Blocking muscle wasting via deletion of the muscle-specific E3 ligase MuRF1 impedes pancreatic tumor growth.

Cancer-induced muscle wasting reduces quality of life, complicates or precludes cancer treatments, and predicts early mortality. Herein, we investigate the requirement of the muscle-specific E3 ubiquitin ligase, MuRF1, for muscle wasting induced by pancreatic cancer. Murine pancreatic cancer (KPC) cells, or saline, were injected into the pancreas of WT and MuRF1-/- mice, and tissues analyzed throughout tumor progression. KPC tumors induces progressive wasting of skeletal muscle and systemic metabolic reprogramming in WT mice, but not MuRF1-/- mice. KPC tumors from MuRF1-/- mice also grow slower, and show an accumulation of metabolites normally depleted by rapidly growing tumors. Mechanistically, MuRF1 is necessary for the KPC-induced increases in cytoskeletal and muscle contractile protein ubiquitination, and the depression of proteins that support protein synthesis. Together, these data demonstrate that MuRF1 is required for KPC-induced skeletal muscle wasting, whose deletion reprograms the systemic and tumor metabolome and delays tumor growth.

Blocking muscle wasting via deletion of the muscle-specific E3 ubiquitin ligase MuRF1 impedes pancreatic tumor growth.

Cancer-induced muscle wasting reduces quality of life, complicates or precludes cancer treatments, and predicts early mortality. Herein, we investigated the requirement of the muscle-specific E3 ubiquitin ligase, MuRF1, for muscle wasting induced by pancreatic cancer. Murine pancreatic cancer (KPC) cells, or saline, were injected into the pancreas of WT and MuRF1-/- mice, and tissues analyzed throughout tumor progression. KPC tumors induced progressive wasting of skeletal muscle and systemic metabolic reprogramming in WT mice, but not MuRF1-/- mice. KPC tumors from MuRF1-/- mice also grew slower, and showed an accumulation of metabolites normally depleted by rapidly growing tumors. Mechanistically, MuRF1 was necessary for the KPC-induced increases in cytoskeletal and muscle contractile protein ubiquitination, and the depression of proteins that support protein synthesis. Together, these data demonstrate that MuRF1 is required for KPC-induced skeletal muscle wasting, whose deletion reprograms the systemic and tumor metabolome and delays tumor growth.

Depleting Ly6G Positive Myeloid Cells Reduces Pancreatic Cancer-Induced Skeletal Muscle Atrophy.

Immune cells can mount desirable anti-cancer immunity. However, some immune cells can support cancer disease progression. The presence of cancer can lead to production of immature myeloid cells from the bone marrow known as myeloid-derived suppressor cells (MDSCs). The immunosuppressive and pro-tumorigenic effects of MDSCs are well understood. Whether MDSCs are involved in promoting cancer cachexia is not well understood. We orthotopically injected the pancreas of mice with KPC cells or PBS. One group of tumor-bearing mice was treated with an anti-Ly6G antibody that depletes granulocytic MDSCs and neutrophils; the other received a control antibody. Anti-Ly6G treatment delayed body mass loss, reduced tibialis anterior (TA) muscle wasting, abolished TA muscle fiber atrophy, reduced diaphragm muscle fiber atrophy of type IIb and IIx fibers, and reduced atrophic gene expression in the TA muscles. Anti-ly6G treatment resulted in greater than 50% Ly6G+ cell depletion efficiency in the tumors and TA muscles. These data show that, in the orthotopic KPC model, anti-Ly6G treatment reduces the number of Ly6G+ cells in the tumor and skeletal muscle and reduces skeletal muscle atrophy. These data implicate Ly6G+ cells, including granulocytic MDSCs and neutrophils, as possible contributors to the development of pancreatic cancer-induced skeletal muscle wasting.

Modulation of torque evoked by wide-pulse, high-frequency neuromuscular electrical stimulation and the potential implications for rehabilitation and training.

