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

The role of extracellular K+ concentration in the propagation velocity of action potential was tested in isolated rat skeletal muscles. Different K+ concentrations were produced by KCl additions to extracellular solution. Action potentials were measured extracellularly by means of two annular platinum electrodes. Fibre bundles of m. soleus (SOL), m. extensor digitorum longus (EDL), red (SMR) and white (SMW) part of m. sternomastoideus were maximum stimulated. The conduction velocity (c.v.) was calculated from the distance between the electrodes and the time delay of the potentials measured at 22 degrees C. In Tyrode solution containing 5 mmol/l K+, the c.v. was close to 1 m.s-1. Bundles of the fast muscle type seemed to have a somewhat higher c.v. The differences observed in these studies were not significant. At higher temperatures, the c.v. increased (Q10 of approx. 2) and a dissociation between SMR and SMW muscles appeared. An elevation of K+ concentration to 10 mmol/l induced a drop of the c.v. by approx. 25% and 15% in EDL and SOL muscles, respectively. After return to normal solution, the recovery was not complete within 30 min. In K+ free solution the c.v. of EDL and SM muscles rose by a factor of 1.5, but less in SOL muscles. The weaker response of SOL to K+ modification was related to the higher resistance of this muscle to fatigue. This suggestion was supported by experiments on fatigued fibre bundles. Immediately after a tetanic stimulation producing fatigue, the c.v. of EDL and SOL muscles dropped similarly as in 10 mmol/l K+; again, the drop was less for SOL muscles. Adrenaline (0.5-10.0 mumol/l) enhanced both the c.v. and the twitch amplitude. The results support the suggestion that extracellular K+ accumulation during activity is an essential factor of muscle fatigue.
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PMID:External potassium and action potential propagation in rat fast and slow twitch muscles. 181 28

The effects of sensory intake and rejection tasks on auditory fatigue were examined in 14 male subjects. Auditory thresholds, psychophysical tuning curves and physiological measures of cardiovascular function were obtained before, during, and following a 7 min 110 dB SPL white noise exposure. Acceleration of heart rate was observed under the sensory rejection (mental arithmetic) condition, and poorer post-exposure auditory thresholds and larger Q10 tip values (measured during late post-exposure intervals) were seen when the task required counting of interruptions in the noise. However, Q10 tip values obtained 1 to 2 min post-exposure, when cochlear effects are maximal, failed to confirm a significant difference as a function of task.
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PMID:Effects of intake and rejection tasks on auditory fatigue. 324 22

Small, random length changes were applied to bundles of intact fibers from rat and mouse extensor digitorum longus (EDL) and soleus muscles, while they were being tetanically stimulated. With increasing frequency of length changes, EDL muscle stiffness (tension change per unit change in length) increased, then decreased and increased again. The decrease was not seen in the soleus muscles. The EDL frequency-response could be well fitted by three exponential components with apparent rate constants of approximately 25, 150, and 500 s-1 at 20 degrees C. All rate constants increased steadily with temperature and for each 10 degrees C increase in temperature, the rates in the mouse EDL increased by a factor (Q10) between 1.8 and 2.4. With tetanic stimulation, force increased nearly exponentially to a steady level with a rate constant of 24 s-1 at 20 degrees C in mouse EDL muscles, and a Q10 of 2.4. These values correspond closely to the lowest frequency rate constant measured with length perturbations, which suggests that this process may limit the rate of rise of force in intact muscle fibers. During fatigue the high frequency and intermediate frequency rate constants declined, but the low frequency rate constant remained unchanged. These results are discussed in relation to current biochemical models for cross-bridge cycling.
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PMID:Measurement of rate constants for the contractile cycle of intact mammalian muscle fibers. 382 52

The relaxation rate from electrically stimulated isometric contractions of human adductor pollicis has been measured during rewarming following cooling, during ischaemia and during ischaemic contractile activity. The Q10 for relaxation rate (25-37 degrees C intramuscular temperature) was estimated as 2 . 3. Relaxation rate was found to decline at a rate of 1 . 5% initial value per minute of ischaemia. Relaxation rate declined more rapidly during ischaemic contractile activity than during ischaemia alone. The rate of decline was more closely related to the force X time performed than the number of excitatory impulses. During a supramaximal 20 Hz tetanus, relaxation rate declined markedly at a time when only slight force fatigue had occurred; hence the decline could not be explained by selective fast muscle fibre fatigue. No recovery occurred during ischaemic rest following ischaemic activity but, following restoration of the circulation, recovery occurred with a half time of one minute being virtually complete at 5-7 min. Changes in relaxation rate during fatiguing contractions and recovery from them follow different time courses from muscle excitability and force production. Change in relaxation rate cannot be simply related to changes in the concentration of major energy metabolites. It is proposed that relaxation rate is related to the rate of energy turnover in the contracting muscle.
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PMID:The effect of temperature, ischaemia and contractile activity on the relaxation rate of human muscle. 689 5

