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

Recordings were made of isometric contractions of single motor units of the cat's m. peroneus longus (PerL). The units were activated by stimulation of dissected filaments of ventral roots. In accordance with the general principles introduced by Burke et al. (1973), the 80 isolated PerL units were classified into three or four type-categories according to their contractile speed and endurance. Three currently used varieties of a "fatigue index" were calculated and found to give equivalent results. Units with a high, intermediate and low resistance to fatigue were responsible for about 22.5, 25.4 and 52.1% respectively of the total muscle force. Two alternative methods for fast/slow categorization were compared: (i) classifying all units as slow that failed to show a "sag" in partly fused contractions ("sag-criterion", Burke et al. 1973) and (ii) classifying all units as slow that had a more prolonged twitch contraction time than that of fatigue-sensitive units ('FF vs. S-criterion'). The relative contribution of slow units to total muscle force was about 2.8 times as great (14 versus 5%) for a classification by the FF vs. S-criterion than for a subdivision according to sagging behaviour. When compared to equivalent data from previously published studies of feline hindlimb muscles, peroneus longus was found to resemble gastrocnemius medialis in relative motor unit composition. The maximum force of individual PerL units was, however, on average less than or equal to 50% of that reported for corresponding types of gastrocnemius units.
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PMID:Motor unit categorization on basis of contractile properties: an experimental analysis of the composition of the cat's m. peroneus longus. 664 56

The passive-mechanical and dynamic properties of the rabbit inferior oblique muscle IO were studied in vitro at 35 degrees C. The influence of length on the resting tension and isometric contractions were determined. Maximum twitch tension and fusion tension were developed at optimum length (Lo) an extension of the muscle to about 1.15 times LR, the resting length of the IO in situ. A linear relation was found between length and tension in the activated muscle. An increase in stimulation frequency induced a parallel shift in the curves to higher tension but the slope of the curves remained unchanged. On an average the IO had in response to direct massive stimulation a twitch contraction time of 6.4 ms and a half-relaxation time of 7.0 ms. At stimulation with 300 Hz or above the tetanus fused. Stimulus frequencies above fusion frequency increased the rate of tension rise but not the maximum tetanic tension. The maximum tetanic tension was about 6.4 N/cm2, and the twitch:tetanus ratio was 0.1. To prolonged tetanic stimulations the IO exhibited a high fatigue resistance. Cooling the muscle to 25 degrees C was followed by an increase in the time parameters of single twitches and tetanic contractions, a decrease of the tension developed in a fused tetanus and a small potentiation of the twitch. Following a repetitive stimulation a small post-tetanic potentiation of the twitch was observed.
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PMID:Mechanical properties of the isolated inferior oblique muscle of the rabbit. 732 45

Between 1923 and 1976, 312 posterior spinal fusions using the methods of Albee, Bosworth and Hibbs were performed on patients with spondylolisthesis at the Orthopaedic University Hospital Balgrist, Zurich. Long-term clinical results were rated as good in 80% of all patients operated on with the Albee method. The results after the Hibbs procedure on the other hand was good in only 65%. Progressive vertebral slipping after operation was seen in about 60% of all patients operated on by Albee's method, particularly in cases with severe spondylolisthesis. This slipping stops as soon as the intervertebral space had fused. Relief of the subjective symptoms also occured at this time. This phenomenon was only rarely seen after the Hibbs operation, in which the better postoperative stability leads to sclerosis of the discs indicating degenerative changes without a significant decrease in the disc height. The greater stability of the Hibb's fusion is more easily overstressed by body movements which can cause persistent symptoms and later lead to fatigue fractures.
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PMID:A comparison of the long term results of three types of posterior fusion of the lumbar spine for spondylolisthesis. 734 14

