The multiple roles of phosphate in muscle fatigue

Enhancing exercise performance and recovery through sodium bicarbonate supplementation: introducing the ingestion recovery framework

Sodium bicarbonate (SB) supplementation is an ergogenic strategy for athletes competing in high-intensity exercise, but the efficacy of SB for accelerating recovery from exercise and thus improving performance during repeated bouts of exercise is not fully understood. In a similar fashion to using SB as a pre-exercise buffer, it is possible accelerated restoration of blood pH and bicarbonate following an exercise bout mechanistically underpins the use of SB as a recovery aid. Physiological mechanisms contributing to beneficial effects for SB during repeated bout exercise could be more far-reaching however, as alterations in strong ion difference (SID) and attenuated cellular stress response might also contribute to accelerated recovery from exercise. From inspection of existing literature, ingestion of 0.3 g kg-1 body mass SB ~60-90 min pre-exercise seems to be the most common dosage strategy, but there is evidence emerging for the potential application of post-exercise supplementation timing, gradual SB doses throughout a competition day, or even ingestion during exercise. Based on this review of literature, an SB ingestion recovery framework is proposed to guide athletes and practitioners on the use of SB to enhance performance for multiple bouts of exercise.

Skeletal Muscle Injury in Chronic Kidney Disease-From Histologic Changes to Molecular Mechanisms and to Novel Therapies

Chronic kidney disease (CKD) is associated with significant reductions in lean body mass and in the mass of various tissues, including skeletal muscle, which causes fatigue and contributes to high mortality rates. In CKD, the cellular protein turnover is imbalanced, with protein degradation outweighing protein synthesis, leading to a loss of protein and cell mass, which impairs tissue function. As CKD itself, skeletal muscle wasting, or sarcopenia, can have various origins and causes, and both CKD and sarcopenia share common risk factors, such as diabetes, obesity, and age. While these pathologies together with reduced physical performance and malnutrition contribute to muscle loss, they cannot explain all features of CKD-associated sarcopenia. Metabolic acidosis, systemic inflammation, insulin resistance and the accumulation of uremic toxins have been identified as additional factors that occur in CKD and that can contribute to sarcopenia. Here, we discuss the elevation of systemic phosphate levels, also called hyperphosphatemia, and the imbalance in the endocrine regulators of phosphate metabolism as another CKD-associated pathology that can directly and indirectly harm skeletal muscle tissue. To identify causes, affected cell types, and the mechanisms of sarcopenia and thereby novel targets for therapeutic interventions, it is important to first characterize the precise pathologic changes on molecular, cellular, and histologic levels, and to do so in CKD patients as well as in animal models of CKD, which we describe here in detail. We also discuss the currently known pathomechanisms and therapeutic approaches of CKD-associated sarcopenia, as well as the effects of hyperphosphatemia and the novel drug targets it could provide to protect skeletal muscle in CKD.

Disuse-Induced Muscle Fatigue: Facts and Assumptions

Skeletal muscle unloading occurs during a wide range of conditions, from space flight to bed rest. The unloaded muscle undergoes negative functional changes, which include increased fatigue. The mechanisms of unloading-induced fatigue are far from complete understanding and cannot be explained by muscle atrophy only. In this review, we summarize the data concerning unloading-induced fatigue in different muscles and different unloading models and provide several potential mechanisms of unloading-induced fatigue based on recent experimental data. The unloading-induced changes leading to increased fatigue include both neurobiological and intramuscular processes. The development of intramuscular fatigue seems to be mainly contributed by the transformation of soleus muscle fibers from a fatigue-resistant, "oxidative" "slow" phenotype to a "fast" "glycolytic" one. This process includes slow-to-fast fiber-type shift and mitochondrial density decline, as well as the disruption of activating signaling interconnections between slow-type myosin expression and mitochondrial biogenesis. A vast pool of relevant literature suggests that these events are triggered by the inactivation of muscle fibers in the early stages of muscle unloading, leading to the accumulation of high-energy phosphates and calcium ions in the myoplasm, as well as NO decrease. Disturbance of these secondary messengers leads to structural changes in muscles that, in turn, cause increased fatigue.

The Effects of Anchor Schemes on Performance Fatigability, Neuromuscular Responses and the Perceived Sensations That Contributed to Task Termination

The present study examined the effect of anchor schemes on the time to task failure (TTF), performance fatigability, neuromuscular responses, and the perceived sensations that contributed to task termination following the sustained, isometric forearm flexion tasks. Eight women completed sustained, isometric forearm flexion tasks anchored to RPE = 8 (RPEFT) and the torque (TRQFT) that corresponded to RPE = 8. The subjects performed pre-test and post-test maximal isometric contractions to quantify performance fatigability and changes in electromyographic amplitude (EMG AMP) and neuromuscular efficiency (NME). In addition, the subjects completed a post-test questionnaire (PTQ) to quantify the contributions of perceived sensations to task termination. Repeated measure ANOVAs were used to assess the mean differences for TTF, performance fatigability, and neuromuscular responses. Wilcoxon Signed Rank Tests were used to assess the differences between anchor schemes for the average values from the PTQ item scores. For TTF, the RPEFT was longer than the TRQFT (174.9 ± 85.6 vs. 65.6 ± 68.0 s; p = 0.006). Collapsed across the anchor scheme, there were decreases in torque (23.7 ± 5.5 Nm vs. 19.6 ± 4.9 Nm; p < 0.001) and NME (1.00 ± 0.00 vs. 0.76 ± 0.15; p = 0.003). There were no significant (p > 0.577) changes for EMG AMP. For the PTQ, there were no differences (p > 0.05) between anchor schemes. There were, however, inter-individual differences in the response scores. The current findings indicated that performance fatigability was likely due to peripheral fatigue (based on NME), not central fatigue (based on EMG AMP). Furthermore, the use of a PTQ may serve as a simple tool to assess the contributions of perceived sensations to task termination.

Utilizing the RPE-Clamp model to examine interactions among factors associated with perceived fatigability and performance fatigability in women and men

PURPOSE

The purpose of the present study was to examine the interactions between perceived fatigability and performance fatigability in women and men by utilizing the RPE-Clamp model to assess the fatigue-induced effects of a sustained, isometric forearm flexion task anchored to RPE = 8 on time to task failure (TTF), torque, and neuromuscular responses.

METHODS

Twenty adults (10 men and 10 women) performed two, 3 s forearm flexion maximal voluntary isometric contractions (MVICs) followed by a sustained, isometric forearm flexion task anchored to RPE = 8 using the OMNI-RES (0-10) scale at an elbow joint angle of 100°. Electromyographic amplitude (EMG AMP) was recorded from the biceps brachii. Torque and EMG AMP values resulting from the sustained task were normalized to the pretest MVIC. Neuromuscular efficiency was defined as NME = normalized torque/normalized EMG AMP. Mixed factorial ANOVAs and Bonferroni corrected dependent t tests and independent t tests were used to examine differences across time and between sex for torque and neuromuscular parameters.