The effectiveness of neuromuscular electrical stimulation (NMES) for rehabilitation is proportional to the evoked torque. The progressive increase in torque (extra torque) that may develop in response to low intensity wide-pulse high-frequency (WPHF) NMES holds great promise for rehabilitation as it overcomes the main limitation of NMES, namely discomfort. WPHF NMES extra torque is thought to result from reflexively recruited motor units at the spinal level. However, whether WPHF NMES evoked force can be modulated is unknown. Therefore, we examined the effect of two interventions known to change the state of spinal circuitry in opposite ways on evoked torque and motor unit recruitment by WPHF NMES. The interventions were high-frequency transcutaneous electrical nerve stimulation (TENS) and anodal transcutaneous spinal direct current stimulation (tsDCS). We show that TENS performed before a bout of WPHF NMES results in lower evoked torque (median change in torque time-integral: - 56%) indicating that WPHF NMES-evoked torque might be modulated. In contrast, the anodal tsDCS protocol used had no effect on any measured parameter. Our results demonstrate that WPHF NMES extra torque can be modulated and although the TENS intervention blunted extra torque production, the finding that central contribution to WPHF NMES-evoked torques can be modulated opens new avenues for designing interventions to enhance WPHF NMES.

FoxP1 is a transcriptional repressor associated with cancer cachexia that induces skeletal muscle wasting and weakness.

BACKGROUND: Skeletal muscle wasting is a devastating consequence of cancer that affects up to 80% of cancer patients and associates with reduced survival. Herein, we investigated the biological significance of Forkhead box P1 (FoxP1), a transcriptional repressor that we demonstrate is up-regulated in skeletal muscle in multiple models of cancer cachexia and in cachectic cancer patients. METHODS: Inducible, skeletal muscle-specific FoxP1 over-expressing (FoxP1iSkmTg/Tg ) mice were generated through crossing conditional Foxp1a transgenic mice with HSA-MCM mice that express tamoxifen-inducible Cre recombinase under control of the skeletal muscle actin promoter. To determine the requirement of FoxP1 for cancer-induced skeletal muscle wasting, FoxP1-shRNA was packaged and targeted to muscles using AAV9 delivery prior to inoculation of mice with Colon-26 Adenocarcinoma (C26) cells. RESULTS: Up-regulation of FoxP1 in adult skeletal muscle was sufficient to induce features of cachexia, including 15% reduction in body mass (P < 0.05), and a 16-27% reduction in skeletal muscle mass (P < 0.05) that was characterized by a 20% reduction in muscle fibre cross-sectional area of type IIX/B muscle fibres (P = 0.020). Skeletal muscles from FoxP1iSkmTg/Tg mice also showed significant damage and myopathy characterized by the presence of centrally nucleated myofibres, extracellular matrix expansion, and were 19-26% weaker than controls (P < 0.05). Transcriptomic analysis revealed FoxP1 as a potent transcriptional repressor of skeletal muscle gene expression, with enrichment of pathways related to skeletal muscle structure and function, growth signalling, and cell quality control. Because FoxP1 functions, at least in part, as a transcriptional repressor through its interaction with histone deacetylase proteins, we treated FoxP1iSkmTg/Tg mice with Trichostatin A, and found that this completely prevented the loss of muscle mass (p = 0.007) and fibre atrophy (P < 0.001) in the tibialis anterior. In the context of cancer, FoxP1 knockdown blocked the cancer-induced repression of myocyte enhancer factor 2 (MEF2)-target genes critical to muscle differentiation and repair, improved muscle ultrastructure, and attenuated muscle fibre atrophy by 50% (P < 0.05). CONCLUSIONS: In summary, we identify FoxP1 as a novel repressor of skeletal muscle gene expression that is increased in cancer cachexia, whose up-regulation is sufficient to induce skeletal muscle wasting and weakness, and required for the normal wasting response to cancer.

MEF2c-Dependent Downregulation of Myocilin Mediates Cancer-Induced Muscle Wasting and Associates with Cachexia in Patients with Cancer.

