial cells. It is not clear yet whether perineurial cell ensheathment of muscle fibers is an occasional feature, or whether it has a pathogenetic role in the clinical picture of both cases. The perineurial sheaths might alter the correct neuromuscular transmission mimicking a myasthenia-like disease, either by interfering with the neuromuscular junction, or by changing the microenvironment, and, thus, altering the general excitability of the muscle fibers.

Biopsy↗
PubMed1994

Fusimotor responses to fatiguing muscle contractions in non-denervated hindlimb of decerebrate cats.

Changes in discharge rate of 21 fusimotor neurons to medial gastrocnemius muscle during long-lasting fatiguing contractions of lateral gastrocnemius and soleus muscles were recorded in decerebrate cats with innervation of the same hindlimb preserved. Both the spontaneous activity and reflex responses of fusimotor neurons differed from those found previously in preparations with denervated hindlimb. Higher proportion of units fired at rest at rates above 20 impulses/s, the initial increase in discharge rate at the onset of muscle contraction was markedly prolonged, lasting in the majority of units throughout the muscle contraction, while the late increase in discharge rate developing with muscle fatigue was either absent or short-lasting. It is suggested that the increase in spontaneous firing rate occurs in dynamic fusimotor neurons being supported by afferent inflow from secondary muscle spindle endings from non-contracting muscles, the enhancement of the early responses to be primarily due to recurrent disinhibition and the differences in changes of early and late responses to reflect their partly different origin. The possibility is raised that the late reflex responses are lacking more often in static than in dynamic fusimotor neurons. From the functional point of view the differences in fusimotor reflex responses in innervated versus denervated hindlimb may indicate their susceptibility to modifications by changes in afferent inflow according, supposedly, to the current demands of motor control of the active and/or fatigued muscle.

Animals↗
PubMed1996

Effect of intracellular and extracellular ion changes on E-C coupling and skeletal muscle fatigue.

The causative factors in muscle fatigue are multiple, and vary depending on the intensity and duration of the exercise, the fibre type composition of the muscle, and the individual's degree of fitness. Regardless of the aetiology, fatigue is characterized by the inability to maintain the required power output and the decline in power can be attributed to a reduced force and velocity. Following high-intensity exercise, peak force has been shown to recover biphasically with an initial rapid (2 min) recovery followed by a slower (50 min) return to the pre-fatigued condition. The resting membrane potential depolarizes by 10-15 mV, while the action potential overshoot declines by a similar magnitude. Following high-frequency stimulation of the frog semitendinous muscle, we observed intracellular potassium [K+]1 decrease from 142 +/- 5 to 97 +/- 8 mM, while sodium [Na+]i rose from 16 +/- 1 to 49 +/- 6 mM. The [K+]i loss was similar to that observed in fatigued mouse and human skeletal muscle, which suggests that there may be a limit to which [K+]i can decrease before the associated depolarization begins to limit the action potential frequency. Fibre depolarization to- 60 mV (a value observed in some cells) caused a significant reduction in the t-tubular charge movement, and the extent of the decline was inversely related to the concentration of extracellular Ca2+. A decrease in intracellular pH (pHi) to 6.0 was observed, and it has been suggested by some that low pH may disrupt E-C coupling by directly inhibiting the SR Ca2+ release channel. However, Lamb at al. (1992) observed that low pH had no effect on Ca2+ release, and we found low pHi to have no effect on t-tubular charge movement (Q) or the Q vs. Vm relationship. The Ca2+ released from the SR plays three important roles in the regulation of E-C coupling. As Ca2+ rises, it binds to the inner surface of the t-tubular charge sensor to increase charge (Q gamma) and thus Ca2+ release, it opens SR Ca2+ channels that are not voltage-regulated, and as [Ca2+]i increases further it feeds back to close the same channels. The late stages of fatigue have been shown to be in part caused by a reduced SR Ca2+ release. The exact cause of the reduced release is unknown, but the mechanism appears to involve a direct inhibition of the SR Ca2+ channel.

