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Cation pumps in skeletal muscle: potential role in muscle fatigue.

Two membrane bound pumps in skeletal muscle, the sarcolemma Na+-K+ adenosine triphosphatase (ATPase) and the sarcoplasmic reticulum Ca2+-ATPase, provide for the maintenance of transmembrane ionic gradients necessary for excitation and activation of the myofibrillar apparatus. The rate at which the pumps are capable of establishing ionic homeostasis depends on the maximal activity of the enzyme and the potential of the metabolic pathways for supplying adenosine triphosphate (ATP). The activity of the Ca2+-ATPase appears to be expressed in a fibre type specific manner with both the amount of the enzyme and the isoform type related to the speed of contraction. In contrast, only minimal differences exist between slow-twitch and fast-twitch fibres in Na+-K+ ATPase activity. Evidence is accumulating that both active transport of Na+ and K+ across the sarcolemma and Ca2+-uptake by the sarcoplasmic reticulum may be impaired in vivo in a task specific manner resulting in loss of contractile function. In contrast to the Ca2+-ATPase, the Na+-K+ ATPase can be rapidly upregulated soon after the onset of a sustained pattern of activity. Similar programmes of activity result in a downregulation of Ca2+-ATPase but at a much later time point. The manner in which the metabolic pathways reorganize following chronic activity to meet the changes in ATP demand by the cation pumps and the degree to which these adaptations are compartmentalized is uncertain.

Animals↗

Quantitation of progressive muscle fatigue during dynamic leg exercise in humans.

There is virtually no published information on muscle fatigue, defined as a gradual decline in force-generating capacity, during conventional dynamic (D) leg exercise. To quantitate progression of fatigue, we developed 1) a model featuring integration of maximal voluntary static contraction (MVC) of knee extension (KE) muscles with ongoing DKE and 2) a device that allows frequent rapid transfer between DKE isolated to the quadriceps femoris muscles and measurement of KE MVC. Eight healthy men performed graded and submaximal constant work rate one-leg DKE to exhaustion while seated. Work rate, a product of a contraction rate (1 Hz), force measured at the ankle, and distance of ankle movement from 90 degrees to 150 degrees of KE, was precisely controlled. Lack of rise in myoelectric activity in biceps femoris of the active leg during DKE and MVC was consistent with restriction of muscle action to quadriceps femoris. The slope of the linear relationship between O2 uptake and work rate was 13.7 ml O2/W (r = 0.93). This slope and the increase of heart rate relative to increasing work intensity agreed with published values for D leg exercise. Test-retest values for O2 uptake were similar (P > 0.05) for matched DKE work rates. To track fatigue, MVC (90 degrees knee angle) was performed every 2 min of DKE. After 4 min of DKE at work rates corresponding to (mean +/- SE) 66 +/- 2, 78 +/- 2, and 100% of peak DKE O2 uptake, MVC fell to 95 +/- 3, 90 +/- 5, and 65 +/- 7%* of MVC of rested muscle, respectively (*P < 0.01 from previous work rates). Virtually identical declines in MVC were observed by the end of graded work rate DKE and submaximal constant work rate DKE tests. Quantitation of progressive muscle fatigue during D leg exercise provides a framework to study the effects of a variety of interventions on the fatigue process and may permit unique insights into the involved mechanisms.

Adolescent↗

Posture, muscle activity and muscle fatigue in prolonged VDT work at different screen height settings.

With the increasing use of video display terminals (VDTs), there is growing concern over the corresponding increase in the number of health problems reported. Although much research has focused on identifying the optimal screen height, there is to date no consensus. This study aimed to investigate the effect of prolonged (89 min) VDT work at four different screen heights on head-neck posture, muscle activity and the development of muscle fatigue. The results show that lowering screen height, starting from 15 cm above the baseline (i.e. top of the screen level with eye height while sitting), decreased the ear-eye angle, increased the viewing angle, increased the viewing angle relative to the ear-eye line, and increased the muscle activity of the neck extensor muscles. There were also some significant time effects on postural angles and muscle activity. In this study there were only rare occurrences of muscle fatigue, defined as a simultaneous increase in EMG amplitude and a shift of the EMG power spectrum to lower frequencies. Muscle activity increased significantly in some muscles and for certain screen heights.