Skeletal muscle wasting is a devastating consequence of cancer that contributes to increased complications and poor survival, but is not well understood at the molecular level. Herein, we investigated the role of Myocilin (Myoc), a skeletal muscle hypertrophy-promoting protein that we showed is downregulated in multiple mouse models of cancer cachexia. Loss of Myoc alone was sufficient to induce phenotypes identified in mouse models of cancer cachexia, including muscle fiber atrophy, sarcolemmal fragility, and impaired muscle regeneration. By 18 months of age, mice deficient in Myoc showed significant skeletal muscle remodeling, characterized by increased fat and collagen deposition compared with wild-type mice, thus also supporting Myoc as a regulator of muscle quality. In cancer cachexia models, maintaining skeletal muscle expression of Myoc significantly attenuated muscle loss, while mice lacking Myoc showed enhanced muscle wasting. Furthermore, we identified the myocyte enhancer factor 2 C (MEF2C) transcription factor as a key upstream activator of Myoc whose gain of function significantly deterred cancer-induced muscle wasting and dysfunction in a preclinical model of pancreatic ductal adenocarcinoma (PDAC). Finally, compared with noncancer control patients, MYOC was significantly reduced in skeletal muscle of patients with PDAC defined as cachectic and correlated with MEF2c. These data therefore identify disruptions in MEF2c-dependent transcription of Myoc as a novel mechanism of cancer-associated muscle wasting that is similarly disrupted in muscle of patients with cachectic cancer. SIGNIFICANCE: This work identifies a novel transcriptional mechanism that mediates skeletal muscle wasting in murine models of cancer cachexia that is disrupted in skeletal muscle of patients with cancer exhibiting cachexia.

Three weeks of sprint interval training improved high-intensity cycling performance and limited ryanodine receptor modifications in recreationally active human subjects.

PURPOSE: Mechanisms underlying the efficacy of sprint interval training (SIT) remain to be understood. We previously reported that an acute bout of SIT disrupts the integrity of the sarcoplasmic reticulum (SR) Ca2+ release channel, the ryanodine receptor 1 (RyR1), in recreationally active human subjects. We here hypothesize that in addition to improving the exercise performance of recreationally active humans, a period of repeated SIT sessions would make the RyR1 protein less vulnerable and accelerate recovery of contractile function after a SIT session. METHODS: Eight recreationally active males participated in a 3-week SIT program consisting of nine sessions of four-six 30-s all-out cycling bouts with 4 min of rest between bouts. RESULTS: Total work performed during a SIT session and maximal power (Wmax) reached during an incremental cycling test were both increased by ~ 7.5% at the end of the training period (P < 0.05). Western blots performed on vastus lateralis muscle biopsies taken before, 1 h, 24 h and 72 h after SIT sessions in the untrained and trained state showed some protection against SIT-induced reduction of full-length RyR1 protein expression in the trained state. SIT-induced knee extensor force deficits were similar in the untrained and trained states, with a major reduction in voluntary and electrically evoked forces immediately and 1 h after SIT (P < 0.05), and recovery after 24 h. CONCLUSIONS: Three weeks of SIT improves exercise performance and provides some protection against RyR1 modification, whereas it does not accelerate recovery of contractile function.

Colon 26 adenocarcinoma (C26)-induced cancer cachexia impairs skeletal muscle mitochondrial function and content.

The present study aimed to determine the impact of colon 26 adenocarcinoma (C26)-induced cancer cachexia on skeletal muscle mitochondrial respiration and content. Twelve male CD2F1 mice were injected with C26-cells (tumor bearing (TB) group), whereas 12 age-matched mice received PBS vehicle injection (non-tumor bearing (N-TB) group). Mitochondrial respiration was studied in saponin-permeabilized soleus myofibers. TB mice showed lower body weight (~ 20%) as well as lower soleus, gastrocnemius-plantaris complex and tibialis anterior masses versus N-TB mice (p < 0.05). Soleus maximal state III mitochondrial respiration was 20% lower (10 mM glutamate, 5 mM malate, 5 mM adenosine diphosphate; p < 0.05) and acceptor control ratio (state III/state II) was 15% lower in the TB vs. N-TB (p < 0.05), with the latter suggesting uncoupling. Lower VDAC protein content suggested reduced mitochondrial content in TB versus N-TB (p < 0.05). Skeletal muscle in C26-induced cancer cachexia exhibits reductions in: maximal mitochondrial respiration capacity, mitochondrial coupling and mitochondrial content.

