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

The contractile properties of a muscle depend on the activation history of its motor units. At the same time as fatigue seems to impair muscle excitation-contraction coupling, post-tetanic potentiation can augment force production. The effects of post-tetanic potentiation on the mechanical muscle properties of the intact human ankle extensor muscles were investigated by a 4 degree dorsiflexion of the ankle joint during a sustained contraction. The contraction was elicited by 10 Hz electrical stimulation of the tibial nerve. The changes in the contraction torque and in the intrinsic muscle stiffness of the ankle extensors before and after prolonged electrically elicited muscle activation were measured. From the onset of continuous synchronized 10 Hz stimulation to the attainment of maximal torque, the ankle joint torque increased by 47%. At matched background contraction, the prolonged electrically elicited contraction increased the intrinsic muscle stiffness by 49%. The first stretch after prolonged stimulation gave rise to a 17% yield in the background contraction and a 73% yield in the torque increment. The findings imply that with fatigue an increase in the intrinsic stiffness of the pre-stretched muscle might operate as a "safety factor" to compensate for a reduced reflex-induced stiffness, keeping the total muscle resistance at a high level in the active muscle.
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PMID:Mechanical properties of human ankle extensors after muscle potentiation. 128 60

1. The contribution to muscle force regulation provided by reflex pathways was studied in the elbow flexor muscles of seven normal human subjects, with the use of voluntary fatigue to induce a deficit in the force-generating capability of these muscles. To estimate the changes in the mechanical state of the muscle and the compensatory actions taken by reflex pathways to minimize the impact of fatigue, stochastic and "step" angular perturbations were applied to the joint, and the resulting joint stiffness and electromyographic (EMG) responses were compared before and after fatigue. 2. The magnitude of contractile fatigue, induced by repeatedly lifting a weight via a pulley system, was quantified by comparing the slope of the isometric torque-EMG relationship before and after fatigue. The exercise routine was quite effective in producing severe and long-lasting fatigue, with average percentage changes in the isometric torque-EMG slope of 210-306% for biceps and 129-205% for brachioradialis, depending on the point in time examined. 3. The torque response to a rapid step stretch of the elbow joint was quite similar before and after fatigue for the time interval before reflex action (less than 20 ms after stretch onset), suggesting that intrinsic muscle stiffness for a given mean torque level was not changed by fatigue. The steady-state torque level attained after completion of the stretch was always decreased after fatigue, indicating a decrease in the reflex component of joint stiffness, but this decrease was small compared with the change in the isometric torque-EMG relationship and was accompanied by a significantly larger incremental EMG response after fatigue. This increase in incremental EMG after fatigue was found to be of reflex origin, with activation-related reflex gain changes apparently playing a significant role only at low contraction levels. 4. Torque and angle responses recorded during stochastic perturbations were used to identify elbow joint compliance impulse responses. A second-order mechanical model was fit to each impulse response, and the parameters representing joint inertia, elastic stiffness, and viscous stiffness were used to summarize changes in joint mechanical properties as the mean contraction level was varied. For a perturbation with a relatively wide bandwidth (0-25 Hz), fatigue had little or no effect on the form of the compliance impulse response, apparently because the stimulus disabled reflex force generation in elbow flexor muscles, whereas a perturbation with a more restricted bandwidth (0-10 Hz) demonstrated consistent decreases in joint stiffness after fatigue.(ABSTRACT TRUNCATED AT 400 WORDS)
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PMID:Neural compensation for fatigue-induced changes in muscle stiffness during perturbations of elbow angle in human. 152 69