RESULTS

There were no differences between the women and men for the fatigue-induced decreases in torque, EMG AMP, or NME, and the mean decreases (collapsed across sex) were 50.3 ± 8.6 to 2.8 ± 2.9% MVIC, 54.7 ± 12.0 to 19.6 ± 5.3% MVIC, and 0.94 ± 0.19 to 0.34 ± 0.16, respectively. Furthermore, there were no differences between the women and men for TTF (251.8 ± 74.1 vs. 258.7 ± 77.9 s).

CONCLUSION

The results suggested that the voluntary reductions in torque to maintain RPE and the decreases in NME were likely due to group III/IV afferent feedback from peripheral fatigue that resulted in excitation-contraction coupling failure.

Environmental-related variation of stoichiometric traits in body and organs of non-native sailfin catfishes Pterygoplichthys spp

Intraspecific variation in stoichiometric traits was thought to be an adaptive response to reduce the elemental imbalance between organism and diet in the habitat. Studying the spatial variation of stoichiometric traits of non-native species and the factors contributing to the variation could help to better understand the invasion mechanism of non-native fish. In this study, stoichiometric traits (i.e. carbon [C], phosphorus [P], calcium [Ca] and their ratios) variation in the body and organs of non-native sailfin catfishes Pterygoplichthys spp. were investigated across 13 river sections in the main river basins of Guangdong province. The relationships between environmental factors and stoichiometric traits were analyzed using a general linear model and an information-theoretic approach. A manipulated feeding experiment was conducted to investigate the impact of food quality on the stoichiometry of sailfin catfishes in a greenhouse. Sailfin catfishes exhibited considerable variability in body and organ elemental composition. Site identity was the main factor contributing to the variation, which could be explained by a combination of environmental factors including climate, diet quality, fish species richness and trophic status in the invaded rivers. Water chemistry (i.e. total nitrogen and phosphorus, ammonia nitrogen and soluble reactive phosphorus) contributed to the most variation of stoichiometric traits. Imbalances of P and Ca between sailfin catfishes and food resources varied among sampling sites, reflecting the spatial heterogeneity of nutrients limitation. Juvenile sailfin catfishes exhibited stoichiometric homeostasis (0 < 1/H < 0.25) for all elemental contents and ratios in the feeding experiment. These findings suggested variation in stoichiometric traits of sailfin catfishes might be attributed to the changes in elemental metabolism to cope with context-specific environments. This study provided heuristic knowledge about environmental-related variation in stoichiometric traits, which could enhance the understanding of the non-native species' adaptation to resource fluctuation in the invaded ecosystems.

Effect of Six-Week Speed Endurance Training on Peripheral Fatigue

(1) Speed endurance training (inducing a high blood lactate concentration) delays excitation-contraction coupling impairment, thus providing more space for high-frequency fatigue to occur in the early stage of maximal concentric actions. This study aimed to test the hypothesis that the maintenance type of speed endurance training may shift peripheral fatigue from low-frequency to high-frequency fatigue after the 15 s long Wingate test. (2) Six students of physical education performed the corresponding training for six weeks. Before and after this period, they were tested for low- and high-frequency fatigue after the 15 s long Wingate test; additionally, their blood lactate concentrations, maximal cycling power, work, fatigue index, and muscle twitch responses were also tested. (3) The training increased the maximal cycling power and work (p < 0.001 and p < 0.01, respectively) with minor changes in the mean fatigue index and blood lactate concentration (both p > 0.05). Low-frequency dominant fatigue before the training showed a trend toward high-frequency dominant fatigue after the training (p > 0.05). (4) The results showed that the 15 s Wingate test failed to induce significant high-frequency fatigue. Even though it displayed a substantial fatigue index, the changes in favor of high-frequency fatigue were too small to be relevant.

Creatine Enhances the Effects of Cluster-Set Resistance Training on Lower-Limb Body Composition and Strength in Resistance-Trained Men: A Pilot Study

Creatine monohydrate (CrM) supplementation has been shown to improve body composition and muscle strength when combined with resistance training (RT); however, no study has evaluated the combination of this nutritional strategy with cluster-set resistance training (CS-RT). The purpose of this pilot study was to evaluate the effects of CrM supplementation during a high-protein diet and a CS-RT program on lower-limb fat-free mass (LL-FFM) and muscular strength. Twenty-three resistance-trained men (>2 years of training experience, 26.6 ± 8.1 years, 176.3 ± 6.8 cm, 75.6 ± 8.9 kg) participated in this study. Subjects were randomly allocated to a CS-RT+CrM (n = 8), a CS-RT (n = 8), or a control group (n = 7). The CS-RT+CrM group followed a CrM supplementation protocol with 0.1 g·kg-1·day-1 over eight weeks. Two sessions per week of lower-limb CS-RT were performed. LL-FFM corrected for fat-free adipose tissue (dual-energy X-ray absorptiometry) and muscle strength (back squat 1 repetition maximum (SQ-1RM) and countermovement jump (CMJ)) were measured pre- and post-intervention. Significant improvements were found in whole-body fat mass, fat percentage, LL-fat mass, LL-FFM, and SQ-1RM in the CS-RT+CrM and CS-RT groups; however, larger effect sizes were obtained in the CS-RT+CrM group regarding whole body FFM (0.64 versus 0.16), lower-limb FFM (0.62 versus 0.18), and SQ-1RM (1.23 versus 0.75) when compared to the CS-RT group. CMJ showed a significant improvement in the CS-RT+CrM group with no significant changes in CS-RT or control groups. No significant differences were found between groups. Eight weeks of CrM supplementation plus a high-protein diet during a CS-RT program has a higher clinical meaningfulness on lower-limb body composition and strength-related variables in trained males than CS-RT alone. Further research might study the potential health and therapeutic effects of this nutrition and exercise strategy.

Perceived mental strain dissociates from perceived physical strain during relative intensity submaximal exercise on ascent from low to high altitude

Perceived fatigability, which has perception of physical strain and of mental strain as its components, can impact exercise tolerance. Upon ascent to high altitude, low landers experience reduced exercise capacity and reduced tolerance for a given absolute submaximal work rate. It is established that perceived physical strain tracks with relative exercise intensity. However, it is not known how altitude ascent affects perceived mental strain relative to perceived physical strain. We tested the hypothesis that when exercising at the same relative exercise intensity perceived physical strain will remain unchanged whereas perceived mental strain will decrease on ascent from low to high altitude in the Everest region in Nepal. Twelve hours after arriving at each of three elevations; 1400 m, 3440 m, and 4240 m, 12 untrained participants used the task effort awareness (TEA) and physical-rating of perceived exertion (P-RPE) scales to report perceived mental and physical strain during a 20 min walking test at a self-monitored heart rate reserve (HRR) range of 40-60% (Polar HR Monitor). TEA and P-RPE were recorded twice during exercise (5-7 min and 14-16 min). Neither P-RPE (1400 m: 11.1 ± 1.8, 3440 m: 10.7 ± 1.2, 4240 m: 11.5 ± 1.5) nor %HRR (1400 m: 55.25 ± 7.34, 3440 m: 51.70 ± 6.70, 4240 m: 50.17 ± 4.02) changed as altitude increased. TEA decreased at 4240 m (2.05 ± 0.71) compared to 1400 m (3.44 ± 0.84)--this change was not correlated with any change in %HRR nor was it due to a change in core affect. These findings support our hypothesis and demonstrate the independence of perceived physical and perceived mental strain components of perceived fatigability. Implications for exercise tolerance remain to be determined.