Neuromuscular adaptations to wide-pulse high-frequency neuromuscular electrical stimulation training.

PURPOSE: No studies have evaluated the potential benefits of wide-pulse high-frequency (WPHF) neuromuscular electrical stimulation (NMES) despite it being an interesting alternative to conventional NMES. Hence, this study evaluated neuromuscular adaptations induced by 3 weeks of WPHF NMES. METHODS: Ten young healthy individuals (training group) completed nine sessions of WPHF NMES training spread over 3 weeks, whereas seven individuals (control group) only performed the first and last sessions. Plantar flexor neuromuscular function (maximal voluntary contraction (MVC) force, voluntary activation level, H reflex, V wave, contractile properties) was evaluated before the first and last training sessions. Each training session consisted of ten 20-s WPHF NMES contractions (pulse duration: 1 ms, stimulation frequency: 100 Hz) interspaced by 40 s of recovery and delivered at an intensity set to initially evoke ~ 5% of MVC force. The averaged mean evoked forces produced during the ten WPHF NMES-evoked contractions of a given session as well as the sum of the ten contractions force time integral (total FTI) were computed. RESULTS: Total FTI (+ 118 ± 98%) and averaged mean evoked forces (+ 96 ± 91%) increased following the 3-week intervention (p < 0.05); no changes were observed in the control group. The intervention did not induce any change (p > 0.05) in parameters used to characterize plantar flexor neuromuscular function. CONCLUSION: Three weeks of WPHF NMES increased electrically evoked forces but induced no other changes in plantar flexor neuromuscular properties. Before introducing WPHF NMES clinically, optimal training program characteristics (such as frequency, duration and intensity) remain to be identified.

Toxic doses of caffeine are needed to increase skeletal muscle contractility.

Discrepant results have been reported regarding an intramuscular mechanism underlying the ergogenic effect of caffeine on neuromuscular function in humans. Here, we reevaluated the effect of caffeine on muscular force production in humans and combined this with measurements of the caffeine dose-response relationship on force and cytosolic free [Ca2+] ([Ca2+]i) in isolated mouse muscle fibers. Twenty-one healthy and physically active men (29 ± 9 yr, 178 ± 6 cm, 73 ± 10 kg, mean ± SD) took part in the present study. Nine participants were involved in two experimental sessions during which supramaximal single and paired electrical stimulations (at 10 and 100 Hz) were applied to the femoral nerve to record evoked forces. Evoked forces were recorded before and 1 h after ingestion of 1) 6 mg caffeine/kg body mass or 2) placebo. Caffeine plasma concentration was measured in 12 participants. In addition, submaximal tetanic force and [Ca2+]i were measured in single mouse flexor digitorum brevis (FDB) muscle fibers exposed to 100 nM up to 5 mM caffeine. Six milligrams of caffeine per kilogram body mass (plasma concentration ~40 µM) did not increase electrically evoked forces in humans. In superfused FDB single fibers, millimolar caffeine concentrations (i.e., 15- to 35-fold above usual concentrations observed in humans) were required to increase tetanic force and [Ca2+]i. Our results suggest that toxic doses of caffeine are required to increase muscle contractility, questioning the purported intramuscular ergogenic effect of caffeine in humans.

Test-retest reliability of wide-pulse high-frequency neuromuscular electrical stimulation evoked force.

INTRODUCTION: We compare forces evoked by wide-pulse high-frequency (WPHF) neuromuscular electrical stimulation (NMES) delivered to a nerve trunk versus muscle belly and assess their test-retest intraindividual and interindividual reliability. METHODS: Forces evoked during 2 sessions with WPHF NMES delivered over the tibial nerve trunk and 2 sessions over the triceps surae muscle belly were compared. Ten individuals participated in 4 sessions involving ten 20-s WPHF NMES contractions interspaced by 40-s recovery. Mean evoked force and force time integral of each contraction were quantified. RESULTS: For both nerve trunk and muscle belly stimulation, intraindividual test-retest reliability was good (intraclass correlation coefficient > 0.9), and interindividual variability was large (coefficient of variation between 140% and 180%). Nerve trunk and muscle belly stimulation resulted in similar evoked forces. DISCUSSION: WPHF NMES locations might be chosen by individual preference because intraindividual reliability was relatively good for both locations. Muscle Nerve 57: E70-E77, 2018.

