Metabolic factors contributing to altered Ca2+ regulation in skeletal muscle fatigue

Competing effects of activation history on force and cytosolic Ca2+ in intact single mice myofibers

The purpose was to investigate the changes in cytosolic Ca2+ and force output during post-tetanic potentiation (PTP) during pre-fatigue and during prolonged low-frequency force depression (PLFFD) following fatigue. Intact single myofibers from the flexor digitorum brevis of mice were electrically stimulated to record force (n = 8) and free cytosolic Ca2+ concentration ([Ca2+]c) with FURA-2 (n = 6) at 32 °C. Initially, force and [Ca2+]c were measured during brief (350 ms) trains of stimuli at 30, 50, 70, and 200 Hz at ~ 2 s intervals (Force-frequency protocol, FFP). Then, a conditioning stimulus (CS) of six 120 Hz stimuli, separated by ~ 3 s, was used to induce PTP, immediately followed by an FFP. Myofiber fatigue was produced by 150 Hz trains every 3 s until peak force decayed 70% of the initial. Thirty minutes after the fatigue, the CS was repeated to assess the effect of PTP on force and [Ca2+]c during PLFFD. The CS in unfatigued myofibers induced PTP as the submaximal force was enhanced and accompanied by increased peak [Ca2+]c with no change in myofilament Ca2+ sensitivity. After fatigue, PLFFD was due to lowered peak [Ca2+]c. Inducing PTP during PLFFD enhanced submaximal force primarily through greater peak [Ca2+]c, mitigating the submaximal force deficits. Despite the impaired force during PLFFD, myofibers remained sensitive to PTP, and this mitigated the submaximal force deficits through increased peak [Ca2+]c without a change in myofilament Ca2+ sensitivity. Therefore, force adjustments of intact single myofibers due to activation history are principally accomplished by opposing adjustments in [Ca2+]c.

Quantifying Active and Passive Stiffness in Plantar Flexor Muscles Following Intermittent Maximal Isometric Contractions Using Shear Wave Elastography

OBJECTIVE

This study aimed: (i) to investigate the impact of fatigue, triggered by maximal isometric contraction exercises, on the active and passive stiffness of plantar flexors (PF), and (ii) to examine the relationship between changes in mechanical parameters and neuromuscular alterations after fatigue.

METHODS

A healthy cohort (n = 12; age = 27.3 ± 5.5 y; BMI = 24.4 ± 2.35 kg/m²) was instructed to perform 60 isometric contractions, each lasting 4 s with a 1-s rest interval, using an ergometer. Several measures were taken before and after the fatigue protocol. First, the stiffness of the PF-tendon complex (PFC) was quantified during passive ankle mobilization both during and after the fatigue protocol using the ergometer. Additionally, from shear wave elastography, the active and passive stiffness of the gastrocnemius medialis (GM) were measured during passive ankle mobilization and isometric maximal voluntary contraction (MVC), respectively. Finally, the peak torque and the rate of torque development (RFD) of PF were assessed during the MVC using the ergometer. Ankle muscle activities (surface electromyograph [SEMG]) were recorded during all evaluations using electromyography.

RESULTS

After the fatigue protocol, the results revealed a decline in active stiffness, peak torque of PF, RFD and SEMG activity of the GM (p < 0.001). Furthermore, significant correlation was identified between the decrease of the peak torque of PF and the active stiffness of the GM (r = 0.6; p < 0.05). A decrease in the PFC stiffness (p < 0.001) and a decrease in the shear modulus of the GM at 20° (p < 0.001) were also observed.

CONCLUSION

Isometric fatiguing exercises modify the mechanical properties of both the contractile and elastic components. Notably, decreases in both passive and active stiffness may be critical for athletes, as these changes could potentially increase the risk of injury.

New insights into the cellular and molecular mechanisms of skeletal muscle fatigue: the Marion J. Siegman Award Lectureships

Skeletal muscle fibers need to have mechanisms to decrease energy consumption during intense physical exercise to avoid devastatingly low ATP levels, with the formation of rigor cross bridges and defective ion pumping. These protective mechanisms inevitably lead to declining contractile function in response to intense exercise, characterizing fatigue. Through our work, we have gained insights into cellular and molecular mechanisms underlying the decline in contractile function during acute fatigue. Key mechanistic insights have been gained from studies performed on intact and skinned single muscle fibers and more recently from studies performed and single myosin molecules. Studies on intact single fibers revealed several mechanisms of impaired sarcoplasmic reticulum Ca2+ release and experiments on single myosin molecules provide direct evidence of how putative agents of fatigue impact myosin's ability to generate force and motion. We conclude that changes in metabolites due to an increased dependency on anaerobic metabolism (e.g., accumulation of inorganic phosphate ions and H+) act to directly and indirectly (via decreased Ca2+ activation) inhibit myosin's force and motion-generating capacity. These insights into the acute mechanisms of fatigue may help improve endurance training strategies and reveal potential targets for therapies to attenuate fatigue in chronic diseases.

