Supramaximal Stimulation Research Articles

Intermittent hypoxia enhances voluntary activation and reduces performance fatigability during repeated lower limb contractions.

Prior research has highlighted the therapeutic benefits of acute intermittent hypoxia (AIH) in enhancing motor performance after motor incomplete spinal cord injury and in able-bodied individuals. While studies in rodents and humans indicate that AIH may facilitate motor excitability, the relationship between excitability changes and functional performance remains unclear. Additionally, discrepancies in the effects of AIH on excitability in able-bodied individuals merit further investigation. Understanding the concurrent impact of repetitive AIH on voluntary activation and spinal reflex excitability may clarify the functional implications of AIH for muscle force production. High voluntary activation is vital for sustaining torque production during activities that require repeated muscle contractions. We hypothesized that repetitive AIH would attenuate declines in both voluntary activation and maximum torque production typically observed during fatiguing contractions. To test this hypothesis, we examined the effects of four consecutive days of AIH on voluntary activation and torque generation during repeated maximal plantar flexion contractions. We assessed changes in voluntary activation using the central activation ratio (CAR) by calculating the ratio of voluntary torque to the torque produced with supramaximal electrical stimulation. Consistent with our hypothesis, we show that repetitive AIH significantly increases both CAR and peak torque during fatiguing contractions. We did not observe any changes in resting spinal reflex excitability or antagonist muscle coactivation during the fatiguing contractions post-AIH. Together, these findings suggest that repetitive AIH reduces performance fatigability through enhanced descending neural drive. Optimizing voluntary activation is critical for facilitating the recovery of functional walking skills after neurological injury.

Effects of cervical transcutaneous spinal direct current stimulation on spinal excitability

ObjectiveTo investigate the effects of transcutaneous spinal direct current stimulation (tsDCS) on spinal cord excitability using neurophysiological methods. MethodsSpinal cord motoneuron excitability was assessed using various neurophysiological techniques in a sham-controlled randomized experiment, which involved delivering 2 mA tsDCS and testing four different montages. Transcranial magnetic stimulation (TMS), F-waves to supramaximal ulnar nerve stimulation and somatosensory evoked potentials to upper limb nerves stimulation were measured in the participants with the electrode configuration that yielded the greatest effect, for a total of about 18 min. 18 young volunteers were recruited. ResultsAmong the tested ones, the most promising tsDCS montage was the one with the anode placed on the 7th cervical spinous process and the cathode on the glottis. With this configuration, a significant enhancement of motor responses in the hand muscles to TMS of the contralateral hand motor area was observed during tsDCS (p<0.00001), reaching a plateau after 6 min. This facilitation rapidly declined within a few minutes after the tsDCS was stopped. ConclusionResults of the different techniques suggest a possible contribution to facilitatory neuromodulation of the motoneurons at the cervical spine level. SignificanceThe occurrence of enhanced excitability after tsDCS suggests potential application in individuals with partial corticospinal fiber impairment affecting hand motor function.

Effects of ischemic pre-conditioning on electrically stimulated contractions.

Ischemic pre-conditioning (IPC) offers protection against future ischemic events and may improve sports performance due to several mechanisms at local and systemic levels. This study investigates the local effects on muscle contractility in electrically induced muscle contractions, thus effectively excluding any uncontrolled change in the motor drive. Twenty-one subjects were divided into two groups: 12 subjects in the IPC group (3 × 5/5min right arm ischemia/reperfusion; cuff pressure 250mmHg) and 9 subjects in the SHAM group (same treatment at 20mmHg). The adductor pollicis was contracted by supramaximal stimulation of the ulnar nerve with single pulses, trains of stimuli (5, 8, 10 and 12Hz, 1-s duration) and bursts (4 pulses, 25Hz), all separated by 5-s intervals. The stimulation sequence was delivered before and 15 and 30min after IPC/SHAM treatment. The isometric contraction force, the superficial electromyographic signal, and tissue oxygenation were continuously monitored. A significant force decrease in time was observed at 8, 10 (p < 0.01) and 12Hz (p < 0.05) along with a decrease in half-relaxation time in single twitches and bursts (p = 0.01), regardless of treatment. This general time-related weakening was more marked in IPC than SHAM at 5-Hz stimulation. No effects were observed on the magnitude of the superficial electromyographic signal. Data indicate that IPC does not increase muscle force during electrically stimulated contractions, supporting the idea that IPC's ergogenic effects are not due to increased muscle contractility.

