Cellular mechanisms of skeletal muscle fatigue

Non-invasive assessment of fatigue and recovery of inspiratory rib cage muscles during endurance test in healthy individuals

INTRODUCTION

Fatigue is defined as loss of capacity to develop muscle force and/or velocity that is reversible at rest. We assessed non-invasively the fatigue and recovery of inspiratory rib cage muscles during two respiratory endurance tests in healthy individuals.

METHODS

The sniff nasal inspiratory pressure (SNIP) was assessed before and after two respiratory endurance tests: normocapnic hyperpnea (NH) and inspiratory pressure threshold loading (IPTL). Contractile (maximum rate of pressure development and time to peak pressure) and relaxation parameters (maximum relaxation rate [MRR], time constant of pressure decay [τ], and half relaxation time) obtained from sniff curves and shortening velocity and mechanical power estimated using optoelectronic plethysmography were analyzed during SNIP maneuvers. Respiratory muscle activity (electromyography) and tissue oxygenation (near-infrared spectroscopy-NIRS) were obtained during endurance tests and SNIP maneuvers. Fatigue development of inspiratory rib cage muscles was assessed according to the slope of decay of median frequency.

RESULTS

Peak pressure during SNIP decreased after both protocols (p <0.05). MRR, shortening velocity, and mechanical power decreased (p <0.05), whereas τ increased after IPTL (p <0.05). The median frequency of inspiratory rib cage muscles (i.e., sum of sternocleidomastoid, scalene, and parasternal) decreased linearly during IPTL and exponentially during NH, mainly due to the sternocleidomastoid.

CONCLUSION

Fatigue development behaved differently between protocols and relaxation properties (MRR and τ), shortening velocity, and mechanical power changed only in the IPTL.

Impaired regenerative capacity contributes to skeletal muscle dysfunction in chronic obstructive pulmonary disease

Locomotor skeletal muscle dysfunction is a relevant comorbidity of chronic obstructive pulmonary disease (COPD) and is strongly associated with worse clinical outcomes including higher mortality. Over the last decades, a large body of literature helped characterize the process, defining the disruptive muscle phenotype caused by COPD that involves reduction in muscle mass, force-generation capacity, fatigue-tolerance, and regenerative potential following injury. A major limitation in the field has been the scarcity of well-calibrated animal models to conduct mechanistic research based on loss- and gain-of-function studies. This article provides an overall description of the process, the tools available to mechanistically investigate it, and the potential role of mitochondrially driven metabolic signals on the regulation muscle regeneration after injury in COPD. Finally, a description of future avenues to further expand on the area is proposed based on very recent evidence involving mitochondrial metabolic cues affecting myogenesis.

Ascorbate attenuates cycling exercise-induced neuromuscular fatigue but fails to improve exertional dyspnea and exercise tolerance in COPD

We examined the effect of intravenous ascorbate (VitC) administration on exercise-induced redox balance, inflammation, exertional dyspnea, neuromuscular fatigue, and exercise tolerance in patients with chronic obstructive pulmonary disease (COPD). Eight COPD patients completed constant-load cycling (∼80% of peak power output, 83 ± 10 W) to task failure after intravenous VitC (2 g) or saline (placebo, PL) infusion. All participants repeated the shorter of the two exercise trials (isotime) with the other infusate. Quadriceps fatigue was determined by pre- to postexercise changes in quadriceps twitch torque (ΔQtw, electrical femoral nerve stimulation). Corticospinal excitability before, during, and after exercise was assessed by changes in motor evoked potentials triggered by transcranial magnetic stimulation. VitC increased superoxide dismutase (marker for endogenous antioxidant capacity) by 129% and mitigated C-reactive protein (marker for inflammation) in the plasma during exercise but failed to alter the exercise-induced increase in lipid peroxidation (malondialdehyde) and free radicals [electron paramagnetic resonance (EPR)-spectroscopy]. Although VitC did, indeed, decrease neuromuscular fatigue (ΔQtw: PL -29 ± 5%, VitC -23 ± 6%, P < 0.05), there was no impact on corticospinal excitability and time to task failure (∼8 min, P = 0.8). Interestingly, in terms of pulmonary limitations to exercise, VitC had no effect on perceived exertional dyspnea (∼8.5/10) and its determinants, including oxygen saturation ([Formula: see text]) (∼92%) and respiratory muscle work (∼650 cmH2O·s·min-1) (P > 0.3). Thus, although VitC facilitated indicators for antioxidant capacity, diminished inflammatory markers, and improved neuromuscular fatigue resistance, it failed to improve exertional dyspnea and cycling exercise tolerance in patients with COPD. As dyspnea is recognized to limit exercise tolerance in COPD, the otherwise beneficial effects of VitC may have been impacted by this unaltered sensation.NEW & NOTEWORTHY We investigated the effect of intravenous vitamin C on redox balance, exertional dyspnea, neuromuscular fatigue, and exercise tolerance in chronic obstructive pulmonary disease (COPD) patients. Acute vitamin C administration increased superoxide dismutase (marker of antioxidant capacity) and attenuated fatigue development but failed to improve exertional dyspnea and exercise tolerance. These findings suggest that a compromised redox balance plays a critical role in the development of fatigue in COPD but also highlight the significance of exertional dyspnea as an important symptom limiting the patients' exercise tolerance.

Type 2 diabetes and reduced exercise tolerance: a review of the literature through an integrated physiology approach

The association between type 2 diabetes mellitus (T2DM) and heart failure (HF) is well established. Early in the course of the diabetic disease, some degree of impaired exercise capacity (a powerful marker of health status with prognostic value) can be frequently highlighted in otherwise asymptomatic T2DM subjects. However, the literature is quite heterogeneous, and the underlying pathophysiologic mechanisms are far from clear. Imaging-cardiopulmonary exercise testing (CPET) is a non-invasive, provocative test providing a multi-variable assessment of pulmonary, cardiovascular, muscular, and cellular oxidative systems during exercise, capable of offering unique integrated pathophysiological information. With this review we aimed at defying the cardiorespiratory alterations revealed through imaging-CPET that appear specific of T2DM subjects without overt cardiovascular or pulmonary disease. In synthesis, there is compelling evidence indicating a reduction of peak workload, peak oxygen assumption, oxygen pulse, as well as ventilatory efficiency. On the contrary, evidence remains inconclusive about reduced peripheral oxygen extraction, impaired heart rate adjustment, and lower anaerobic threshold, compared to non-diabetic subjects. Based on the multiparametric evaluation provided by imaging-CPET, a dissection and a hierarchy of the underlying mechanisms can be obtained. Here we propose four possible integrated pathophysiological mechanisms, namely myocardiogenic, myogenic, vasculogenic and neurogenic. While each hypothesis alone can potentially explain the majority of the CPET alterations observed, seemingly different combinations exist in any given subject. Finally, a discussion on the effects -and on the physiological mechanisms-of physical activity and exercise training on oxygen uptake in T2DM subjects is also offered. The understanding of the early alterations in the cardiopulmonary response that are specific of T2DM would allow the early identification of those at a higher risk of developing HF and possibly help to understand the pathophysiological link between T2DM and HF.