Adult↗

Masseter muscle fatigue in man objectively quantified by analysis of myoelectric signals.

Localized muscle fatigue in the masseter muscle was studied with a method based on power spectrum analysis of myoelectric signals. Under the influence of fatiguing contractions, a gradual shift of the spectral curve occurred; the rate of change was taken as a measure of the development of fatigue. The fatigue was dependent on the bite force. The existence of a threshold value of force, below which significant myoelectric fatigue changes do not develop, was shown.

Adult↗

The role of O2 supply in muscle fatigue.

It is well established that altering O2 delivery to contracting skeletal muscle affects human performance. In this respect, a reduced O2 supply (e.g., hypoxia) increases the rate of muscle fatigue, whereas increasing O2 supply (e.g., hyperoxia) reduces the rate of fatigue. Interestingly, the faster onset of fatigue in moderate hypoxia does not appear to be a consequence of mitochondrial O2 limitation because these effects occur at submaximal rates of O2 consumption for these conditions and at O2 tensions well above that which impairs mitochondrial O2 uptake in vitro. Alterations in O2 supply modulate the regulation of cellular respiration and may affect the onset of impaired Ca2+ handling with fatigue. Specifically, changes in O2 supply alter the coupling between phosphocreatine hydrolysis and O2 uptake in contracting muscles, which by determining the rate of inorganic phosphate (Pi) accumulation may affect Ca2+ release. Partial ischemia differs somewhat in that the reduction in force could be due to reduced O2 supply and/or impaired removal of metabolic by-products secondary to insufficient blood flow. Nonetheless, recent evidence shows a parallel decline and restoration of force with alterations in O2 supply but not blood flow alone during submaximal contractions. Furthermore, the causes of fatigue are similar when O2 is plentiful and when it is reduced.

Humans↗

Upper and lower extremity muscle fatigue after a baseball pitching performance.

BACKGROUND: Previous studies have estimated joint torques and electromyogram activity associated with the pitching motion. Although previous studies have investigated the influence of extended pitching (fatigue) on kinematic and kinetic parameters, no attempts have been made to quantify the fatigue associated with a pitching performance. PURPOSE: Considering previous investigations on muscle activity during pitching, this study investigated muscle fatigue in upper and lower extremity muscle groups after a pitching performance. STUDY DESIGN: Descriptive laboratory study. METHODS: Thirteen baseball pitchers from 4 universities and 1 independent minor league team were tested before and after 19 games. Pitchers threw an average of 99 pitches during an average of 7 innings. Shoulder, scapular, and lower extremity muscle strengths were assessed using a handheld dynamometer before and after the pitching performances. RESULTS: Baseline strength tests revealed that the pitching arm was 12% weaker (P=.02) in the empty can test (supraspinatus) compared to the contralateral side. Postgame shoulder strength tests revealed selective fatigue of 15% in shoulder flexion (P=.02), 18% fatigue in internal rotation (P=.03), and 11% fatigue in shoulder adduction (P = .01). Minimal fatigue was noted in the empty can test, scapular stabilizers, and hip musculature. CONCLUSIONS: A trend toward significant baseline strength in internal rotation together with significant selective postgame fatigue on internal rotation of the dominant upper extremity indicate that the internal rotators experience a high performance demand during pitching. Weakness in the empty can test on the dominant arm combined with minimal postgame fatigue was surprising given that studies and injury patterns have indicated a high performance demand on the supraspinatus during pitching.

Adult↗

Reversal of muscle fatigue in intact rabbits by intravenous potassium chloride.