Physiology of intracellular calcium buffering

Calcium signaling underlies much of physiology. Almost all the Ca2+ in the cytoplasm is bound to buffers, with typically only ∼1% being freely ionized at resting levels in most cells. Physiological Ca2+ buffers include small molecules and proteins, and experimentally Ca2+ indicators will also buffer calcium. The chemistry of interactions between Ca2+ and buffers determines the extent and speed of Ca2+ binding. The physiological effects of Ca2+ buffers are determined by the kinetics with which they bind Ca2+ and their mobility within the cell. The degree of buffering depends on factors such as the affinity for Ca2+, the Ca2+ concentration, and whether Ca2+ ions bind cooperatively. Buffering affects both the amplitude and time course of cytoplasmic Ca2+ signals as well as changes of Ca2+ concentration in organelles. It can also facilitate Ca2+ diffusion inside the cell. Ca2+ buffering affects synaptic transmission, muscle contraction, Ca2+ transport across epithelia, and the killing of bacteria. Saturation of buffers leads to synaptic facilitation and tetanic contraction in skeletal muscle and may play a role in inotropy in the heart. This review focuses on the link between buffer chemistry and function and how Ca2+ buffering affects normal physiology and the consequences of changes in disease. As well as summarizing what is known, we point out the many areas where further work is required.

Neuromuscular Fatigue and Metabolic Stress during the 15 Minutes of Rest after Carrying Out a Bench Press Exercise Protocol

Background: Velocity loss (VL) at 1 m·s−1 can help to determine neuromuscular fatigue after performing an exercise protocol. The aim of this study was to analyse muscle fatigue and metabolic stress during the 15 min that follow the execution of a bench press (BP) exercise protocol. Methods: Forty-four healthy male students of sports science performed two exercise sessions separated by one week of rest. In the first week, the participants carried out a test with progressive loads in the (BP) exercise until reaching the one-repetition maximum (1RM) in order to obtain the load−velocity relationship of each participant. In the second week, each participant conducted three BP exercise sets at an intensity of 70% of 1RM, determining this load through the mean propulsive velocity (MPV) obtained from the individual load−velocity relationship, with the participants performing the maximum number of repetitions (MNR) to muscle failure. Two minutes of rest were allocated between sets. MPV at 1 m·s−1 and blood lactate concentrations were recorded before executing the exercise and at minute 0, 5, 10 and 15 after performing the exercise protocol. Results: A two-factor repeated measures ANOVA was performed. MPV at 1 m·s−1 in minute 0 post-exercise was −33.3% (p < 0.05), whereas in minute 10 and 15 post-exercise, it was ≈−9% (p > 0.05). Regarding the blood lactate levels, significant differences were observed in all measurements before and after the exercise protocol (p < 0.001), obtaining ≈7 mmol·L−1 at minute 10 post-exercise and 4.3 mmol·L−1 after 15 min of recovery. Conclusions: MPV with medium or moderate loads shows a certain recovery from minute 10 of rest. However, the blood lactate levels are still high (>5 mmol·L−1). Therefore, although there seem to be certain conditions to reach a similar maximum MPV peak, the residual fatigue at the neuromuscular level and the non-recovery of metabolic homeostasis would hinder the reproduction of these protocols, both at the level of applied stimulus and from a methodological perspective, since a long recovery time would be required between sets and exercises.

Physiological Responses at 15 Minutes of Recovery after a Session of Functional Fitness Training in Well-Trained Athletes

Background: the aim of this study was to analyse muscle fatigue and metabolic stress at 15 min of recovery after performing two independent sessions of functional fitness training (FFT): a session of strength functional fitness training (FFTstrength) and a session of endurance functional fitness training (FFTendurance). Methods: eighteen well-trained men conducted two protocols, separated by one week of rest: FFTstrength (3 sets of 21, 15 and 9 repetitions of Thruster with bar + Pull ups) and FFTendurance (3 sets × (30 kcal rowing + 15 kcal assault air bike)). Neuromuscular fatigue and metabolic stress were measured right before, right after and at 10 and 15 min after completing the FFT workout, as well as the mean heart rate (HRmean) and the rating of perceived exertion (RPE) at the end of the FFT. Results: FFTendurance recovered the velocity loss values after 15 min of recovery. On the other hand, FFTstrength only recovered velocity in the 1 m·s⁻¹ Tests in squat (SQ), since the velocity levels were 7% lower in the 1 m·s⁻¹ Tests in military press exercise (MP) after 15 min. Conclusions: These data indicate that there are specific recovery patterns not only as a function of the exercise and the body regions involved, but also regarding the recovery of neuromuscular and metabolic factors, since both FFT workouts obtained high blood lactate concentrations.