LATEST ADVANCES IN FACIAL NERVE INTRAOPERATIVE MONITORING FOR VESTIBULAR SCHWANNOMA - UP TO DATE 2024

Introduction: Vestibular schwannoma, a benign tumor of the eighth cranial nerve, can be treated with radiosurgery, microsurgery, or observation. This study aims to present advances in intraoperative monitoring techniques for preserving facial nerve function during acoustic neuroma surgeries. By sharing the latest evidence, we hope to influence clinical practices for the benefit of patients. Methodology: Thus, we conducted a comprehensive literature review using the Scopus database, we searched for articles published from 2018 to 2023 using the keywords "facial paralysis," "acoustic neuroma," and "intraoperative monitoring." Results: The use of electromyographic parameters in the pre, intra, and post-resection monitoring of vestibular schwannoma provides valuable indicators of both the functional state of the facial nerve and the tumor adherence. This information plays a crucial role in making safe decisions regarding the extent of tumor resection while minimizing the risk of facial nerve damage during surgery. Supramaximal stimulation of the facial nerve in proximity to the tumor is of utmost importance to preserve facial function. However, the presence of a separate intermediate nerve poses challenges in accurately detecting false positives. Therefore, monitoring the facial nerve during surgery becomes essential to prevent potential injuries and to predict both short- and long-term outcomes. Conclusion: Intraoperative electrophysiological facial nerve monitoring is a crucial strategy to reduce postoperative facial paralysis. While the best technique is still under debate, several options exist to enhance monitoring efficacy and patient safety. By staying informed, clinicians can make better decisions to improve patient outcomes.

F wave analysis based on the compound muscle action potential scan.

Conventional F wave analysis involves a relatively uniform physiological environment induced by supramaximal stimulations. The F wave characteristics in a dynamic physiological condition, however, are rarely investigated. This study aimed to improve understanding of F wave properties in the more dynamic process by introducing a novel method to analyze F waves based on the compound muscle action potential (CMAP) scan technique. Twenty four healthy subjects participated in the study. The CMAP scan was applied to record muscle responses in the abductor pollicis brevis (APB) and abductor digiti minimi (ADM) muscles, respectively. F wave characteristics including mean F wave amplitude and latency (F-M latency), persistence and activating threshold were quantified. An average of 200 F waves per muscle were obtained from the CMAP scan recording. Weak to moderate correlations between F wave amplitude and stimulating intensity were observed in most of the APB (19 muscles; r = 0.33 ± 0.14, all p < .05) and ADM (23 muscles, r = 0.46 ± 0.16, all p < .05) muscles. Significantly longer mean F latency and lower activating F-threshold were found in the ADM muscles (F-M latency: APB: 25.43 ± 2.39 ms, ADM: 26.15 ± 2.32 ms, p < .05; F-threshold: APB: 7.68 ± 8.96% CMAP, ADM: 2.35 ± 2.42% CMAP, p < .05). This study introduces new features of F waves using the CMAP scan technique and identifies differences of F wave characteristics between the hand muscles. The CMAP scan based F waves analysis can be combined with the motor unit number estimation to assess functional alterations in motor neurons in neurological disorders.

Nerve conduction study on the split-hand plus index in Amyotrophic lateral sclerosis: correlations with lower motor neuron impairment.