1. We studied isometric twitch peak force (TPF) and twitch contraction time (TCT) of 249 motor units of the masseter muscle in 41 rabbits after extracellular electrical stimulation of single trigeminal motoneurons in the brain stem. In 41 of these units we determined the amount of tension decrease during a partially fused tetanus (sag) and the ratio between peak tetanic force after 2 min of intermittent tetanic stimulation and initial tension (fatigue index). Muscle fibers of 24 motor units were identified by the glycogen depletion method and characterized in serial sections with monoclonal antibodies against type IID, IIA, "cardiac" alpha, and I isoforms of myosin heavy chain (MHC). 2. The motor units had TCTs ranging from 13 to 32 ms. The majority of the units showed forces < 35 mN. The TPFs were larger and varied more for motor units with short and intermediate TCTs than for units with long TCTs. There is a small but statistically significant negative correlation between the motor unit TPF and the TCT. 3. All units exhibited "sag" and, with the exeption of one, had fatigue indexes > 0.75. The studied rabbit masseter motor units can therefore be classified as fast, fatigue-resistant, except for one that belonged to the FF (fast, fatigable) category. No slow units were represented in the sample pool. Significant correlations were not found either between TCT and the amount of sag or between TCT and the fatigue index. 4. Immunohistochemical analysis showed that the FF unit had fibers containing only IID-MHC. Five other units were found with a single MHC--three with IIA-MHC and two with alpha-MHC. In three other units all fibers showed one combination of two MHCs (1 IIA/IID, 1 IIA/alpha, and 1 alpha/I). The remaining 15 units contained two MHCs spread unevenly over the constituting fibers. Large variations in myosin composition of fibers within one motor unit cast doubts on the presumed dominant neuronal influence on myosin expression in the adult animal. 5. We found a close, statistically significant correlation between the TCT and the estimated MHC content of the units: the TCT was 13 ms for the IID unit, 18 ms for the pure IIA units, and 28 ms for the pure alpha units. Units with two MHCs had intermediate TCTs; units with alpha/I-MHC mixtures had TCTs of 29-30 ms. No pure MHC-I units were identified.(ABSTRACT TRUNCATED AT 400 WORDS)
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PMID:Contraction characteristics and myosin heavy chain composition of rabbit masseter motor units. 753 59

Six forelimb specimens from three adult horses had the fetlock joint fused by application of a dorsal plate and by a screw placed in lag fashion through the metacarpus to each proximal sesamoid bone. Five specimens were instrumented on the central dorsal surface of the plate with a single rosette strain gage, and the plate of the sixth specimen was instrumented with four longitudinally oriented single-axis strain gages. The specimens were loaded axially in compression to 4,000 N in a cast (test 1), in a cast with a heel block (test 2), and uncast (test 3). The principal angle of strain in all specimens, in all tests, closely approximated the vertical axis at loads < 1,000 N. The principal angle in uncast specimens was significantly different at loads > 1,000 N than the cast specimens (P < .05). At loads > 3,000 N, the principal angle in test 3 closely approximated the horizontal axis, indicating a change from tension to compression on the dorsal surface of the plate, whereas the principal angle of the cast specimens was unchanged. Specimens in a cast (tests 1 and 2) suffered less surface deformation than did uncast specimens (test 3). Therefore, the cast changed the direction and extent of bending at the point of fixation, and thereby decreased the deformation of the plate. This effect would lead to greater fatigue life of the implant in the cast specimens compared with the uncast specimens.
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PMID:Stress protection afforded by a cast on plate fixation of the distal forelimb in the horse in vitro. 770 70

In recent years, titanium has become a material of major interest in prosthetic dentistry. Due to its chemical properties, titanium has to be processed differently from conventional alloys. In this paper, two different methods of welding were investigated. Specimens machined from pure titanium rods were fused either by laser welding or plasma welding. Hardness profiles and light microscopy images were taken in the region of the weld. The mechanical properties were tested by alternating bending fatigue tests up to 3 million cycles. Light microscopy images and hardness profiles showed a larger heat-affected zone after plasma welding compared to laser welding. No significant differences comparing fatigue strength could be found between the two methods of welding. However, extreme loads led to earlier fatigue in the plasma-welded specimens. SEM images of the laser-welded joints showed fractures in the welding zone, while the plasma-welded specimens fractured mostly beyond the heat-affected zone. From these results, it can be assumed that both methods are suitable for welding titanium. At the moment, laser welding is the more suitable technique in dentistry because of its lower thermal alteration of the workpieces.
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PMID:Studies on laser- and plasma-welded titanium. 859 37

Fatigue and shortening-induced deactivation, two conditions that both lead to reversible depression of the mechanical performance of striated muscle are briefly reviewed. Fatigue. Isolated fibres from frog skeletal muscle (1-3 degrees C) that are stimulated to produce a 1 s fused tetanus at 15 s intervals are brought into a state of myofibrillar fatigue, (tetanic force reduced to 70-75% of the control) that is attributable to reduced performance of the myofibrils with no significant change in activation of the contractile system. A more intense stimulation programme (a single stimulus applied at 1-2 s intervals) reduces the tetanic force below 70% of the rested-state level. Under these conditions, failure of activation becomes increasingly important as a cause of the force decline. Deficient inward spread of activation is likely to account for at least part of the force decline after a period of intense fatiguing stimulation. Shortening-induced deactivation. Striated muscle that is allowed to shorten during activity loses some of its capacity to produce force, full restoration of the contractile strength being attained 1-2 s after the shortening phase. The depressant effect of shortening is demonstrable in skinned preparations as well as in intact muscle fibres and the magnitude of the effect is dependent on the state of activation of the muscle fibre when the movement occurs. The experimental evidence supports the view that sliding of the thick and thin filaments during activity reduces the affinity for calcium at the regulatory sites on the thin filament, leading to a transitory deactivation of the contractile system.
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PMID:Fatigue vs. shortening-induced deactivation in striated muscle. 872 78