A Single Dose of Oral ATP Supplementation Improves Performance and Physiological Response During Lower Body Resistance Exercise in Recreational Resistance-Trained Males

Freitas, MC, Cholewa, JM, Gerosa-Neto, J, Gonçalves, DC, Caperuto, EC, Lira, FS, and Rossi, FE. A single dose of oral ATP supplementation improves performance and physiological response during lower body resistance exercise in recreational resistance-trained males. J Strength Cond Res 33(12): 3345-3352, 2019-The aim of this study was to investigate the acute effect of adenosine-5'-triphosphate (ATP) supplementation on performance and physiological responses during resistance exercise in recreationally resistance-trained males. Eleven men (age = 27.5 ± 5.5 years, mass = 83.4 ± 9.8 kg, height = 182 ± 0.04 cm) completed 2 randomized, double-blind trials: ATP supplement condition (ATP = 400 mg) or a placebo condition. Thirty minutes after supplement consumption, subjects performed 4 sets of half-squats until momentary muscular failure at 80% of the 1 repetition maximum with 2 minutes of recovery between sets. The total number of repetitions, blood pressure, heart rate, blood lactate, and oxygen consumption were evaluated. The total weight lifted were higher for the ATP condition compared with placebo (Placebo = 3,995.7 ± 1,137.8, ATP = 4,967.4 ± 1,497.9 kg; p = 0.005). Heart rate was higher at set-4 for ATP compared with placebo (p < 0.001) and oxygen consumption during exercise was greater for ATP (p = 0.021). There were no differences between conditions for lactate and blood pressure. In summary, a single oral dose of ATP supplementation improved lower-body resistance training performance and energy expenditure in recreational resistance-trained males.

Molecular Basis for Exercise-Induced Fatigue: The Importance of Strictly Controlled Cellular Ca2+ Handling

The contractile function of skeletal muscle declines during intense or prolonged physical exercise, that is, fatigue develops. Skeletal muscle fibers fatigue acutely during highly intense exercise when they have to rely on anaerobic metabolism. Early stages of fatigue involve impaired myofibrillar function, whereas decreased Ca²⁺ release from the sarcoplasmic reticulum (SR) becomes more important in later stages. SR Ca²⁺ release can also become reduced with more prolonged, lower intensity exercise, and it is then related to glycogen depletion. Increased reactive oxygen/nitrogen species can cause long-lasting impairments in SR Ca²⁺ release resulting in a prolonged force depression after exercise. In this article, we discuss molecular and cellular mechanisms of the above fatigue-induced changes, with special focus on multiple mechanisms to decrease SR Ca²⁺ release to avoid energy depletion and preserve muscle fiber integrity. We also discuss fatigue-related effects of exercise-induced Ca²⁺ fluxes over the sarcolemma and between the cytoplasm and mitochondria.

The Sick and the Weak: Neuropathies/Myopathies in the Critically Ill

Critical illness polyneuropathies (CIP) and myopathies (CIM) are common complications of critical illness. Several weakness syndromes are summarized under the term intensive care unit-acquired weakness (ICUAW). We propose a classification of different ICUAW forms (CIM, CIP, sepsis-induced, steroid-denervation myopathy) and pathophysiological mechanisms from clinical and animal model data. Triggers include sepsis, mechanical ventilation, muscle unloading, steroid treatment, or denervation. Some ICUAW forms require stringent diagnostic features; CIM is marked by membrane hypoexcitability, severe atrophy, preferential myosin loss, ultrastructural alterations, and inadequate autophagy activation while myopathies in pure sepsis do not reproduce marked myosin loss. Reduced membrane excitability results from depolarization and ion channel dysfunction. Mitochondrial dysfunction contributes to energy-dependent processes. Ubiquitin proteasome and calpain activation trigger muscle proteolysis and atrophy while protein synthesis is impaired. Myosin loss is more pronounced than actin loss in CIM. Protein quality control is altered by inadequate autophagy. Ca(2+) dysregulation is present through altered Ca(2+) homeostasis. We highlight clinical hallmarks, trigger factors, and potential mechanisms from human studies and animal models that allow separation of risk factors that may trigger distinct mechanisms contributing to weakness. During critical illness, altered inflammatory (cytokines) and metabolic pathways deteriorate muscle function. ICUAW prevention/treatment is limited, e.g., tight glycemic control, delaying nutrition, and early mobilization. Future challenges include identification of primary/secondary events during the time course of critical illness, the interplay between membrane excitability, bioenergetic failure and differential proteolysis, and finding new therapeutic targets by help of tailored animal models.

In vivo calcium regulation in diabetic skeletal muscle

In skeletal muscle, dysfunctional contractile activity has been linked to impaired intracellular Ca(2+) concentration ([Ca(2+)]i) regulation. Muscle force production is impaired and fatigability and muscle fragility deteriorate with diabetes. Use of a novel in vivo model permits investigation of [Ca(2+)]i homeostasis in diabetic skeletal muscle. Within this in vivo environment we have shown that diabetes perturbs the Ca(2+) regulatory system such that resting [Ca(2+)]i homeostasis following muscle contractions is compromised and elevations of [Ca(2+)]i are exacerbated. This review considers the impact of diabetes on the capacity of skeletal muscle to regulate [Ca(2+)]i, following muscle contractions and, in particular, the relationship between muscle fatigue and elevated [Ca(2+)]i in a highly ecologically relevant circulation-intact environment. Importantly, the role of mitochondria in calcium sequestration and the possibility that diabetes impacts this process is explored. Given the profound microcirculatory dysfunction in diabetes this preparation offers the unique opportunity to study the interrelationships among microvascular function, blood-myocyte oxygen flux and [Ca(2+)]i as they relate to enhanced muscle fatigability and exercise intolerance.