The rate of intramuscular temperature rise (dT/dt) has been measured in ischaemic isometric contractions of adductor pollicis in man as an index of the rate of energy turnover. Experiments were designed so as to compare dT/dt in contractions made at a constant force but at differing intramuscular temperatures or degrees of ischaemic fatigue. In the same experiments relaxation rate was measured. As intramuscular temperature was raised from 25 to 37 degrees C, dT/dt increased with a Q10 of 1 . 8 and was closely correlated with relaxation rate. During constant force submaximal ischaemic contractions dT/dt fell, failed to recover with ischaemic rest but did so within 10 min of restoration of the circulation. During the fatiguing contraction and recovery thereafter dT/dt was linearly correlated with relaxation rate. The results show that the rate of energy turnover at a constant force is susceptible to change in association with alteration in the contractile speed of the muscle. It is suggested that relaxation rate may be used as an index of the capacity of the muscle to liberate energy.
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PMID:Metabolic heat production in isometric ischaemic contractions of human adductor pollicis. 689 6

The isometric and isotonic contractile properties of the cross-striated adductor muscle of the bay scallop (Argopecten irradians) were measured in vitro at 10, 15 and 20 degrees C. The length at which twitch force was maximal as a function of the closed length in situ (L0/Lcl) averaged 1.38 +/- 0.01 (mean +/- S.E.M.) at 10 degrees C. This length is very close to the typical length at maximum gape during natural swimming at this temperature. Passive force was very low over the range of lengths measured here; at L0, passive force averaged approximately 0.08 N cm-2, or only 0.5% of the corresponding peak twitch force. The mean peak isometric twitch force (Ptw,max) at 10 degrees C was 21.43 +/- 0.68 N cm-2 (S.E.M.), and the ratio of peak twitch force to tetanic force (Ptw,max/P0) averaged 0.89 +/- 0.01. Temperature did not affect either twitch force (Ptw), once fatigue was taken into account, or Ptw,max/P0. In contrast, the time-related properties of twitch contractions (latent period, tL; time to peak tension, tPtw; and time from peak tension to half-relaxation, t50%R) were positively modified by temperature at all temperatures measured (Q10 > 1.8). All three properties were more temperature-sensitive over the range 10-15 degrees C than over the range 15-20 degrees C. The force-velocity relationships of the striated adductor muscle were fitted to the hyperbolic-linear (HYP-LIN) equation. The force-velocity curves of the striated adductor muscle of the scallop were strongly influenced by temperature. Maximal velocity at zero force (Vmax), and therefore maximal power output, increased significantly with temperature. The Q10 over the temperature range 10-15 degrees C (1.42) was significantly lower than that over the range 15-20 degrees C (2.41). The shape of the force-velocity relationship, assessed through comparisons of the power ratio (Wmax/VmaxP0), was not influenced by temperature.
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PMID:Contractile properties of the striated adductor muscle in the bay scallop Argopecten irradians at several temperatures. 847 1

1. Raising the intracellular [Ca2+] for 10 s at 23 degrees C abolished depolarization-induced force responses in mechanically skinned muscle fibres of toad and rat (half-maximal effect at 10 and 23 microM, respectively), without affecting the ability of caffeine or low [Mg2+] to open the ryanodine receptor (RyR)/Ca2+ release channels. Thus, excitation-contraction coupling was lost, even though the Ca2+ release channels were still functional. Coupling could not be restored in the duration of an experiment (up to 1 h). 2. The Ca(2+)-dependent uncoupling had a Q10 > 3.5, and was three times slower at pH 5.8 than at pH 7.1. Sr2+ caused similar uncoupling at twenty times higher concentration, but Mg2+, even at 10 mM, was ineffective. Uncoupling was not noticeably affected by removal of ATP or application of protein kinase or phosphatase inhibitors. 3. Confocal laser scanning microscopy showed that the transverse tubular system was sealed in its entirety in mechanically skinned fibres and that its integrity was maintained in uncoupled fibres. Electron microscopy revealed distorted or severed triad junctions and Z-line aberrations in uncoupled fibres. 4. Only when uncoupling was induced at a relatively slow rate (e.g. over 60 s with 2.5 microM Ca2+) could it be prevented by the protease inhibitor leupeptin (1 mM). Immunostaining of Western blots showed no evidence of proteolysis of the RyR, the alpha 1-subunit of dihydropyridine receptor (DHPR) or triadin in uncoupled fibres. 5. Fibres which, whilst intact, were stimulated repeatedly by potassium depolarization with simultaneous application of 30 mM caffeine showed reduced responsiveness after skinning to depolarization but not to caffeine. Rapid release of endogenous Ca2+, or raised [Ca2+] under conditions which minimized the loss of endogenous diffusible myoplasmic molecules from the skinned fibre, caused complete uncoupling. Taken together, these results suggest that Ca(2+)-dependent uncoupling can also occur in intact fibres. 6. This Ca(2+)-dependent loss of depolarization-induced Ca2+ release may play an important feedback role in muscle by stopping Ca2+ release in localized areas where it is excessive and may be responsible for long-lasting muscle fatigue after severe exercise, as well as contributing to muscle weakness in various dystrophies.
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PMID:Raised intracellular [Ca2+] abolishes excitation-contraction coupling in skeletal muscle fibres of rat and toad. 884 31