Ergogenic Effects of Photobiomodulation on Performance in the 30-Second Wingate Test: A Randomized, Double-Blind, Placebo-Controlled, Crossover Study

Molina Correa, JC, Padoin, S, Varoni, PR, Demarchi, MC, Flores, LJ, Nampo, FK, and de Paula Ramos, S. Ergogenic effects of photobiomodulation on performance in the 30-second Wingate test: A randomized, double-blind, placebo-controlled, crossover study. J Strength Cond Res XX(X): 000-000, 2020-The purpose of this study was to evaluate the ergogenic effects of red light (630 nm) photobiomodulation on anaerobic capacity in the Wingate test. Sixteen healthy and physically active male volunteers (21.71 ± 2.49 years of age, body mass index between 18.5 and 24.9 kg/m) participated in this randomized, double-blind, placebo-controlled, crossover study. The subjects performed 3 Wingate test sessions, with a 48-hour interval between tests. In the first session (baseline session, BS), a Wingate test was performed to evaluate the initial performance. Subjects were paired by performance in the BS and allocated through a draw to receive either the phototherapy (630 nm, 4.6 J/cm, 6 J per point, 16 points, light-emitting diode [LED] session) or placebo intervention (PLA session) in the second test session. In the third test session, a crossover intervention was performed. The repeated-measures analysis of variance test, followed by Bonferroni post hoc test or Friedman test with Dunn's post hoc test (p < 0.05) and Cohen's d statistic were used for comparisons. The LED session with phototherapy promoted an increase in performance in peak power (p < 0.05), relative power (p < 0.05), RPMpeak (p < 0.05), and peak velocity (p < 0.05), as well as total displacement (p < 0.01) compared with PLA. The mean power (p < 0.05), relative power (p < 0.05), RPMmean (p < 0.01), and mean velocity (p < 0.01) were higher in the LED session than those of BS. We concluded that phototherapy improves performance in Wingate anaerobic exercise, possibly due to large effects on the anaerobic alactic metabolism.

The Relationship between CrossFit® Performance and Laboratory-Based Measurements of Fitness

To date, research has examined the physiological determinants of performance in standardized CrossFit^® (CF) workouts but not without the influence of CF familiarity. Therefore, the purpose of this present study was to examine the predictive value of aerobic fitness, body composition, and total body strength on performance of two standardized CF workouts in CF-naïve participants. Twenty-two recreationally trained individuals (males = 13, females = 9) underwent assessments of peak oxygen consumption (VO₂ peak), ventilatory thresholds, body composition, and one repetition maximum tests for the back squat, deadlift, and overhead press in which the sum equaled the CF Total. Participants also performed two CF workouts: a scaled version of the CF Open workout 19.1 and a modified version of the CF Benchmark workout Fran to determine scores based on total repetitions completed and time-to-completion, respectively. Simple Pearson’s r correlations were used to determine the relationships between CF performance variables (19.1 and modified Fran) and the independent variables. A forward stepwise multiple linear regression analysis was performed and significant variables that survived the regression analysis were used to create a predictive model of CF performance. Absolute VO₂ peak was a significant predictor of 19.1 performance, explaining 39% of its variance (adjusted R² = 0.39, p = 0.002). For modified Fran, CF Total was a significant predictor and explained 33% of the variance in performance (adjusted R² = 0.33, p = 0.005). These results suggest, without any influence of CF familiarity or experience, that performance in these two CF workouts could be predicted by distinct laboratory-based measurements of fitness.

Effect of eccentric exercise on markers of muscle damage in patients with chronic obstructive pulmonary disease

Background: Eccentric exercise may be considered as an attractive alternative to conventional exercise in pulmonary rehabilitation (PR) for patients with chronic obstructive pulmonary disease (COPD). However, due to muscle damage associated with eccentric exercise, there has been reluctance in using this exercise form in PR.Objective: The aim of the present study was to investigate the effect of eccentric exercise on markers of muscle damage in patients with COPD.Methods: We analyzed 14 patients with moderate-severe COPD and 14 age-matched healthy controls. Both groups performed submaximal eccentric exercise of the elbow flexors. Muscle soreness (MS), maximum voluntary isometric contraction (MVC) of the elbow flexors, elbow range of motion (ROM), upper arm circumference (CIR), and biochemical markers such as creatine Kinase (CK) and lactate Dehydrogenase (LDH) were measured at pre-exercise, 24 h, 48 h, and 72 h following submaximal eccentric exercise.Results: There was a significant difference in markers of muscle damage, MS (p = .002), MVC (p < .001), ROM (p = .010), CIR (p < .001), and LDH (p = .001). However, no significant differences were observed in the activity of CK (p = .261) between COPD and control group following eccentric exercise which indicates greater degree of muscle damage in COPD as compared with control.Conclusion: Sub-maximal eccentric exercise causes significantly greater muscle damage in elderly COPD patients than healthy controls. Therefore, initial exercise should be progressed with lower intensities to prevent undue muscle damage in these patients.

The regulation of skeletal muscle fatigability and mitochondrial function by chronically elevated interleukin-6

NEW FINDINGS

What is the central question of this study? Interleukin-6 has been associated with muscle mass and metabolism in both physiological and pathological conditions. A causal role for interleukin-6 in the induction of fatigue and disruption of mitochondrial function has not been determined. What is the main finding and its importance? We demonstrate that chronically elevated interleukin-6 increased skeletal muscle fatigability and disrupted mitochondrial content and function independent of changes in fibre type and mass.

ABSTRACT

Interleukin-6 (IL-6) can initiate intracellular signalling in skeletal muscle by binding to the IL-6-receptor and interacting with the transmembrane gp130 protein. Circulating IL-6 has established effects on skeletal muscle mass and metabolism in both physiological and pathological conditions. However, the effects of circulating IL-6 on skeletal muscle function are not well understood. The purpose of this study was to determine whether chronically elevated systemic IL-6 was sufficient to disrupt skeletal muscle force, fatigue and mitochondrial function. Additionally, we examined the role of muscle gp130 signalling during overexpression of IL-6. Systemic IL-6 overexpression for 2 weeks was achieved by electroporation of an IL-6 overexpression plasmid or empty vector into the quadriceps of either C57BL/6 (WT) or skeletal muscle gp130 knockout (KO) male mice. Tibialis anterior muscle in situ functional properties and mitochondrial respiration were determined. Interleukin-6 accelerated in situ skeletal muscle fatigue in the WT, with a 18.5% reduction in force within 90 s of repeated submaximal contractions and a 7% reduction in maximal tetanic force after 5 min. There was no difference between fatigue in the KO and KO+IL-6. Interleukin-6 reduced WT muscle mitochondrial respiratory control ratio by 36% and cytochrome c oxidase activity by 42%. Interleukin-6 had no effect on either KO respiratory control ratio or cytochrome c oxidase activity. Interleukin-6 also had no effect on body weight, muscle mass or tetanic force in either genotype. These results provide evidence that 2 weeks of elevated systemic IL-6 is sufficient to increase skeletal muscle fatigability and decrease muscle mitochondrial content and function, and these effects require muscle gp130 signalling.

Exercise intolerance in Type 2 diabetes: is there a cardiovascular contribution?

Physical activity is critically important for Type 2 diabetes management, yet adherence levels are poor. This might be partly due to disproportionate exercise intolerance. Submaximal exercise tolerance is highly sensitive to muscle oxygenation; impairments in exercising muscle oxygen delivery may contribute to exercise intolerance in Type 2 diabetes since there is considerable evidence for the existence of both cardiac and peripheral vascular dysfunction. While uncompromised cardiac output during submaximal exercise is consistently observed in Type 2 diabetes, it remains to be determined whether an elevated cardiac sympathetic afferent reflex could sympathetically restrain exercising muscle blood flow. Furthermore, while deficits in endothelial function are common in Type 2 diabetes and are often cited as impairing exercising muscle oxygen delivery, no direct evidence in exercise exists, and there are several other vasoregulatory mechanisms whose dysfunction could contribute. Finally, while there are findings of impaired oxygen delivery, conflicting evidence also exists. A definitive conclusion that Type 2 diabetes compromises exercising muscle oxygen delivery remains premature. We review these potentially dysfunctional mechanisms in terms of how they could impair oxygen delivery in exercise, evaluate the current literature on whether an oxygen delivery deficit is actually manifest, and correspondingly identify key directions for future research.