In this paper we develop a theory for calculating the chemical energy liberation and heat production of a skeletal muscle subjected to an arbitrary history of stimulation, loading, and length variation. This theory is based on and complements the distribution-moment (DM) model of muscle [Zahalak and Ma, J. biomech. Engng 112, 52-62 (1990)]. The DM model is a mathematical approximation of the A. F. Huxley cross-bridge theory and represents a muscle in terms of five (normalized) state variables: A, the muscle length, c, the sarcoplasmic free calcium concentration, and Q0, Q1, Q2, the first three moments of the actin-myosin bond-distribution function (which, respectively, have macroscopic interpretations as the muscle stiffness, force, and elastic energy stored in the contractile tissue). From this model are derived two equations which predict the chemical energy liberation and heat production rates in terms of the five DM state variables, and which take account of the following factors: (1) phosphocreatine hydrolysis associated with cross-bridge cycling; (2) phosphocreatine hydrolysis associated with sarcoplasmic-reticulum pumping of calcium; (3) passive calcium flux across the sarcoplasmic-reticulum membrane; (4) calcium-troponin bonding; (5) cross-bridge bonding at zero strain; (6) cross-bridge strain energy; (7) tendon strain energy; and (8) external work. Using estimated parameters appropriate for a frog sartorius at 0 degree C, the energy rates are calculated for several experiments reported in the literature, and reasonable agreement is found between our model and the measurements. (The selected experiments are confined to the plateau of the isometric length-tension curve, although our theory admits arbitrary length variations.) The two most important contributions to the energy rates are phosphocreatine hydrolysis associated with cross-bridge cycling and with sarcoplasmic-reticulum calcium pumping, and these two contributions are approximately equal under tetanic, isometric, steady-state conditions. The contribution of the calcium flux across the electrochemical potential gradient at the sarcoplasmic-reticulum membrane was found to be small under all conditions examined, and can be neglected. Long-term fatigue and oxidative recovery effects are not included in this theory. Also not included is the so-called 'unexplained energy' presumably associated with reactions which have not yet been identified. Within these limitations our model defines clear quantitative interrelations between the activation, mechanics, and energetics in muscle, and permits rational estimates of the energy production to be calculated for arbitrary programs of muscular work.
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PMID:A distribution-moment model of energetics in skeletal muscle. 849 84

In Sweden, several studies have been performed in patients with fibromyalgia to study muscle morphology, chemistry and physiology in order to understand the origin of the most prominent symptoms in fibromyalgia: muscle pain, muscle fatigue and muscle stiffness. These studies have shown changes indicating disturbed microcirculation, mitochondrial damage and a reduced content of high energy phosphates. Thus, there may be an energy deficiency state in the resting painful muscle in fibromyalgia. Pain analysis has supported the idea that there is a nociceptive origin of the pain. Our hypothesis is that any condition that could lead to constant muscle hypoxia, e.g., through establishment of abnormal motor patterns, might be a possible cause of fibromyalgic pain.
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PMID:The muscle in fibromyalgia--a review of Swedish studies. 269 74

The increase in stiffness of cat peroneus longus muscle elicited by the contraction of homogeneous groups of motor units (slow, fast fatigue-resistant and fast fatigable) was measured during sinusoidal stretches (20-80 Hz) of small amplitude (40-100 micron). For comparable tetanic tensions, slow units increase muscle stiffness more than fast unit.
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PMID:[Comparison of the increase of stiffness produced during sinusoidal stretch of the long fibula muscle in the cat by the contraction of slow or fast motor units]. 312 35

Dynamic and static stretch responses in muscle spindles were investigated in fatigued muscle to determine if acute adaptations do occur in receptor discharge as has been shown after contractions of short duration. Fatigue to 60-50% maximum tetanic tension was induced in the isolated gastrocnemius muscle in 16 cats by sustained, 7 X threshold electrical stimulation (100 Hz) of the cut L7 ventral root and S1 ventral root. Group Ia and II afferent fiber responses to slow ramp stretches (5 mm X s-1) and vibration (100 Hz) applied to the Achilles tendon were monitored before and immediately after muscle tetany to fatigue. Changes in firing characteristics were similar when results from faster (25-30 mm X s-1) ramp stretches were contrasted. During muscle fatigue, decreases in response latency to displacement and increases in resting discharge, mean frequency during stretch, and frequency of firing to vibration were predominant in both afferent fiber types. Static responses were generally lower, indicating a decrease in position sensitivity. Resting muscle force and passive peak muscle stiffness were consistently higher following contraction. The sum effects of these proprioceptive afferent and mechanical muscle responses would be to increase muscle stiffness and thus resist yield in muscle length to perturbations at lower muscle forces. The magnitude of these adaptations in proprioceptive discharge appears dependent on intrafusal muscle fiber activation.
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PMID:Dynamic and static stretch responses in muscle spindle receptors in fatigued muscle. 316 75

