Muscle fatigue due to changes beyond the neuromuscular junction

History dependence of human muscle-fiber conduction velocity during voluntary isometric contractions

The conduction velocity (CV) of a muscle fiber is affected by the fiber's discharge history going back ∼1 s. We investigated this dependence by measuring CV fluctuations during voluntary isometric contractions of the human brachioradialis muscle. We recorded electromyogram (EMG) signals simultaneously from multiple intramuscular electrodes, identified potentials belonging to the same motor unit using EMG decomposition, and estimated the CV of each discharge from the interpotential interval. In 12 of 14 subjects, CV increased by ∼10% during the first second after recruitment and then fluctuated by about ±2% in a way that mirrored the fluctuations in the instantaneous firing rate. The CV profile could be precisely described in terms of the discharge history by a simple mathematical model. In the other two subjects, and one subject retested after cooling the arm, the CV fluctuations were inversely correlated with instantaneous firing rate. In all subjects, CV was additionally affected by very short interdischarge intervals (<25 ms): it was increased in doublets at recruitment, but decreased in doublets during continuous firing and after short interdischarge intervals in doubly innervated fibers. CV also exhibited a slow trend of about -0.05%/s that did not depend on the immediate discharge history. We suggest that measurements of CV fluctuations during voluntary contractions, or during stimulation protocols that involve longer and more complex stimulation patterns than are currently being used, may provide a sensitive approach for estimating the dynamic characteristics of ion channels in the human muscle-fiber membrane.

EMG and fatigue of human voluntary and stimulated contractions

During a 60s maximal voluntary isometric contraction (MVC) of the adductor pollicis muscle the loss of force is accompanied by a parallel decline in both the integrated surface electromyogram (EMG) and the single muscle fibre spike counts recorded intramuscularly. This decline is not due to neuromuscular block since the muscle mass action potential (M wave) evoked by single maximal shocks to the nerve is well maintained; nor does the size of the single fibre spike change. It must, therefore, reflect a decline in the firing pattern of the motor neuron pool. The force of a sustained MVC continues to match that from maximal tetanic nerve stimulation; thus, all motor units remain active. Continuous nerve stimulation at the frequency required to match the voluntary force of unfatigued muscle leads to a progressive failure of the M wave, and a more rapid force loss than in an MVC. Both are largely restored by reducing the stimulus frequency. The decline in neural firing rate correlates well with the rate of muscle contractile slowing. It thus optimizes force by maintaining a relatively constant degree of tetanic fusion, while avoiding peripheral failure of electrical propagation.

Clearance of extracellular K+ during muscle contraction--roles of membrane transport and diffusion

Excitation of muscle often leads to a net loss of cellular K⁺ and a rise in extracellular K⁺ ([ K⁺ ]_o), which in turn inhibits excitability and contractility. It is important, therefore, to determine how this K⁺ is cleared by diffusion into the surroundings or by reaccumulation into the muscle cells. The inhibitory effects of the rise in [K⁺ ]_o may be assessed from the time course of changes in tetanic force in isolated muscles where diffusional clearance of K⁺ is eliminated by removing the incubation medium and allowing the muscles to contract in air. Measurements of tetanic force, endurance, and force recovery showed that in rat soleus and extensor digitorum longus (EDL) muscles there was no significant difference between the performance of muscles contracting in buffer or in air for up to 8 min. Ouabain-induced inhibition of K⁺ clearance via the Na⁺,K⁺ pumps markedly reduced contractile endurance and force recovery in air. Incubation in buffer containing 10 mM K⁺ clearly inhibited force development and endurance, and these effects were considerably reduced by stimulating Na⁺,K⁺ pumps with the β₂-agonist salbutamol. Following 30–60 s of continuous stimulation at 60 Hz, the amount of K⁺ released into the extracellular space was assessed from washout experiments. The release of intracellular K⁺ per pulse was fourfold larger in EDL than in soleus, and in the two muscles, the average [K⁺ ]_o reached 52.4 and 26.0 mM, respectively, appreciably higher than previously detected. In conclusion, prevention of diffusion of K⁺ from the extracellular space of isolated working muscles causes only modest interference with contractile performance. The Na⁺,K⁺ pumps play a major role in the clearance of K⁺ and the maintenance of force. This new information is important for the evaluation of K⁺-induced inhibition in muscles, where diffusional clearance of K⁺ is reduced by tension development sufficient to suppress circulation.

Diminished fatigue at reduced muscle length in human skeletal muscle

Understanding muscle fatigue properties at different muscle lengths is essential to improve electrical stimulation applications in which impaired muscle is activated to produce function or to serve as an orthotic assist. This study examined the effects of muscle length on fatigue in human quadriceps muscle. Twelve healthy subjects were tested at short and long muscle lengths (15 degrees and 90 degrees of knee flexion, respectively) before and after a fatigue-producing protocol using low-, high-, and variable-frequency testing trains. Greater fatigue was observed at the longer muscle length, supporting the notion that fatigue is largely dependent upon metabolic factors. Fatigue, however, was characterized by greater attenuation of low- than high-frequency responses (i.e., low-frequency fatigue, LFF) at the long length. This observation, accompanied by the fact that variable-frequency trains produced greater augmentations in force production than comparable low-frequency trains at the longer length, suggests that excitation-contraction coupling impairment is also a contributing factor to fatigue and plays a greater role at the more fatigue-susceptible longer muscle length.

