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

Contractile responses of motor units in the gastrocnemius, soleus and pretibial flexor muscles of adult cats were elicited by intracellular stimulation of motoneurones. The motor units were classified into types FF, FR and S (Burke et al. 1971) and their responses to the same stimulation patterns as those used in a previous investigation of whole muscles (Hammarberg and Kellerth 1975 a) were studied. The duration of motoneurone afterhyperpolarization was short in both the fast twitch FF and FR units; it was longer in the soleus S units than in the S units of the pale muscles. Twitch time-to-peak was less than 30 ms in the FF and FR units, but exceeded 40 ms in the S units. Soleus S units were slower than S units of the pale muscles. Potentiation was observed in the gastrocnemius units, but not in the soleus S units. A short rest allowed fatigued extensor units of the FF and FR types to regain some contractile strength. This was less evident in the S units which, on the other hand, were extremely resistant to fatigue. Differences in response patterns between corresponding motor unit types of the flexor and extensor muscles were observed. A few fast twitch units were identified in the slow soleus muscle.
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PMID:Studies of some twitch and fatigue properties of different motor unit types in the ankle muscles of the adult cat. 17 16

The mechanical properties of two extraocular muscles (superior oblique and superior rectus muscles) of the frog were studied and compared with those of a frog's skeletal muscle (iliofibularis muscle) which contains the same types of muscle fibres as the oculorotatory muscles. The extraocular muscles are very fast twitching muscles. They exhibit a smaller contraction time, a smaller half-relaxation time, a higher fusion frequency, and a lower twitch-tetanus ratio than the skeletal muscles. The maximum isometric tetanic tension produced per unit cross-sectional area is lower in the extraocular muscles than in skeletal muscles. However, the extraocular muscles show a higher fatigue resistance than the skeletal muscles. With respect to the dynamic properties there are some differences between the various oculorotatory muscles of the frog. The superior rectus muscle exhibits a faster time-course of the contraction, a higher fusion frequency, and a higher fatigability than the superior oblique muscle. An increase of the extracellular K+-concentration evokes sustained contractures not only in the extraocular muscles but also in the iliofibularis muscle; between these muscles there are no striking differences in the mechanical threshold of the whole muscle preparation. The mechanical threshold depends on the Ca++-concentration of the bathing solution and it is found in a range between 12.5 and 17.5 mM K+ in a normal Ringer solution containing 1.8 mM Ca++. The static-mechanical properties of the extraocular muscles of the frog and the dependence of the active developed tension on the muscle extension are very similar to those which are known to exist in the extraocular muscles of other vertebrates. In tetanic activated frog's oculorotatory muscles a linear relationship exists between length and tension. A variation of the stimulation frequency does not change the slope of this curve but causes parallel shifts of the curve. The peculiar properties of the extraocular muscles of the frog are discussed with respect to the muscle fibre types in these muscles and to the diameter of the muscle fibres.
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PMID:The properties of the extraocular muscles of the frog. I. Mechanical properties of the isolated superior oblique and superior rectus muscles. 30 53

The contractile properties of frog sartorius muscles were evaluated during development of fatigue and recovery. Muscles were stimulated under anaerobic conditions at a rate of 30 stimuli/min. Twitch tension (Pt) decreased 59% and tetanic tension (Po) decreased 52% during 15 min of stimulation. Contraction time and one-half relaxation time increased markedly during the first 6 min and then leveld off. The peak rate of twitch tension development (dP/dt) progressively declined to 33% of its initial value. Maximum shortening velocity did not change significantly. Large increases in Pt and twitch dP/dt occurred during the first 15 s of recovery while Po did not increase significantly in this time. Thereafter, Pt, Po, and, dP/dt all progressively returned towards normal over 30 min. These results show that the initial rapid recovery in Pt is not due to an increase in the capacity to generate force. Po is a better indicator of the force-generating capacity of a muscle because, unlike Pt, it is not affected by changes in the rate of tension development.
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PMID:Effects of fatigue and recovery on contractile properties of frog muscle. 31 Apr 31

We have studied two cases of the syndrome of myokymia and impaired muscular relaxation with continuous motor unit activity. Both patients complained of muscle twitching, weakness, stiffness, and hyperhydrosis during their illness. Myokymia was present over the entire body in both. On repetitive testing of muscle strength each patient showed initial fatigue followed by increasing strength as he continued his efforts. Both patinets improved on phenytoin therapy at high blood levels. Nerve conduction velocities were decreased. Electromyograms showed continuous electrical activity at rest which persisted during sleep and spinal anaesthesia but was diminished by curare. Intravital staining with methylene blue in one case demonstrated sprouting and beading of motor nerve terminals with multiple innervation of muscle fibres. The neurophysiological and pathological findings in these two cases indicate an abnormality of peripheral nerve in this disorder.
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PMID:Myokymia and impaired muscular relaxation with continuous motor unit activity. 46 63