Animals↗
PubMed2002

Electromyography and ratings of lumbar muscle fatigue using a four-level staircase protocol.

OBJECTIVE: The purpose of the study was to explore whether a relationship existed between subject's experience and objective measurements of back-muscle fatigue in healthy subjects. This may be used as reference material later. DESIGN: Muscle fatigue in the lower back was measured in healthy subjects using a staircase protocol. BACKGROUND: Muscle fatigue measurement in the lower back is of importance when assessing patients with low-back pain. The subject's experience of muscle fatigue needs to be explored further. METHODS: Twenty healthy subjects with informed consent participated. Borg CR-10 scale responses and electromyography measurements were obtained from low-back muscles of the subjects following a staircase protocol with sustained contractions at 20%, 40%, 60%, and 80% of maximum voluntary contraction. RESULTS: A strong correlation was found between Borg ratings and force (r=0.83) and between slope and force (r= -0.72) but not between Borg ratings and slopes which was at best 0.43 (/r/< or =0.43). CONCLUSIONS: Although subjective ratings and slopes showed a low correlation; increases in force with a concomitant increase in fatigue was reflected both by the subjective ratings and the slope. One cannot use subjective ratings for prediction of the slope for this type of protocol. However, a more complete description of muscle fatigue needs both the slope and the Borg ratings. RELEVANCE: In clinical testing, both subjective and objective measures of muscle fatigue must be considered since they measure different aspects of muscle fatigue. The median frequency slope probably reflects "physical discomfort" but the Borg ratings add a factor of "lack of energy".

Adult↗
PubMed2001

Movement-related cortical potentials associated with progressive muscle fatigue in a grasping task.

OBJECTIVE: The present research was aimed to further address the general empirical question regarding the behavioral and neurophysiological indices and mechanisms that contribute to and/or compensate for muscle fatigue. In particular, we examined isometric force production, EMG, and EEG correlates of progressive muscle fatigue while subjects performed a grasping task. METHODS: Six neurologically healthy subjects were instructed to produce and maintain 70% of maximum voluntary contraction (MVC) for a total of 5 s in a sequence of 120 trials using a specially designed grip dynamometer. Three components of movement-related potentials (Bereitschaftspotential, BP, Motor potential, MP, and Movement-monitoring potential, MMP) were extracted from continuous EEG records and analyzed with reference to behavioral indicators of muscle fatigue. RESULTS: Experimental manipulations induced muscle fatigue that was demonstrated by decreases in both MVC values and mean force levels produced concomitant to increases in EMG root mean square (RMS) amplitude with respect to baseline levels, and EMG slope. EEG data revealed a significant increase in MP amplitude at precentral (Cz and FCz) and contralateral (C3) electrode sites, and increases in BP amplitude at precentral (Cz and FCz) electrode sites. CONCLUSIONS: The increases in EMG amplitude, EMG slope, and MP amplitudes suggest a possible link between the control signal originating in the motor cortex and activity level of the alpha-motoneuron pool as a function of progressive muscle fatigue. Overall, the data demonstrate that progressive muscle fatigue induced a systematic increase in the electrocortical activation over the supplementary motor and contralateral sensorimotor areas as reflected in the amplitude of movement-related EEG potentials.

Adolescent↗
PubMed2005

Neural and metabolic mechanisms of excessive muscle fatigue in maintenance hemodialysis patients.