Exercising muscle mass influences neuromuscular, cardiorespiratory, and perceptual responses during and following ramp-incremental cycling to task failure

Neuromuscular (NM), cardiorespiratory, and perceptual responses to maximal-graded exercise using different amounts of active muscle mass remain unclear. We hypothesized that during dynamic exercise, peripheral NM fatigue (declined twitch force) and muscle pain would be greater using smaller muscle mass, whereas central fatigue (declined voluntary activation) and ventilatory variables would be greater using larger muscle mass. Twelve males (29.8 ± 4.7 years) performed two ramp-incremental cycling tests until task failure: 1) single-leg (SL) with 10 W·min^-1 ramp and 2) double-leg (DL) with 20 W·min^-1 ramp. NM fatigue was assessed at baseline, task failure (post), and after 1, 4, and 8 min of recovery. Cardiorespiratory and perceptual variables [i.e., ratings of perceived exertion (RPE), pain, and dyspnea] were measured throughout cycling. Exercise duration was similar between sessions (SL: 857.7 ± 263.6 s; DL: 855.0 ± 218.8 s; P = 0.923), and higher absolute peak power output was attained in DL (SL: 163.2 ± 43.8 W; DL: 307.0 ± 72.0 W; P < 0.001). Although central fatigue did not differ between conditions (SL: -6.6 ± 6.5%; DL: -3.5 ± 4.8%; P = 0.091), maximal voluntary contraction (SL: -41.6 ± 10.9%; DL: -33.7 ± 8.5%; P = 0.032) and single twitch forces (SL: -59.4 ± 18.8%; DL: -46.2 ± 16.2%; P = 0.003) declined more following SL. DL elicited higher peak oxygen uptake (SL: 42.1 ± 10.0 mL·kg^-1·min^-1; DL: 50.3 ± 9.3 mL·kg^-1·min^-1; P < 0.001), ventilation (SL: 137.1 ± 38.1 L·min^-1; DL: 171.5 ± 33.2 L·min^-1; P < 0.001), and heart rate (SL: 167 ± 21 bpm; DL: 187 ± 8 bpm; P = 0.005). Dyspnea (P = 0.025) was higher in DL; however, RPE (P = 0.005) and pain (P < 0.001) were higher in SL. These results suggest that interplay between NM, cardiorespiratory, and perceptual determinants of exercise performance during ramp-incremental cycling to task failure is muscle mass dependent.

Muscle Ionic Shifts During Exercise: Implications for Fatigue and Exercise Performance

Exercise causes major shifts in multiple ions (e.g., K⁺ , Na⁺ , H⁺ , lactate^- , Ca²⁺ , and Cl^- ) during muscle activity that contributes to development of muscle fatigue. Sarcolemmal processes can be impaired by the trans-sarcolemmal rundown of ion gradients for K⁺ , Na⁺ , and Ca²⁺ during fatiguing exercise, while changes in gradients for Cl^- and Cl^- conductance may exert either protective or detrimental effects on fatigue. Myocellular H⁺ accumulation may also contribute to fatigue development by lowering glycolytic rate and has been shown to act synergistically with inorganic phosphate (Pi) to compromise cross-bridge function. In addition, sarcoplasmic reticulum Ca²⁺ release function is severely affected by fatiguing exercise. Skeletal muscle has a multitude of ion transport systems that counter exercise-related ionic shifts of which the Na⁺ /K⁺ -ATPase is of major importance. Metabolic perturbations occurring during exercise can exacerbate trans-sarcolemmal ionic shifts, in particular for K⁺ and Cl^- , respectively via metabolic regulation of the ATP-sensitive K⁺ channel (K_ATP ) and the chloride channel isoform 1 (ClC-1). Ion transport systems are highly adaptable to exercise training resulting in an enhanced ability to counter ionic disturbances to delay fatigue and improve exercise performance. In this article, we discuss (i) the ionic shifts occurring during exercise, (ii) the role of ion transport systems in skeletal muscle for ionic regulation, (iii) how ionic disturbances affect sarcolemmal processes and muscle fatigue, (iv) how metabolic perturbations exacerbate ionic shifts during exercise, and (v) how pharmacological manipulation and exercise training regulate ion transport systems to influence exercise performance in humans. © 2021 American Physiological Society. Compr Physiol 11:1895-1959, 2021.

Patient-Reported Outcomes from a Randomized, Active-Controlled, Open-Label, Phase 3 Trial of Burosumab Versus Conventional Therapy in Children with X-Linked Hypophosphatemia

Changing to burosumab, a monoclonal antibody targeting fibroblast growth factor 23, significantly improved phosphorus homeostasis, rickets, lower-extremity deformities, mobility, and growth versus continuing oral phosphate and active vitamin D (conventional therapy) in a randomized, open-label, phase 3 trial involving children aged 1-12 years with X-linked hypophosphatemia. Patients were randomized (1:1) to subcutaneous burosumab or to continue conventional therapy. We present patient-reported outcomes (PROs) from this trial for children aged ≥ 5 years at screening (n = 35), using a Patient-Reported Outcomes Measurement Information System (PROMIS) questionnaire and SF-10 Health Survey for Children. PROMIS pain interference, physical function mobility, and fatigue scores improved from baseline with burosumab at weeks 40 and 64, but changed little with continued conventional therapy. Pain interference scores differed significantly between groups at week 40 (- 5.02, 95% CI - 9.29 to - 0.75; p = 0.0212) but not at week 64. Between-group differences were not significant at either week for physical function mobility or fatigue. Reductions in PROMIS pain interference and fatigue scores from baseline were clinically meaningful with burosumab at weeks 40 and 64 but not with conventional therapy. SF-10 physical health scores (PHS-10) improved significantly with burosumab at week 40 (least-squares mean [standard error] + 5.98 [1.79]; p = 0.0008) and week 64 (+ 5.93 [1.88]; p = 0.0016) but not with conventional therapy (between-treatment differences were nonsignificant). In conclusion, changing to burosumab improved PRO measures, with statistically significant differences in PROMIS pain interference at week 40 versus continuing with conventional therapy and in PHS-10 at weeks 40 and 64 versus baseline.Trial registration: ClinicalTrials.gov NCT02915705.