Muscle fatigue is the most disabling symptom in primary fibromyalgia (PF), which in addition is characterized by generalised pain and muscle stiffness. In order to assess whether the fatigue is of central and/or peripheral origin, skeletal muscle function was studied by measuring maximum voluntary hand grip strength, and by measuring various contraction characteristics in the adductor pollicis muscle after electrical stimulation of the ulnar nerve. The PF-patients were also studied after a regional sympathetic blockade of the forearm with guanethidine. A lower hand grip strength was found in the PF-patients compared to the controls, before as well as during the sympathetic blockade. The developed force, measured during electrical stimulation, did not differ between patients and controls. A lower muscle relaxation rate was found in the PF-patients. The relaxation rate increased in the PF-patients during the sympathetic blockade. The results indicate both a central and a peripheral cause of muscle dysfunction. Activity in the muscle sympathetic system may be one link in the chain of events that leads to muscular symptoms in PF.
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PMID:Skeletal muscle function in primary fibromyalgia. Effect of regional sympathetic blockade with guanethidine. 337 44

For long-term cementless fixation of prostheses compression stress transfer between bone and prosthesis is desirable. Shear stress and relative movement between implant and bone lead to bone resorption and implant loosening. A compression stress can be achieved by a suitable force transmission, a form-closed press fit and a suitable prosthesis surface. A ripped, rough or porous surface can transfer shear and tensile stress into compression stress at the interface. A force locking anchorage of the prosthesis can produce a prestressed fixation which supports the growing of the bone into the prosthesis surface. Stiff prostheses lead to a stronger stress protection and atrophy of the bone which favors the fatigue fracture of bone trabeculae.
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PMID:[Theoretical principles of the cement-free prosthetic technic]. 343 Dec 54

Stretch-shortening cycle (SSC), which is a normal contraction behavior of muscle, was used as a model to investigate muscular fatigue. Nine male volunteers were subjected to 100 repeated and exhaustive SSC contractions of the forearm extensors using a special sledge apparatus incorporating a force plate system. The fatigue contractions were performed on submaximal levels but the before-after comparison also included maximal drop-jump condition on the sledge as well as falls on to the floor. The results indicated that in the 100 submaximal SSCs the fatigue was characterized by increases in the contact times for both the eccentric and concentric phases of SSC, but the influence was more pronounced on the concentric part. The force-time curves during contact on the platform were influenced by fatigue so that the initial force peak became higher and the subsequent initial drop of force more pronounced. During submaximal and maximal drops, the angular velocities changed in the two phases of SSC. With progressing fatigue, the eccentric maximal angular velocity increased and the corresponding concentric velocities decreased. These changes were accompanied by slight changes in the elbow joint mechanism with respect to the contact, release, and maximal flexion angles. The results suggest that repeated SSC induces fatigue and the fatigue effects on the mechanical behavior of the muscle are very much similar to those induced by either isometric or concentric fatigue contractions. However, the transfer of the energy between eccentric and concentric phases was drastically reduced and this implies that SSCs can be used effectively to examine the fatiguability of the system regulating muscle stiffness during exercise.
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PMID:Fatigue during stretch-shortening cycle exercises: changes in mechanical performance of human skeletal muscle. 359 79

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


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