Dialysis patients have severe exercise limitations related to metabolic disturbances, but muscle fatigue has not been well studied in this population. We investigated the magnitude and mechanisms of fatigue of the ankle dorsiflexor muscles in patients on maintenance hemodialysis. Thirty-three dialysis patients and twelve healthy control subjects performed incremental isometric dorsiflexion exercise, beginning at 10% of their maximal voluntary contraction (MVC) and increasing by 10% every 2 min. Muscle fatigue (fall of MVC), completeness of voluntary activation, and metabolic responses to exercise were measured. Before exercise, dialysis subjects exhibited reduced strength and impaired peripheral activation (lower compound muscle activation potential amplitude) but no metabolic perturbation. During exercise, dialysis subjects demonstrated threefold greater fatigue than controls with evidence of central activation failure but no change in peripheral activation. All metabolic parameters were significantly more perturbed at end exercise in dialysis subjects than in controls, including lower phosphocreatine (PCr) and pH, and higher P(i), P(i)/PCr, and H(2)PO(4)(-). Oxidative potential was markedly lower in patients than in controls [62.5 (SD 27.2) vs. 134.6 (SD 31.7), P < 0.0001]. Muscle fatigue was negatively correlated with oxidative potential among dialysis subjects (r = -0.52, P = 0.04) but not controls. Changes in central activation ratio were also correlated with muscle fatigue in the dialysis subjects (r = 0.59, P = 0.001) but not the controls. This study provides new information regarding the excessive muscular fatigue of dialysis patients and demonstrates that the mechanisms of this fatigue include both intramuscular energy metabolism and central activation failure.

Ankle↗
PubMed2002

Does wearing a functional knee brace affect hamstring reflex time in subjects with anterior cruciate ligament deficiency during muscle fatigue?

OBJECTIVE: To evaluate the effects of wearing a functional knee brace and muscle fatigue on hamstring reflex time in subjects with anterior cruciate ligament (ACL) deficiency. DESIGN: Repeated-measures clinical trial. SETTING: Outpatient physical therapy department. PARTICIPANTS: Sixteen subjects with ACL deficiency. INTERVENTION: Subjects tested with and without a functional knee brace before and after an exercise protocol designed to fatigue the knee muscles. MAIN OUTCOME MEASURE: Latency of hamstring reflex muscle activity after sudden perturbation of the knee. RESULTS: Wearing a knee brace shortened the hamstring reflex latency regardless of fatigue (F(1,15)=20.62, P<.001). Muscle fatigue lengthened the hamstring reflex time regardless of the bracing condition (F(1,15)=7.57, P<.015). CONCLUSION: Wearing a functional knee brace facilitated hamstring muscle reflex, but muscle fatigue lengthened the hamstring reflex latency. Subjects with ACL deficiency should not rely on the knee brace to facilitate hamstring reflex for joint protection during prolonged sporting activities when muscles are fatigued.

Achilles Tendon↗
PubMed1992

Detection of skeletal muscle fatigue in patients with heart failure using electromyography.

Patients with heart failure frequently report that leg fatigue limits maximal exercise capacity. However, objective documentation of muscle fatigue has not been obtained in such patients. In normal subjects, muscle fatigue during constant work load exercise is associated with an increase in electrical activity generated per contraction due to use of additional muscle fibers to compensate for fiber fatigue. The present study was performed to determine if this approach can be used to document muscle fatigue in patients with heart failure. Vastus lateralis surface electromyograms were monitored in 8 ambulatory patients with nonedematous heart failure and 6 normal subjects during maximal bicycle exercise (20 W increments every 2 minutes). The electromyogram was stored on tape and subsequently analyzed for integrated root-mean-square voltage/contraction (iRMSV). At each work load, the iRMSV of the first and last 30 seconds of the work load were compared. The maximal work load achieved by patients with heart failure was significantly lower (73 +/- 22 W) than that by normal subjects (150 +/- 15 W; p less than 0.01). Both groups had no significant difference between the initial and final iRMSV at submaximal work loads. However, during the 2 highest work loads, both groups reported leg fatigue and had significant increases in iRMSV, consistent with muscle fiber fatigue (maximal work load: 259 +/- 59 to 279 +/- 58 mv.ms [normals] vs 258 +/- 94 to 283 +/- 93 mv.ms [heart failure]; p less than 0.03). The data indicate that the surface electromyogram can be used to detect skeletal muscle fatigue in patients with heart failure.(ABSTRACT TRUNCATED AT 250 WORDS)

Aged↗
PubMed1982

Muscle fatigue simulator.