Neuromuscular responses to fatiguing locomotor exercise

Over the last two decades, an abundance of research has explored the impact of fatiguing locomotor exercise on the neuromuscular system. Neurostimulation techniques have been implemented prior to and following locomotor exercise tasks of a wide variety of intensities, durations, and modes. These techniques have allowed for the assessment of alterations occurring within the central nervous system and the muscle, while techniques such as transcranial magnetic stimulation and spinal electrical stimulation have permitted further segmentalization of locomotor exercise-induced changes along the motor pathway. To this end, the present review provides a comprehensive synopsis of the literature pertaining to neuromuscular responses to locomotor exercise. Sections of the review were divided to discuss neuromuscular responses to maximal, severe, heavy and moderate intensity, high-intensity intermittent exercise, and differences in neuromuscular responses between exercise modalities. During maximal and severe intensity exercise, alterations in neuromuscular function reside primarily within the muscle. Although post-exercise reductions in voluntary activation following maximal and severe intensity exercise are generally modest, several studies have observed alterations occurring at the cortical and/or spinal level. During prolonged heavy and moderate intensity exercise, impairments in contractile function are attenuated with respect to severe intensity exercise, but are still widely observed. While reductions in voluntary activation are greater during heavy and moderate intensity exercise, the specific alterations occurring within the central nervous system remain unclear. Further work utilizing stimulation techniques during exercise and integrating new and emerging techniques such as high-density electromyography is warranted to provide further insight into neuromuscular responses to locomotor exercise.

Acute effect of tendon vibration applied during isometric contraction at two knee angles on maximal knee extension force production

The aim of the current study was to investigate the effect of a single session of prolonged tendon vibration combined with low submaximal isometric contraction on maximal motor performance. Thirty-two young sedentary adults were assigned into two groups that differed based on the knee angle tested: 90° or 150° (180° = full knee extension). Participants performed two fatigue-inducing exercise protocols: one with three 10 min submaximal (10% of maximal voluntary contraction) knee extensor contractions and patellar tendon vibration (80 Hz) another with submaximal knee extensor contractions only. Before and after each fatigue protocol, maximal voluntary isometric contractions (MVC), voluntary activation level (assessed by the twitch interpolation technique), peak-to-peak amplitude of maximum compound action potentials of vastus medialis and vastus lateralis (assessed by electromyography with the use of electrical nerve stimulation), peak twitch amplitude and peak doublet force were measured. The knee extensor fatigue was significantly (P<0.05) greater in the 90° knee angle group (-20.6% MVC force, P<0.05) than the 150° knee angle group (-8.3% MVC force, P = 0.062). Both peripheral and central alterations could explain the reduction in MVC force at 90° knee angle. However, tendon vibration added to isometric contraction did not exacerbate the reduction in MVC force. These results clearly demonstrate that acute infrapatellar tendon vibration using a commercial apparatus operating at optimal conditions (i.e. contracted and stretched muscle) does not appear to induce knee extensor neuromuscular fatigue in young sedentary subjects.

Importance of Dietary Phosphorus for Bone Metabolism and Healthy Aging

Inorganic phosphate (Pi) plays a critical function in many tissues of the body: for example, as part of the hydroxyapatite in the skeleton and as a substrate for ATP synthesis. Pi is the main source of dietary phosphorus. Reduced bioavailability of Pi or excessive losses in the urine causes rickets and osteomalacia. While critical for health in normal amounts, dietary phosphorus is plentiful in the Western diet and is often added to foods as a preservative. This abundance of phosphorus may reduce longevity due to metabolic changes and tissue calcifications. In this review, we examine how dietary phosphorus is absorbed in the gut, current knowledge about Pi sensing, and endocrine regulation of Pi levels. Moreover, we also examine the roles of Pi in different tissues, the consequences of low and high dietary phosphorus in these tissues, and the implications for healthy aging.

Calcium entry units (CEUs): perspectives in skeletal muscle function and disease

In the last decades the term Store-operated Ca²⁺ entry (SOCE) has been used in the scientific literature to describe an ubiquitous cellular mechanism that allows recovery of calcium (Ca²⁺) from the extracellular space. SOCE is triggered by a reduction of Ca²⁺ content (i.e. depletion) in intracellular stores, i.e. endoplasmic or sarcoplasmic reticulum (ER and SR). In skeletal muscle the mechanism is primarily mediated by a physical interaction between stromal interaction molecule-1 (STIM1), a Ca²⁺ sensor located in the SR membrane, and ORAI1, a Ca²⁺-permeable channel of external membranes, located in transverse tubules (TTs), the invaginations of the plasma membrane (PM) deputed to propagation of action potentials. It is generally accepted that in skeletal muscle SOCE is important to limit muscle fatigue during repetitive stimulation. We recently discovered that exercise promotes the assembly of new intracellular junctions that contains colocalized STIM1 and ORAI1, and that the presence of these new junctions increases Ca²⁺ entry via ORAI1, while improving fatigue resistance during repetitive stimulation. Based on these findings we named these new junctions Ca²⁺ Entry Units (CEUs). CEUs are dynamic organelles that assemble during muscle activity and disassemble during recovery thanks to the plasticity of the SR (containing STIM1) and the elongation/retraction of TTs (bearing ORAI1). Interestingly, similar structures described as SR stacks were previously reported in different mouse models carrying mutations in proteins involved in Ca²⁺ handling (calsequestrin-null mice; triadin and junctin null mice, etc.) or associated to microtubules (MAP6 knockout mice). Mutations in Stim1 and Orai1 (and calsequestrin-1) genes have been associated to tubular aggregate myopathy (TAM), a muscular disease characterized by: (a) muscle pain, cramping, or weakness that begins in childhood and worsens over time, and (b) the presence of large accumulations of ordered SR tubes (tubular aggregates, TAs) that do not contain myofibrils, mitochondria, nor TTs. Interestingly, TAs are also present in fast twitch muscle fibers of ageing mice. Several important issues remain un-answered: (a) the molecular mechanisms and signals that trigger the remodeling of membranes and the functional activation of SOCE during exercise are unclear; and (b) how dysfunctional SOCE and/or mutations in Stim1, Orai1 and calsequestrin (Casq1) genes lead to the formation of tubular aggregates (TAs) in aging and disease deserve investigation.

Force-frequency relationship during fatiguing contractions of rat medial gastrocnemius muscle

The force–frequency relationship presents the amount of force a muscle can produce as a function of the frequency of activation. During repetitive muscular contractions, fatigue and potentiation may both impact the resultant contractile response. However, both the apparent fatigue observed, and the potential for activity-dependent potentiation can be affected by the frequency of activation. Thus, we wanted to explore the effects that repetitive stimulation had on the force–frequency relationship. The force–frequency relationship of the rat medial gastrocnemius muscle was investigated during consecutive bouts of increasing fatigue with 20 to 100 Hz stimulation. Force was measured prior to the fatiguing protocol, during each of three levels of fatigue, and after 30 min of recovery. Force at each frequency was quantified relative to the pre-fatigued 100 Hz contractions, as well as the percentage reduction of force from the pre-fatigued level at a given frequency. We observed less reduction in force at low frequencies compared to high frequencies, suggesting an interplay of fatigue and potentiation, in which potentiation can “protect” against fatigue in a frequency-dependent manner. The exact mechanism of fatigue is unknown, however the substantial reduction of force at high frequency suggests a role for reduced force per cross-bridge.