Intramuscular Contributions to Low-Frequency Force Potentiation Induced by a High-Frequency Conditioning Stimulation.

Electrically-evoked low-frequency (submaximal) force is increased immediately following high-frequency stimulation in human skeletal muscle. Although central mechanisms have been suggested to be the major cause of this low-frequency force potentiation, intramuscular factors might contribute. Thus, we hypothesized that two intramuscular Ca2+-dependent mechanisms can contribute to the low-frequency force potentiation: increased sarcoplasmic reticulum Ca2+ release and increased myofibrillar Ca2+ sensitivity. Experiments in humans were performed on the plantar flexor muscles at a shortened, intermediate, and long muscle length and electrically evoked contractile force and membrane excitability (i.e., M-wave amplitude) were recorded during a stimulation protocol. Low-frequency force potentiation was assessed by stimulating with a low-frequency tetanus (25 Hz, 2 s duration), followed by a high-frequency tetanus (100 Hz, 2 s duration), and finally followed by another low-frequency (25 Hz, 2 s duration) tetanus. Similar stimulation protocols were performed on intact mouse single fibers from flexor digitorum brevis muscle, whereby force and myoplasmic free [Ca2+] ([Ca2+]i) were assessed. Our data show a low-frequency force potentiation that was not muscle length-dependent in human muscle and it was not accompanied by any increase in M-wave amplitude. A length-independent low-frequency force potentiation could be replicated in mouse single fibers, supporting an intramuscular mechanism. We show that at physiological temperature (31°C) this low-frequency force potentiation in mouse fibers corresponded with an increase in sarcoplasmic reticulum (SR) Ca2+ release. When mimicking the slower contractile properties of human muscle by cooling mouse single fibers to 18°C, the low-frequency force potentiation was accompanied by minimally increased SR Ca2+ release and hence it could be explained by increased myofibrillar Ca2+ sensitivity. Finally, introducing a brief 200 ms pause between the high- and low-frequency tetanus in human and mouse muscle revealed that the low-frequency force potentiation is abolished, arguing that increased myofibrillar Ca2+ sensitivity is the main intramuscular mechanism underlying the low-frequency force potentiation in humans.

Neuromuscular Fatigue After Repeated Jumping With Concomitant Electrical Stimulation.

PURPOSE: To evaluate the etiology and extent of neuromuscular fatigue induced by 50 squat jumps performed with and without neuromuscular electrical stimulation (NMES) of the knee extensors. METHODS: Nine healthy, recreationally active men (24 ± 2 y) took part in 2 experiments. These consisted of 50 squat jumps performed with stimulation (NMES) or without (CON). Maximal voluntary contraction (MVC) force, maximal voluntary activation level (VAL), and forces evoked by single and double (10 and 100 Hz) stimulations were recorded before and after the 50 jumps. NMES was delivered at the maximal tolerated intensity. RESULTS: Despite average jump height being ∼16% lower in the NMES than in the CON session, a reduction over time in jump height was only found in the NMES condition (-6%). After the 50 jumps, MVC force was reduced to a greater extent in NMES than in CON (-25% ± 11% vs -11% ± 12%). Similarly, forces evoked by single stimulations, as well as by 10-Hz and 100-Hz paired stimulations, were reduced to a greater extent in NMES (-33% ± 12%, -42% ± 15%, and -25% ± 13%) than in CON (-21% ± 6%, -30% ± 9%, and -14% ± 11%). VAL was not significantly altered by either condition. CONCLUSION: Performing repeated squat jumps with concomitant NMES induced a greater fatigue than squat jumps performed alone and might potentially represent a stronger training stimulus.

Plantar flexor muscle weakness and fatigue in spastic cerebral palsy patients.