Potassium, Na+,K+-pumps and fatigue in rat muscle

During contractile activity, skeletal muscles undergo a net loss of cytoplasmic K(+) to the interstitial space. During intense exercise, plasma K(+) in human arterial blood may reach 8 mm, and interstitial K(+) 10-12 mm. This leads to depolarization, loss of excitability and contractile force. However, little is known about the effects of these physiological increases in extracellular K(+) ([K(+)](o)) on contractile endurance. Soleus muscles from 4-week-old rats were mounted on transducers for isometric contractions in Krebs-Ringer bicarbonate buffer containing 4-10 mm K(+), and endurance assessed by recording the rate of force decline during continuous stimulation at 60 Hz. Increasing [K(+)](o) from 4 to 8 or 10 mm and equilibrating the muscles for 40 or 20 min augmented the rate of force decline 2.4-fold and 7.2-fold, respectively (P < 0.001). The marked loss of endurance elicited by exposure to 8 or 10 mm K(+) was alleviated or significantly reduced by stimulating the Na(+),K(+)-pumps by intracellular Na(+) loading, the beta(2)-agonist salbutamol, adrenaline, calcitonin gene related peptide, insulin or repeated excitation. In conclusion, excitation-induced increase in [K(+)](o) is an important cause of high-frequency fatigue, and the Na(+),K(+)-pumps are essential for the maintenance of contractile force in the physiological range of [K(+)](o). Recordings of contractile force during continuous stimulation at 8-10 mm K(+) may be used to analyse the effects of agents or conditions influencing the excitability of working isolated muscles.

Motor unit conduction velocity during sustained contraction of the vastus medialis muscle

The aim of the study was to analyze motor unit conduction velocity at varying force of the vastus medialis muscle during sustained contraction. Surface (8-electrode array) and intramuscular (two wire electrodes) EMG signals were recorded from the distal part of the dominant vastus medialis muscle of ten healthy male subjects. The subjects sat on a chair with the knee 90 degrees flexed and performed seven 180-s long contractions at forces in the range 2.5-30% of the maximal voluntary contraction force. For each force level, the discharge patterns of the newly recruited motor units with respect to the previous force level were identified from the intramuscular recordings and used as trigger for averaging the surface EMG signals. Motor unit conduction velocity was estimated from the averaged surface EMG. Average discharge rate at which motor units were analyzed was the same for each force level (mean +/- SD, 8.3 +/- 0.8 pulses per second). Motor unit conduction velocity at the beginning of the contraction and its rate of change over time increased with force (P < 0.05). Conduction velocity at the beginning of the contraction estimated from the interference surface EMG (4.44 +/- 0.66 m/s) and from single motor units (4.75 +/- 0.56 m/s) were positively correlated (R (2) = 0.46; P < 0.0001) but significantly different (P < 0.05). The results indicate that single motor unit conduction velocity and its rate of change during sustained contraction, assessed at a fixed discharge rate, depend on force level.

Feedback-controlled stimulation enhances human paralyzed muscle performance

Chronically paralyzed muscle requires extensive training before it can deliver a therapeutic dose of repetitive stress to the musculoskeletal system. Neuromuscular electrical stimulation, under feedback control, may subvert the effects of fatigue, yielding more rapid and extensive adaptations to training. The purposes of this investigation were to 1) compare the effectiveness of torque feedback-controlled (FDBCK) electrical stimulation with classic open-loop constant-frequency (CONST) stimulation, and 2) ascertain which of three stimulation strategies best maintains soleus torque during repetitive stimulation. When torque declined by 10%, the FDBCK protocol modulated the base stimulation frequency in three ways: by a fixed increase, by a paired pulse (doublet) at the beginning of the stimulation train, and by a fixed decrease. The stimulation strategy that most effectively restored torque continued for successive contractions. This process repeated each time torque declined by 10%. In fresh muscle, FDBCK stimulation offered minimal advantage in maintaining peak torque or mean torque over CONST stimulation. As long-duration fatigue developed in subsequent bouts, FDBCK stimulation became most effective ( approximately 40% higher final normalized torque than CONST). The high-frequency strategy was selected approximately 90% of the time, supporting that excitation-contraction coupling compromise and not neuromuscular transmission failure contributed to fatigue of paralyzed muscle. Ideal stimulation strategies may vary according to the site of fatigue; this stimulation approach offered the advantage of online modulation of stimulation strategies in response to fatigue conditions. Based on stress-adaptation principles, FDBCK-controlled stimulation may enhance training effects in chronically paralyzed muscle.