PURPOSE: Skeletal muscle fatigue has been associated with potassium efflux from the myocytes, resulting in endogenous increases in blood potassium concentration ([K+]). Conversely, exogenous increases in extracellular [K+] potentiates contraction in isolated muscle preparations. The mechanisms responsible for these contradictory effects of [K+] on skeletal muscle function are unknown. Moreover, little is known about the effect of exogenous increases in [K+] on force generation by intact animals, given potassium's deleterious effect on cardiac function. METHODS: We compared the response to exogenous increases in blood [K+] in rabbits given an infusion of potassium chloride (KCl) intravenously (IV) (0.2 mol/L; KCl group; n = 7) to a group given 0.9% sodium chloride (NaCl) (control; n = 7). The rabbits underwent low-frequency, isometric twitch stimulation of the left hindlimb (square wave pulses 100 microseconds, 40V, 0.25 Hz) throughout the experiment. Both groups received 0.9% NaCl (25 mL/h) during the first hour of twitch stimulation and experienced similar decreases in hindlimb forces to 70% of initial force. A continuous infusion of KCl or of saline (60 mL/h) was started, and hindlimb stimulation continued for 2 hours. RESULTS: There were no changes in [K+] in the control group, and twitch forces progressively declined during the next 2 hours (369 +/- 47 g to 279 +/- 34 g, P < .01). Arterial [K+] increased in the KCl group from 2.6 +/- 0.1 to 10.1 +/- 0.5 mmol/L (P < .01), and hindlimb twitch forces almost doubled (418 +/- 49 g to 756 +/- 55 g, P < .01). Force frequency curves showed improved contractility in the KCl group at stimulation frequencies below 30 Hz. CONCLUSIONS: Exogenous increases in blood [K+] potentiate skeletal muscle contraction in intact animals and reverse low-frequency twitch fatigue. A possible mechanism may be the maintenance of intracellular [K+] by hindering K+ efflux from skeletal muscle cells.

Animals↗

Levocarnitine administration in elderly subjects with rapid muscle fatigue: effect on body composition, lipid profile and fatigue.

AIM: Levocarnitine is an important contributor to cellular energy metabolism. This study aims to evaluate the effects of levocarnitine supplementation on body composition, lipid profile and fatigue in elderly subjects with rapid muscle fatigue. METHOD: This was a placebo-controlled, randomised, double-blind, two-phase study. Eighty-four elderly subjects with onset of fatigue following slight physical activity were recruited to the study. Prior to randomisation all patients entered a 2-week normalisation phase where they were given an 'ad libitum'diet, according to the National Cholesterol Education Program (Step 2). Subjects were asked to record their daily food intake every 2 days. Before the 30-day treatment phase, subjects were randomly assigned to two groups (matched for male/female ratio, age and body mass index). One group received levocarnitine 2g twice daily (n = 42) and the other placebo (n = 42). Efficacy measures included changes in total fat mass, total muscle mass, serum triglyceride, total cholesterol, high-density lipoprotein-cholesterol (HDL-C), low-density lipoprotein-cholesterol (LDL-C), apolipoprotein (apo)A1, and apoB levels. The Wessely and Powell scale was used to evaluate physical and mental fatigue. Subjects were assessed at the beginning and end of the study period. RESULTS: At the end of the study, compared with placebo, the levocarnitine-treated patients showed significant improvements in the following parameters: total fat mass (-3.1 vs -0.5 kg), total muscle mass (+2.1 vs +0.2 kg), total cholesterol (-1.2 vs +0.1 mmol/L), LDL-C (-1.1 vs -0.2 mmol/L), HDL-C (+0.2 vs +0.01 mmol/L), triglycerides (-0.3 vs 0.0 mmol/L), apoA1 (-0.2 vs 0.0 g/L), and apoB (-0.3 vs -0.1 g/L). Wessely and Powell scores decreased significantly by 40% (physical fatigue) and 45% (mental fatigue) in subjects taking levocarnitine, compared with 11% and 8%, respectively, in the placebo group (p < 0.001 vs placebo for both parameters). No adverse events were reported in any treatment group. CONCLUSION: Administration of levocarnitine to healthy elderly subjects resulted in a reduction of total fat mass, an increase of total muscle mass, and appeared to exert a favourable effect on fatigue and serum lipids.

Aged↗

Effects of activation frequency on dynamic performance of human fresh and fatigued muscles.

The force-frequency relationship for an individual muscle depends on the fatigue state, the length at which it is activated, and the muscle's activation history. The relationship among stimulation frequency and dynamic (nonisometric) muscle performance measurements (e.g., excursion, work, peak power, and average power) has not been reported. The purpose of this study was to identify the relationship between stimulation frequency and dynamic performance measurements for fresh and fatigued muscles. Constant-frequency and catchlike-inducing trains (CFT and CIT, respectively) were tested. When fresh, interpulse intervals of 40-50 ms [20-25 pulses/s (pps)] produced maximum performance for CFTs. For CITs, maximum performance occurred at interpulse intervals of 50-60 ms ( approximately 16-20 pps). Generally, CFTs produced slightly greater performance than did CITs. When fatigued, however, CITs produced greater performance than did CFTs. Maximum performance for CFTs occurred at interpulse intervals of 20-40 ms (25-50 pps) and at 30-50 ms (20-33 pps) for CITs. Enhancement of performance by CITs when fatigued may be due to less susceptibility to impairments in excitation-contraction coupling and greater ability to maintain rates of rise of force than CFTs.