The purpose of the present study was to investigate the effect of temperature on the rates of isometric force development and relaxation in electrically activated fresh and fatigued human adductor pollicis muscle. Following immersion of the lower arm for 20 min in water baths of four different temperatures, muscle temperatures were approximately 37, 31, 25 and 22 C. Maximal isometric force was reduced by 16.8 +/- 1.5 % at 22 C. The stimulation frequency-force and -rate of force development relationships were shifted to the left at lower temperatures. Q10 values for the maximal rates of force development and relaxation, and the times for 100 to 50 % and 50 to 25 % force relaxation, were about 2.0 between 37 and 25 C and about 3.8 between 25 and 22 C. However, the time for 50 to 25 % force relaxation had a relatively high Q10 value between 25 and 22 C (6.9) and this parameter also appeared to be more sensitive to fatigue compared to the other indices of relaxation. Nevertheless, the effect of fatigue on all parameters decreased with cooling over the entire (37-22 C) temperature range.
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PMID:Temperature effect on the rates of isometric force development and relaxation in the fresh and fatigued human adductor pollicis muscle. 1056 10

The purpose of the present study was to investigate the effect of muscle temperature on the force/velocity relationship of electrically activated human adductor pollicis muscle. Following immersion of the lower arm for 20 min in water baths of four different temperatures, the calculated muscle temperatures were 37.1, 31.4, 25.6 and 22.2 degrees C. At 22.2 degrees C maximal isometric force was reduced to 79.3+/-2.9% of the force obtained at 37.1 degrees C. Q10 values for the maximal rates of force development and relaxation, and relaxation times, were about 2.0 between 37.1 and 25.6 degrees C and increased to about 3.5 below 25.6 degrees C. The Q10 values of the maximal shortening velocity and the velocity for maximal power production were similar to those of the isometric speed parameters. The Q10 for maximal power production increased from 2.0 above 31.4 degrees C to 6.9 between 25.6 and 22.2 degrees C. Following repetitive isometric contractions maximal power production was reduced to 60.0+/-1.7 and 90.5+/-1.0% at 37.1 and 22.2 degrees C respectively. Fatigue decreased with cooling of the muscle over the entire (37.1-22.2 degrees C) temperature range.
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PMID:Temperature effect on the force/velocity relationship of the fresh and fatigued human adductor pollicis muscle. 1086 11

A 23-year-old woman and a 13-year-old boy were diagnosed with mitochondrial respiratory chain disease. The woman had muscle pain, fatigue and bilateral ophthalmoplegia--symptoms consistent with Kearns-Sayre syndrome. The boy had aspecific symptoms; eventually, reduced activity of complex 1 was found to be the cause of the mitochondrial respiratory chain disease in the boy and his mother, who had suffered from unexplained fatigue and muscle pain for 15 years. Mitochondrial diseases often involve several organ systems. Diagnosis can be difficult, because laboratory tests such as serum and urinary lactate and creatine kinase have low sensitivity and specificity. Biochemical assessment of muscle biopsy can reveal reduced oxidation ATP synthesis and sometimes specific abnormalities in individual protein complexes. DNA analysis may be helpful in demonstrating mitochondrial or nuclear mutations or deletions. The goal of treatment is to increase mitochondrial ATP production, improve clinical symptoms and enhance stamina. Replacement of the following substances (also referred to as cofactors) may be attempted: co-enzyme Q10, antioxidants (lipoic acid, vitamins C and E), riboflavin, thiamine, creatine and carnitine. Evidence regarding the optimal treatment approach is lacking; one usually has to rely on observing effects in the individual patient.
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PMID:[Two patients with mitochondrial respiratory chain disease]. 1900 76


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