Na+-K+ Pump Regulation and Skeletal Muscle Contractility

Clausen, Torben. Na+-K+ Pump Regulation and Skeletal Muscle Contractility. Physiol Rev 83: 1269-1324, 2003; 10.1152/physrev.00011.2003.—In skeletal muscle, excitation may cause loss of K+, increased extracellular K+ ([K+]o), intracellular Na+ ([Na+]i), and depolarization. Since these events interfere with excitability, the processes of excitation can be self-limiting. During work, therefore, the impending loss of excitability has to be counterbalanced by prompt restoration of Na+-K+ gradients. Since this is the major function of the Na+-K+ pumps, it is crucial that their activity and capacity are adequate. This is achieved in two ways: 1) by acute activation of the Na+-K+ pumps and 2) by long-term regulation of Na+-K+ pump content or capacity. 1) Depending on frequency of stimulation, excitation may activate up to all of the Na+-K+ pumps available within 10 s, causing up to 22-fold increase in Na+ efflux. Activation of the Na+-K+ pumps by hormones is slower and less pronounced. When muscles are inhibited by high [K+]o or low [Na+]o, acute hormone- or excitation-induced activation of the Na+-K+ pumps can restore excitability and contractile force in 10-20 min. Conversely, inhibition of the Na+-K+ pumps by ouabain leads to progressive loss of contractility and endurance. 2) Na+-K+ pump content is upregulated by training, thyroid hormones, insulin, glucocorticoids, and K+ overload. Downregulation is seen during immobilization, K+ deficiency, hypoxia, heart failure, hypothyroidism, starvation, diabetes, alcoholism, myotonic dystrophy, and McArdle disease. Reduced Na+-K+ pump content leads to loss of contractility and endurance, possibly contributing to the fatigue associated with several of these conditions. Increasing excitation-induced Na+ influx by augmenting the open-time or the content of Na+ channels reduces contractile endurance. Excitability and contractility depend on the ratio between passive Na+-K+ leaks and Na+-K+ pump activity, the passive leaks often playing a dominant role. The Na+-K+ pump is a central target for regulation of Na+-K+ distribution and excitability, essential for second-to-second ongoing maintenance of excitability during work.

Muscular, skeletal, and neural adaptations following spinal cord injury

Spinal cord injury is associated with adaptations to the muscular, skeletal, and spinal systems. Experimental data are lacking regarding the extent to which rehabilitative methods may influence these adaptations. An understanding of the plasticity of the muscular, skeletal, and spinal systems after paralysis may be important as new rehabilitative technologies emerge in the 21st century. Moreover, individuals injured today may become poor candidates for future scientific advancements (cure) if their neuromusculoskeletal systems are irreversibly impaired. The primary purpose of this paper is to explore the physiological properties of skeletal muscle as a result of spinal cord injury; secondarily, to consider associated changes at the skeletal and spinal levels. Muscular adaptations include a transformation to faster myosin, increased contractile speeds, shift to the right on the torque-frequency curve, increased fatigue, and enhanced doublet potentiation. These muscular adaptations may be prevented in individuals with acute paralysis and partially reversed in individuals with chronic paralysis. Moreover, the muscular changes may be coordinated with motor unit and spinal circuitry adaptations. Concurrently, skeletal adaptations, as measured by bone mineral density, show extensive loss within the first six months after paralysis. The underlying science governing neuromusculoskeletal adaptations after paralysis will help guide professionals as new rehabilitation strategies evolve in the future.

Fatigue-related changes in electomyogram activity of the cat gastrocnemius during frequency-modulated efferent stimulation

Changes in the compound muscle action potentials of cat gastrocnemius muscle were studied during low- and high-frequency fatigue. Fatiguing session consisted of 25-28 repetitions of the standard single fatigue tests (1.5min interval between the tests) that included the part of continuous frequency-modulated stimulation preceded and followed by single stimuli evoking twitch contractions in the muscle. The rate of the continuous part was changed in accordance with symmetrical double-trapezoidal signal, including three successive phases of constant rate at 10, 40 and 10s(-1); between these phases of 4s duration the rate changed linearly within a 2s interval. During fatigue relative changes in compound muscle action potential waves were usually smaller than changes in tension. Within the same fatigue procedure applied to a fresh muscle, the drop in tension was as much as 35% for high-rate stimulation and 59-71% for low-rate stimulation, whereas the decrease of the peak-to-peak compound muscle action potential waves amplitudes did not exceed 10-20%. Compound muscle action potential waves underwent the most pronounced depression during high-rate stimulation, the decrease proceeding during the following phase of low-rate stimulation. The tension changes during long-lasting activation were different for low- and high-frequency fatigue, with more pronounced depression during low-rate stimulation. As a rule, compound muscle action potential waves changes followed opposite patterns. Compound muscle action potential waves progressively split up, which was probably associated with a continuous slowing of the action potentials in the most fatigable motor units and the subsequent disappearance of the reactions at least in part of the motor units. Hysteresis effects in muscle contraction seem to be able, at least in part, to compensate for some of the depressive effects appearing during conduction of action potentials in muscle fibres. Changes in the compound muscle action potentials were studied during development of the muscle fatigue. These changes showed pronounced dependency on stimulation rate allowing differentiating effects of low- and high-frequency stimulation of the efferents supplying muscle under study. At the same time the fatigue-related changes in the action potentials were noticeably smaller than changes in tension, thus supporting existing concepts in the field arguing that fatigue effects are mainly connected with corresponding activity-dependent changes in muscle contraction machinery.

Relations between excitability and contractility in rat soleus muscle: role of the Na+-K+ pump and Na+/K+ gradients

1. The effects of reduced Na+/K+ gradients and Na+-K+ pump stimulation on compound action potentials (M waves) and contractile force were examined in isolated rat soleus muscles stimulated through the nerve. 2. Exposure of muscles to buffer containing 85 mM Na+ and 9 mM K+ (85 Na+/9 K+ buffer) produced a 54% decrease in M wave area and a 50 % decrease in tetanic force compared with control levels in standard buffer containing 147 mM Na+ and 4 mM K+. Subsequent stimulation of active Na+-K+ transport, using the beta2-adrenoceptor agonist salbutamol, induced a marked recovery of M wave area and tetanic force (to 98 and 87% of the control level, respectively). Similarly, stimulation of active Na+-K+ transport with insulin induced a significant recovery of M wave area and tetanic force. 3. During equilibration with 85 Na+/9 K+ buffer and after addition of salbutamol there was a close linear correlation between M wave area and tetanic force (r = 0.92, P < 0.001). Similar correlations were found in muscles where tetrodotoxin was used to reduce excitability and in muscles fatigued by 120 s of continuous stimulation at a frequency of 30 Hz. 4. These results show a close correlation between excitability and tetanic force. Furthermore, in muscles depressed by a reduction in the Na+/K+ gradients, beta-adrenergic stimulation of the Na+-K+ pump induces a recovery of excitability which can fully explain the previously demonstrated recovery of tetanic force following Na+-K+ pump stimulation. Moreover, the data indicate that loss of excitability is an important factor in fatigue induced by high-frequency (30 Hz) stimulation.