In the arms of patients with Amyotrophic lateral sclerosis (ALS) two peculiar patterns of dissociated muscular atrophy have been described: the split-hand sign (with predominant atrophy of the lateral aspect of the hand, compared to hypothenar eminence) and the split-hand-plus sign (SHPS), a predominant abductor pollicis brevis (ABP) atrophy with sparing of flexor pollicis longus (FPL). In this case-control study, we evaluated the diagnostic utility of a neurophysiological indicator of SHPS and assessed its association with clinical features. We prospectively studied 59 incident ALS patients, 61 patients with ALS-mimic disorders (OND) and 61 non-neurological controls (NNCs). ABP and FPL compound muscle action potentials (CMAP) amplitudes were obtained by supramaximal stimulation of median nerve at elbow. Split-hand plus index (SHPI) was calculated according to the formula: APB-CMAP/FPL-CMAP. SHPI was significantly lower in ALS compared to OND patients and NNCs (p < 0.0001). SHPI value < 1 was observed in 2% of NNCs and 9% of OND patients and demonstrated an accuracy of 71% in differentiating ALS from OND and an accuracy of 74% in differentiating ALS from NNC. SHPI was associated with higher LMN score, and higher disease severity as quantified by the ALSFRS-r. Our results indicate that SHPI is a reliable indicator to distinguish ALS patients from ONDs and NNCs. SHPI was significantly associated to the degree of lower motor neuron impairment but showed no association with upper motoneuron impairment.

Clinical characteristics of late-onset myasthenia gravis

ObjectiveLate-onset myasthenia gravis (LOMG) often has comorbidities, and its initial symptoms may be ignored or misdiagnosed as other diseases. There were few large surveys on LOMG. Our study aimed to summarize clinical characteristics of LOMG to improve the rate of correct MG diagnosis. MethodsA retrospective cohort study included 240 LOMG patients with onset age ≥65 years old who were treated at PLA General Hospital from January 1, 2003 to January 1, 2023. ResultsThe male to female ratio was 1:1.2 (P = 0.699). MGFA clinical classification: Class I 31.3%, Class IIa 12.9%, Class IIb 51.3%, Class IIIa 0.8%, Class IIIb 0.8%, Class IV 0.4%, Class V2.5%. The onset symptom was ptosis in 78.8% and diplopia was in 18.8%. Swallowing dysfunction in the stage of LOMG was in 41.7%. The incidence of thymoma in LOMG was 14.2%. 85.4% of patients antibodies against the muscle acetylcholine receptor (AChR) are detected. The overall incidence of supramaximal repetitive nerve stimulation (Jolly test) was 57.1%, among which the highest positive rate (50.7%) was in the facial nerve. Jolly test of Class IIb was tested in the highest positive rate and Class I was in the lowest one (χ2 = 7.023, P = 0.030). ConclusionThere was no significant difference in the incidence of LOMG between males and females. The clinical manifestations were mainly Class I and Class II, and severe MG was rare. The most common onset symptom was ptosis. The incidence of LOMG with thymoma was low. Supramaximal repetitive nerve stimulation (Jolly test) of the facial nerve was the easiest to detect and Jolly test of Class IIb was tested in the highest positive rate and Class I was in the lowest one.

Synergistic difference in the effect of stretching on electromechanical delay components.

This study investigated the synergistic difference in the effect of stretching on electromechanical delay (EMD) and its components, using a simultaneous recording of electromyographic, mechanomyographic, and force signals. Twenty-six healthy men underwent plantar flexors passive stretching. Before and after stretching, the electrochemical and mechanical components of the EMD and the relaxation EMD (R-EMD) were calculated in gastrocnemius medialis (GM), lateralis (GL) and soleus (SOL) during a supramaximal motor point stimulation. Additionally, joint passive stiffness was assessed. At baseline, the mechanical components of EMD and R-EMD were longer in GM and GL than SOL (Cohen's d from 1.78 to 3.67). Stretching decreased joint passive stiffness [-22(8)%, d = -1.96] while overall lengthened the electrochemical and mechanical EMD. The mechanical R-EMD components were affected more in GM [21(2)%] and GL [22(2)%] than SOL [12(1)%], with d ranging from 0.63 to 1.81. Negative correlations between joint passive stiffness with EMD and R-EMD mechanical components were found before and after stretching in all muscles (r from -0.477 to -0.926; P from 0.007 to <0.001). These results suggest that stretching plantar flexors affected GM and GL more than SOL. Future research should calculate EMD and R-EMD to further investigate the mechanical adaptations induced by passive stretching in synergistic muscles.