Neuromuscular fatigue is not different between constant and variable frequency stimulation

This study compared fatigue development of the triceps surae induced by two electrical stimulation protocols composed of constant and variable frequency trains (CFTs, VFTs, 450 trains, 30 Hz, 167 ms ON, 500 ms OFF and 146 ms ON, 500 ms OFF respectively). For the VFTs protocol a doublet (100 Hz) was used at the beginning of each train. The intensity used evoked 30% of a maximal voluntary contraction (MVC) and was defined using CFTs. Neuromuscular tests were performed before and after each protocol. Changes in excitation-contraction coupling were assessed by analysing the M-wave [at rest (M_max) and during MVC (M_sup)] and associated peak twitch (Pt). H-reflex [at rest (H_max) and during MVC (H_sup)] and the motor evoked potential (MEP) during MVC were studied to assess spinal and corticospinal excitability of the soleus muscle. MVC decrease was similar between the protocols (−8%, P<0.05). M_max, M_sup and Pt decreased after both protocols (P<0.01). H_max/M_max was decreased (P<0.05), whereas H_sup/M_sup and MEP/M_sup remained unchanged after both protocols. The results indicate that CFTs and VFTs gave rise to equivalent neuromuscular fatigue. This fatigue resulted from alterations taking place at the muscular level. The finding that cortical and spinal excitability remained unchanged during MVC indicates that spinal and/or supraspinal mechanisms were activated to compensate for the loss of spinal excitability at rest.

In vivo imaging of intracellular Ca2+ after muscle contractions and direct Ca2+ injection in rat skeletal muscle in diabetes

The effects of muscle contractions on the profile of postcontraction resting intracellular Ca2+ ([Ca2+]i) accumulation in Type 1 diabetes are unclear. We tested the hypothesis that, following repeated bouts of muscle contractions, the rise in resting [Ca2+]i evident in healthy rats would be increased in diabetic rats and that these changes would be associated with a decreased cytoplasmic Ca2+ -buffering capacity. Adult male Wistar rats were divided randomly into diabetic (DIA; streptozotocin, ip) and healthy control (CONT) groups. Four weeks later, animals were anesthetized and spinotrapezius muscle contractions (10 sets of 50 contractions) were elicited by electrical stimulation (100 Hz). Ca2+ imaging was achieved using Fura-2 AM in the spinotrapezius muscle in vivo (i.e., circulation intact). The ratio (340/380 nm) was determined from fluorescence images following each set of contractions for estimation of [Ca2+]i. Also, muscle Ca2+ buffering was studied in individual myocytes microinjected with 2 mM Ca2+ solution. After muscle contractions, resting [Ca2+]i in DIA increased earlier and more rapidly than in CONT (P < 0.05 vs. precontraction). Peak [Ca2+]i in response to the Ca2+ injection was significantly higher in CONT (25.8 ± 6.0% above baseline) than DIA (10.2 ± 1.1% above baseline). Subsequently, CONT [Ca(2+)]i decreased rapidly (<15 s) to plateau 9-10% above baseline, whereas DIA remained elevated throughout the 60-s measurement window. No differences in SERCA1 and SERCA2 (Ca2+ uptake) protein levels were evident between CONT and DIA, whereas ryanodine receptor (Ca2+ release) protein level and mitochondrial oxidative enzyme activity (succinate dehydrogenase) were decreased in DIA (P < 0.05). In conclusion, diabetes impairs resting [Ca2+]i homeostasis following muscle contractions. Markedly different responses to Ca2+ injection in DIA vs. CONT suggest fundamentally deranged Ca2+ handling.

Effects of taurine supplementation following eccentric exercise in young adults

The purpose of the present study was to investigate the effects of taurine supplementation on muscle performance, oxidative stress, and inflammation response after eccentric exercise (EE) in males. Twenty-one participants (mean age, 21 ± 6 years; weight, 78.2 ± 5 kg; height, 176 ± 7 cm) were selected and randomly divided into two groups: placebo (n = 10) and taurine (n = 11). Fourteen days after starting supplementation, subjects performed EE (3 sets until exhaustion, with EE of the elbow flexors on the Scott bench, 80% 1 repetition maximum (RM)). Blood samples were collected and muscle performance was measured on days 1, 14, 16, 18, and 21 after starting the supplements. Then, performance, muscle damage, oxidative stress, and inflammatory markers were analyzed. The taurine supplementation resulted in increased strength levels and thiol total content and decreased muscle soreness, lactate dehydrogenase level, creatine kinase activity, and oxidative damage (xylenol and protein carbonyl). Antioxidant enzymes (superoxide dismutase, catalase, and gluthatione peroxidase) and inflammatory markers (tumor necrosis factor, interleukin-1β (IL-1β), and interleukin-10 (IL-10)) were not altered during the recovery period compared with the placebo group. The results suggest that taurine supplementation represents an important factor in improving performance and decreasing muscle damage and oxidative stress but does not decrease the inflammatory response after EE.