A thermodynamic function of glycogen in brain and muscle

Brain and muscle glycogen are generally thought to function as local glucose reserves, for use during transient mismatches between glucose supply and demand. However, quantitative measures show that glucose supply is likely never rate-limiting for energy metabolism in either brain or muscle under physiological conditions. These tissues nevertheless do utilize glycogen during increased energy demand, despite the availability of free glucose, and despite the ATP cost of cycling glucose through glycogen polymer. This seemingly wasteful process can be explained by considering the effect of glycogenolysis on the amount of energy obtained from ATP (ΔG'ATP). The amount of energy obtained from ATP is reduced by elevations in inorganic phosphate (Pi). Glycogen utilization sequesters Pi in the glycogen phosphorylase reaction and in downstream phosphorylated glycolytic intermediates, thereby buffering Pi elevations and maximizing energy yield at sites of rapid ATP consumption. This thermodynamic effect of glycogen may be particularly important in the narrow, spatially constrained astrocyte processes that ensheath neuronal synapses and in cells such as astrocytes and myocytes that release Pi from phosphocreatine during energy demand. The thermodynamic effect may also explain glycolytic super-compensation in brain when glycogen is not available, and aspects of exercise physiology in muscle glycogen phosphorylase deficiency (McArdle disease).

Does Post-Activation Performance Enhancement Occur During the Bench Press Exercise under Blood Flow Restriction?

Background: The aim of the present study was to evaluate the effects of post-activation performance enhancement (PAPE) during successive sets of the bench press (BP) exercise under blood flow restriction (BFR). Methods: The study included 10 strength-trained males (age = 29.8 ± 4.6 years; body mass = 94.3 ± 3.6 kg; BP 1-repetition maximum (1RM) = 168.5 ± 26.4 kg). The experiment was performed following a randomized crossover design, where each participant performed two different exercise protocols: under blood flow restriction (BFR) and control test protocol (CONT) without blood flow restriction. During the experimental sessions, the study participants performed 3 sets of 3 repetitions of the BP exercise at 70%1RM with a 5 min rest interval between sets. The differences in peak power output (PP), mean power output (MP), peak bar velocity (PV), and mean bar velocity (MV) between the CONT and BFR conditions were examined using 2-way (condition × set) repeated measures ANOVA. Furthermore, t-test comparisons between conditions were made for the set 2-set 1, set 3-set 1, and set 3-set 2 delta values for all variables. Results: The post hoc results for condition × set interaction in PP showed a significant increase in set 2 compared to set 1 for BFR (p < 0.01) and CONT (p = 0.01) conditions, a significant increase in set 3 compared to set 1 for the CONT (p = 0.01) condition, as well as a significant decrease in set 3 compared to set 1 for BFR condition occurred (p < 0.01). The post hoc results for condition × set interaction in PV showed a significant increase in set 2 compared to set 1 for BFR (p < 0.01) and CONT (p = 0.01) conditions, a significant increase in set 3 compared to set 1 for CONT (p = 0.03) condition, as well as a significant decrease in set 3 compared to set 1 for BFR condition (p < 0.01). The t-test comparisons showed significant differences in PP (p < 0.01) and PV (p = 0.01) for set 3-set 2 delta values between BFR and CONT conditions. Conclusion: The PAPE effect was analyzed through changes in power output and bar velocity that occurred under both the CONT and BFR conditions. However, the effects of PAPE have different kinetics in successive sets for BFR and for CONT conditions.

Muscle fibre activation and fatigue with low-load blood flow restricted resistance exercise-An integrative physiology review

Blood flow-restricted resistance exercise (BFRRE) has been shown to induce increases in muscle size and strength, and continues to generate interest from both clinical and basic research points of view. The low loads employed, typically 20%-50% of the one repetition maximum, make BFRRE an attractive training modality for individuals who may not tolerate high musculoskeletal forces (eg, selected clinical patient groups such as frail old adults and patients recovering from sports injury) and/or for highly trained athletes who have reached a plateau in muscle mass and strength. It has been proposed that achieving a high degree of muscle fibre recruitment is important for inducing muscle hypertrophy with BFRRE, and the available evidence suggest that fatiguing low-load exercise during ischemic conditions can recruit both slow (type I) and fast (type II) muscle fibres. Nevertheless, closer scrutiny reveals that type II fibre activation in BFRRE has to date largely been inferred using indirect methods such as electromyography and magnetic resonance spectroscopy, while only rarely addressed using more direct methods such as measurements of glycogen stores and phosphocreatine levels in muscle fibres. Hence, considerable uncertainity exists about the specific pattern of muscle fibre activation during BFRRE. Therefore, the purpose of this narrative review was (1) to summarize the evidence on muscle fibre recruitment during BFRRE as revealed by various methods employed for determining muscle fibre usage during exercise, and (2) to discuss reported findings in light of the specific advantages and limitations associated with these methods.

Isoproterenol enhances force production in mouse glycolytic and oxidative muscle via separate mechanisms

Fight or flight is a biologic phenomenon that involves activation of β-adrenoceptors in skeletal muscle. However, how force generation is enhanced through adrenergic activation in different muscle types is not fully understood. We studied the effects of isoproterenol (ISO, β-receptor agonist) on force generation and energy metabolism in isolated mouse soleus (SOL, oxidative) and extensor digitorum longus (EDL, glycolytic) muscles. Muscles were stimulated with isometric tetanic contractions and analyzed for metabolites and phosphorylase activity. Under conditions of maximal force production, ISO enhanced force generation markedly more in SOL (22%) than in EDL (8%). Similarly, during a prolonged tetanic contraction (30 s for SOL and 10 s for EDL), ISO-enhanced the force × time integral more in SOL (25%) than in EDL (3%). ISO induced marked activation of phosphorylase in both muscles in the basal state, which was associated with glycogenolysis (less in SOL than in EDL), and in EDL only, a significant decrease (16%) in inorganic phosphate (P_i). ATP turnover during sustained contractions (1 s EDL, 5 s SOL) was not affected by ISO in EDL, but essentially doubled in SOL. Under conditions of maximal stimulation, ISO has a minor effect on force generation in EDL that is associated with a decrease in P_i, whereas ISO has a marked effect on force generation in SOL that is associated with an increase in ATP turnover. Thus, phosphorylase functions as a phosphate trap in ISO-mediated force enhancement in EDL and as a catalyzer of ATP supply in SOL.

