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Query: UMLS:C0015672 (fatigue)
 51,768 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

A set-up for percutaneous electrical stimulation of the forearm extensor muscles and measurement of wrist extension force is described. The frequency-force relationship and pulse duration-force relationship are described together with an experimental protocol showing that brief electrical test stimulations do not produce fatigue. In another set of experiments carried out a few weeks later, the subjects performed handgrip contractions: protocol A at 25% of maximal voluntary contraction (MVC) continuously until exhaustion, protocol B at 25% MVC intermittent (contraction + relaxation = 10 + 2 s) until exhaustion, and protocol C at 25% MVC intermittent until half the time to exhaustion. In all experiments, brief electrical stimulations were used to test the degree of fatigue during and up to 24 h after the experiments. There were marked changes in the force during stimulation at 20 and 100 Hz and these changes did not correlate with the increase in intramuscular temperature. Low frequency fatigue persisted for at least 24 h after protocol A and 1 h after protocols B and C. The significance of this is discussed and it is suggested that low frequency fatigue could be used as a sensitive indicator of muscle dysfunction after low and medium intensity exercise.
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PMID:Electrical stimulation of human forearm extensor muscles as an indicator of handgrip fatigue and recovery. 187 44

We examined expiratory muscle fatigue during expiratory resistive loading in 11 normal subjects. Subjects breathed against expiratory resistances at their own breathing frequency and tidal volume until exhaustion or for 60 min. Respiratory muscle strength was assessed from both the maximum static expiratory and inspiratory mouth pressures (PEmax and PImax). At the lowest resistance, PEmax and PImax measured after completion of the expiratory loaded breathing were not different from control values. With higher resistance, both PEmax and PImax were decreased (P less than 0.05), and the decrease lasted for greater than or equal to 60 min. The electromyogram high-to-low frequency power ratio for the rectus abdominis muscle decreased progressively during loading (P less than 0.01), but the integrated EMG activity did not change during recovery. Transdiaphragmatic pressure during loading was increased 3.6-fold compared with control (P less than 0.05). These findings suggest that expiratory resistive loaded breathing induces muscle fatigue in both expiratory and inspiratory muscles. Fatigue of the expiratory muscles can be attributed directly to the high work load and that of the inspiratory muscles may be related to increased work due to shortened inspiratory time.
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PMID:Expiratory muscle fatigue in normal subjects. 188 58

The purpose of this investigation was to determine the oxygen consumption, heart rate and plasma lactate responses at the fatigue threshold (FT) and estimate the length of time the FT could be maintained. Ten adult males (mean age +/- SD = 21.1 +/- 1.3 yrs) volunteered to perform a maximal treadmill test and FT test. During the maximal test, VO2 heart rate and plasma lactate measurements were taken. The results of the investigation indicated that the FT (14.0 +/- 1.2 km.hr-1, 197 +/- 8 bpm; 47.5 +/- 5.7 ml/kg.min-1, 5.4 +/- 1.3 mM) was very close to a maximal effort (VO2max = 14.4 +/- 1.2 km.hr-1, 203 +/- 10 bpm; 49.5 +/- 6.1 ml/kg.min-1, 7.4 +/- 2.1 mM) and could be maintained for only 0.16 to 0.28 hrs. These findings do not support the validity of the FT as a measure of the maximal running velocity that can be continued for an extended period of time without exhaustion.
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PMID:Physiological responses at the fatigue threshold. 188 40

Hypothermia has altered the course of military history. Military casualties tend to occur in epidemics, associated with cold weather combat or maneuvers, trauma, immobilization, improper clothing, exhaustion, and underlying illness. Symptoms of hypothermia begin subtly with fatigue and loss of concentration, but progress to stupor, coma, and resemble rigor mortis. Treatment of mild hypothermia is with body heat and warm clothes and fluids. Moderate and severe cases require gentle evacuation and active core rewarming method(s). Inhalation of warm (40 degrees C, 104 degrees F) humidified oxygen is safe, effective, and can be begun in the field. Recognition of risk factors and active measures can lessen the menance of cold weather for military personnel.
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PMID:Hypothermia: threat to military operations. 190 77