Adolescent↗

Effect of a selected amino acid mixture on the recovery from muscle fatigue during and after eccentric contraction exercise training.

The effect of an amino acid mixture on the recovery from muscle fatigue after eccentric exercise (ECEX) training was examined in twenty-two male college students. The administration of 5.6 g of the amino acid mixture twice daily resulted faster recovery of muscle strength than that with a placebo. The oral administration of the amino acid mixture was proved to effective for muscle strength recovery after the eccentric exercise.

Administration, Oral↗

Assessment of muscle fatigue during biking.

The analysis of the surface myoelectric signal recorded while a muscle is performing a sustained contraction is a valuable tool for assessing the progression of localized fatigue. It is well known that the modifications of the spectral content of the myoelectric signal are mainly related to changes in the interstitial fluid pH, which, in turn, affect the membrane excitability of the active muscle fibers. This paper describes the effects of muscle fatigue on the surface myoelectric signal recorded from three thigh and leg muscles during biking, on a population consisting of 22 young healthy volunteers. The purpose of this study was to obtain normative data relative to an exercise protocol mild enough to be applicable, in the future, to pathological subjects as well. Each subject was asked to exercise 30 min on a cycloergometer at a constant velocity and against a constant torque. While subjects were biking, the surface myoelectric signal was recorded from the rectus femoris, the biceps femoris, and the gastrocnemius muscles. In this study, we considered two different aspects of muscle fatigue: first, the localized muscle fatigue as shown by the decrement of the instantaneous frequency of the myoelectric signal during the exercise; second, the modifications of the muscle ON-OFF timing, which could be explained as a strategy for increasing endurance by modifying the role of different muscles during the exercise. The first aspect was studied by obtaining the spectral characteristics of the signals by means of bilinear time-frequency transforms and by applying an original estimator of the instantaneous frequency of stochastic processes based on cross time-frequency transforms. Our results demonstrated that none of the subjects showed significant signs of localized muscle fatigue, since the decrement of the instantaneous frequency during the exercise was always lower than 5% of its initial value. Muscle ON-OFF timing was obtained by applying to the raw myoelectric signal a double threshold statistical detector to identify the time intervals during which the observed muscles were active. This demonstrated that the subjective feeling of fatigue each subject reported during the exercise did not cause a change of the activation strategy of the observed muscles. It is concluded that the experimental protocol herein described and the signal processing procedures adopted are appropriate for monitoring different effects of muscle fatigue during biking. Moreover, data obtained from our sample population can be considered as a reference for studying the manifestations of muscle fatigue in pathological subjects asked to follow a similar experimental protocol.

Adaptation, Physiological↗

Energy supply and muscle fatigue in humans.

Limitations in energy supply is a classical hypothesis of muscle fatigue. The present paper reviews the evidence available from human studies that energy deficiency is an important factor in fatigue. The maximal rate of energy expenditure determined in skinned fibres is close to the rate of adenosine triphosphate (ATP) utilisation observed in vivo and data suggest that performance during short bursts of exercise (<5 s duration) primarily is limited by other factors than energy supply (e.g. Vmax of myosine adenosine triphosphatase (ATPase), motor unit recruitment, engaged muscle mass). Within 10 s of exercise maximal power output decreases considerably and coincides with depletion of phosphocreatine. During recovery, maximal force and power output is restored with a similar time course as the resynthesis of phosphocreatine. Increases in muscle store of phosphocreatine through dietary supplementation with creatine increases performance during high-intensity exercise. These findings support the hypothesis that energy supply limits performance during high-intensity exercise. It is well documented that pre-exercise muscle glycogen content is related to performance during moderate intensity exercise. Recent data indicates that the interfibre variation in phosphocreatine is large after prolonged exercise to fatigue and that some fibres are depleted to the same extent as after high-intensity exercise. Despite relatively small decreases in ATP, the products of ATP hydrolysis (Pi and free ADP) may increase considerably. Free ADP calculated from the creatine kinase reaction increases 10-fold both after high-intensity exercise and after prolonged exercise to fatigue. It is suggested that local increases in ADP may reach inhibitory levels for the contraction process.