Bioenergetic basis for the increased fatigability with ageing

KEY POINTS

The mechanisms for the age-related increase in fatigability during dynamic exercise remain elusive. We tested whether age-related impairments in muscle oxidative capacity would result in a greater accumulation of fatigue causing metabolites, inorganic phosphate (P_i ), hydrogen (H⁺ ) and diprotonated phosphate (H₂ PO₄ ^- ), in the muscle of old compared to young adults during a dynamic knee extension exercise. The age-related increase in fatigability (reduction in mechanical power) of the knee extensors was closely associated with a greater accumulation of metabolites within the working muscle but could not be explained by age-related differences in muscle oxidative capacity. These data suggest that the increased fatigability in old adults during dynamic exercise is primarily determined by age-related impairments in skeletal muscle bioenergetics that result in a greater accumulation of metabolites.

ABSTRACT

The present study aimed to determine whether the increased fatigability in old adults during dynamic exercise is associated with age-related differences in skeletal muscle bioenergetics. Phosphorus nuclear magnetic resonance spectroscopy was used to quantify concentrations of high-energy phosphates and pH in the knee extensors of seven young (22.7 ± 1.2 years; six women) and eight old adults (76.4 ± 6.0 years; seven women). Muscle oxidative capacity was measured from the phosphocreatine (PCr) recovery kinetics following a 24 s maximal voluntary isometric contraction. The fatiguing exercise consisted of 120 maximal velocity contractions (one contraction per 2 s) against a load equivalent to 20% of the maximal voluntary isometric contraction. The PCr recovery kinetics did not differ between young and old adults (0.023 ± 0.007 s^-1  vs. 0.019 ± 0.004 s^-1 , respectively). Fatigability (reductions in mechanical power) of the knee extensors was ∼1.8-fold greater with age and was accompanied by a greater decrease in pH (young = 6.73 ± 0.09, old = 6.61 ± 0.04) and increases in concentrations of inorganic phosphate, [P_i ], (young = 22.7 ± 4.8 mm, old = 32.3 ± 3.6 mm) and diprotonated phosphate, [H₂ PO₄ ^- ], (young = 11.7 ± 3.6 mm, old = 18.6 ± 2.1 mm) at the end of the exercise in old compared to young adults. The age-related increase in power loss during the fatiguing exercise was strongly associated with intracellular pH (r = -0.837), [P_i ] (r = 0.917) and [H₂ PO₄ ^- ] (r = 0.930) at the end of the exercise. These data suggest that the age-related increase in fatigability during dynamic exercise has a bioenergetic basis and is explained by an increased accumulation of metabolites within the muscle.

Bioenergetic basis of skeletal muscle fatigue

Energetic demand from high-intensity exercise can easily exceed ATP synthesis rates of mitochondria leading to a reliance on anaerobic metabolism. The reliance on anaerobic metabolism results in the accumulation of intracellular metabolites, namely inorganic phosphate (P_i) and hydrogen (H⁺), that are closely associated with exercise-induced reductions in power. Cellular and molecular studies have revealed several steps where these metabolites impair contractile function demonstrating a causal role in fatigue. Elevated P_i or H⁺ directly inhibits force and power of the cross-bridge and decreases myofibrillar Ca²⁺ sensitivity, whereas P_i also inhibits Ca²⁺ release from the sarcoplasmic reticulum (SR). When both metabolites are elevated, they act synergistically to cause marked reductions in power, indicating that fatigue during high-intensity exercise has a bioenergetic basis.

Fatigue-independent alterations in muscle activation and effort perception during forearm exercise: role of local oxygen delivery

The oxygen-conforming response (OCR) of skeletal muscle refers to a downregulation of muscle force for a given muscle activation when oxygen delivery (O₂D) is reduced, which is rapidly reversed when O₂D is restored. We tested the hypothesis that the OCR exists in voluntary human exercise and results in compensatory changes in muscle activation to maintain force output, thereby altering perception of effort. In eight men and eight women, electromyography (EMG), oxyhemoglobin (O₂Hb) and deoxyhemoglobin (HHb), forearm blood flow (FBF), and task effort awareness (TEA) were measured. Participants completed two nonfatiguing rhythmic handgrip tests consisting of 5-min steady state (SS) followed by two bouts of 2-min brachial artery compression to reduce FBF by ~50% of SS (C1 and C2), separated by 2 min of no compression (NC1) and ending with 2 min of no compression (NC2). When FBF was compromised during C1, EMG/Force (1.58 ± 0.39) increased compared with SS (1.31 ± 0.33, P = 0.001). However, EMG/Force was not restored upon FBF restoration at NC1 (1.48 ± 0.38, P = 0.479), consistent with C1 evoking skeletal muscle fatigue. When FBF was compromised during C2, EMG/Force increased (1.73 ± 0.50) compared with NC1 (1.48 ± 0.38, P = 0.013). EMG/Force returned to NC1 levels during NC2 (1.50 ± 0.39, P = 0.016), consistent with an OCR in C2. TEA (SS 2.2 ± 2.3, C1 3.9 ± 2.5, NC1 3.4 ± 2.7, C2 4.6 ± 2.7, NC2 3.9 ± 2.8) mirrored changes in EMG. It is noteworthy that during the second compromise and then restoration of muscle oxygenation EMG and TEA were rapidly restored to precompromise levels. We interpreted these findings to support the existence of an OCR and its ability to rapidly modify perception of effort during voluntary exercise. NEW & NOTEWORTHY In healthy individuals, when force output is maintained during rhythmic handgrip exercise, muscle activation and perception of effort rapidly increase with compromised muscle oxygen delivery (O₂D) and then return to precompromised levels when muscle O₂D is restored. These findings suggest that an oxygen-conforming response (OCR) exists and is able to modify perception of effort during voluntary exercise. Therefore, similar to fatigue, an OCR may have implications for exercise tolerance.

Heating after intense repeated contractions inhibits glycogen accumulation in mouse EDL muscle: role of phosphorylase in postexercise glycogen metabolism

The effects of heating on glycogen synthesis (incorporation of [¹⁴C]glucose into glycogen) and accumulation after intense repeated contractions were investigated. Isolated mouse extensor digitorum longus muscle (type II) was stimulated electrically to perform intense tetanic contractions at 25°C. After 120 min recovery at 25°C, glycogen accumulated to almost 80% of basal, whereas after recovery at 35°C, glycogen remained low (~25% of basal). Glycogen synthesis averaged 0.97 ± 0.07 µmol·30 min^-1·g wet wt^-1 during recovery at 25°C and 1.48 ± 0.08 during recovery at 35°C ( P < 0.001). There were no differences in phosphorylase and glycogen synthase total activities nor in phosphorylase fractional activity, whereas glycogen synthase fractional activity was increased by ~50% after recovery at 35°C vs. 25°C. Inorganic phosphate (P_i, substrate for phosphorylase) was markedly increased (~300% of basal) following contraction but returned to control levels after 120 min recovery at 25°C. In contrast, P_i remained elevated after recovery at 35°C (>2-fold higher than recovery at 25°C). Estimates of glycogen breakdown indicated that phosphorylase activity (either via inhibition at 25°C or activation at 35°C) was responsible for ~60% of glycogen accumulation during recovery at 25°C and ~45% during recovery at 35°C. These data demonstrate that despite the enhancing effect of heating on glycogen synthesis during recovery from intense contractions, glycogen accumulation is inhibited owing to P_i-mediated activation of phosphorylase. Thus phosphorylase can play a quantitatively important role in glycogen biogenesis during recovery from repeated contractions in isolated type II muscle.