Muscle metabolic and neuromuscular determinants of fatigue during cycling in different exercise intensity domains

The gas exchange threshold and the critical power demarcate discrete exercise intensity domains. For the first time, we show that the limit of tolerance during whole body exercise within these domains is characterized by distinct metabolic and neuromuscular responses. Fatigue development during exercise greater than critical power is associated with the attainment of consistent “limiting” values of muscle metabolites, whereas substrate availability and limitations to muscle activation may constrain performance at lower intensities.

Lactate or gas exchange threshold (GET) and critical power (CP) are closely associated with human exercise performance. We tested the hypothesis that the limit of tolerance (T_lim) during cycle exercise performed within the exercise intensity domains demarcated by GET and CP is linked to discrete muscle metabolic and neuromuscular responses. Eleven men performed a ramp incremental exercise test, 4–5 severe-intensity (SEV; >CP) constant-work-rate (CWR) tests until T_lim, a heavy-intensity (HVY; <CP but >GET) CWR test until T_lim, and a moderate-intensity (MOD; <GET) CWR test until T_lim. Muscle biopsies revealed that a similar (P > 0.05) muscle metabolic milieu (i.e., low pH and [PCr] and high [lactate]) was attained at T_lim (approximately 2–14 min) for all SEV exercise bouts. The muscle metabolic perturbation was greater at T_lim following SEV compared with HVY, and also following SEV and HVY compared with MOD (all P < 0.05). The normalized M-wave amplitude for the vastus lateralis (VL) muscle decreased to a similar extent following SEV (−38 ± 15%), HVY (−68 ± 24%), and MOD (−53 ± 29%), (P > 0.05). Neural drive to the VL increased during SEV (4 ± 4%; P < 0.05) but did not change during HVY or MOD (P > 0.05). During SEV and HVY, but not MOD, the rates of change in M-wave amplitude and neural drive were correlated with changes in muscle metabolic ([PCr], [lactate]) and blood ionic/acid-base status ([lactate], [K⁺]) (P < 0.05). The results of this study indicate that the metabolic and neuromuscular determinants of fatigue development differ according to the intensity domain in which the exercise is performed.

NEW & NOTEWORTHY The gas exchange threshold and the critical power demarcate discrete exercise intensity domains. For the first time, we show that the limit of tolerance during whole-body exercise within these domains is characterized by distinct metabolic and neuromuscular responses. Fatigue development during exercise greater than critical power is associated with the attainment of consistent “limiting” values of muscle metabolites, whereas substrate availability and limitations to muscle activation may constrain performance at lower intensities.

Cytosolic calcium transients are a determinant of contraction-induced HSP72 transcription in single skeletal muscle fibers

The intrinsic activating factors that induce transcription of heat shock protein 72 (HSP72) in skeletal muscle following exercise remain unclear. We hypothesized that the cytosolic Ca(2+) transient that occurs with depolarization is a determinant. We utilized intact, single skeletal muscle fibers from Xenopus laevis to test the role of the cytosolic Ca(2+) transient and several other exercise-related factors (fatigue, hypoxia, AMP kinase, and cross-bridge cycling) on the activation of HSP72 transcription. HSP72 and HSP60 mRNA levels were assessed with real-time quantitative PCR; cytosolic Ca(2+) concentration ([Ca(2+)]cyt) was assessed with fura-2. Both fatiguing and nonfatiguing contractions resulted in a significant increase in HSP72 mRNA. As expected, peak [Ca(2+)]cyt remained tightly coupled with peak developed tension in contracting fibers. Pretreatment with N-benzyl-p-toluene sulfonamide (BTS) resulted in depressed peak developed tension with stimulation, while peak [Ca(2+)]cyt remained largely unchanged from control values. Despite excitation-contraction uncoupling, BTS-treated fibers displayed a significant increase in HSP72 mRNA. Treatment of fibers with hypoxia (Po2: <3 mmHg) or AMP kinase activation had no effect on HSP72 mRNA levels. These results suggest that the intermittent cytosolic Ca(2+) transient that occurs with skeletal muscle depolarization provides a sufficient activating stimulus for HSP72 transcription. Metabolic or mechanical factors associated with fatigue development and cross-bridge cycling likely play a more limited role.

Development of bacoside enriched date syrup juice and its evaluation for physical endurance

Bacoside rich juice (BRJ) was developed using date syrup as base. BRJ was evaluated for physicochemical, sensory attributes and its effect on physical endurance. Overall acceptability of BRJ and date syrup juice (DSJ) was good according to hedonic scale/ratings. Twenty four adult male Wistar rats were divided into 4 groups (n = 6). Sedentary (Group I) and control (Group II) group rats were allowed to drink water whereas DSJ and BRJ group rats were provided free access to drink DSJ (Group III) and BRJ (Group IV) for 14 days and were subjected to weight-loaded forced swim test (WFST) for every alternate day in order to evaluate the physical endurance. Both BRJ and DSJ group rats swimming efficiency was improved by 3 and 2 folds respectively in comparison with control group on day- 15. Improved physical endurance in BRJ group is due to reduced malondialdehyde levels in brain, liver and muscle tissues by 16.50 %, 17.88 % and 30.20 %, respectively, compared to DSJ group (p < 0.01). In addition, administration of BRJ significantly protected the hepatic and muscle glycogen levels and reduced the levels of lactic acid in comparison to DSJ group. Hence, the present study clearly indicates that BRJ is an effective anti-fatigue drink ameliorates the various impairments associated with physical endurance.

Creatine, L-carnitine, and ω3 polyunsaturated fatty acid supplementation from healthy to diseased skeletal muscle

Myopathies are chronic degenerative pathologies that induce the deterioration of the structure and function of skeletal muscle. So far a definitive therapy has not yet been developed and the main aim of myopathy treatment is to slow the progression of the disease. Current nonpharmacological therapies include rehabilitation, ventilator assistance, and nutritional supplements, all of which aim to delay the onset of the disease and relieve its symptoms. Besides an adequate diet, nutritional supplements could play an important role in the treatment of myopathic patients. Here we review the most recent in vitro and in vivo studies investigating the role supplementation with creatine, L-carnitine, and ω3 PUFAs plays in myopathy treatment. Our results suggest that these dietary supplements could have beneficial effects; nevertheless continued studies are required before they could be recommended as a routine treatment in muscle diseases.

An official American Thoracic Society/European Respiratory Society statement: update on limb muscle dysfunction in chronic obstructive pulmonary disease

BACKGROUND

Limb muscle dysfunction is prevalent in chronic obstructive pulmonary disease (COPD) and it has important clinical implications, such as reduced exercise tolerance, quality of life, and even survival. Since the previous American Thoracic Society/European Respiratory Society (ATS/ERS) statement on limb muscle dysfunction, important progress has been made on the characterization of this problem and on our understanding of its pathophysiology and clinical implications.

PURPOSE

The purpose of this document is to update the 1999 ATS/ERS statement on limb muscle dysfunction in COPD.

METHODS

An interdisciplinary committee of experts from the ATS and ERS Pulmonary Rehabilitation and Clinical Problems assemblies determined that the scope of this document should be limited to limb muscles. Committee members conducted focused reviews of the literature on several topics. A librarian also performed a literature search. An ATS methodologist provided advice to the committee, ensuring that the methodological approach was consistent with ATS standards.

RESULTS

We identified important advances in our understanding of the extent and nature of the structural alterations in limb muscles in patients with COPD. Since the last update, landmark studies were published on the mechanisms of development of limb muscle dysfunction in COPD and on the treatment of this condition. We now have a better understanding of the clinical implications of limb muscle dysfunction. Although exercise training is the most potent intervention to address this condition, other therapies, such as neuromuscular electrical stimulation, are emerging. Assessment of limb muscle function can identify patients who are at increased risk of poor clinical outcomes, such as exercise intolerance and premature mortality.