Energy Metabolism↗

Changes in fusimotor discharge rate provoked by isotonic fatiguing muscle contractions in decerebrate cats.

Changes in discharge rate of 26 fusimotor neurones to medial gastrocnemius muscle were studied during isotonic fatiguing contractions of lateral gastrocnemius and soleus muscles in decerebrate cats. Muscle contractions were elicited by either continuous or repetitive electrical stimulation of the muscle nerves. The muscles were considered to be fatigued when, against a load equal to one third of the tension developed at the onset of an isometric contraction, i.e. to the tension indicating isometrically-induced fatigue, they returned to the length at which the isometric contraction was elicited. At the onset of muscle contraction an increase in discharge rate, lasting for 5-220 s, occurred in all except one of the neurones. In 73% of the units a late increase developed in addition in parallel with muscle fatigue outlasting the contraction for 5-180 s. All but one of the remaining neurones exhibited a short lasting burst of spike discharges coincident with the end of contraction. Enhancement of the late increase by muscle ischaemia indicates contribution of chemosensitive small-diameter muscle afferents, while the short lasting burst is supposed to be elicited rather by the mechanosensitive units sensitized by metabolic products liberated during contraction and/or fatigue. Differences of the fusimotor reflex responses to isotonic vs. isometric contraction and/or fatigue and their possible functional role are discussed.

Animals↗

Historical perspective: a framework for interpreting pathobiological ideas on human muscle fatigue.

The flow of ideas on the causes of human muscle fatigue appear to have been established in the literature during the last century. Critical analysis had to await innovative experimental designs or techniques. Progress has come particularly from the recognition of inconsistencies, particularly in clinical conditions in which there are alterations in the supply of energy or contractile function. While there is a continuing search for a unique cause of fatigue, much evidence points to there being different causes according to the type of muscular activity or clinical condition. "Nature's experiments" (patients exhibiting isolated defects of function or metabolism) offer unique opportunities for understanding the relative importance of particular levels of metabolic organization or physiological control. Theories of limiting biochemical processes and the Ca- kinetic basis of electromechanical coupling defects are both essentially "single-cell" models of fatigue. Functional requirements appear to determine the diversity of structure and organization of motor units. This would suggest that a "muscle cell population" approach would take into account the consequences of disease altering the number, type of functioning fibers or intrinsic strength of individual fibers. A graphical model is offered to allow a possible interpretation of the cause of fatigue in different forms of exercise and clinical conditions.

History, 18th Century↗

[Muscle fatigue and dyspnea].

Calm breathing requires little effort on the part of the inspiratory muscles to overcome the elastic and non-elastic resistances of the chest-lung system. Spontaneous expiration is passive and does not engage the respiratory muscles. In the event of restriction or obstruction, however, there are changes in these two types of resistance and their distribution. Extra work is required and the blood flow through the muscles has a decisive influence on their performance. Histologically detectable differences in the number of fast or slow, glycolytic or oxidative fibres in the respiratory muscles correspond to functional differences. Some muscles are required to supply the force or energy of contraction, others are concerned with endurance and others with intermediate situations. The diaphragm is mainly composed of slow-oxidative fibres. It is thus characterised by its strength, low tension, high repetitive capacity and incredible endurance. This brief review describes the main concepts of the physiology of the respiratory muscles. Reference is also made to the mechanisms of muscle fatigue and dyspnea, two independent or associated clinical situations that point to the risk of progression to respiratory insufficiency.

Dyspnea↗

Muscle fatigue and endurance during repetitive intermittent static efforts: development of prediction models.