Extrapolation of time series of EMG power spectrum parameters in isometric endurance tests of trunk extensor muscles

The aim of the present study was to test the viability of using short isometric contractions of trunk extensor muscles to perform an assessment of their endurance capacity. To this aim two types of analysis were performed. First, electromyographic (EMG) mean power frequency (MPF) slopes with respect to time as estimated over shorter fixed periods were compared to slopes estimated over the full contraction period of a contraction sustained until the endurance time. Second, the relationship between MPF slope estimates as estimated over various periods and the endurance time of the muscle group was evaluated. Five subjects performed three isometric trunk endurance tests at 25%, 50% and 75% of their maximum voluntary contraction (MVC), respectively. EMG signals of the left and right multifidus, iliocostalis and longissimus muscles were continuously recorded and spectral parameters were calculated. The MPF appeared to decrease consistently during all endurance tests. The extrapolation from a MPF time series of half the estimated contraction period to the time series of the complete contraction period gave reasonable results at all force levels, when data from several electrode locations were incorporated in a single slope estimate (mean or steepest slope). The accuracy of the prediction of trunk extensor endurance on the basis of these parameters describing the MPF time series over half the estimated contraction period was satisfactory. Endurance time predictions from yet shorter periods were unreliable.

Fatigue-induced changes in group IV muscle afferent activity: differences between high- and low-frequency electrically induced fatigues

Recordings of group IV afferent activity of tibialis anterior muscle were performed in paralysed rabbits during runs of electrically induced fatigue produced by direct muscle stimulation at a high (100 Hz, high-frequency fatigue HFF) or a low rate (10 Hz, low-frequency fatigue LFF). In addition to analysis of afferent nerve action potentials, muscle force and compound muscle action potentials (M waves) elicited by direct muscle stimulation with single shocks were recorded. Changes in M wave configuration were used as an index of the altered propagation of membrane potentials and the associated efflux of potassium from muscle fibers. The data show that increased group IV afferent activity occurred during LFF as well as HFF trials and developed parallel with force failure. Enhanced afferent activity was significantly higher during LFF (maximal delta f(impulses) = 249 +/- 35%) than HFF (147 +/- 45%). No correlation was obtained between the responses of group IV afferents to LFF or to pressure exerted on tibialis anterior muscle. On the other hand, decreased M wave amplitude was minimal with LFF while it was pronounced with HFF. Close correlations were found between fatigue-induced activation of group IV afferents and decreases in force or M wave amplitude, but their strength was significantly higher with LFF compared to HFF. Thus, electrically induced fatigue activates group IV muscle afferents with a prominent effect of low-frequency stimulation. The mechanism of muscle afferent stimulation does not seem to be due to the sole increase in extracellular potassium concentration, but also by the efflux of muscle metabolites, present during fatiguing contractions at low rate of stimulation.

Variable-frequency stimulation patterns for the optimization of force during muscle fatigue. Muscle wisdom and the catch-like property

Muscle wisdom is the process whereby the activation rates of motor units are modulated by the central nervous system to optimize the force during sustained voluntary contractions. During maximal voluntary contractions the activation rates decline as the muscle fatigues. No similar decline has been observed during submaximal contractions. Subsequent chapters explore the potential mechanisms for muscle wisdom. In this chapter, a historical background on the development of ideas on muscle wisdom is first presented. Next, artificial wisdom, the procedure used to optimize force during an electrically imposed tetanus by progressively reducing the stimulation frequency as the muscle fatigues, is discussed. Finally, recent studies are described in which fatigue was delayed and reduced by the use of variable-frequency stimulus trains that elicit the catch-like property of muscle.

Maximum relaxation rate of the diaphragm during weaning from mechanical ventilation

BACKGROUND

The maximum relaxation rate (MRR; percentage fall in pressure/10 ms) of oesophageal (POES) and transdiaphragmatic (PDI) pressure slows under conditions of loaded breathing, and has been measured previously in normal subjects. MRR has not been measured in intubated patients weaning from mechanical ventilation.

METHODS

Five postoperative patients who were expected to wean and nine patients who had previously failed were studied. POES and PDI MRR, peak oesophageal pressure during spontaneous breathing, maximum oesophageal pressure, and the inspiratory duty cycle were measured at rest during mechanical ventilation, in the first two minutes of spontaneous breathing, and after reventilation in those patients who failed, or before extubation in those patients who succeeded.

RESULTS

At rest POES MRR in intubated patients had a range of 5.6-11 and PDI MRR 6.9-10.0, with a coefficient of variation of 9.9% and 7.3% respectively. POES and PDI MRR were similar before and after extubation in five postoperative patients, and POES MRR was reflected by endotracheal MRR measured at the airway. In five patients who failed to wean POES and PDI MRR slowed by 47% and 44%, and fully recovered after 10 minutes reventilation. In four patients who were successfully weaned MRR was unchanged during spontaneous breathing. At the time when MRR decreased, the respiratory muscles were heavily loaded in relation to their strength.