CONCLUSIONS

Limb muscle dysfunction is a key systemic consequence of COPD. However, there are still important gaps in our knowledge about the mechanisms of development of this problem. Strategies for early detection and specific treatments for this condition are also needed.

A novel model incorporating two variability sources for describing motor evoked potentials

OBJECTIVE

Motor evoked potentials (MEPs) play a pivotal role in transcranial magnetic stimulation (TMS), e.g., for determining the motor threshold and probing cortical excitability. Sampled across the range of stimulation strengths, MEPs outline an input-output (IO) curve, which is often used to characterize the corticospinal tract. More detailed understanding of the signal generation and variability of MEPs would provide insight into the underlying physiology and aid correct statistical treatment of MEP data.

METHODS

A novel regression model is tested using measured IO data of twelve subjects. The model splits MEP variability into two independent contributions, acting on both sides of a strong sigmoidal nonlinearity that represents neural recruitment. Traditional sigmoidal regression with a single variability source after the nonlinearity is used for comparison.

RESULTS

The distribution of MEP amplitudes varied across different stimulation strengths, violating statistical assumptions in traditional regression models. In contrast to the conventional regression model, the dual variability source model better described the IO characteristics including phenomena such as changing distribution spread and skewness along the IO curve.

CONCLUSIONS

MEP variability is best described by two sources that most likely separate variability in the initial excitation process from effects occurring later on. The new model enables more accurate and sensitive estimation of the IO curve characteristics, enhancing its power as a detection tool, and may apply to other brain stimulation modalities. Furthermore, it extracts new information from the IO data concerning the neural variability-information that has previously been treated as noise.

Muscle metabolism during fatiguing isometric quadriceps exercise in adolescents and adults

Children and adolescents are less susceptible to muscle fatigue during repeated bouts of high-intensity exercise than adults, but the physiological basis for these differences is not clear. The purpose of the current investigation was to investigate the muscle metabolic responses, using 31-phosphorus magnetic resonance spectroscopy, during fatiguing isometric quadriceps exercise in 13 adolescents (7 females) and 14 adults (8 females). Participants completed 30 maximal voluntary contractions (6-s duration) separated by 6 s of rest. Fatigue was quantified as the relative decrease in force over the test. Fatigue was not significantly different with age (p = 0.20) or sex (p = 0.63). Metabolic perturbation (change in phosphocreatine, inorganic phosphate, and ADP concentrations) was significantly greater in adults compared with adolescents; no sex effects were present. Muscle pH did not differ with age or sex. Phosphocreatine recovery following exercise was not significantly different with age (p = 0.27) or sex (p = 0.97) but a significant interaction effect was present (p = 0.04). Recovery tended to be faster in boys than men but slower in girls than women, though no significant group differences were identified. The results of this study show that at a comparable level of muscle fatigue, the metabolic profile is profoundly different between adolescents and adults.

Ascorbate infusion increases skeletal muscle fatigue resistance in patients with chronic obstructive pulmonary disease

Chronic obstructive pulmonary disease (COPD) is associated with systemic oxidative stress and skeletal muscle dysfunction. The purpose of this study was to examine the impact of intravenous ascorbate administration (AO) on biological markers of antioxidant capacity and oxidative stress, and subsequently skeletal muscle function during dynamic, small muscle mass exercise in patients with COPD. Ten patients with spirometric evidence of COPD performed single-leg knee extensor (KE) trials matched for intensity and time (isotime) following intravenous ascorbate (2 g) or saline infusion (PL). Quadriceps fatigue was quantified by changes in force elicited by maximal voluntary contraction (MVC) and magnetic femoral nerve stimulation (Qtw,pot). AO administration significantly increased antioxidant capacity, as measured by the ferric-reducing ability of plasma (PL: 1 ± 0.1 vs. AO: 5 ± 0.2 mM), and significantly reduced malondialdehyde levels (PL: 1.16 ± 0.1 vs. AO: 0.97 ± 0.1 mmol). Additionally, resting blood pressure was significantly reduced (PL: 104 ± 4 vs. AO: 93 ± 6 mmHg) and resting femoral vascular conductance was significantly elevated after AO (PL: 2.4 ± 0.2 vs. AO: 3.6 ± 0.4 ml·min(-1)·mmHg(-1)). During isotime exercise, the AO significantly attenuated both the ventilatory and metabolic responses, and patients accumulated significantly less peripheral quadriceps fatigue, as illustrated by less of a fall in MVC (PL: -11 ± 2% vs. AO: -5 ± 1%) and Qtw,pot (PL: -37 ± 1% vs. AO: -30 ± 2%). These data demonstrate a beneficial role of AO administration on skeletal muscle fatigue in patients with COPD and further implicate systemic oxidative stress as a causative factor in the skeletal muscle dysfunction observed in this population.

Muscle mass and peripheral fatigue: a potential role for afferent feedback?

AIM

The voluntary termination of exercise has been hypothesized to occur at a sensory tolerance limit, which is affected by feedback from group III and IV muscle afferents, and is associated with a specific level of peripheral quadriceps fatigue during whole body cycling. Therefore, the purpose of this study was to reduce the amount of muscle mass engaged during dynamic leg exercise to constrain the source of muscle afferent feedback to the central nervous system (CNS) and examine the effect on peripheral quadriceps fatigue.

METHOD

Eight young males performed exhaustive large (cycling - BIKE) and small (knee extensor - KE) muscle mass dynamic exercise at 85% of the modality-specific maximal workload. Pre- vs. post-exercise maximal voluntary contractions (MVC) and supramaximal magnetic femoral nerve stimulation (Q(tw,pot)) were used to quantify peripheral quadriceps fatigue.

RESULT

Significant quadriceps fatigue was evident following both exercise trials; however, the exercise-induced changes in MVC (-28 ± 1% vs. -16 ± 2%) and Q(tw,pot) (-53 ± 2% vs. -34 ± 2%) were far greater following KE compared to BIKE exercise, respectively. The greater degree of quadriceps fatigue following KE exercise was in proportion to the greater exercise time (9.1 ± 0.4 vs. 6.3 ± 0.5 min, P < 0.05), suggestive of a similar rate of peripheral fatigue development.

CONCLUSION

These data suggest that when the source of skeletal muscle afferent feedback is confined to a small muscle mass, the CNS tolerates a greater magnitude of peripheral fatigue and likely a greater intramuscular metabolic disturbance. An important implication of this finding is that the adoption of small muscle mass exercise may facilitate greater exercise-induced muscular adaptation.

Neuromuscular fatigue recovery following rapid and slow stretch–shortening cycle movements

The purpose of this study was to investigate underlying mechanisms and neuromuscular recovery patterns following rapid and slow stretch–shortening cycle (SSC) movements performed to fatigue. Fourteen (10 moderately trained (MT) and four highly trained (HT)) subjects completed rapid and slow SSC movements to fatigue. The rapid SSC movement consisted of continuous drop jumps from a 30 cm platform until a predetermined jump height was no longer maintained, and the slow SSC movement consisted of continuous squats to 90° of knee flexion at a load of 65% of subject’s one-repetition maximum until no further repetitions could be completed. Although blood lactate measures were significantly (p < 0.002) higher after the rapid SSC condition versus after the slow SSC condition, the recovery of neuromuscular properties (maximum voluntary contractions, twitch force, muscle compound action potential) following the two conditions to fatigue did not differ. The duration of the rapid SSC movement was dependent on the training status of the subject; HT subjects performed the rapid SSC longer (68.2%) than the MT subjects until fatigued. Thus, the neuromuscular fatigue recovery patterns were independent of the type of SSC movement, condition duration, and subject training status. Because rapid and slow SSC exercises induce similar fatigue patterns, training programs incorporating rapid SSC exercises can be developed similar to that prescribed in traditional slow SSC resistance training programs.