The changes with postnatal age in post-tetanic potentiation (PTP) and fatigue of the inferior oblique muscle have been studied in the cat. PTP of the twitch amplitude increased steadily with age up to 20 weeks after birth. Twitch contraction time (ct) and half relaxation time (hrt) was not significantly changed. The potentiation of the tetanic response also became more prominent with age. The young muscles (10 weeks and below) were slightly more susceptible to fatigue than older muscles, but recovery was rapid in muscles of all ages. Intensive stimulation induced post-tetanic depression of twitch responses in muscles older than 1 week, but none in the muscles of new-born cats. Ct and hrt were greatly prolonged in muscles above six weeks of age. In these muscles, but never in the very youngest, repetitive firing could be observed in response to post-tetanic single nerve stimulation. The results are compared with those obtained in similar experiments on hind-limb muscles by other workers. They further support the idea, raised in a previous paper, that slow eye muscle fibres develop quicker and reach maturity earlier than fast fibres.
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PMID:The postnatal development of the inferior oblique muscle of the cat. II. Effects of repetitive stimulation on isometric tension responses. 67 66

1. Polyneuronal innervation is known to occur in several types of vertebrate muscle, including mammalian muscle spindle (Gray, 1957). Previous work had led to the suggestion that the multiply innervated cat extraocular muscles were similarly polyneuronally innervated. 2. The presence of polyneuronal innervation in fibres that show propagated conduction was explored in the two muscles innervated by the abducens nerve: the lateral rectus (which contains multiply innervated fibres) and the retractor bulbus (which does not). 3. Under conditions of twitch and tetanic stimulation, the sum of individual tensions from the two nerve branches was compared with the tension elicited from the whole nerve. An analysis of variance indicated that the lateral rectus and the retractor bulbus did not differ significantly in terms of tension excess. The twitch condition yielded a small but statistically significant (P smaller than 0-01) amount of tension excess, whereas the tetanic condition did not. 4. Twitches elicited by stimulation of one branch of the abducens nerve showed slight potentiation when preceded by a tetanic stimulation to the other nerve branch. This effect could be ready differentiated from the post-tetanic potentiation elicited by applying both twitch and tetanic stimuli to the whole nerve. 5. Muscle fatigue produced by intensive stimulation of one nerve branch did not decrease the amplitude of the tetanic contraction elicited by stimulation of the other branch. In fact, instead of crossed fatigue, a small but significant (P smaller than 0-05) potentiation was observed. 6. Analysis of the results of the three tests led to the conclusion that polyneuronal innervation could not be demonstrated in cat extraocular muslce fibers that showed propagated conduction. Thus, the end-to-end muscle fibre junctions (Floyd, 1970) apparently do not transmit conducted impulses.
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PMID:Absence of polyneuronal innervation in cat extraocular muscles. 113 73

Twitch potentiation and the associated changes in contraction time and 1/2 relaxation time have been studied in a sample of 78 medial gastrocnemius motor units from 8 cats. Potentiation was produced by repetitive stimulation of the motor units every 10 sec with a brief tetanus followed 2 sec later by a twitch. Fast twitch fatigue resistant units were found to potentiate more strongly than either slow twitch (contraction time greater than 45 msec) or fast twitch fatigable units. It is concluded that Type C muscle fibres are more susceptible to potentiation than either Type A or Type B fibres. In a sample of 88 motor units from the same experiments, values for twitch/tetanus ratio were compared amongst units sharing similar mechanical properties. Slow contracting units developed small tetanic tensions and had small twitch/tetanus ratios. Fast twitch non fatigable units had intermediate values for contraction strength and twitch/tetanus ratio overlapping the ranges found for both the slow twitch and the more powerful fast twitch fatigable units. It is concluded that differences in twitch/tetanus ratio for medial gastrocnemius motor units are primarily related to motor unit contraction strength rather than differences in muscle fibre type.
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PMID:The motor units of cat medial gastrocnemius. Twitch potentiation and twitch-tetanus ratio. 117 49