CONCLUSIONS

Weaning failure occurs when the applied load exceeds the capacity of the respiratory muscles, and this is associated with a slowing of respiratory muscle MRR.

Exercise-induced diaphragmatic fatigue in healthy humans

1. Twelve healthy subjects (33 +/- 3 years) with a variety of fitness levels (maximal oxygen uptake (VO2, max) = 61 +/- 4 ml kg-1 min-1, range 40-80), exercised at 95 and 85% VO2, max to exhaustion (mean time = 14 +/- 3 and 31 +/- 8 min, expired ventilation (VE) over final minute of exercise = 149 +/- 9 and 126 +/- 10 l min-1). 2. Bilateral transcutaneous supramaximal phrenic nerve stimulation (BPNS) was performed before and immediately after exercise at four lung volumes, and 400 ms tetanic stimulations were performed at 10 and 20 Hz. The coefficients of variation of repeated measurements for the twitch transdiaphragm pressures (Pdi) were +/- 7-10% and for compound muscle action potentials (M wave) +/- 10-15%. 3. Following exercise at 95% of VO2, max, group mean Pdi twitch values were reduced at all lung volumes (range -8 +/- 3 to -32 +/- 5%) and tetanically stimulated Pdi values were reduced at both 10 and 20 Hz (-21 +/- 3 and -13 +/- 2%, respectively) (P = 0.001-0.047). Following exercise at 85% VO2, max, stimulated Pdi values were reduced at all lung volumes and stimulating frequencies, but only significantly so with the twitch at functional residual capacity (-15 +/- 5%). Stimulated Pdi values recovered partially by 30 min post-exercise and almost completely by an average time of 70 min. 4. The fall in stimulated Pdi values post-exercise was significantly correlated with the percentage increase in diaphragmatic work (integral of Pdi min-1) from rest to end-exercise and the relative intensity of the exercise. 5. The integral of Pdi min-1 and the integral of Po min-1 (Po, esophageal pressure) rose together from rest through the fifth to tenth minute of exercise, after which integral of Pdi min-1 plateaued even though integral of Po min-1, VE and inspiratory flow rate all continued to rise substantially until exercise terminated. Thus, the relative contribution of the diaphragm to total respiratory motor output was progressively reduced with exercise duration. 6. We conclude that significant diaphragmatic fatigue is caused by the ventilatory requirements imposed by heavy endurance exercise in healthy persons with a variety of fitness levels. The magnitude of the fatigue and the likelihood of its occurrence increases as the relative intensity of the exercise exceeds 85% of VO2, max.

Effects of selective beta 2-adrenoceptor blockade on serum potassium and exercise performance in normal men

1. The differential effects of beta-adrenoceptor subtypes on potassium fluxes and exercise capacity were compared in eight healthy young men using single oral doses of the selective beta 2-adrenoceptor antagonist ICI-118551, the selective beta 1-adrenoceptor antagonist atenolol or the non-selective beta-adrenoceptor antagonist propranolol. The study was randomized, double-blind and placebo controlled. 2. Potassium in the venous effluent from the exercising muscles increased progressively with increasing exercise intensity. This response was augmented by propranolol, whereas neither atenolol nor ICI-118551 modified the response. After exercise potassium concentration fell exponentially with no difference between the treatment regimens. 3. Cumulative work was significantly reduced by ICI-118551 (6.4%, P = 0.04) and by propranolol (12.4%, P less than 0.01), whereas the reduction with atenolol (5.6%) did not reach statistical significance. 4. Atenolol and propranolol reduced peak heart rate by 23% and 29%, and peak systolic blood pressure by 9% and 11% respectively during maximal exercise. ICI-118551 caused a non-significant reduction in heart rate during submaximal exercise, with a significant reduction at maximum exercise (6% reduction), whereas systolic blood pressure was not different from placebo. Diastolic blood pressures were similar across all treatment regimens. 5. Similar glucose concentrations were obtained at baseline and at exhaustion during all treatment regimens. Lactate concentrations were comparable for any given exercise intensity irrespective of treatment regimens. Propranolol reduced lactate concentrations from the exercising muscles at maximum exercise in proportion to the reduction of maximal exercise capacity. 6. The subjective perception of fatigue was not affected by either beta 1- or beta 2-adrenoceptor blockade.(ABSTRACT TRUNCATED AT 250 WORDS)

Force decline due to fatigue and intracellular acidification in isolated fibres from mouse skeletal muscle