Hemodynamic and neuromuscular basis of reduced exercise capacity in patients with end-stage renal disease.

The present study aimed to characterize the exercise-induced neuromuscular fatigue and its possible links with cerebral and muscular oxygen supply and utilization to provide mechanistic insights into the reduced exercise capacity characterizing patients with end-stage renal disease (ESRD). Thirteen patients with ESRD and thirteen healthy males (CTR group) performed a constant-force sustained isometric contraction at 50% of their maximal voluntary isometric contraction (MVC) until exhaustion. Quadriceps muscle activation during exercise was estimated from vastus lateralis, vastus medialis, and rectus femoris EMG. Central and peripheral fatigue were quantified via changes in pre- to postexercise quadriceps voluntary activation (ΔVA) and quadriceps twitch force (ΔQtw,pot) evoked by supramaximal electrical stimulation, respectively. To assess cerebral and muscular oxygenation, throughout exercise, near-infrared spectroscopy allowed investigation of changes in oxyhemoglobin (∆O2Hb), deoxyhemoglobin (∆HHb), and total hemoglobin (∆THb) in the prefrontal cortex and in the vastus lateralis muscle. ESRD patients demonstrated lower exercise time to exhaustion than that of CTR (88.8 ± 15.3s and 119.9 ± 14.6s, respectively, P < 0.01). Following the exercise, MVC, Qtw,pot, and VA reduction were similar between the groups (P > 0.05). There was no significant difference in muscle oxygenation (∆O2Hb) between the two groups (P > 0.05). Cerebral and muscular blood volume (∆THb) and oxygen extraction (∆HHb) were significantly blunted in the ESRD group (P < 0.05). A significant positive correlation was observed between time to exhaustion and cerebral blood volume (∆THb) in both groups (r2 = 0.64, P < 0.01). These findings support cerebral hypoperfusion as a factor contributing to the reduction in exercise capacity characterizing ESRD patients.

The potassium-glycogen interaction on force and excitability in mouse skeletal muscle: implications for fatigue.

A reduced muscle glycogen content and potassium (K+ ) disturbances across muscle membranes occur concomitantly during repeated intense exercise and together may contribute to skeletal muscle fatigue. Therefore, we examined whether raised extracellular K+ concentration ([K+ ]o ) (4 to 11 mM) interacts with lowered glycogen to reduce force production. Isometric contractions were evoked in isolated mouse soleus muscles (37°C) using direct supramaximal field stimulation. (1) Glycogen declined markedly in non-fatigued muscle with >2h exposure in glucose-free physiological saline compared with control solutions (11 mM glucose), i.e. to <45% control. (2) Severe glycogen depletion was associated with increased 5'-AMP-activated protein kinase activity, indicative of metabolic stress. (3) The decline of peak tetanic force at 11mM [K+ ]o was exacerbated from 67% initial at normal glycogen to 22% initial at lowered glycogen. This was due to a higher percentage of inexcitable fibres (71% vs. 43%), yet without greater sarcolemmal depolarisation or smaller amplitude action potentials. (4) Returning glucose while at 11mM [K+ ]o increased both glycogen and force. (5) Exposure to 4mM [K+ ]o glucose-free solutions (15 min) did not increase fatiguability during repeated tetani; however, after recovery there was a greater force decline at 11mM [K+ ]o at lower than normal glycogen. (6) An important exponential relationship was established between relative peak tetanic force at 11mM [K+ ]o and muscle glycogen content. These findings provide direct evidence of a synergistic interaction between raised [K+ ]o and lowered muscle glycogen as the latter shifts the peak tetanic force-resting EM relationship towards more negative resting EM due to lowered sarcolemmal excitability, which hence may contribute to muscle fatigue. KEY POINTS: Diminished muscle glycogen levels and raised extracellular potassium concentrations ([K+ ]o ) occur simultaneously during intense exercise and together may contribute to muscle fatigue. Prolonged exposure of isolated non-fatigued soleus muscles of mice to glucose-free physiological saline solutions markedly lowered muscle glycogen levels, as does fatigue then recovery in glucose-free solutions. For both approaches, the subsequent decline of maximal force at 11mM[K+ ]o , which mimics interstitial [K+ ] levels during intense exercise, was exacerbated at lowered compared with normal glycogen. This was mainly due to many more muscle fibres becoming inexcitable. We established an important relationship that provides evidence of a synergistic interaction between raised [K+ ]o and lowered glycogen content to reduce force production. This paper indicates that partially lowered muscle glycogen (and/or metabolic stress) together with elevated interstitial [K+ ] interactively lowers muscle force, and hence may diminish performance especially during repeated high-intensity exercise.