Effects of cyclic static stretch on fatigue recovery of triceps surae in female basketball players

Static stretch is a safe and feasible method which usually is used before exercise to avoid muscle injury and to improve muscle performance. The purpose of this study was to determine the effects of cyclic static stretch (CSS) on fatigue recovery of triceps surae (TS) in female basketball players. Nine athlete volunteers between 20 and 30 years participated in this study containing two sessions. After warm-up a pressure cuff was fastened above the knee joint and its pressure was increased to 140 mmHg. The subjects were asked to perform one maximum voluntary contraction (MVC) followed by a fatigue test including maximum isometric fatiguing contraction of TS. These steps were similar in both sessions. Then, a two-minute rest was included in the first session while 4 static stretches were performed to TS in the second session. After interventions, one MVC was done and the pressure cuff was released. During these steps, peak torque (PT) and electromyography (EMG) were recorded. The amount of lower leg pain was determined by the visual analogue scale (VAS). The value of PT increased significantly after CSS but its increase was not significant after rest. It seems that the effects of rest and CSS on the EMG parameters, PT and pain are similar.

Different recovery methods and muscle performance after exhausting exercise: comparison of the effects of electrical muscle stimulation and massage

In this study we assessed the influence of the three different recovery interventions massage (MSG), electrical muscle stimulation (EMS), and passive rest (PR) on lactate disappearance and muscle recovery after exhausting exercise bouts. Twelve healthy male sport students participated in the study. They attended the laboratory on five test days. After measurement of V.O₂max and a baseline Wingate test (WG_b), the three recovery interventions were tested in random counterbalanced order. High intensity exercise, which consisted of six exhausting exercise bouts (interspersed with active recovery), was followed by MSG, EMS or PR application (24 minutes); then the final Wingate test (WG_f) was performed. Lactate, heart rate, peak and mean power, rating of perceived exertion (RPE), and total quality of recovery (TQR) were recorded. In WG_f mean power was significantly higher than in WG_b for all three recovery modalities (MSG 6.29%, EMS 5.33%, PR 4.84% increase, p < 0.05), but no significant differences in mean and peak power were observed between the three recovery modes (p > 0.05). The heart rate response and the changes in blood lactate concentration were identical in all three interventions during the entire protocol (p = 0.817, p = 0.493, respectively). RPE and TQR scores were also not different among the three interventions (p > 0.05). These results provide further evidence that MSG and EMS are not more effective than PR in the process of recovery from high intensity exercise.

Fatigue after short (100-m), medium (200-m) and long (400-m) treadmill sprints

The aim of this study was to compare the aetiology of neuromuscular fatigue following maximal sprints of different distances. It was hypothesized that increasing the distance would modify the type of peripheral and induce central fatigue. 11 subjects performed 100-, 200- and 400-m sprints on a motorized instrumented treadmill. Neuromuscular function, evaluated before (Pre), 30 s after (Post), and 5 and 30 min after the sprints (Post5 and Post30), consisted in determining maximal voluntary knee extensors torque (MVC), maximal voluntary activation of the knee extensors (%AL), maximal compound muscle action potential amplitude and duration on vastus lateralis, single twitch (Tw), and low- (Db10) and high-frequency torque. Compared with peak values, running speed decreased by 8%, (P < 0.01), 20% (P < 0.001) and 39% (P < 0.001) at the end of the 100-, 200- and 400-m sprints, respectively. MVC was not altered following 100 and 200 m, but decreased by 14% (P < 0.001) after the 400 m, was still depreciated Post5 (-11%, P < 0.01) and went back to initial values Post30. A decrease in %AL (-6.0%, P < 0.01) was observed Post5 for the 400 m. Tw, Db10 and low-to-high doublets ratio decreased Post-sprints and were not recovered Post30 after all sprints. Single maximal sprints of 100-400 m did not alter sarcolemmal excitability but induced progressive and substantial low-frequency fatigue and a slight reduction in neural drive with increasing sprint duration. Despite altered single or paired stimulations, MVC strength loss was detected only after the 400 m.

Store-operated Ca2+ entry in malignant hyperthermia-susceptible human skeletal muscle

In malignant hyperthermia (MH), mutations in RyR1 underlie direct activation of the channel by volatile anesthetics, leading to muscle contracture and a life-threatening increase in core body temperature. The aim of the present study was to establish whether the associated depletion of sarcoplasmic reticulum (SR) Ca(2+) triggers sarcolemmal Ca(2+) influx via store-operated Ca(2+) entry (SOCE). Samples of vastus medialis muscle were obtained from patients undergoing assessment for MH susceptibility using the in vitro contracture test. Single fibers were mechanically skinned, and confocal microscopy was used to detect changes in [Ca(2+)] either within the resealed t-system ([Ca(2+)](t-sys)) or within the cytosol. In normal fibers, halothane (0.5 mM) failed to initiate SR Ca(2+) release or Ca(2+)(t-sys) depletion. However, in MH-susceptible (MHS) fibers, halothane induced both SR Ca(2+) release and Ca(2+)(t-sys) depletion, consistent with SOCE. In some MHS fibers, halothane-induced SR Ca(2+) release took the form of a propagated wave, which was temporally coupled to a wave of Ca(2+)(t-sys) depletion. SOCE was potently inhibited by "extracellular" application of a STIM1 antibody trapped within the t-system but not when the antibody was denatured by heating. In conclusion, (i) in human MHS muscle, SR Ca(2+) depletion induced by a level of volatile anesthetic within the clinical range is sufficient to induce SOCE, which is tightly coupled to SR Ca(2+) release; (ii) sarcolemmal STIM1 has an important role in regulating SOCE; and (iii) sustained SOCE from an effectively infinite extracellular Ca(2+) pool may contribute to the maintained rise in cytosolic [Ca(2+)] that underlies MH.