BACKGROUND: Patients with cerebral palsy develop an important muscle weakness which might affect the aetiology and extent of exercise-induced neuromuscular fatigue. AIM: This study evaluated the aetiology and extent of plantar flexor neuromuscular fatigue in patients with cerebral palsy. METHODS: Ten patients with cerebral palsy and 10 age- and sex-matched healthy individuals (∼20 years old, 6 females) performed four 30-s maximal isometric plantar flexions interspaced by a resting period of 2-3s to elicit a resting twitch. Maximal voluntary contraction force, voluntary activation level and peak twitch were quantified before and immediately after the fatiguing task. RESULTS: Before fatigue, patients with cerebral palsy were weaker than healthy individuals (341±134N vs. 858±151N, p<0.05) and presented lower voluntary activation (73±19% vs. 90±9%, p<0.05) and peak twitch (100±28N vs. 199±33N, p<0.05). Maximal voluntary contraction force was not significantly reduced in patients with cerebral palsy following the fatiguing task (-10±23%, p>0.05), whereas it decreased by 30±12% (p<0.05) in healthy individuals. CONCLUSIONS: Plantar flexor muscles of patients with cerebral palsy were weaker than their healthy peers but showed greater fatigue resistance. WHAT THIS PAPER ADDS: Cerebral palsy is a widely defined pathology that is known to result in muscle weakness. The extent and origin of muscle weakness were the topic of several previous investigations; however some discrepant results were reported in the literature regarding how it might affect the development of exercise-induced neuromuscular fatigue. Importantly, most of the studies interested in the assessment of fatigue in patients with cerebral palsy did so with general questionnaires and reported increased levels of fatigue. Yet, exercise-induced neuromuscular fatigue was quantified in just a few studies and it was found that young patients with cerebral palsy might be more fatigue resistant that their peers. Thus, it appears that (i) conflicting results exist regarding objectively-evaluated fatigue in patients with cerebral palsy and (ii) the mechanisms underlying this muscle fatigue - in comparison to those of healthy peers - remain poorly understood. The present study adds important knowledge to the field as it shows that when young adults with cerebral palsy perform sustained maximal isometric plantar flexions, they appear less fatigable than healthy peers. This difference can be ascribed to a better preservation of the neural drive to the muscle. We suggest that the inability to drive their muscles maximally accounts for the lower extent of exercise-induced neuromuscular fatigue in patients with cerebral palsy.

Are There Critical Fatigue Thresholds? Aggregated vs. Individual Data.

The mechanisms underlying task failure from fatiguing physical efforts have been the focus of many studies without reaching consensus. An attractive but debated model explains effort termination with a critical peripheral fatigue threshold. Upon reaching this threshold, feedback from sensory afferents would trigger task disengagement from open-ended tasks or a reduction of exercise intensity of closed-ended tasks. Alternatively, the extant literature also appears compatible with a more global critical threshold of loss of maximal voluntary contraction force. Indeed, maximal voluntary contraction force loss from fatiguing exercise realized at a given intensity appears rather consistent between different studies. However, when looking at individual data, the similar maximal force losses observed between different tasks performed at similar intensities might just be an "artifact" of data aggregation. It would then seem possible that such a difference observed between individual and aggregated data also applies to other models previously proposed to explain task failure from fatiguing physical efforts. We therefore suggest that one should be cautious when trying to infer models that try to explain individual behavior from aggregated data.

Muscle Fatigue Affects the Interpolated Twitch Technique When Assessed Using Electrically-Induced Contractions in Human and Rat Muscles.

The interpolated twitch technique (ITT) is the gold standard to assess voluntary activation and central fatigue. Yet, its validity has been questioned. Here we studied how peripheral fatigue can affect the ITT. Repeated contractions at submaximal frequencies were produced by supramaximal electrical stimulations of the human adductor pollicis muscle in vivo and of isolated rat soleus fiber bundles; an extra stimulation pulse was given during contractions to induce a superimposed twitch. Human muscles fatigued by repeated 30-Hz stimulation trains (3 s on-1 s off) showed an ~80% reduction in the superimposed twitch force accompanied by a severely reduced EMG response (M-wave amplitude), which implies action potential failure. Subsequent experiments combined a less intense stimulation protocol (1.5 s on-3 s off) with ischemia to cause muscle fatigue, but which preserved M-wave amplitude. However, the superimposed twitch force still decreased markedly more than the potentiated twitch force; with ITT this would reflect increased "voluntary activation." In contrast, the superimposed twitch force was relatively spared when a similar protocol was performed in rat soleus bundles. Force relaxation was slowed by >150% in fatigued human muscles, whereas it was unchanged in rat soleus bundles. Accordingly, results similar to those in the human muscle were obtained when relaxation was slowed by cooling the rat soleus muscles. In conclusion, our data demonstrate that muscle fatigue can confound the quantification of central fatigue using the ITT.