Plantar Flexor Weakness and Pain Sensitivity Cannot Be Assumed in Midportion Achilles Tendinopathy.

The purpose of this study was to determine the following in persons with midportion Achilles tendinopathy (AT): 1) maximal strength and power; 2) neural drive during maximal contractions and contractile function during electrically evoked resting contractions; and 3) whether pain, neural drive, and contractile mechanisms contribute to differences in maximal strength. Twenty-eight volunteers (14 AT, 14 controls) completed isometric, concentric, and eccentric maximal voluntary contractions (MVCs) of the plantar flexors in a Biodex™ dynamometer. Supramaximal electrical stimulation of the tibial nerve was performed to quantify neural drive and contractile properties of the plantar flexors. Pain sensitivity was quantified as the pressure-pain thresholds of the Achilles tendon, medial gastrocnemius, and upper trapezius. There were no differences in plantar flexion strength or power between AT and controls (isometric MVC: P = 0.95; dynamic MVC: P = 0.99; power: P = 0.98), nor were there differences in neural drive and contractile function (P = 0.55 and P = 0.06, respectively). However, the mechanisms predicting maximal strength differed between groups: neural drive predicted maximal strength in controls (P = 0.02) and contractile function predicted maximal strength in AT (P = 0.001). Although pain did not mediate these relationships (i.e., between maximal strength and its contributing mechanisms), pressure-pain thresholds at the upper trapezius were higher in AT (P = 0.02), despite being similar at the calf (P = 0.24) and Achilles tendon (P = 0.40). There were no deficits in plantar flexion strength or power in persons with AT, whether evaluated isometrically, concentrically, or eccentrically. However, the mechanisms predicting maximal plantar flexor strength differed between groups, and systemic pain sensitivity was diminished in AT.

FM latency, FM ratio, F-wave latency versus F-wave index in the diagnosis of diabetic neuropathy

BackgroundDiabetes is one of the most common causes for peripheral neuropathy which warrants early diagnosis and treatment. The conventional nerve conduction studies appear to be normal if few of the nerve roots are escaping the lesion. In view of identifying a better clinical testing method, a late response index comprising most of the parameter of F wave was constructed and compared with the diagnosing ability of other parameters.MethodsThis hospital-based cross-sectional study was conducted among male diabetics aged 30 to 50 years in a tertiary care hospital. Maintaining an ambient temperature of 28 °C and following a supramaximal stimulation of the median nerve of both upper limbs, using a digitalized nerve conduction/electromyography/EP machine, the F-wave tracing was obtained.ResultsThe F-wave index detected a greater number of subjects with diabetic peripheral neuropathy when compared to F-wave mean latency, F M latency and F–M ratio.ConclusionF-wave index can be used as a surrogate electrophysiological study to detect peripheral neuropathy at an early stage and aids in better prognosis.

The potential of electromyography, diagnostic transcranial magnetic stimulation, and multiparametric magnetic resonance imaging in the combinatory assessment of facial nerve disorders: a literature review and clinical case series