Quadriceps low-frequency fatigue and muscle pain are contraction-type-dependent

Eccentric contractions are thought to induce greater low-frequency fatigue (LFF) and delayed-onset muscle soreness (DOMS) than concentric contractions. In this study we induced a similar amount of eccentric quadriceps muscle fatigue during either a concentric or eccentric fatigue task to compare LFF and DOMS. Subjects (n = 22) performed concentric or eccentric fatigue tasks using 75% of the pre-fatigue maximal voluntary contraction (MVC) torque, and both tasks ended when the MVC eccentric torque decreased by 25% pre-fatigue. When subjects reached the failure criterion during the eccentric and concentric tasks, the concentric MVC was 78 +/- 9.8% and 64 +/- 8.4% of initial, respectively. LFF was greater after the concentric than the eccentric protocols (22 +/- 12.4% and 15 +/- 7.6% increase, respectively; P < 0.01). DOMS was over 100% greater for the eccentric protocol. These results indicate that DOMS is not dependent on the events that contribute to LFF.

Anticipatory postural control following fatigue of postural and focal muscles

OBJECTIVE

To investigate the effect of fatigue of postural and focal muscles on anticipatory postural adjustments (APAs).

METHODS

Nine healthy adults performed rapid bilateral arm raising movements before and after isometric hamstring (postural) and deltoid (focal/prime mover) muscle fatigue. Muscle force and peak acceleration of the arm movements were recorded to assess the presence of fatigue. Ground reaction forces, EMG activity of trunk and leg muscles and center of pressure (COP) displacements were recorded and quantified within the time intervals typical of APAs.

RESULTS

Early APA onset was seen in erector spinae and semitendinosus muscles post-deltoid fatigue. Anticipatory EMG integrals were reduced in the semitendinosus muscle post-hamstring fatigue, and were increased in the gastrocnemius muscle post-deltoid fatigue. No changes in COP displacement were observed following fatigue of both muscle groups.

CONCLUSION

A common pattern of APA adaptations seen following fatigue of either muscle groups along with no changes observed in COP displacements emphasizes the efficiency of the CNS in maintaining dynamic postural stability in the presence of fatigue.

SIGNIFICANCE

The outcome of the study is important for better understanding of the effect of muscular fatigue on feedforward mechanisms of postural control with possible implications for the elderly and individuals with neurological disorders.

Impaired calcium release during fatigue

Impaired calcium release from the sarcoplasmic reticulum (SR) has been identified as a contributor to fatigue in isolated skeletal muscle fibers. The functional importance of this phenomenon can be quantified by the use of agents, such as caffeine, which can increase SR Ca2+release during fatigue. A number of possible mechanisms for impaired calcium release have been proposed. These include reduction in the amplitude of the action potential, potentially caused by extracellular K+accumulation, which may reduce voltage sensor activation but is counteracted by a number of mechanisms in intact animals. Reduced effectiveness of SR Ca2+channel opening is caused by the fall in intracellular ATP and the rise in Mg2+concentrations that occur during fatigue. Reduced Ca2+available for release within the SR can occur if inorganic phosphate enters the SR and precipitates with Ca2+. Further progress requires the development of methods that can identify impaired SR Ca2+release in intact, blood-perfused muscles and that can distinguish between the various mechanisms proposed.

Skeletal Muscle Fatigue: Cellular Mechanisms

Repeated, intense use of muscles leads to a decline in performance known as muscle fatigue. Many muscle properties change during fatigue including the action potential, extracellular and intracellular ions, and many intracellular metabolites. A range of mechanisms have been identified that contribute to the decline of performance. The traditional explanation, accumulation of intracellular lactate and hydrogen ions causing impaired function of the contractile proteins, is probably of limited importance in mammals. Alternative explanations that will be considered are the effects of ionic changes on the action potential, failure of SR Ca2+release by various mechanisms, and the effects of reactive oxygen species. Many different activities lead to fatigue, and an important challenge is to identify the various mechanisms that contribute under different circumstances. Most of the mechanistic studies of fatigue are on isolated animal tissues, and another major challenge is to use the knowledge generated in these studies to identify the mechanisms of fatigue in intact animals and particularly in human diseases.

Reduced sarcoplasmic reticulum content of releasable Ca2+ in rat soleus muscle fibres after eccentric contractions

AIM

The purpose was to evaluate the effects of fatiguing eccentric contractions (EC) on calcium (Ca2+) handling properties in mammalian type I muscles. We hypothesized that EC reduces both endogenous sarcoplasmic reticulum (SR) content of releasable Ca2+ (eSRCa2+) and myofibrillar Ca2+ sensitivity.

METHODS

Isolated rat soleus muscles performed 30 EC bouts. Single fibres were isolated from the muscle and after mechanical removal of sarcolemma used to measure eSRCa2+, rate of SR Ca2+ loading and myofibrillar Ca2+ sensitivity.