Role of phosphate sensing in bone and mineral metabolism

Inorganic phosphate (P_i) is essential for signal transduction and cell metabolism, and is also an essential structural component of the extracellular matrix of the skeleton. P_i is sensed in bacteria and yeast at the plasma membrane, which activates intracellular signal transduction to control the expression of P_i transporters and other genes that control intracellular P_i levels. In multicellular organisms, P_i homeostasis must be maintained in the organism and at the cellular level, requiring an endocrine and metabolic P_i-sensing mechanism, about which little is currently known. This Review will discuss the metabolic effects of P_i, which are mediated by P_i transporters, inositol pyrophosphates and SYG1-Pho81-XPR1 (SPX)-domain proteins to maintain cellular phosphate homeostasis in the musculoskeletal system. In addition, we will discuss how P_i is sensed by the human body to regulate the production of fibroblast growth factor 23 (FGF23), parathyroid hormone and calcitriol to maintain serum levels of P_i in a narrow range. New findings on the crosstalk between iron and P_i homeostasis in the regulation of FGF23 expression will also be outlined. Mutations in components of these metabolic and endocrine phosphate sensors result in genetic disorders of phosphate homeostasis, cardiomyopathy and familial basal ganglial calcifications, highlighting the importance of this newly emerging area of research.

Mechanomyographic responses for the biceps brachii are associated with failure times during isometric force tasks

In order to characterize the physiological adjustments within the neuromuscular system that contribute to task failure, this study examined the surface mechanomyographic (MMG) response during maximal and submaximal isometric force tasks of the elbow flexors sustained to failure. The time and frequency components of the MMG signal have shown to be influenced by motor unit activation patterns as well as tetanus. Therefore, it was hypothesized that the rate of change for the MMG response would associate with failure times and would be reduced to a similar degree between the two tasks. The isometric force tasks were performed by the dominant elbow flexors of twenty healthy males (age: 25 ± 4 years) and MMG was collected from the biceps brachii. Regression analyses were used to model the relationships between the rates of change for MMG versus failure times. There were high levels of interindividual variability in the response patterns, yet the models demonstrated significant negative associations between the rate of change for the MMG responses and failure times during both tasks (R2  = 0.41-0.72, P < 0.05). Similarly, the mean MMG amplitude and frequency values were reduced to comparable levels at the failure point of the two tasks. The results of this study demonstrated that force failure is associated with the rate of diminution in the properties of the muscle force twitch.

Muscle fatigue: general understanding and treatment

Muscle fatigue is a common complaint in clinical practice. In humans, muscle fatigue can be defined as exercise-induced decrease in the ability to produce force. Here, to provide a general understanding and describe potential therapies for muscle fatigue, we summarize studies on muscle fatigue, including topics such as the sequence of events observed during force production, in vivo fatigue-site evaluation techniques, diagnostic markers and non-specific but effective treatments.

Modulators of actin-myosin dissociation: basis for muscle type functional differences during fatigue

The muscle types present with variable fatigue tolerance, in part due to the myosin isoform expressed. However, the critical steps that define “fatigability” in vivo of fast vs. slow myosin isoforms, at the molecular level, are not yet fully understood. We examined the modulation of the ATP-induced myosin subfragment 1 (S1) dissociation from pyrene-actin by inorganic phosphate (P_i), pH, and temperature using a specially modified stopped-flow system that allowed fast kinetics measurements at physiological temperature. We contrasted the properties of rabbit psoas (fast) and bovine masseter (slow) myosins (obtained from samples collected from New Zealand rabbits and from a licensed abattoir, respectively, according to institutional and national ethics permits). To identify ATP cycling biochemical intermediates, we assessed ATP binding to a preequilibrated mixture of actomyosin and variable [ADP], pH (pH 7 vs. pH 6.2), and P_i (zero, 15, or 30 added mM P_i) in a range of temperatures (5 to 45°C). Temperature and pH variations had little, if any, effect on the ADP dissociation constant (K_ADP) for fast S1, but for slow S1, K_ADP was weakened with increasing temperature or low pH. In the absence of ADP, the dissociation constant for phosphate (K_Pi) was weakened with increasing temperature for fast S1. In the presence of ADP, myosin type differences were revealed at the apparent phosphate affinity, depending on pH and temperature. Overall, the newly revealed kinetic differences between myosin types could help explain the in vivo observed muscle type functional differences at rest and during fatigue.

Superhero physiology: the case for Captain America

Using pop icons in the science classroom represents a creative way to engage often-distracted students in a relevant and, perhaps more importantly, fun way. When the pop icon is as universally known as Captain America, the pedagogical stage is set. However, when the movies can also be employed to link dramatic references to the science concepts at hand, we may have a very powerful tool by which linkages between fiction and science can be forged. In this regard, Captain America's performances in several movies to date can be used to explain actual science. Granted, script writers and movie directors may or may not be interested in whether the physical performances they depict can be explained, but that is irrelevant. The point is to make a connection using science to explain how the superhero can run faster, jump higher, or lift more than is humanly possible. If a teachable moment has occurred and an important concept has been communicated, the educator has accomplished his or her job well.

Role of physiological ClC-1 Cl- ion channel regulation for the excitability and function of working skeletal muscle

Electrical membrane properties of skeletal muscle fibers have been thoroughly studied over the last five to six decades. This has shown that muscle fibers from a wide range of species, including fish, amphibians, reptiles, birds, and mammals, are all characterized by high resting membrane permeability for Cl⁻ ions. Thus, in resting human muscle, ClC-1 Cl⁻ ion channels account for ∼80% of the membrane conductance, and because active Cl⁻ transport is limited in muscle fibers, the equilibrium potential for Cl⁻ lies close to the resting membrane potential. These conditions—high membrane conductance and passive distribution—enable ClC-1 to conduct membrane current that inhibits muscle excitability. This depressing effect of ClC-1 current on muscle excitability has mostly been associated with skeletal muscle hyperexcitability in myotonia congenita, which arises from loss-of-function mutations in the CLCN1 gene. However, given that ClC-1 must be drastically inhibited (∼80%) before myotonia develops, more recent studies have explored whether acute and more subtle ClC-1 regulation contributes to controlling the excitability of working muscle. Methods were developed to measure ClC-1 function with subsecond temporal resolution in action potential firing muscle fibers. These and other techniques have revealed that ClC-1 function is controlled by multiple cellular signals during muscle activity. Thus, onset of muscle activity triggers ClC-1 inhibition via protein kinase C, intracellular acidosis, and lactate ions. This inhibition is important for preserving excitability of working muscle in the face of activity-induced elevation of extracellular K⁺ and accumulating inactivation of voltage-gated sodium channels. Furthermore, during prolonged activity, a marked ClC-1 activation can develop that compromises muscle excitability. Data from ClC-1 expression systems suggest that this ClC-1 activation may arise from loss of regulation by adenosine nucleotides and/or oxidation. The present review summarizes the current knowledge of the physiological factors that control ClC-1 function in active muscle.