Facial neuropathy (FN) is a complex multicausal problem that, with a seemingly obvious clinical picture, might be challenging to diagnose. Up to 5% of FN cases could be caused by neoplastic or otogenic processes, necessitating an interdisciplinary approach to its treatment by various specialties and in some cases a surgical intervention. In addition, in the early stages of FN, it is difficult to predict its outcomes. Therefore, beyond usual neurological exam and widely used electromyography (EMG), other additional diagnostic tools are used to ensure extended diagnosis, including cancer awareness.&#x0D; In this paper we have analyzed the principles, role and value of computed tomography, magnetic resonance imaging (MRI), diagnostic transcranial magnetic stimulation combined with EMG, and ultrasound assessment with a high-frequency linear transducer in acute FN. We present our own clinical cases of pediatric patients with FN, who were assessed with EMG and multiparametric MRI including diffusion tensor imaging. These cases illustrate both the abnormalities found in the typical course of Bell's palsy, as well as the abnormalities in neoplasm-associated FN that clinically fully mimic the Bell's palsy. Based on the world experience in multiparametric MRI, including the use of extended protocols in the Pediatric Research and Clinical Center for Infectious Diseases, in case of suspected FN, the most important are high-resolution structural submillimeter sequences based on the gradient echo (SSFP) and diffusion tensor imaging (DTI). Measurement and assessment of fractional anisotropy at the motor nuclei of the facial nerves in the pons look promising for further research. The paper is the first to describe a modified combination diagnostic approach to Bell's palsy with the use of diagnostic transcranial magnetic stimulation with round coil, supramaximal stimulation with identification of the motor evoked response threshold (minimal inducer power to register a reproducible evoked motor response of 50-100 mV in amplitude) in the occipito-parietal area of the ipsilateral muscle.

Influence of temperature on twitch potentiation following submaximal voluntary contractions in human plantar flexor muscles.

This study aimed to clarify the influence of temperature on post-activation twitch potentiation, a possible indicator of fast-twitch fiber activation during a preceding voluntary contraction. Ten healthy males immersed their left lower leg in water of different temperatures (cold: 0°C, neutral: 32-35°C, hot: ~43°C) for 20 min each. In each temperature condition, they performed submaximal (10%-50% of maximal voluntary contraction torque measured before water immersion) and maximal plantar flexions. Immediately after each voluntary contraction, twitch contractions were evoked with supramaximal stimulation of the posterior tibial nerve. The magnitude of twitch potentiation, defined as a percent increase in twitch torque following a voluntary contraction, increased with the intensity of the preceding voluntary contraction. The magnitude of twitch potentiation after the maximal voluntary contraction was smaller in Cold than in the other temperature conditions. However, temperature had no influence on the relative magnitude of twitch potentiation following the submaximal contractions. In addition, there was no difference in electromyographic activity between the gastrocnemius and soleus muscles in any temperature conditions. Collectively, the temperature dependence was not observed when using twitch potentiation or electromyographic amplitude as an indicator of fast-twitch fiber activation during brief submaximal voluntary contractions.

Neuromuscular electrical stimulation during maximal voluntary contraction: a Delphi survey with expert consensus

PurposeThe use of electrical stimulation to assess voluntary activation of muscle/s is a popular method employed in numerous exercise science and health research settings. This Delphi study aimed to collate expert opinion and provide recommendations for best practice when using electrical stimulation during maximal voluntary contractions.MethodsA two-round Delphi study was undertaken with 30 experts who completed a 62-item questionnaire (Round 1) comprising of open- and closed-ended questions. Consensus was assumed if ≥ 70% of experts selected the same response; such questions were removed from the subsequent Round 2 questionnaire. Responses were also removed if they failed to meet a 15% threshold. Open-ended questions were analysed and converted into closed-ended questions for Round 2. It was assumed there was no clear consensus if a question failed to achieve a ≥ 70% response in Round 2.ResultsA total of 16 out of 62 (25.8%) items reached consensus. Experts agreed that electrical stimulation provides a valid assessment of voluntary activation in specific circumstances, such as during maximal contraction, and this stimulation can be applied at either the muscle or the nerve. Experts recommended using doublet stimuli, self-adhesive electrodes, a familiarisation session, real-time visual or verbal feedback during the contraction, a minimum current increase of + 20% to ensure supramaximal stimulation, and manually triggering stimuli.ConclusionThe results of this Delphi consensus study can help researchers make informed decisions when considering technical parameters when designing studies involving electrical stimulation for the assessment of voluntary activation.

High-intensity interval training in the form of isometric contraction improves fatigue resistance in dystrophin-deficient muscle.