The objective of the present study was to investigate whether isometric contraction of the right triceps brachii muscle, of maximal duration and at 25% of the maximal voluntary contraction (MVC), would reduce mean fibre conduction velocity (CV) for the active motor units (MU). In addition to the cross-correlation of surface electromyograms (EMG) for CV determination, median frequency (fm) and root-mean-square amplitude (rms-amplitude) were calculated. The initial 5 min of the recovery of the three parameters was also investigated. The MVC were performed before and after the sustained contraction. Seven males-six in their twenties and one aged 43-participated in the investigation. Mean CV for the unfatigued muscle was 4.5 m.s-1, SD 0.38. On average, CV decreased less than 10% during the sustained contraction (P less than 0.05). The fm decreased almost linearly (46%) during the endurance time, while three quarters of the 250% increase in rms-amplitude took place during the last 50% of the contraction (P less than 0.001, both parameters). The MVC was reduced by 39% immediately after exhaustion (P less than 0.05). During the 1st min of recovery the rms-amplitude decreased by 50%, and the fm increased from 54% to 82% of the initial value (both P less than 0.05). No measurable simultaneous CV restitution occurred. A parallel 15% increase in fm and CV took place during the last 4 min of recovery (both P less than 0.001), while the amplitude remained constant. Since mean CV was essentially unchanged during the last 50% of the endurance time where large changes in fm and rms-amplitude occurred, factors supplementary to CV probably caused the striking changes in fatigue EMG, notably-MU recruitment, synchronization of MU activity, and lowering of MU firing frequencies. Nevertheless, during the last 4 min of recovery the entire increase in fm could be accounted for by the simultaneous increase in CV.
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PMID:Changes in conduction velocity, median frequency, and root mean square-amplitude of the electromyogram during 25% maximal voluntary contraction of the triceps brachii muscle, to limit of endurance. 191 35

Symptomatic postpolio patients report improved function when they pace their activities, but this has never been objectively assessed. The purpose of this study was to determine whether seven symptomatic postpolio subjects would (1) have less evidence of local muscle fatigue and recover strength more readily when they pace their activity (via work-rest intervals) than when they work at a constant rate to exhaustion, and (2) be able to perform more work with less fatigue by pacing. Subjects were evaluated on three separate days with at least one week between tests. On the first test day (constant exercise), isometric peak torque (MVC) of the quadriceps was determined and an isometric endurance test was performed to exhaustion at 40% of MVC. Thirty seconds after exhaustion the subject performed an MVC. One psychophysiologic and two electrophysiologic variables associated with fatigue were measured during the testing procedure. Work capacity (TTI) was determined as torque x time. In test 2 (quartile exercise), the same TTI was performed at 40% of MVC, but in this instance the work was performed in quartiles with two-minute rest breaks between work quartiles rather than continuously to exhaustion. In test 3 (interval exercise), exercise was performed at 40% of MVC in 20-second bouts with two-minute rest breaks until rating of perceived exertion exceeded 17 (very hard), or until six-minutes of work were performed. Analyses demonstrated that both work-rest interval programs resulted in less evidence of local muscle fatigue, increased capacity to perform work, and increased ability to recover strength after activity in symptomatic postpolio patients.
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PMID:Intermittent isometric activity: its effect on muscle fatigue in postpolio subjects. 195 20

This study was performed to determine whether deficits in muscle strength, work capacity, and ability to recover strength after exhaustion in symptomatic postpolio subjects were due to central inhibition (lassitude) or other physiologic differences. We studied 34 symptomatic polio subjects, 16 asymptomatic polio subjects, and 41 control subjects. Root mean squared electromyography (RMS-EMG) and the median frequency (Fm) of the power spectrum of the quadriceps muscle was determined with surface electrodes during maximal volitional contraction (MVC) of isometric knee extension, an endurance test to exhaustion at 40% of maximal torque, and strength recovery testing that was performed at regular intervals for 10 minutes postexhaustion. Initial Fm during MVC and during the 40% MVC endurance test were similar in all three groups. The Fm decreased during the endurance test and increased during recovery similarly in all three groups. The RMS-EMG also changed in parallel fashion in all three groups during the same procedures. During recovery, a similar degree of RMS-EMG facilitation was seen in all three groups. Thus, reduction in strength, work capacity, and ability to recover from fatiguing exercise do not appear to be related to central factors (lassitude); they are probably related to other factors, such as local muscle fatigue. Each group appears to fatigue and recover in a similar manner electrophysiologically, although symptomatic polio subjects have a deficit in strength recovery, apparently due to local muscular fatigue.
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PMID:Electrophysiologic study of the quadriceps muscles during fatiguing exercise and recovery: a comparison of symptomatic and asymptomatic postpolio patients and controls. 195 24