Lactate as a Metabolite and a Regulator in the Central Nervous System

More than two hundred years after its discovery, lactate still remains an intriguing molecule. Considered for a long time as a waste product of metabolism and the culprit behind muscular fatigue, it was then recognized as an important fuel for many cells. In particular, in the nervous system, it has been proposed that lactate, released by astrocytes in response to neuronal activation, is taken up by neurons, oxidized to pyruvate and used for synthesizing acetyl-CoA to be used for the tricarboxylic acid cycle. More recently, in addition to this metabolic role, the discovery of a specific receptor prompted a reconsideration of its role, and lactate is now seen as a sort of hormone, even involved in processes as complex as memory formation and neuroprotection. As a matter of fact, exercise offers many benefits for our organisms, and seems to delay brain aging and neurodegeneration. Now, exercise induces the production and release of lactate into the blood which can reach the liver, the heart, and also the brain. Can lactate be a beneficial molecule produced during exercise, and offer neuroprotection? In this review, we summarize what we have known on lactate, discussing the roles that have been attributed to this molecule over time.

Caloric restriction induces energy-sparing alterations in skeletal muscle contraction, fiber composition and local thyroid hormone metabolism that persist during catch-up fat upon refeeding

Weight regain after caloric restriction results in accelerated fat storage in adipose tissue. This catch-up fat phenomenon is postulated to result partly from suppressed skeletal muscle thermogenesis, but the underlying mechanisms are elusive. We investigated whether the reduced rate of skeletal muscle contraction-relaxation cycle that occurs after caloric restriction persists during weight recovery and could contribute to catch-up fat. Using a rat model of semistarvation-refeeding, in which fat recovery is driven by suppressed thermogenesis, we show that contraction and relaxation of leg muscles are slower after both semistarvation and refeeding. These effects are associated with (i) higher expression of muscle deiodinase type 3 (DIO3), which inactivates tri-iodothyronine (T₃), and lower expression of T₃-activating enzyme, deiodinase type 2 (DIO2), (ii) slower net formation of T₃ from its T₄ precursor in muscles, and (iii) accumulation of slow fibers at the expense of fast fibers. These semistarvation-induced changes persisted during recovery and correlated with impaired expression of transcription factors involved in slow-twitch muscle development. We conclude that diminished muscle thermogenesis following caloric restriction results from reduced muscle T₃ levels, alteration in muscle-specific transcription factors, and fast-to-slow fiber shift causing slower contractility. These energy-sparing effects persist during weight recovery and contribute to catch-up fat.

Effects of stimulation frequency, amplitude, and impulse width on muscle fatigue

INTRODUCTION

We investigated the effect of stimulation intensity (in percent of maximal tolerated stimulation current, mTSC), frequency, and impulse width on muscle fatigue.

METHODS

Using a randomized crossover design, 6 parameter combinations (80% mTSC, 80 Hz, 400 μs; 60% mTSC, 80 Hz, 400 μs; 80% mTSC, 20 Hz, 400 μs; 60% mTSC, 20 Hz, 400 μs; 80% mTSC, 80 Hz, 150 μs; 60% mTSC, 80 Hz, 150 μs) were tested in both legs of 13 athletic men (age 26 ± 2.3). The slope of the linear regression line over all tetani (FIS) and the number of tetani whose force was above 50% of the initial tetanus (FIN) were used to quantify fatigue.

RESULTS

FIS and FIN were significantly lower in high-frequency protocols. No effects on FIS and FIN were found for intensity and impulse width.

CONCLUSIONS

Stimulation frequency, but not impulse width or intensity, affected fatigue kinetics.

The effect of passive versus active recovery on power output over six repeated wingate sprints

PURPOSE

The aim of this study was to examine the effect of active versus passive recovery on 6 repeated Wingate tests (30-s all-out cycling sprints on a Velotron ergometer).

METHOD

Fifteen healthy participants aged 29 (SD = 8) years old (body mass index = 23 [3] kg/m(2)) participated in 3 sprint interval training sessions separated by 3 to 7 days between each session during a period of 1 month. The 1st visit was familiarization to 6 cycling sprints; the 2nd and 3rd visits involved a warm-up followed by 6 30-s cycling sprints. Each sprint was followed by 4 min of passive (resting still on the ergometer) or active recovery (pedaling at 1.1 W/kg). The same recovery was used within each visit, and recovery type was randomized between visits.

RESULTS

Active recovery resulted in a 0.6 W/kg lower peak power output in the second sprint (95% confidence interval [CI] [ - 0.2, - 0.8 W/kg], effect size = 0.50, p < .01) and a 0.4 W/kg greater average power output in the 5th and 6th sprints (95% CI [+0.2,+0.6 W/kg], effect size = 0.50, p < .01) compared with passive recovery. There was little difference between fatigue index, total work, or accumulated work between the 2 recovery conditions.

CONCLUSIONS

Passive recovery is beneficial when only 2 sprints are completed, whereas active recovery better maintains average power output compared with passive recovery when several sprints are performed sequentially (partial eta squared between conditions for multiple sprints = .38).

Effect of temperature on crossbridge force changes during fatigue and recovery in intact mouse muscle fibers

Repetitive or prolonged muscle contractions induce muscular fatigue, defined as the inability of the muscle to maintain the initial tension or power output. In the present experiments, made on intact fiber bundles from FDB mouse, fatigue and recovery from fatigue were investigated at 24°C and 35°C. Force and stiffness were measured during tetani elicited every 90 s during the pre-fatigue control phase and recovery and every 1.5 s during the fatiguing phase made of 105 consecutive tetani. The results showed that force decline could be split in an initial phase followed by a later one. Loss of force during the first phase was smaller and slower at 35°C than at 24°C, whereas force decline during the later phase was greater at 35°C so that total force depression at the end of fatigue was the same at both temperatures. The initial force decline occurred without great reduction of fiber stiffness and was attributed to a decrease of the average force per attached crossbridge. Force decline during the later phase was accompanied by a proportional stiffness decrease and was attributed to a decrease of the number of attached crossbridge. Similarly to fatigue, at both 24 and 35°C, force recovery occurred in two phases: the first associated with the recovery of the average force per attached crossbridge and the second due to the recovery of the pre-fatigue attached crossbridge number. These changes, symmetrical to those occurring during fatigue, are consistent with the idea that, i) initial phase is due to the direct fast inhibitory effect of [Pi]i increase during fatigue on crossbridge force; ii) the second phase is due to the delayed reduction of Ca(2+) release and /or reduction of the Ca(2+) sensitivity of the myofibrils due to high [Pi]i.