Four weeks of normobaric "live high-train low" do not alter muscular or systemic capacity for maintaining pH and K⁺ homeostasis during intense exercise

It was investigated if athletes subjected to 4 wk of living in normobaric hypoxia (3,000 m; 16 h/day) while training at 800-1,300 m ["live high-train low" (LHTL)] increase muscular and systemic capacity for maintaining pH and K(+) homeostasis as well as intense exercise performance. The design was double-blind and placebo controlled. Mean power during 30-s all-out cycling was similar before and immediately after LHTL (650 ± 31 vs. 628 ± 32 W; n = 10) and placebo exposure (658 ± 22 vs. 660 ± 23 W; n = 6). Supporting the performance data, arterial plasma pH, lactate, and K(+) during submaximal and maximal exercise were also unaffected by the intervention in both groups. In addition, muscle buffer capacity (in mmol H(+)·kg dry wt(-1)·pH(-1)) was similar before and after in both the LHTL (140 ± 12 vs. 140 ± 16) and placebo group (145 ± 2 vs. 140 ± 3). The expression of sarcolemmal H(+) transporters (Na(+)/H(+) exchanger 1, monocarboxylate transporters 1 and 4), as well as expression of Na(+)-K(+) pump subunits-α(1), -α(2), and -β(1) was also similar before and after the intervention. In conclusion, muscular and systemic capacity for maintaining pH and K(+) balance during exercise is similar before and after 4 wk of placebo-controlled normobaric LHTL. In accordance, 30-s all-out sprint ability was similar before and after LHTL.

Physiological changes in jeju crossbred riding horses by swim training

The changes in physiologic parameters by swim exercise duration were examined in five female well-trained Jeju crossbred riding horses that had riding experience of more than three years without swim training experience. The horses were performed with swim exercise for 10 min (60.0 m/min) once a day for 14 days. Physiologic characteristics and haematic parameters were measured before swimming, immediately after swimming, and after a 10 min rest at first day (D0), 7 days (D7), and 14 days (D14) of training. After 14 days of swim training, heart rate (p<0.05), blood glucose (p<0.05), lactate concentration (p<0.001), packed cell volume (p<0.01), and hemoglobin (p<0.01) measured immediately after swim and after 10 min rest showed significant lower values than those of D0. The results illustrate the benefits of swim training for riding horses and the need for the establishment of swimming routines of appropriate duration and intensity to maximize the advantages of swim training.

Influence of moderate hypoxia on tolerance to high-intensity exercise

It remains uncertain as how the reduction in systemic oxygen transport limits high-intensity exercise tolerance. 11 participants (5 males; age 35 ± 10 years; peak [Formula: see text] 3.5 ± 0.4 L min(-1)) performed cycle ergometry to the limit of tolerance: (1) a ramp test to determine ventilatory threshold (VT) and peak [Formula: see text]; (2) three to four constant-load tests in order to model the linear P-t (-1) relationship for estimation of intercept (critical power; CP) and slope (AWC). All tests were performed in a random order under moderate hypoxia (FiO(2) = 0.15) and normoxia. The linearity of the P-t (-1) relationship was retained under hypoxia, with a systematic reduction in CP (220 ± 25 W vs. 190 ± 28 W; P < 0.01) but no significant difference in AWC (11.7 ± 5.5 kJ vs. 12.1 ± 4.4 kJ; P > 0.05). However, large individual variations in the change of the latter were observed (-36 to +66%). A significant relationship was found between the % change in CP (r = 0.80, P < 0.01) and both peak [Formula: see text] (CP: r = -0.65, P < 0.05) and VT values recorded under normoxia (CP: r = -0.65, P < 0.05). The present study demonstrates the aerobic nature of the intercept of the P-t (-1) relationship, i.e. CP. However, the extreme within-individual changes in AWC do not support the original assumption that AWC reflects a finite energy store. Lower hypoxia-induced decrements in CP were observed in aerobically fitter participants. This study also demonstrates the greater ability these participants have to exercise at supra-CP but close to CP workloads under moderate hypoxia.

Acute and chronic effects of dietary nitrate supplementation on blood pressure and the physiological responses to moderate-intensity and incremental exercise

Dietary nitrate (NO3−) supplementation with beetroot juice (BR) over 4–6 days has been shown to reduce the O2cost of submaximal exercise and to improve exercise tolerance. However, it is not known whether shorter (or longer) periods of supplementation have similar (or greater) effects. We therefore investigated the effects of acute and chronic NO3−supplementation on resting blood pressure (BP) and the physiological responses to moderate-intensity exercise and ramp incremental cycle exercise in eight healthy subjects. Following baseline tests, the subjects were assigned in a balanced crossover design to receive BR (0.5 l/day; 5.2 mmol of NO3−/day) and placebo (PL; 0.5 l/day low-calorie juice cordial) treatments. The exercise protocol (two moderate-intensity step tests followed by a ramp test) was repeated 2.5 h following first ingestion (0.5 liter) and after 5 and 15 days of BR and PL. Plasma nitrite concentration (baseline: 454 ± 81 nM) was significantly elevated (+39% at 2.5 h postingestion; +25% at 5 days; +46% at 15 days; P < 0.05) and systolic and diastolic BP (baseline: 127 ± 6 and 72 ± 5 mmHg, respectively) were reduced by ∼4% throughout the BR supplementation period ( P < 0.05). Compared with PL, the steady-state V̇o2during moderate exercise was reduced by ∼4% after 2.5 h and remained similarly reduced after 5 and 15 days of BR ( P < 0.05). The ramp test peak power and the work rate at the gas exchange threshold (baseline: 322 ± 67 W and 89 ± 15 W, respectively) were elevated after 15 days of BR (331 ± 68 W and 105 ± 28 W; P < 0.05) but not PL (323 ± 68 W and 84 ± 18 W). These results indicate that dietary NO3−supplementation acutely reduces BP and the O2cost of submaximal exercise and that these effects are maintained for at least 15 days if supplementation is continued.

Dietary nitrate supplementation enhances muscle contractile efficiency during knee-extensor exercise in humans

The purpose of this study was to elucidate the mechanistic bases for the reported reduction in the O2cost of exercise following short-term dietary nitrate (NO3−) supplementation. In a randomized, double-blind, crossover study, seven men (aged 19–38 yr) consumed 500 ml/day of either nitrate-rich beetroot juice (BR, 5.1 mmol of NO3−/day) or placebo (PL, with negligible nitrate content) for 6 consecutive days, and completed a series of low-intensity and high-intensity “step” exercise tests on the last 3 days for the determination of the muscle metabolic (using31P-MRS) and pulmonary oxygen uptake (V̇o2) responses to exercise. On days 4–6, BR resulted in a significant increase in plasma [nitrite] (mean ± SE, PL 231 ± 76 vs. BR 547 ± 55 nM; P < 0.05). During low-intensity exercise, BR attenuated the reduction in muscle phosphocreatine concentration ([PCr]; PL 8.1 ± 1.2 vs. BR 5.2 ± 0.8 mM; P < 0.05) and the increase in V̇o2(PL 484 ± 41 vs. BR 362 ± 30 ml/min; P < 0.05). During high-intensity exercise, BR reduced the amplitudes of the [PCr] (PL 3.9 ± 1.1 vs. BR 1.6 ± 0.7 mM; P < 0.05) and V̇o2(PL 209 ± 30 vs. BR 100 ± 26 ml/min; P < 0.05) slow components and improved time to exhaustion (PL 586 ± 80 vs. BR 734 ± 109 s; P < 0.01). The total ATP turnover rate was estimated to be less for both low-intensity (PL 296 ± 58 vs. BR 192 ± 38 μM/s; P < 0.05) and high-intensity (PL 607 ± 65 vs. BR 436 ± 43 μM/s; P < 0.05) exercise. Thus the reduced O2cost of exercise following dietary NO3−supplementation appears to be due to a reduced ATP cost of muscle force production. The reduced muscle metabolic perturbation with NO3−supplementation allowed high-intensity exercise to be tolerated for a greater period of time.