1. Single, intact muscle fibres from the flexor brevis foot muscle of the mouse have been fatigued at 25 degrees C by 350 ms, 70 Hz stimulation trains, initially delivered every 3.8 s and then at stepwise decreasing intervals until tension was down to about 30% of the original (Po). Rested fibres generated a specific force of 372 +/- 8.4 kPa (mean +/- S.E.M., n = 25). 2. Endurance, defined as time to attain 0.5 Po, varied from 2.5 to 24 min, with the majority of fibres falling in the range 4-8 min, corresponding to 70-160 tetani. In all fibres where it was followed, tension recovery after cessation of stimulation was 90% or better. 3. Tetanic force declined in a characteristic way during fatiguing stimulation: initially tension fell to about 0.85 Po during eight to fourteen tetani (phase 1), then followed a long period of nearly steady tension generation (phase 2) and finally there was a rapid force decline (phase 3). 4. Caffeine (15 or 25 mM) caused a slight potentiation of tetanic force in the rested state (4.7 +/- 0.9%, n = 21) and slowed relaxation. No change in resting tension was seen with caffeine at concentrations up to 25 mM. 5. Caffeine (15-25 mM) caused a rapid and dramatic increase in tetanic force when applied to severely fatigued fibres: force output rose from 29.8 +/- 1.5 to 82.5 +/- 1.2% (n = 13) of Po. During phase 2 force potentiation with caffeine was much smaller. 6. A 10 s pause resulted in a large, transient force increase when imposed during phase 3 but had little effect on force production during phase 2. 7. Intracellular acidosis, induced by superfusion with Tyrode solution gassed with 30% CO2 instead of the normal 5% (extracellular pH 6.5 vs. 7.3), resulted in a fall in tetanic tension to about 0.85 Po (n = 7). This depression could to some extent be counteracted by 15 mM-caffeine, which brought tension back to about 0.90 Po. 8. It is concluded that there are at least two mechanisms for force decline during fatiguing stimulation: one which manifests itself early and is likely to be related to cross-bridge function and another representing deficient Ca2+ handling which becomes prominent at a later stage. For severe fatigue (0.3 Po) the latter mechanism is dominant.

Apparent upregulation of Na+,K+ pump sites in SHR skeletal muscle with reduced transport capacity

Slow-twitch, oxidative skeletal muscles in SHR exhibit several physiological defects, including a reduced ability to maintain force during high frequency repetitive stimulation (1). Muscle fatigue may be produced by one of a variety of factors acting at different levels of the neuromuscular system. Several lines of evidence, however, suggest that SHR soleus fatigues more rapidly than WKY soleus because SHR muscles allow more K+ to accumulate in the extracellular space during repetitive muscle activity. An increase in extracellular K+ can lead to a failure in the generation or conduction of muscle action potentials. Comparison of the compound action potentials recorded from SHR and WKY muscles during repetitive stimulation provided evidence for a decrease in excitability of SHR soleus. Since the K+ released from muscle fibers during exercise is returned to the fiber principally via the activity of the Na+, K+ pump, the increase in extracellular K+ in SHR muscle may reflect a decrease in pump capacity. Measurements including intracellular K+ and Na+ content at rest, the level of hyperpolarization produced by the addition of epinephrine and insulin to SHR soleus and the post-exercise recovery of resting membrane potentials all appear to indicate that Na+, K+ pump capacity is reduced in SHR soleus muscles. Nonetheless, ouabain binding studies show a significantly greater number of pump sites in SHR muscles. The data suggest that Na+ pump activity is decreased in SHR soleus muscles without an apparent reduction in either the number of pump sites or in pump binding affinity.

The importance of potassium and lactate for maximal exercise performance during beta blockade

Changes in femoral vein pH, lactate, glucose and potassium were studied in a double-blind randomized, short-term, dynamic cycle ergometry exercise test on six healthy male subjects after administration of non-selective (timolol), beta-1-selective (atenolol) beta blocker or placebo. The exercise intensity was increased in steps of 200 kpm/min every 2 min until exhaustion. During submaximal exercise, potassium concentrations in blood from the exercising leg muscles increased progressively with increasing exercise intensity, and was significantly higher for any given exercise level following timolol as compared to placebo administration. The potassium concentrations following atenolol were in-between those of timolol and placebo. Despite reduced working capacity after non-selective beta blockade, almost identical potassium concentrations were reached at exhaustion irrespective of treatment regimens (placebo: 6.3, range 5.8-6.8 mmol/l; atenolol: 6.5, range 6.1-7.3 mmol/l and timolol: 6.4, range 6.2-6.8 mmol/l). The increase in s-lactate concentrations was similar across all treatments, and rose in proportion to the increase in the exercise intensity. A biphasic increase in lactate was observed with identical breaking points (anaerobic threshold) irrespective of treatment regimens. There was no difference in glucose concentrations between the treatment regimens. The marked increase in serum potassium during maximal exercise coincides with leg muscle fatigue and may, by its effect on the muscle cell membrane potential, limit the maximal working capacity following beta blockers. The rise in serum potassium may curtail the use of maximal exercise test as an index of cardiac performance in healthy young subjects.

Beta-receptor properties in soleus muscles from spontaneously hypertensive rats

We have compared the properties of beta-adrenergic receptors in slow-twitch, oxidative skeletal muscles (soleus) from spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats at three different ages. The investigation was based on the hypothesis that the increase in Na+ content and decrease in fatigue resistance observed previously in the soleus of SHR might be the result of a down regulation of muscle beta-receptors. Activation of beta-adrenergic receptors in skeletal muscle stimulates sarcolemmal sodium-potassium adenosine triphosphatase, which produces an efflux of Na+ and an influx of K+. Receptor down-regulation would be expected to reduce hormonal stimulation of Na+ pump activity, particularly during exercise. The results of receptor binding studies, however, and an investigation of cyclic adenosine monophosphate (cAMP) production in response to applied epinephrine indicated that there were no significant differences in receptor properties in the soleus muscles of SHR and WKY rats. Receptor number and affinity were the same in the two strains, and the rate, magnitude, and duration of the increase in cAMP in response to 10(-6) M epinephrine were also similar. beta-Adrenergic receptor down-regulation does not appear to be a generalized phenomenon in tissues of SHR, despite the appearance of other physiological changes in the tissue.