Duchenne muscular dystrophy is a genetic muscle-wasting disorder characterized by progressive muscle weakness and easy fatigability. Here we examined whether high-intensity interval training (HIIT) in the form of isometric contraction improves fatigue resistance in skeletal muscle from dystrophin-deficient mdx52mice. Isometric HIIT was performed on plantar flexor muscles in vivo with supramaximal electrical stimulation every other day for 4weeks (a total of 15sessions). In the non-trained contralateral gastrocnemius muscle from mdx52 mice, the decreased fatigue resistance was associated with a reduction in the amount of peroxisome proliferator-activated receptor γ coactivator 1-α, citrate synthase activity, mitochondrial respiratory complex II, LC3B-II/I ratio, and mitophagy-related gene expression (i.e. Pink1, parkin, Bnip3 and Bcl2l13) as well as an increase in the phosphorylation levels of Src Tyr416 and Akt Ser473, the amount of p62, and the percentage of Evans Blue dye-positive area. Isometric HIIT restored all these alterations and markedly improved fatigue resistance in mdx52muscles. Moreover, an acute bout of HIIT increased the phosphorylation levels of AMP-activated protein kinase (AMPK) Thr172, acetyl CoA carboxylase Ser79, unc-51-like autophagy activating kinase 1 (Ulk1) Ser555, and dynamin-related protein 1 (Drp1) Ser616 in mdx52muscles. Thus, our data show that HIIT with isometric contractions significantly mitigates histological signs of pathology and improves fatigue resistance in dystrophin-deficient muscles. These beneficial effects can be explained by the restoration of mitochondrial function via AMPK-dependent induction of the mitophagy programme and de novo mitochondrial biogenesis. KEY POINTS: Skeletal muscle fatigue is often associated with Duchenne muscular dystrophy (DMD) and leads to an inability to perform daily tasks, profoundly decreasing quality of life. We examined the effect of high-intensity interval training (HIIT) in the form of isometric contraction on fatigue resistance in skeletal muscle from the mdx52mouse model of DMD. Isometric HIIT counteracted the reduced fatigue resistance as well as dystrophic changes in skeletal muscle of mdx52mice. This beneficial effect could be explained by the restoration of mitochondrial function via AMP-activated protein kinase-dependent mitochondrial biogenesis and the induction of the mitophagy programme in the dystrophic muscles.

M-wave changes caused by brief voluntary and stimulated isometric contractions

IntroductionUnder isometric conditions, the increase in muscle force is accompanied by a reduction in the fibers’ length. The effects of muscle shortening on the compound muscle action potential (M wave) have so far been investigated only by computer simulation. This study was undertaken to assess experimentally the M-wave changes caused by brief voluntary and stimulated isometric contractions.MethodsTwo different methods of inducing muscle shortening under isometric condition were adopted: (1) applying a brief (1 s) tetanic contraction and (2) performing brief voluntary contractions of different intensities. In both methods, supramaximal stimulation was applied to the brachial plexus and femoral nerves to evoke M waves. In the first method, electrical stimulation (20 Hz) was delivered with the muscle at rest, whereas in the second, stimulation was applied while participants performed 5-s stepwise isometric contractions at 10, 20, 30, 40, 50, 60, 70, and 100% MVC. The amplitude and duration of the first and second M-wave phases were computed.ResultsThe main findings were: (1) on application of tetanic stimulation, the amplitude of the M-wave first phase decreased (~ 10%, P < 0.05), that of the second phase increased (~ 50%, P < 0.05), and the M-wave duration decreased (~ 20%, P < 0.05) across the first five M waves of the tetanic train and then plateaued for the subsequent responses; (2) when superimposing a single electrical stimulus on muscle contractions of increasing forces, the amplitude of the M-wave first phase decreased (~ 20%, P < 0.05), that of the second phase increased (~ 30%, P < 0.05), and M-wave duration decreased (~ 30%, P < 0.05) as force was raised from 0 to 60–70% MVC force.ConclusionsThe present results will help to identify the adjustments in the M-wave profile caused by muscle shortening and also contribute to differentiate these adjustments from those caused by muscle fatigue and/or changes in Na+–K+ pump activity.