The above description of the structure and operation of the Muscle Fatigue Simulator was necessarily presented in an abridged fashion (a more complete description to be found in the report (3). The main goal of the construction of the model was creation of a system simulating the changes occurring in the EMG signal under the impact of local muscle fatigue. The performed verification of the model in the aspect of time, frequency and dynamic changes of simulated parameters confirmed simulating faithfulness of the model and thereby the initial assumptions and the general idea of the model may be considered correct. The muscle fatigue simulator has the capability to display some changes occurring in myoelectric signals during fatiguing contractions, and it is believed to have a great merit in demonstration of the signal processes within the muscle. For that reason, the Muscle Fatigue Simulator can be of use in teaching and in demonstration of particulars of the myoelectric signal during isometric contraction. It may also be used in instrument calibrating and in checking the operation of the EMG-signal measuring and analysing equipment applied in ergonomic studies.

Action Potentials↗
PubMed2006

Effect of frequency and pulse duration on human muscle fatigue during repetitive electrical stimulation.

Different combinations of stimulation frequency and intensity can generate a targeted force during functional electrical stimulation (FES). This study compared isometric performance and muscle fatigue during repetitive stimulation with three different combinations of frequency and pulse duration that produced the same initial peak forces: protocol 1 used long pulse duration (fixed at 600 micros) and 11.5 +/- 1.2 Hz (low frequency); protocol 2 used 30 Hz (medium frequency) and medium pulse duration (150 +/- 21 micros); and protocol 3 used 60 Hz (high frequency) and short pulse duration (131 +/- 24 micros). Twenty and 60 Hz pre- and postfatigue testing trains were delivered at the pulse duration used by the fatiguing trains and at 600 micros pulse duration. The percentage decline in peak force between the first and last fatiguing train of each protocol was the measure of muscle performance. The declines in peak force of the 60 Hz testing trains were used to measure muscle fatigue. The 20 Hz:60 Hz peak force ratio was used as a measure of low-frequency fatigue. The results showed that protocol 1 produced the least decline in peak force in response to the fatiguing trains, as well as the least muscle fatigue and low-frequency fatigue when the pulse duration was maintained at the level used by the fatiguing trains. Interestingly, protocol 2 produced the least muscle fatigue, and there were no differences in the levels of low-frequency fatigue across protocols when a comparable motor unit population was tested using 600 micros pulse duration. The results suggest that if the frequency and intensity are kept constant during FES, using the lowest frequency and longest pulse duration may maximize performance.

Adult↗
PubMed1980

Effect of pH on muscle fatigue in isolated frog sartorius muscle.

Isolated frog sartorius muscle was incubated for 3 h in Ringer solution (5 mM phosphate buffer) at pH 7 or pH 8. Each muscle was stimulated supramaximally at 2 per second continuously. Muscles at pH 7 fatigued more quickly than those at pH 8. Muscles at pH 8 released acid whereas those at pH 7 did not.

Acids↗
PubMed2006

Effects of abdominal muscle fatigue on anticipatory postural adjustments associated with arm raising.

Anticipatory postural adjustments (APAs) are postulated to ameliorate the effects of the disturbance to posture caused by voluntary movement. The primary hypothesis tested in our study was that the magnitude of anticipatory trunk muscle activity is altered by abdominal muscle fatigue. A subsidiary aim of the present study was to examine the directional nature of APAs and use this information to elucidate the central or peripheral nature of changes in postural muscle activity associated with abdominal muscle fatigue. The present study was a within subject design, where abdominal muscle fatigue was induced by a static abdominal curl. Surface EMG was used to assess postural muscle activity in the following trunk muscles; rectus abdominis, erector spinae and internal oblique. Following abdominal muscle fatigue, the magnitude of muscle activity during APAs was significantly reduced by 20% in both the rectus abdominis (fatigued muscle) and the erector spinae (not fatigued) indicating a central rather than peripheral fatigue effect on muscle activity. Abdominal muscle fatigue also induced a 30% increase in the baseline muscle activity of the internal oblique. The changes in magnitude of APA muscle activity may reflect a change in system gain or a change in postural control perhaps related to a change in perceived postural stability. An increase in baseline muscle activity in the internal oblique may compensate partially for the reduction in APAs.

Abdominal Muscles↗