Tetanic responses of electrically stimulated paralyzed muscle at varying interpulse intervals

The influence of stimulus interpulse interval (IPI) on torque output during electrically-evoked contractions was investigated for the knee extensor muscles of paralyzed subjects. The parameters measured were the rise time, magnitude, and relaxation time of the contraction at stimulus IPI's ranging from 62 to 7 ms. Torque output increased as IPI's were decreased from 62 to 15 ms. Peak torques were recorded at IPI's of 12-15 ms; IPI's less than these resulted in an insignificant loss of torque. Rise times decreased as IPI's were decreased. Relaxation time generally increased as IPI's were decreased with the longest relaxation times occurring with stimulation at an IPI of 12 ms. To demonstrate the influence of IPI on muscle fatigue, the effect of prolonged stimulation at short (12 ms) and long (50 ms) IPI's was also compared. After 30 s of stimulation with an IPI of 12 ms, mean torque had declined to 5 +/- 3 percent and after 30 s of stimulation with an IPI of 50 ms, mean torque had declined to 82 +/- 4 percent of the initial value. Knowledge of how stimulus IPI influences the response of paralyzed muscle to electrical stimulation may assist in the development of rehabilitation devices which utilize these technologies.

31P nuclear magnetic resonance studies of high energy phosphates and pH in human muscle fatigue. Comparison of aerobic and anaerobic exercise

The goal of these experiments was to investigate the relationship of ATP, phosphocreatine (PCr), inorganic phosphate (Pi), monobasic phosphate (H2PO4-), and pH to human muscle fatigue. Phosphates and pH were measured in adductor pollicis using 31P nuclear magnetic resonance at 2.0 Tesla. The force of muscle contraction was simultaneously measured with a force transducer. The effects of aerobic and anaerobic exercise were compared using two exercise protocols: 4 min sustained maximal voluntary contraction (MVC) and 40 min of repeated intermittent contractions (75% MVC). The sustained maximal contraction produced a rapid decline of MVC and PCr, and was accompanied by a rapid rise of Pi, H+, and H2PO4-. Intermittent exercise produced steady state changes of MVC, pH, and phosphates. No significant changes of ATP were found in either protocol. During fatiguing exercise, PCr and Pi had a nonlinear relationship with MVC. H+ showed a more linear correlation, while H2PO4- showed the best correlation with MVC. Furthermore, the correlations between MVC and H2PO4- were similar in sustained (r = 0.70) and intermittent (r = 0.73) exercise. The highly significant linear relationship between increases of H+ and H2PO4- and the decline of MVC strongly suggests that both H+ and H2PO4- are important determinants of human muscle fatigue.

Absence of excess peripheral muscle fatigue during beta-adrenoceptor blockade

1. In eight normal volunteers, the adductor pollicis (AP) was fatigued using intermittent trains of programmed, supramaximal stimulation at 1, 10, 20, 50, 100 and 1 Hz. Activity protocols were performed both with and without circulatory occlusion, both without and during propranolol 80 mg thrice daily in order to investigate the effects of beta-adrenoceptor blockade on 'peripheral' fatigue mechanisms. 2. The degree of beta-adrenoceptor blockade was assessed by the reduction of exercise tachycardia during cycle ergometry, e.g. pulse rates at 210 watts were reduced from 190 +/- 15 to 127 +/- 5 beats min-1 (mean +/- 1 s.d.) indicating that beta-adrenoceptor blockade was substantial and highly significant (P less than 0.001). 3. Before, during and following fatiguing activity with circulatory occlusion force declines were identical during and without beta-adrenoceptor blockade. During and following activity without occlusion, there were slight declines in force which were questionably significantly different at 20 Hz (P less than 0.05). 4. The compound muscle action potential (CMAP) amplitude, measured from the skin surface over the muscle, was unaltered by beta-adrenoceptor blockade before, during or after activity whether with or without circulatory occlusion. 5. The maximal relaxation rate (MRR) was not significantly reduced in previously unfatigued muscle during beta-adrenoceptor blockade. During activity, both with and without circulatory occlusion, there was no evidence that MRR was reduced significantly more during beta-adrenoceptor blockade. 6. The absence of a convincing effect of beta-adrenoceptor blockade on peripheral fatigue mechanisms may indicate that central mechanisms are involved or that impairments of peripheral force production, of a specific nature or as a result of exacerbation of limitations of circulatory oxygen transport, though small are detected during voluntary exercise and give rise to increases in motor unit recruitment and/or firing rates, and hence increased perception of fatigue.

Fatigue of lateral rectus and retractor bulbi motor units in cat

The fatigue properties of lateral rectus, retractor bulbi and split lateral rectus-retractor bulbi motor units were studied in the cat. Lateral rectus motor units showed a range in resistance to fatigue while retractor bulbi motor units were all fatigable. Within the abducens nucleus, axons of split lateral rectus-retractor bulbi motor units are found. These motor units are unique in that one motoneuron projects to two separate muscles. Split motor units were studied to determine if both the lateral rectus and retractor bulbi muscle fibers of split units would show uniform fatigue properties. The results showed that the fatigue resistance of the separate muscle components of these motor units are different, suggesting that the muscle fibers of a motor unit may be physiologically dissimilar.