1. The functional properties of tibialis anterior muscles of normal adult (C57BL/10) and age-matched dystrophin-deficient (C57BL/10 mdx) mice have been investigated in situ. Comparisons were made between tibialis anterior muscle strength, rates of force development and relaxation, force-frequency responses and fatiguability. Subjecting mdx and C57 muscles to a regimen of eccentric exercise allowed the hypothesis to be tested that dystrophin-deficient muscles are more susceptible to exercise-induced muscle damage. 2. mdx muscles were, on average, 30% stronger than C57 muscles and almost 80% heavier, but both had similar muscle lengths. Thus, although mdx muscles were stronger in absolute terms, their estimated force per unit cross-sectional area was significantly less than that of C57 muscles. 3. The force-frequency relationships of C57 and mdx muscles differed in that whilst, at 40 Hz, the former developed 70% of the force developed at 100 Hz, the latter developed only 55% of the maximal force. Twitch force was normal in mdx muscles, but contraction time was shortened, and the consequent reduction in fusion frequency probably explains the force-frequency differences observed between the two groups. 4. mdx muscles were less fatiguable than normal muscles when stimulated repeatedly at a frequency of 40 Hz. It is possible that the lower relative force at 40 Hz in mdx muscles entailed a lower energy demand and thus a slower rate of fatigue than seen in normal muscles. 5. Eccentrically exercised C57 muscles showed a large loss of maximal force for up to 12 days after exercise. Maximal force loss occurred 3 days after exercise (55% of non-exercised tibialis anterior muscle), which also corresponded with the period of greatest fibre necrosis. C57 muscles showed a significantly reduced 40 Hz/100 Hz force-frequency ratio at 1 and 3 days after exercise. This was primarily due to a reduced twitch amplitude rather than to a change in the time course of the twitch. It is unlikely, therefore, that the altered contractile characteristics of mdx muscle were a result of the presence of damaged but otherwise normal fibres. 6. C57 and mdx tibialis anterior muscles displayed similar degrees of force loss after exercise. Furthermore, the rate of recovery after the nadir of force loss was very similar for the two groups. By 12 days after exercise, force recovered to 76% and 80% of control in C57 and mdx muscles, respectively. Our findings do not support the hypothesis that dystrophin-deficient muscle is more susceptible to exercise-induced muscle damage.
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PMID:Contractile properties and susceptibility to exercise-induced damage of normal and mdx mouse tibialis anterior muscle. 131 62

This investigation examined the mechanical responses of malignant hyperthermic (MH) and normal porcine skeletal muscle to repetitive stimulation. Twitch and maximal tetanic tensions were not significantly different between muscle types. Tensions produced during stimulation at 20-80 Hz were significantly less in MH muscle than in normal muscle. In addition, MH muscle showed significantly greater force decline (tetanic fade) at the end of contractions evoked by 20-80 Hz stimulation. When stimulated to fatigue, both normal and MH muscle exhibited similar rates of tension decline during the initial minutes. Further stimulation caused additional decline in normal muscle, but a tension plateau in MH muscle. In all cases, normal muscle had greater magnitudes of fatigue than did MH muscle. Results show that there are marked differences between MH and normal muscle in the mechanical responses to repetitive stimulation. Due to its inability to properly regulate intracellular Ca2+ exchange, it is possible that MH muscle might be a useful tool for identifying the mechanisms of muscle fatigue in normal muscle.
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PMID:Altered mechanical responses of malignant hyperthermic skeletal muscle during repetitive stimulation. 134 1

beta,beta'-Iminodipropionitrile (IDPN) impairs axonal transport of neurofilaments; their accumulation leads to the formation of proximal swellings in motor axons. Similar proximal swellings are a feature of some cases of motor neuron disease such as amyotrophic lateral sclerosis (ALS). Motor units in IDPN-treated animals were assessed to determine their relative susceptibilities to impaired function and whether the functional changes resulting from proximal axonal swellings share certain electromyographic features with ALS. Intrinsic properties of medial gastrocnemius motoneurones (MN) and contractile responses of their motor units were examined during the evolution of proximal axonal swellings in cats administered IDPN (50 mg/kg once weekly) for 7, 14 or 35 days. While conduction velocities were significantly decreased in all motor unit types by 35 days, the conduction slowing was greater in fast fatigable (types FF and FI) motor units than in fatigue resistant (types FR and S) motor units. Normal correlations between axonal conduction velocity and MN input resistance (Rin) and the inverse relationship between Rin and rheobase were lost with progression of the neuropathy. Twitch and maximum tetanic tension developed by fast-fatigable motor units declined early in the neuropathy, whereas fatigue-resistant units did not show similar changes until later stages of the intoxication. In some motor units, irregular and abnormal tetanic tensions were elicited by repetitive MN discharge. At 14 and 35 days, a novel, intermediate motor unit response classified as slow and fatigable (SF) was observed. Conduction block, characterized by repetitive MN firing without a corresponding contractile response, was observed in some type FF and S units by 35 days. Morphometric analysis of muscle fiber types showed significant atrophy, particularly in the type I fibers at 14-35 days; the atrophy reversed following cessation of IDPN administration. The influence of proximal axonal swellings on motor unit function in IDPN neuropathy is discussed in terms of reported electrophysiological alterations in motoneurone disease.
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PMID:Motor unit function during evolution of proximal axonal swellings. 138 10


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