Effects of membrane depolarization and changes in extracellular [K(+)] on the Ca (2+) transients of fast skeletal muscle fibers. Implications for muscle fatigue

Repetitive activation of skeletal muscle fibers leads to a reduced transmembrane K⁺ gradient. The resulting membrane depolarization has been proposed to play a major role in the onset of muscle fatigue. Nevertheless, raising the extracellular K⁺ (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ {\text{K}}_{\text{o}}^{ + } $$\end{document}) concentration (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ [ {\text{K}}^{ + } ]_{\text{o}} $$\end{document}) to 10 mM potentiates twitch force of rested amphibian and mammalian fibers. We used a double Vaseline gap method to simultaneously record action potentials (AP) and Ca²⁺ transients from rested frog fibers activated by single and tetanic stimulation (10 pulses, 100 Hz) at various \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ [ {\text{K}}^{ + } ]_{\text{o}} $$\end{document} and membrane potentials. Depolarization resulting from current injection or raised \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ [ {\text{K}}^{ + } ]_{\text{o}} $$\end{document} produced an increase in the resting [Ca²⁺]. Ca²⁺ transients elicited by single stimulation were potentiated by depolarization from −80 to −60 mV but markedly depressed by further depolarization. Potentiation was inversely correlated with a reduction in the amplitude, overshoot and duration of APs. Similar effects were found for the Ca²⁺ transients elicited by the first pulse of 100 Hz trains. Depression or block of Ca²⁺ transient in response to the 2nd to 10th pulses of 100 Hz trains was observed at smaller depolarizations as compared to that seen when using single stimulation. Changes in Ca²⁺ transients along the trains were associated with impaired or abortive APs. Raising \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ [ {\text{K}}^{ + } ]_{\text{o}} $$\end{document} to 10 mM potentiated Ca²⁺ transients elicited by single and tetanic stimulation, while raising \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ [ {\text{K}}^{ + } ]_{\text{o}} $$\end{document} to 15 mM markedly depressed both responses. The effects of 10 mM \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ {\text{K}}_{\text{o}}^{ + } $$\end{document} on Ca²⁺ transients, but not those of 15 mM \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ {\text{K}}_{\text{o}}^{ + } $$\end{document}, could be fully reversed by hyperpolarization. The results suggests that the force potentiating effects of 10 mM \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ {\text{K}}_{\text{o}}^{ + } $$\end{document} might be mediated by depolarization dependent changes in resting [Ca²⁺] and Ca²⁺ release, and that additional mechanisms might be involved in the effects of 15 mM \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ {\text{K}}_{\text{o}}^{ + } $$\end{document} on force generation.

Influence of hyperoxia on muscle metabolic responses and the power-duration relationship during severe-intensity exercise in humans: a 31P magnetic resonance spectroscopy study

Severe-intensity constant-work-rate exercise results in the attainment of maximal oxygen uptake, but the muscle metabolic milieu at the limit of tolerance (T(lim)) for such exercise remains to be elucidated. We hypothesized that T(lim) during severe-intensity exercise would be associated with the attainment of consistently low values of intramuscular phosphocreatine ([PCr]) and pH, as determined using (31)P magnetic resonance spectroscopy, irrespective of the work rate and the inspired O(2) fraction. We also hypothesized that hyperoxia would increase the asymptote of the hyperbolic power-duration relationship (the critical power, CP) without altering the curvature constant (W). Seven subjects (mean +/- s.d., age 30 +/- 9 years) completed four constant-work-rate knee-extension exercise bouts to the limit of tolerance (range, 3-10 min) both in normoxia (N) and in hyperoxia (H; 70% O(2)) inside the bore of 1.5 T superconducting magnet. The [PCr] (approximately 5-10% of resting baseline) and pH (approximately 6.65) at the limit of tolerance during each of the four trials was not significantly different either in normoxia or in hyperoxia. At the same fixed work rate, the overall rate at which [PCr] fell with time was attenuated in hyperoxia (mean response time: N, 59 +/- 20 versus H, 116 +/- 46 s; P < 0.05). The CP was higher (N, 16.1 +/- 2.6 versus H, 18.0 +/- 2.3 W; P < 0.05) and the W was lower (N, 1.92 +/- 0.70 versus H, 1.48 +/- 0.31 kJ; P < 0.05) in hyperoxia compared with normoxia. These data indicate that T(lim) during severe-intensity exercise is associated with the attainment of consistently low values of muscle [PCr] and pH. The CP and W parameters of the power-duration relationship were both sensitive to the inspiration of hyperoxic gas.

Swimming performance after passive and active recovery of various durations

PURPOSE

To examine the effects of active and passive recovery of various durations after a 100-m swimming test performed at maximal effort.

METHODS

Eleven competitive swimmers (5 males, 6 females, age: 17.3 +/- 0.6 y) completed two 100-m tests with a 15-min interval at a maximum swimming effort under three experimental conditions. The recovery between tests was 15 min passive (PAS), 5 min active, and 10 min passive (5ACT) or 10 min active and 5 min passive (10ACT). Self-selected active recovery started immediately after the first test, corresponding to 60 +/- 5% of the 100-m time. Blood samples were taken at rest, 5, 10, and 15 min after the first as well as 5 min after the second 100-m test for blood lactate determination. Heart rate was also recorded during the corresponding periods.

RESULTS

Performance time of the first 100 m was not different between conditions (P > .05). The second 100-m test after the 5ACT (64.49 +/- 3.85 s) condition was faster than 10ACT (65.49 +/- 4.63 s) and PAS (65.89 +/- 4.55 s) conditions (P < .05). Blood lactate during the 15-min recovery period between the 100-m efforts was lower in both active recovery conditions compared with passive recovery (P < .05). Heart rate was higher during the 5ACT and 10ACT conditions compared with PAS during the 15-min recovery period (P < .05).

CONCLUSION

Five minutes of active recovery during a 15-min interval period is adequate to facilitate blood lactate removal and enhance performance in swimmers. Passive recovery and/or 10 min of active recovery is not recommended.

Effect of caffeine on the neuromuscular system--potential as an ergogenic aid

The ergogenic effect of caffeine on endurance exercise performance is multifactorial; however, there is evidence for an effect on both the central nervous system and the excitation-contraction coupling of skeletal muscle. The increase in exercise performance seen following intracerebroventrical caffeine injection in rats provides strong evidence for a central ergogenic effect. The central ergogenic effect is not likely related to the ability of caffeine to promote wakefulness, but could be due to an increase in the pain and effort perception threshold. There is no evidence that caffeine alters peripheral nerve conduction velocity or neuromuscular transmission, and 1 study showed that motor unit synchronization was not altered by caffeine. Studies have also shown that caffeine can have a direct effect on skeletal muscle that could be ergogenic. For example, patients with high cervical spinal cord lesions showed improvements in stimulated contractile force during cycling, in spite of the fact that they have no peripheral pain input and no sympathetic nervous system response. Two studies have found a potentiation of force production during submaximal stimulation intensities, and 1 found that the M-wave amplitude was not altered by caffeine. Together, these studies suggest that caffeine can enhance contractile force during submaximal contractions by potentiating calcium release from the ryanodine receptor, not by altering sarcoplasmic excitability. Furthermore, the potentiation of force during submaximal electrical stimulation is identical in habitual and nonhabitual caffeine consumers. In summary, the ergogenic effects of caffeine during endurance activity are mediated partly by enhanced contractile force and partly by a reduction in perceived exertion, possibly though a blunting of effort and (or) pain.