RESULTS

Following EC maximal force in whole muscle was reduced by 30% and 16/100 Hz force ratio by 33%. The eSRCa2+ in fibres from non-stimulated muscles was 45 +/- 5% of the maximal loading capacity. After EC, eSRCa2+ per fibre CSA decreased by 38% (P = 0.05), and the maximal capacity of SR Ca2+ loading was depressed by 32%. There were no effects of EC on either myofibrillar Ca2+ sensitivity, maximal Ca2+ activated force per cross-sectional area and rate of SR Ca2+ loading, or in SR vesicle Ca2+ uptake and release.

CONCLUSIONS

We conclude that EC reduces endogenous SR content of releasable Ca2+ but that myofibrillar Ca2+ sensitivity and SR vesicle Ca2+ kinetics remain unchanged. The present data suggest that the long-lasting fatigue induced by EC, which was more pronounced at low frequencies (low frequency fatigue), is caused by reduced Ca2+ release occurring secondary to reduced SR content of releasable Ca2+.

Comparison of recovery strategies on muscle performance after fatiguing exercise

OBJECTIVE

The objective of this study was to assess the influence of different relaxation modes: stretching (ST), active recovery (AR), and passive recovery (PR) on muscle relaxation after dynamic exercise of the quadriceps femoris.

DESIGN

Ten healthy male volunteers between 24 and 38 yrs of age participated in this study. After the warm-up, subjects performed three sets of dynamic leg extension and flexion (at an angle of 20-110 degrees) at 50% of previously determined maximal voluntary contraction (MVC), with 30 secs. of rest between sets. Immediately after completing the leg exercise, one of the relaxation methods was applied, in a randomized order (AR, PR, ST). Then, subjects performed isometric knee extension at 50% of MVC to the point of fatigue, and surface electromyogram (EMG) of the vastus lateralis muscle was measured.

RESULTS

After AR, the mean MVC was significantly (P < 0.05) higher than after PR and ST. Moreover, there was no difference in MVC between AR and baseline (P > 0.05). Total time of the effort during EMG measurement was significantly lower for all three recovery modes than at baseline. During the effort after both PR and ST, there was no significant increase in motor unit activation, but a significant increase was noted after AR (P < 0.05). There was no difference in frequency between any of the recovery modes and baseline (P > 0.05).

CONCLUSION

The results of this study suggest that the most appropriate and effective recovery mode after dynamic muscle fatigue involves light, active exercises, such as cycling with minimal resistance.

The effect of taurine depletion on the contractile properties and fatigue in fast-twitch skeletal muscle of the mouse

Taurine increases force production in skeletal muscle, and taurine levels may fall during exercise. The contractile properties and fatigability of extensor digitorum longus (EDL) muscles depleted of taurine by guanodinoethane sulfonate (GES) treatment were investigated. GES treatment decreased muscle taurine levels to <40% of controls. Peak twitch force levels were 23% of controls in GES treated EDL muscles (p < 0.05), but maximal specific force was unaffected. The force-frequency relationship was examined and significantly less force was produced by the GES treated muscles compared to controls at stimulation frequencies from 50 to 100 Hz (p < 0.05). GES treated EDL muscles exhibited significantly slower rates of fatigue than controls (p < 0.05). In skinned fibres, 20 mM GES had a small but significant effect on force production, indicating that GES may have some minor taurine-like effects. In this study, a fall in taurine levels decreased force output, and increased the endurance of EDL skeletal muscles.

Calcium phosphate precipitation in the sarcoplasmic reticulum reduces action potential-mediated Ca2+release in mammalian skeletal muscle

During vigorous exercise, Piconcentration levels within the cytoplasm of fast-twitch muscle fibers may reach ≥30 mM. Cytoplasmic Pimay enter the sarcoplasmic reticulum (SR) and bind to Ca2+to form a precipitate (CaPi), thus reducing the amount of releasable Ca2+. Using mechanically skinned rat fast-twitch muscle fibers, which retain the normal action potential-mediated Ca2+release mechanism, we investigated the consequences of Piexposure on normal excitation-contraction coupling. The total amount of Ca2+released from the SR by a combined caffeine/low-Mg2+concentration stimulus was reduced by ∼20%, and the initial rate of force development slowed after 2-min exposure to 30 mM Pi(with or without the presence creatine phosphate). Peak (50 Hz) tetanic force was also reduced (by ∼25% and ∼45% after 10 and 30 mM Piexposure, respectively). Tetanic force responses produced after 30 mM Piexposure were nearly identical to those observed in the same fiber after depletion of total SR Ca2+by ∼35%. Ca2+content assays revealed that the total amount of Ca2+in the SR was not detectably changed by exposure to 30 mM Pi, indicating that Ca2+had not leaked from the SR but instead formed a precipitate with the Pi, reducing the amount of available Ca2+for rapid release. These results suggest that CaPiprecipitation that occurs within the SR could contribute to the failure of Ca2+release observed in the later stages of metabolic muscle fatigue. They also demonstrate that the total amount of Ca2+stored in the SR cannot drop substantially below the normal endogenous level without reducing tetanic force responses.