Localized muscle fatigue has received growing attention as a potential design variable and exposure metric in research towards prevention of musculoskeletal disorders in the workplace. While fatigue during sustained static work has been investigated extensively, effects during tasks comprising work-rest cycles are less clear. Work-rest models for static intermittent work have been presented in several reports, but the applicability is often limited to specific conditions. A study was conducted that facilitated a description of the relationships between static intermittent efforts and muscle endurance and fatigue. Exercises consisted of 1 h (maximum) of repetitive static arm abductions, involving a range of muscle contraction levels (10-30% maximum voluntary exertion), duty cycles (0.2-0.8) and cycle times (20-180 s). A between-subject central composite experimental design was used and 15 different exercise conditions were examined with six participants (three females and three males) for each. Along with endurance times, temporal changes related to fatigue were monitored using muscle strength, ratings of discomfort and electromyography (EMG) obtained from the middle-deltoid muscle during the contraction phase of the work cycles. The results of this study showed the influence of contraction level and duty cycle on the majority of fatigue measures used, while cycle time tended to affect EMG spectral measures. Using a response surface methodology, several fatigue prediction models and contour plots were developed that can be employed as an aid for design and evaluation of light repetitive static tasks. Good correspondence was generally found between discomfort rating and other measures of fatigue, suggesting the usefulness of this measure for rapid assessments of local fatigue in the workplace.

Adult↗

Ischemia causes muscle fatigue.

The purpose of this investigation was to determine whether ischemia, which reduces oxygenation in the extensor carpi radialis (ECR) muscle, causes a reduction in muscle force production. In eight subjects, muscle oxygenation (TO2) of the right ECR was measured noninvasively and continuously using near infrared spectroscopy (NIRS) while muscle twitch force was elicited by transcutaneous electrical stimulation (1 Hz, 0.1 ms). Baseline measurements of blood volume, muscle oxygenation and twitch force were recorded continuously, then a tourniquet on the upper arm was inflated to one of five different pressure levels: 20, 40, 60 mm Hg (randomized order) and diastolic (69 +/- 9.8 mm Hg) and systolic (106 +/- 12.8 mm Hg) blood pressures. Each pressure level was maintained for 3-5 min, and was followed by a recovery period sufficient to allow measurements to return to baseline. For each respective tourniquet pressure level, mean TO2 decreased from resting baseline (100% TO2) to 99 +/- 1.2% (SEM), 96 +/- 1.9%, 93 +/- 2.8%, 90 +/- 2.5%, and 86 +/- 2.7%, and mean twitch force decreased from resting baseline (100% force) to 99 +/- 0.7% (SEM), 96 +/- 2.7%, 93 +/- 3.1%, 88 +/- 3.2%, and 86 +/- 2.6%. Muscle oxygenation and twitch force at 60 mm Hg tourniquet compression and above were significantly lower (P < 0.05) than baseline value. Reduced twitch force was correlated in a dose-dependent manner with reduced muscle oxygenation (r = 0.78, P < 0.001). Although the correlation does not prove causation, the results indicate that ischemia leading to a 7% or greater reduction in muscle oxygenation causes decreased muscle force production in the forearm extensor muscle. Thus, ischemia associated with a modest decline in TO2 causes muscle fatigue.

Adult↗

The magnitude of the effect of calf muscles fatigue on postural control during bipedal quiet standing with vision depends on the eye-visual target distance.

The purpose of the present experiment was to investigate whether, with vision, the magnitude of the effect of calf muscles fatigue on postural control during bipedal quiet standing depends on the eye-visual target distance. Twelve young university students were asked to stand upright as immobile as possible in three visual conditions (No vision, Vision 1m and Vision 4m) executed in two conditions of No fatigue and Fatigue of the calf muscles. Centre of foot pressure displacements were recorded using a force platform. Similar increased variances of the centre of foot pressure displacements were observed in the fatigue relative to the No fatigue condition for both the No vision and Vision 4m conditions. Interestingly, in the vision 1m condition, fatigue yielded: (1) a similar increased variance of the centre of foot pressure displacements to those observed in the No vision and Vision 4m conditions along the medio-lateral axis and (2) a weaker destabilising effect relative to the No vision and Vision 4m conditions along the antero-posterior axis. These results evidence that the ability to use visual information for postural control during bipedal quiet standing following calf muscles fatigue is dependent on the eye-visual target distance. More largely, in the context of the multisensory control of balance, the present findings suggest that the efficiency of the sensory reweighting of visual sensory cues as the neuro-muscular constraints acting on the subject change is critically linked with the quality of the information the visual system obtains.

Adult↗