Comparison between slow sodium channel inactivation in rat slow- and fast-twitch muscle

1. Voltage-clamp Na+ currents (INa) were studied in rat soleus slow-twitch muscle fibres at about 18 degrees C using the loose-patch-clamp technique. The maximum inward current density was produced by depolarizations to about -19 mV. 2. Fast inactivation was studied utilizing 20 ms conditioning potentials. INa was reduced by 50% with conditioning potentials to about -70 mV. 3. Changes in the conditioning membrane potential produced slow changes in the peak INa due to a slow inactivation process. INa was reduced by 50% at about -86 mV due to slow inactivation. 4. The mean maximum inward INa when slow inactivation was fully removed was 6.83 mA cm-2. 5. Due to the slow inactivation process, slow-twitch fibres were less susceptible to reduction in INa than fast-twitch fibres.

Studies of the unitary properties of adenosine-5'-triphosphate-regulated potassium channels of frog skeletal muscle

1. Patch-clamp techniques were used to study adenosine-5'-triphosphate (ATP)-dependent K+ channels in sarcolemmal vesicles from frog skeletal muscle. In addition to its ATP dependence, opening of these channels was voltage dependent, the open-state probability (P open) increasing with depolarization. 2. The reversal potential of unitary currents changed with external K+ concentration, [K+]o, as expected if the Na-K permeability ration (pNa/pK) equals 0.015. Unitary conductance increased with increasing [K+]o from 14.8 +/- 0.5 pS (n = 5) in 2.5 mM-K+ to 42.3 +/- 1.0 pS (n = 8) in 60 mM-K+. This increase was less than that expected from independence. 3. Replacement of 60 mM-external K+ by 60 mM-external Rb+ shifted the reversal potential of unitary currents by -6.7 mV, suggesting that Rb+ enters channels nearly as easily as does K+ (Rb-K permeability ration, pRb/pK = 0.76). Unitary currents were much smaller in Rb+, consistent with Rb+ binding within the channel. 4. The ATP-regulated K+ channel was blocked by both internal and external tetraethylammonium ions (TEA+). 2 mM-TEA+, applied to the cytoplasmic face of membrane patches, interrupted channel openings. Higher concentrations reduced unitary current amplitude, suggesting an increase in the rapidity of TEA+ block. 5. The reduction in P open by ATP was consistent with 1:1 binding and a dissociation constant of 0.135 mM. ATP appeared not to be hydrolysed to close channels. Adenosine 5'-diphosphate (ADP) and adenosine 5'-monophosphate (AMP) were less effective than ATP, but retained channel closing properties. Substitution of adenine with other purines or with pyrimidine bases substantially reduced activity, as did substitution of ribose by 2'-deoxyribose or by ribose 2',3'-dialdehyde. 6. Sarcoplasmic Ca2+ did not influence P open. 7. Myotubes, grown from thigh muscles of new-born rats, appeared to lack ATP-dependent K+ channels. Adult frog muscle appeared to lack high-conductance Ca2+-dependent K+ channels, at least in the surface membrane. Such channels were found in myotube membranes. 8. Open- and closed-time histograms were constructed and were consistent with at least two open and at least three closed states. Channel openings were grouped in bursts. Open times, burst lengths and the number of openings per burst were reduced by ATP. 9. The effects of [K+]o on unitary conductance and of K+ replacement with Rb+ are discussed in terms of a simple Eyring rate theory formulation.(ABSTRACT TRUNCATED AT 400 WORDS)

Influence of human muscle length on fatigue

The effect of muscle length on susceptibility to fatigue has been examined in human ankle dorsiflexor muscles. The fatiguing procedure consisted of either indirect tetanic stimulation at 20 Hz or maximal voluntary contraction; each procedure lasted 90 s. The amplitude of the evoked muscle compound action potential (M-wave) increased during the first 30 s or so of the tetanic fatiguing procedure and then decreased. The torque developed by the dorsiflexor muscles declined throughout the period of tetanization. A significantly greater reduction in twitch and tetanic torque was found after the fatiguing procedure had been conducted at the optimum muscle length rather than with the muscle in a shortened position. Relaxation after tetanic stimulation was slower after fatigue had been induced at the optimum muscle length. It is concluded that muscle fatigue is related to the number of actin-myosin cross-bridge interactions and is unlikely to be accounted for solely on the basis of changes in the ionic composition of the transverse tubular fluid.

Effect of aminophylline on the human diaphragm

The effect of intravenous aminophylline on the contractile function of the diaphragm was studied in four normal subjects. The contractility of the diaphragm was assessed by the measurement of transdiaphragmatic pressure (Pdi) after right phrenic nerve stimulation at 1 Hz. Pdi was measured before and during aminophylline infusion (6 mg/kg over 30 minutes), during which therapeutic concentrations of theophylline were attained (mean 13.8 mg/l, range 8.5-20.2). The Pdi achieved was not affected by aminophylline. This result suggests that theophylline at therapeutic concentrations has little effect on the contractility of the normal human diaphragm.

Electromyogram, force and relaxation time during and after continuous electrical stimulation of human skeletal muscle in situ

A technique for recording the surface electromyogram (e.m.g.) simultaneously with electrical stimulation of human skeletal muscle is described. During a fatiguing electrical stimulation the e.m.g. amplitude and force decreased in the same proportion. During recovery the e.m.g. was quickly normalized whereas force remained at a reduced level. Relaxation time increased during the stimulation and helps substantially in keeping up tension during a fatiguing contraction. In the recovery phase the rapid normalization of relaxation time counteracts recovery of tension. It is concluded that e.m.g. measurements alone can be misleading as an index of contraction force and that muscular fatigue during electrical stimulation can be attributed to excitation failure only to a lesser extent.