Muscle cramp is a common, painful, physiological disturbance of skeletal muscle. Many athletes are regularly frustrated by exercise-induced muscle cramp yet the pathogenesis remains speculative with little scientific research on the subject. This has resulted in a perpetuation of myths as to the cause and treatment of it. There is a need for scientifically based protocols for the management of athletes who suffer exercise-related muscle cramp. This article reviews the literature and neurophysiology of muscle cramp occurring during exercise. Disturbances at various levels of the central and peripheral nervous system and skeletal muscle are likely to be involved in the mechanism of cramp and may explain the diverse range of conditions in which cramp occurs. The activity of the motor neuron is subject to a multitude of influences including peripheral receptor sensory input, spinal reflexes, inhibitory interneurons in the spinal cord, synaptic and neurotransmitter modulation and descending CNS input. The muscle spindle and golgi tendon organ proprioceptors are fundamental to the control of muscle length and tone and the maintenance of posture. Disturbance in the activity of these receptors may occur through faulty posture, shortened muscle length, intense exercise and exercise to fatigue, resulting in increased motor neuron activity and motor unit recruitment. The relaxation phase of muscle contraction is prolonged in a fatigued muscle, raising the likelihood of fused summation of action potentials if motor neuron activity delivers a sustained high firing frequency. Treatment of cramp is directed at reducing muscle spindle and motor neuron activity by reflex inhibition and afferent stimulation. There are no proven strategies for the prevention of exercise-induced muscle cramp but regular muscle stretching using post-isometric relaxation techniques, correction of muscle balance and posture, adequate conditioning for the activity, mental preparation for competition and avoiding provocative drugs may be beneficial. Other strategies such as incorporating plyometrics or eccentric muscle strengthening into training programmes, maintaining adequate carbohydrate reserves during competition or treating myofascial trigger points are speculative and require investigation.
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PMID:Exercise-induced muscle cramp. Proposed mechanisms and management. 878 61

Changes in tension of slow motor units in the medial gastrocnemius muscle of the rat were measured and compared in three fatigue tests, each of 4 min duration. In one of these tests motor unit tetani were evoked by repeated trains of stimuli of the standard frequency, 40 Hz, while in other tests frequencies of 20 and 60 Hz were used. Tetanic tension initially decreased in the majority of units during these three tests, then potentiated and, in the last part of the fatigue test, the tension was normally stable. Initial changes in tension were greatest in the fatigue tests with the lowest stimulation frequency (20 Hz) and smallest or even absent during tests with the stimulation frequency of 60 Hz. Moreover, initial changes in tension occurred for the longest period of time when the lowest frequency of stimulation (20 Hz) was used. Some of the motor units showed a slow decrease in tension in the last part of tests, especially when frequencies of 60 and 40 Hz were used. These results show that the tension of repeated unfused tetani of a slow motor unit is not constant, even during constant frequency stimulation, and changes in tension are greater in less fused tetani.
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PMID:Changes in tension of slow motor units in rat medial gastrocnemius during constant-rate stimulation at different frequencies. 896 10

In 57 motor units of rats' medial gastrocnemius muscle the influence of activity on the course of 10 successively repeated tension-frequency curves (T-f curves) was observed. In slow motor units the steep parts of successive T-f curves usually shifted slightly towards higher frequencies. However, in some slow units of relatively short contraction these T-f curves shifted towards lower frequencies. In fast units, especially those of the FF type, more marked changes were observed in T-f curves. At first, a shift of the second to third curve towards lower frequencies was observed. Curves then shifted progressively towards higher frequencies. In all motor units greater changes in tension could be observed in unfused tetani than in fused tetani. In some motor units the tension of fused tetani showed a progressive decrease. Changes in the course of T-f curves related to changes in contraction and relaxation times. The results presented here explain how the duration of the activity can change the sensitivity of motor unit tension to the frequency of stimuli. The importance of the changes observed in the T-f relation in the process of fatigue is discussed.
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PMID:Changes in tension-frequency relationship of motor units induced by their activity in rat muscle. 917 26


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