Reversal of muscle fatigue during 16 h of heavy intermittent cycle exercise

This study examined the effects of extended sessions of heavy intermittent exercise on quadriceps muscle fatigue and weakness. Twelve untrained volunteers (10 men and 2 women), with a peak oxygen consumption of 44.3 ± 2.3 ml·kg−1·min−1, exercised at ∼91% peak oxygen consumption for 6 min once per hour for 16 h. Muscle isometric properties assessed before and after selected repetitions (R1, R2, R4, R7, R12, and R15) were used to quantitate fatigue (before vs. after repetitions) and weakness (before vs. before repetitions). Muscle fatigue at R1 was indicated by reductions ( P < 0.05) in peak twitch force (135 ± 13 vs. 106 ± 11 N) and by a reduction ( P < 0.05) in the force-frequency response, which ranged between ∼53% at 10 Hz (113 ± 12 vs. 52.6 ± 7.4 N) and ∼17% at 50 Hz (324 ± 27 vs. 270 ± 30 N). No recovery of force, regardless of stimulation frequency, was observed during the 54 min between R1 and R2. At R2 and for all subsequent repetitions, no reduction in force, regardless of stimulation frequency, was generally found after the exercise. The only exception was for R2, where, at 20 Hz, force was reduced ( P < 0.05) by 18%. At R15, force before repetitions for high frequencies (i.e., 100 Hz) returned to R1 (333 ± 29 vs. 324 ± 27 N), whereas force at low frequency (i.e., 10 Hz) was only partially ( P < 0.05) recovered (113 ± 12 vs. 70 ± 6.6 N). It is concluded that multiple sessions of heavy exercise can reverse the fatigue noted early and reduce or eliminate weakness depending on the frequency of stimulation.

Effect of low cytoplasmic [ATP] on excitation-contraction coupling in fast-twitch muscle fibres of the rat

In this study we investigated the roles of cytoplasmic ATP as both an energy source and a regulatory molecule in various steps of the excitation-contraction (E-C) coupling process in fast-twitch skeletal muscle fibres of the rat. Using mechanically skinned fibres with functional E-C coupling, it was possible to independently alter cytoplasmic [ATP] and free [Mg2+]. Electrical field stimulation was used to elicit action potentials (APs) within the sealed transverse tubular (T-) system, producing either twitch or tetanic (50 Hz) force responses. Measurements were also made of the amount of Ca2+ released by an AP in different cytoplasmic conditions. The rate of force development and relaxation of the contractile apparatus was measured using rapid step changes in [Ca2+]. Twitch force decreased substantially (approximately 30%) at 2 mm ATP compared to the level at 8 mm ATP, whereas peak tetanic force only declined by approximately 10% at 0.5 mm ATP. The rate of force development of the twitch and tetanus was slowed only slightly at [ATP] > or = 0.5 mm, but was slowed greatly (> 6-fold) at 0.1 mm ATP, the latter being due primarily to slowing of force development by the contractile apparatus. AP-induced Ca2+ release was decreased by approximately 10 and 20% at 1 and 0.5 mm ATP, respectively, and by approximately 40% by raising the [Mg2+] to 3 mm. Adenosine inhibited Ca2+ release and twitch responses in a manner consistent with its action as a competitive weak agonist for the ATP regulatory site on the ryanodine receptor (RyR). These findings show that (a) ATP is a limiting factor for normal voltage-sensor activation of the RyRs, and (b) large reductions in cytoplasmic [ATP], and concomitant elevation of [Mg2+], substantially inhibit E-C coupling and possibly contribute to muscle fatigue in fast-twitch fibres in some circumstances.

Effects of modulators of sarcoplasmic Ca2+ release on the development of skeletal muscle fatigue

The reduced release of Ca2+ from sarcoplasmic reticulum (SR) is considered a major determinant of muscle fatigue. In the present study, we investigated whether the presence of dantrolene, an established inhibitor of SR Ca2+ release, or caffeine, a drug facilitating SR Ca2+ release, modifies muscle fatigue development. Accordingly, the effects of Ca2+ release modulators were analyzed in vitro in mouse fast-twitch [extensor digitorum longus (EDL)] and slow-twitch (soleus) muscles, fatigued by repeated short tetani (40 Hz for 300 ms, 0.5 s-1 in soleus and 60 Hz for 300 ms, 0.3 s-1 in EDL, for 6 min). Caffeine produced a substantial increase of tetanic tension of both EDL and soleus muscles, whereas dantrolene decreased tetanic tension only in EDL muscle. In both EDL and soleus muscles, 5 μM dantrolene did not affect fatigue development, whereas 20 μM dantrolene produced a positive staircase during the first 3 min of stimulation in EDL muscle and a slowing of fatigue development in soleus muscle. The development of the positive staircase was abolished by the addition of 15 μM ML-7, a selective inhibitor of myosin light chain kinase. On the other hand, caffeine caused a larger and faster loss of tension in both EDL and soleus muscles. The results seem to indicate that the changes in fatigue profile induced by caffeine or dantrolene are mainly due to the changes in the initial tetanic tension caused by the drugs, with the resulting changes in the level of contraction-dependent factors of fatigue, rather than to changes in the SR Ca2+ release during fatigue development.