In 15 conscripts, venous plasma potassium was followed during exercise on a training bicycle before and after 10 weeks of moderate physical training and a putative relationship with skeletal muscle Na,K-ATPase was evaluated. Peak plasma potassium concentration obtained at exhaustion was 6.1 +/- 0.2 and 5.6 +/- 0.2 mmol l-1 (mean +/- SEM, n = 14, P less than 0.05) before and after training, respectively. Throughout the exercise period and within the first minutes of rest plasma potassium concentration was 0.2-0.5 mmol l-1 higher before than after training. Neither peak values nor peak rises in plasma potassium concentration before nor after training were correlated to the 3H-ouabain binding site (Na,K-ATPase) concentration in vastus lateralis muscle. The results indicate that net loss of potassium from the skeletal muscle pool during exercise is reduced after training, that the heart during exercise may be exposed to a smaller rise in plasma potassium concentration after training than before, and that moderate improvement of capacity to clear extracellular potassium during exercise may be due to increased activity of existing Na,K-pumps in resting skeletal muscle fibres. This may reduce muscle fatigue, increase physical performance and explain the paradoxical observation that, despite an increased catecholamine response, there is a reduced risk of cardiac events after training.
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PMID:Exercise-induced hyperkalaemia can be reduced in human subjects by moderate training without change in skeletal muscle Na,K-ATPase concentration. 196 26

Fatigue rates of the vastus medialis oblique and vastus lateralis muscles were compared from static and dynamic exercise. Based upon clinical observations, it was expected that the vastus medialis oblique would demonstrate greater fatigability than the vastus lateralis. However, based upon physiological and morphological considerations, it was expected that the vastus lateralis would exhibit greater fatigability than the vastus medialis oblique. In the static exercise condition, nine subjects--having no history of knee problems--maintained an isometric knee extension torque at 30 and 60% of their maximum value until exhaustion, during which time electromyography (EMG) data were collected every 10 s. The knee angle was 20 degrees flexion. In the dynamic exercise condition, a different group of seven subjects performed exercise sets consisting of eight cycles of concentric-eccentric knee extension, with a resistance equal to 40% of the maximum isometric value. The range of motion was restricted to the terminal 30 degrees of knee extension. Each set of eight repetitions was followed by a 1 s 50% maximum isometric knee extension, during which time EMG data were collected. The raw EMG data were numerically processed to extract the median frequency of the power density spectrum, which has been shown to reflect the metabolic processes associated with fatigue. Linear regression generated a slope coefficient representing the rate of change of the median frequency, with respect to contraction duration for each subject, muscle, and condition. Analyses of variance, with repeated measures from both exercise conditions suggest that "short-arc" quadriceps exercise did not selectively fatigue either the vastus medialis oblique or vastus lateralis, thereby supporting neither clinical nor physiological expected outcomes. Therefore, the clinical contention that short-arc quadriceps exercises selectively strengthen the vastus medialis oblique is questioned.
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PMID:Fatigue rates of vastus medialis oblique and vastus lateralis during static and dynamic knee extension. 201 Aug 43

Nine male marathon runners, 24 to 39 years of age, were studied during steady state and maximal graded treadmill exercise under control conditions (C) and immediately after a paced outdoor 21.1-km run averaging 89.5 min (E). The half-marathon run and both treadmill trials were performed at 239 +/- 33 m/min. Oxygen uptake (VO2), respiratory exchange ratio (RER), heart rate (HR), plasma lactate concentration (PLa), and rating of perceived exertion (RPE) were measured in the steady state at 0% grade and at the fatigue end point. Compared to C, mean values in E were significantly lower (p less than 0.05) for time to exhaustion (6.0 vs 4.1 min), VO2max (60.0 vs 56.3 ml/kg/min), peak RER (1.18 vs 1.06), and PLa (9.7 vs 7.8 mM/L), whereas maximal HR (184 vs 184 b/min) and peak RPE (19.6 vs 19.9) were not significantly different between trials. Submaximal VO2 during steady-state runs was similar between C and E (44.4 vs 45.0 ml/kg/min; p = NS). Since the attainable VO2max decreased after E, the percentage of VO2max utilized during steady-state runs was higher, averaging 74% in C and 80% in E (p less than 0.05). In the steady state during E, HR (153 vs 161 b/min) and RPE (13.2 vs 14.8) were higher (p less than 0.05), and the increase in PLa from rest (2.7 vs 1.9 mM/L) was lower (p less than 0.05). Submaximal HR during graded exercise in E was 7 to 8 b/min higher (p less than 0.05) at a given VO2, indicating reduced heart rate reserve.(ABSTRACT TRUNCATED AT 250 WORDS)
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PMID:Acute reduction in maximal oxygen uptake after long-distance running. 203 56


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