Myosin regulatory light chain phosphorylation inhibits shortening velocities of skeletal muscle fibers in the presence of the myosin inhibitor blebbistatin

Phosphorylation of skeletal myosin regulatory light chain (RLC) occurs in fatigue and may play a role in the inhibition of shortening velocities observed in vivo. Forces and shortening velocities were measured in permeabilized rabbit psoas fibers with either phosphorylated or dephosphorylated RLCs and in the presence or absence of the myosin inhibitor blebbistatin. Addition of 20 microM blebbistatin decreased tensions by approximately 80% in fibers, independent of phosphorylation. In blebbistatin maximal shortening velocities (V(max)) at 30 degrees C, were decreased by 45% (3.2 +/- 0.34 vs. 5.8 +/- 0.18 lengths/s) in phosphorylated fibers but were not inhibited in dephosphorylated fibers (6.0 +/- 0.30 vs. 5.4 +/- 0.30). In the presence of 20 microM blebbistatin, K(m) for V(max) as a function of [ATP] was lower for phosphorylated fibers than for dephosphorylated fibers (50 +/- 20 vs. 330 +/- 84 microM) indicating that the apparent binding of ATP is stronger in these fibers. Phosphorylation of RLC in situ during fiber preparation or by addition of myosin light chain kinase yielded similar data. RLC phosphorylation inhibited velocity in blebbistatin at both 30 and 10 degrees C, unlike previous reports where RLC phosphorylation only affected shortening velocities at higher temperatures.

Amyloid-β protein impairs Ca2+ release and contractility in skeletal muscle

Inclusion body myositis (IBM), the most common muscle disorder in the elderly, is partly characterized by dysregulation of β-amyloid precursor protein (βAPP) expression and abnormal, intracellular accumulation of full-length βAPP and β-amyloid epitopes. The present study examined the effects of β-amyloid accumulation on force generation and Ca(2+) release in skeletal muscle from transgenic mice harboring human βAPP and assessed the consequence of Aβ(1-42) modulation of the ryanodine receptor Ca(2+) release channels (RyRs). β-Amyloid laden muscle produced less peak force and exhibited Ca(2+) transients with smaller amplitude. To determine whether modification of RyRs by β-amyloid underlie the effects observed in muscle, in vitro Ca(2+) release assays and RyR reconstituted in planar lipid bilayer experiments were conducted in the presence of Aβ(1-42). Application of Aβ(1-42) to RyRs in bilayers resulted in an increased channel open probability and changes in gating kinetics, while addition of Aβ(1-42) to the rabbit SR vesicles resulted in RyR-mediated Ca(2+) release. These data may relate altered βAPP metabolism in IBM to reductions in RyR-mediated Ca(2+) release and muscle contractility.

Muscle function in men and women during maximal eccentric exercise

This study assessed muscle fatigue patterns of the elbow flexors in untrained men and women to determine if sex differences exist during acute maximal eccentric exercise. High-intensity eccentric exercise is often used by athletes to elicit gains in muscle strength and size gains. Development of fatigue during this type of exercise can increase risk of injury; therefore, it is important to understand fatigue patterns during eccentric exercise to minimize injury risk exposure while still promoting training effects. While many isometric exercise studies have demonstrated that women show less fatigue, the patterns of fatigue during purely eccentric exercise have not been assessed in men and women. Based on the lack of sex differences in overall strength loss immediately post-eccentric exercise, it was hypothesized that women and men would have similar relative fatigue pattern responses (i.e., change from baseline) during a single bout of maximal eccentric exercise. Forty-six subjects (24 women and 22 men) completed 5 sets of 10 maximal eccentric contractions on an isokinetic dynamometer. Maximal voluntary isometric contraction strength was assessed at baseline and immediately following each exercise set. Maximal eccentric torque and contractile properties (i.e., contraction time, work, half relaxation time, and maximal rate of torque development) were calculated for each contraction. Men and women demonstrated similar relative isometric (32% for men and 39% for women) and eccentric (32% for men and 39% for women) fatigue as well as similar deficits in work done and rates of torque development and relaxation during exercise (p > 0.05). Untrained men and women displayed similar relative responses in all measures of muscle function during a single bout of maximal eccentric exercise of the elbow flexors. Thus, there is no reason to suspect that women may be more vulnerable to fatigue-related injury.

Elevation in heat shock protein 72 mRNA following contractions in isolated single skeletal muscle fibers

The purpose of the present study was 1) to develop a stable model for measuring contraction-induced elevations in mRNA in single skeletal muscle fibers and 2) to utilize this model to investigate the response of heat shock protein 72 (HSP72) mRNA following an acute bout of fatiguing contractions. Living, intact skeletal muscle fibers were microdissected from lumbrical muscle of Xenopus laevis and either electrically stimulated for 15 min of tetanic contractions (EX; n=26) or not stimulated to contract (REST; n=14). The relative mean developed tension of EX fibers decreased to 29+/-7% of initial peak tension at the stimulation end point. Following treatment, individual fibers were allowed to recover for 1 (n=9), 2 (n=8), or 4 h (n=9) prior to isolation of total cellular mRNA. HSP72, HSP60, and cardiac alpha-actin mRNA content were then assessed in individual fibers using quantitative PCR detection. Relative HSP72 mRNA content was significantly (P<0.05) elevated at the 2-h postcontraction time point relative to REST fibers when normalized to either HSP60 (18.5+/-7.5-fold) or cardiac alpha-actin (14.7+/-4.3-fold), although not at the 1- or 4-h time points. These data indicate that 1) extraction of RNA followed by relative quantification of mRNA of select genes in isolated single skeletal muscle fibers can be reliably performed, 2) HSP60 and cardiac alpha-actin are suitable endogenous normalizing genes in skeletal muscle following contractions, and 3) a significantly elevated content of HSP72 mRNA is detectable in skeletal muscle 2 h after a single bout of fatiguing contractions, despite minimal temperature changes and without influence from extracellular sources.

Modulation of the actomyosin interaction during fatigue of skeletal muscle

Fatigue of skeletal muscle involves many systems beginning with the central nervous system and ending with the contractile machinery. This review concentrates on those factors that directly affect the actomyosin interaction: the build-up of metabolites; myosin phosphorylation; and oxidation of the myofibrillar proteins by free radicals. The decrease in [ATP] and increase in [ADP] appear to play little role in modulating function. The increase in phosphate inhibits tension. The decrease in pH, long thought to be a major factor, is now known to play a more minor role. Myosin phosphorylation potentiates the force achieved in a twitch, and a further role in inhibiting velocity is proposed. Protein oxidation can both potentiate and inhibit the actomyosin interaction. It is concluded that these factors, taken together, do not fully explain the inhibition of the actomyosin interaction observed in living fibers, and thus additional modulators of this interaction remain to be discovered.