Ryanodine receptor fragmentation and sarcoplasmic reticulum Ca2+ leak after one session of high-intensity interval exercise.

High-intensity interval training (HIIT) is a time-efficient way of improving physical performance in healthy subjects and in patients with common chronic diseases, but less so in elite endurance athletes. The mechanisms underlying the effectiveness of HIIT are uncertain. Here, recreationally active human subjects performed highly demanding HIIT consisting of 30-s bouts of all-out cycling with 4-min rest in between bouts (≤3 min total exercise time). Skeletal muscle biopsies taken 24 h after the HIIT exercise showed an extensive fragmentation of the sarcoplasmic reticulum (SR) Ca(2+) release channel, the ryanodine receptor type 1 (RyR1). The HIIT exercise also caused a prolonged force depression and triggered major changes in the expression of genes related to endurance exercise. Subsequent experiments on elite endurance athletes performing the same HIIT exercise showed no RyR1 fragmentation or prolonged changes in the expression of endurance-related genes. Finally, mechanistic experiments performed on isolated mouse muscles exposed to HIIT-mimicking stimulation showed reactive oxygen/nitrogen species (ROS)-dependent RyR1 fragmentation, calpain activation, increased SR Ca(2+) leak at rest, and depressed force production due to impaired SR Ca(2+) release upon stimulation. In conclusion, HIIT exercise induces a ROS-dependent RyR1 fragmentation in muscles of recreationally active subjects, and the resulting changes in muscle fiber Ca(2+)-handling trigger muscular adaptations. However, the same HIIT exercise does not cause RyR1 fragmentation in muscles of elite endurance athletes, which may explain why HIIT is less effective in this group.

Comparison of electrical nerve stimulation, electrical muscle stimulation and magnetic nerve stimulation to assess the neuromuscular function of the plantar flexor muscles.

INTRODUCTION: As it might lead to less discomfort, magnetic nerve stimulation (MNS) is increasingly used as an alternative to electrical stimulation methods. Yet, MNS and electrical nerve stimulation (ENS) and electrical muscle stimulation (EMS) have not been formally compared for the evaluation of plantar flexor neuromuscular function. METHODS: We quantified plantar flexor neuromuscular function with ENS, EMS and MNS in 10 volunteers in fresh and fatigued muscles. Central alterations were assessed through changes in voluntary activation level (VAL) and peripheral function through changes in M-wave, twitch and doublet (PS100) amplitudes. Discomfort associated with 100-Hz paired stimuli delivered with each method was evaluated on a 10-cm visual analog scale. RESULTS: VAL, agonist and antagonist M-wave amplitudes and PS100 were similar between the different methods in both fresh and fatigued states. Potentiated peak twitch was lower in EMS compared to ENS, whereas no difference was found between ENS and MNS for any parameter. Discomfort associated with MNS (1.5 ± 1.4 cm) was significantly less compared to ENS (5.5 ± 1.9 cm) and EMS (4.2 ± 2.6 cm) (p < 0.05). CONCLUSION: When PS100 is used to evaluate neuromuscular properties, MNS, EMS and ENS can be used interchangeably for plantar flexor neuromuscular function assessment as they provide similar evaluation of central and peripheral factors in unfatigued and fatigued states. Importantly, electrical current spread to antagonist muscles was similar between the three methods while discomfort from MNS was much less compared to ENS and EMS. MNS may be potentially employed to assess neuromuscular function of plantar flexor muscles in fragile populations.