Muscle metabolic responses to exercise above and below the "critical power" assessed using 31P-MRS

We tested the hypothesis that the asymptote of the hyperbolic relationship between work rate and time to exhaustion during muscular exercise, the "critical power" (CP), represents the highest constant work rate that can be sustained without a progressive loss of homeostasis [as assessed using (31)P magnetic resonance spectroscopy (MRS) measurements of muscle metabolites]. Six healthy male subjects initially completed single-leg knee-extension exercise at three to four different constant work rates to the limit of tolerance (range 3-18 min) for estimation of the CP (mean +/- SD, 20 +/- 2 W). Subsequently, the subjects exercised at work rates 10% below CP (<CP) for 20 min and 10% above CP (>CP) for as long as possible, while the metabolic responses in the contracting quadriceps muscle, i.e., phosphorylcreatine concentration ([PCr]), P(i) concentration ([P(i)]), and pH, were estimated using (31)P-MRS. All subjects completed 20 min of <CP exercise without duress, whereas the limit of tolerance during >CP exercise was 14.7 +/- 7.1 min. During <CP exercise, stable values for [PCr], [P(i)], and pH were attained within 3 min after the onset of exercise, and there were no further significant changes in these variables (end-exercise values = 68 +/- 11% of baseline [PCr], 314 +/- 216% of baseline [P(i)], and pH 7.01 +/- 0.03). During >CP exercise, however, [PCr] continued to fall to the point of exhaustion and [P(i)] and pH changed precipitously to values that are typically observed at the termination of high-intensity exhaustive exercise (end-exercise values = 26 +/- 16% of baseline [PCr], 564 +/- 167% of baseline [P(i)], and pH 6.87 +/- 0.10, all P < 0.05 vs. <CP exercise). These data support the hypothesis that the CP represents the highest constant work rate that can be sustained without a progressive depletion of muscle high-energy phosphates and a rapid accumulation of metabolites (i.e., H(+) concentration and [P(i)]), which have been associated with the fatigue process.

Children are more susceptible to central fatigue than adults

Performance in high-intensity exercise is dependent on the ability to activate motor units. The main aim of this study was to test the hypothesis that adult men and women (age 19-27 years) are able to maintain higher levels of voluntary activation (VA) in knee extensor muscles than boys and girls (age 12-14 years). The volunteers (n = 7 in each group) performed three 5-s maximal voluntary contractions (MVCs) and a continuous 2-min MVC. The VA and fatigue of the muscles was assessed by applying 250-ms 100-HZ test tetani (TT100HZ). During brief MVCs girls showed lower VA than women, but the difference between boys and men was not significant. During the 2-min MVC, VA in boys and girls was more depressed than in adults. The end-exercise values of the relative TT100HZ torque correlated with the average VA during the exercise. Thus, the results of the study support the hypothesis that children are more susceptible to central fatigue than adults. This should be taken into account when evaluating results of fitness tests that require high levels of motor unit activation.

Effect of different intensities of active recovery on sprint swimming performance

Active recovery reduces blood lactate concentration faster than passive recovery and, when the proper intensity is applied, a positive effect on performance is expected. The purpose of the study was to investigate the effect of different intensities of active recovery on performance during repeated sprint swimming. Nine male well-trained swimmers performed 8 repetitions of 25 m sprints (8 x 25 m) interspersed with 45 s intervals, followed by a 50 m sprint test 6 min later. During the 45 s and 6 min interval periods, swimmers either rested passively (PAS) or swam at an intensity corresponding to 50% (ACT50) and 60% (ACT60) of their individual 100 m velocity. Blood lactate was higher during PAS compared with ACT50 and ACT60 trials (p < 0.05), whereas plasma ammonia and glycerol concentration were not different between trials (p > 0.05). Mean performance time for the 8 x 25 m sprints was better in the PAS compared with the ACT50 and ACT60 trials (PAS: 13.10 +/- 0.07 vs. ACT50: 13.43 +/- 0.10 and ACT60: 13.47 +/- 0.10s, p < 0.05). The first 25 m sprint was not different across trials (p > 0.05), but performance decreased after sprint 2 during active recovery trials (ACT50 and ACT60) compared with the passive recovery (PAS) trial (p < 0.05). Performance time for the 50 m sprint performed 6 min after the 8 x 25 m sprints was no different between trials (p > 0.05). These results indicate that active recovery at intensities corresponding to 50% and 60% of the 100 m velocity during repeated swimming sprints decreases performance. Active recovery reduces blood lactate concentration, but does not affect performance on a 50 m sprint when 6 min recovery is provided. Passive recovery is advised during short-interval repeated sprint training in well-trained swimmers.

Effect of hypoxia on fatigue development in rat muscle composed of different fibre types

This study investigated the relationship between hypoxia and the rate of fatigue development in contracting rat hindlimb muscles composed primarily of different fibre types. Hindlimb muscles of 11 rats were exposed, and the soleus (SOL) and gastrocnemius/plantaris (GP) were each isolated with circulation intact and attached to individual force transducers. Rats were then equilibrated with either normoxic (N; arterial partial pressure of O(2) 87.7 +/- 1.5 mmHg) or hypoxic conditions (H; arterial partial pressure of O(2) 30.0 +/- 2.4 mmHg) using an inspired O(2) fraction of 0.21 and 0.10, respectively. The stimulation protocol consisted of 2 min each at 0.125, 0.25, 0.33 and 0.5 tetanic contractions s(-1) sequentially for both conditions. Following the 8 min stimulation period, relative developed muscle tension (% of maximal) was nearly identical for both H and N in SOL (54.2 +/- 3.5 versus 54.3 +/- 4.2%), but was significantly (P < 0.05) lower in H than N (10.8 +/- 0.9 versus 43.0 +/- 8.9%) in GP, indicating a greater amount of fatigue during hypoxia only in the GP. Soleus phosphocreatine (PCr) content fell to similar levels (24.1 +/- 1.6 versus 21.1 +/- 4.9 mmol (kg dry weight (dw))(-1)) during both H and N, but in the white portion of the gastrocnemius (WG), PCr was significantly lower following H than N (14.3 +/- 1.5 versus 34.0 +/- 6.0 mmol (kg dw)(-1)). Similarly, muscle lactate increased in both fibre types at fatigue, but only in WG was the increase significantly greater with H (SOL 7.1 +/- 2.0 versus 5.3 +/- 1.1 mmol (kg dw)(-1); WG 13.7 +/- 4.5 versus 5.3 +/- 2.2 mmol (kg dw)(-1)). Increases in calculated muscle [H(+)], free ADP and free AMP were similar between N and H in SOL but were significantly greater during H compared with N in WG. These data demonstrate that hypoxia induces greater fatigue and disruption of cellular homeostasis in rat hindlimb muscle composed primarily of fibres with low oxidative capacity compared with those of a more oxidative type.

Influence of endurance training on muscle [PCr] kinetics during high-intensity exercise

We hypothesized that a period of endurance training would result in a speeding of muscle phosphocreatine concentration ([PCr]) kinetics over the fundamental phase of the response and a reduction in the amplitude of the [PCr] slow component during high-intensity exercise. Six male subjects (age 26 +/- 5 yr) completed 5 wk of single-legged knee-extension exercise training with the alternate leg serving as a control. Before and after the intervention period, the subjects completed incremental and high-intensity step exercise tests of 6-min duration with both legs separately inside the bore of a whole-body magnetic resonance spectrometer. The time-to-exhaustion during incremental exercise was not changed in the control leg [preintervention group (PRE): 19.4 +/- 2.3 min vs. postintervention group (POST): 19.4 +/- 1.9 min] but was significantly increased in the trained leg (PRE: 19.6 +/- 1.6 min vs. POST: 22.0 +/- 2.2 min; P < 0.05). During step exercise, there were no significant changes in the control leg, but end-exercise pH and [PCr] were higher after vs. before training. The time constant for the [PCr] kinetics over the fundamental exponential region of the response was not significantly altered in either the control leg (PRE: 40 +/- 13 s vs. POST: 43 +/- 10 s) or the trained leg (PRE: 38 +/- 8 s vs. POST: 40 +/- 12 s). However, the amplitude of the [PCr] slow component was significantly reduced in the trained leg (PRE: 15 +/- 7 vs. POST: 7 +/- 7% change in [PCr]; P < 0.05) with there being no change in the control leg (PRE: 13 +/- 8 vs. POST: 12 +/- 10% change in [PCr]). The attenuation of the [PCr] slow component might be mechanistically linked with enhanced exercise tolerance following endurance training.