Exacerbated central fatigue and reduced exercise capacity in early-stage breast cancer patients treated with chemotherapy

PurposeThe present study aimed to characterize the etiology of exercise-induced neuromuscular fatigue and its consequences on the force-duration relationship to provide mechanistic insights into the reduced exercise capacity characterizing early-stage breast cancer patients.MethodsFifteen early-stage breast cancer patients and fifteen healthy women performed 60 maximal voluntary isometric quadriceps contractions (MVCs, 3 s of contraction, 2 s of relaxation). The critical force was determined as the mean force of the last six contractions, while W’ was calculated as the force impulse generated above the critical force. Quadriceps muscle activation during exercise was estimated from vastus lateralis, vastus medialis and rectus femoris EMG. Central and peripheral fatigue were quantified via changes in pre- to postexercise quadriceps voluntary activation (ΔVA) and quadriceps twitch force (ΔQTw) evoked by supramaximal electrical stimulation, respectively.ResultsEarly-stage breast cancer patients demonstrated lower MVC than controls preexercise (− 15%, P = 0.022), and this reduction persisted throughout the 60-MVC exercise (− 21%, P = 0.002). The absolute critical force was lower in patients than in controls (144 ± 29N vs. 201 ± 47N, respectively, P < 0.001), while W’ was similar (P = 0.546), resulting in lower total work done (− 23%, P = 0.001). This was associated with lower muscle activation in the vastus lateralis (P < 0.001), vastus medialis (P = 0.003) and rectus femoris (P = 0.003) in patients. Immediately following exercise, ΔVA showed a greater reduction in patients compared to controls (− 21.6 ± 13.3% vs. − 12.6 ± 7.7%, P = 0.040), while ΔQTw was similar (− 60.2 ± 13.2% vs. − 52.8 ± 19.4%, P = 0.196).ConclusionThese findings support central fatigue as a primary cause of the reduction in exercise capacity characterizing early-stage breast cancer patients treated with chemotherapy.Clinical trials registration No. NCT04639609—November 20, 2020.

Motor Unit Number Estimation in Spastic Biceps Brachii Muscles of Chronic Stroke Survivors Before and After BoNT Injection.

The study aims to characterize the motor unit (MU) loss in spastic biceps brachii muscle (BBM) of chronic stroke survivors before and after botulinum neurotoxin (BoNT) injection. High-density weighted average (HDWA) motor unit number estimation (MUNE) was employed to estimate the number of functioning motor units of BBMs of eight chronic stroke survivors 1-week before (1st visit) and 3-week after (2nd visit) BoNT injection based on the surface electromyography (sEMG) signals recorded during voluntary contraction and supramaximal electrical stimulation. Significant lower MUNE was estimated from the spastic BBMs compared to the non-spastic MUNEs during two visits. A surprisingly higher MUNE was obtained from the spastic side during the 2nd visit after BoNT injection. The HDWA MUNE technique can be employed to characterize the motor unit loss in spastic muscle caused by upper motor neuro lesions at contraction level up to 30% MVC, but may fail to detect the MU loss caused by the chemodenervation effect of BoNT due to the non-uniform denervation of small and large size MUs. This study presents the first effort to evaluate the applicability of HDWA MUNE technique to characterize the MU loss in the spastic muscle following stroke and the subsequent BoNT injection for the treatment of post-stroke spasticity. The finding of this study suggests that HDWA MUNE can be a sensitive approach to detect the MU loss in spastic muscles after stroke, but the large inter-subject MUNE variability after the BoNT injection should be interpreted with caution.

Electromagnetic field for supramaximal muscle stimulation: A retrospective study of safety, efficacy, and patient satisfaction in Brazil.

Currently, even individuals who do physical activity regularly have some degree of dissatisfaction with their own bodies. The electromagnetic field for supramaximal muscle contraction has been the subject of research. High-intensity supramaximal muscle stimulation (HI-SMS) is a non-invasive technology used to strengthen, firm, and tone the abdominal muscles, arms, buttocks, and thighs and has been indicated for aesthetic purposes. The present study aimed to examine the safety and efficacy of HI-SMS used in the abdominal muscles of patients through the analysis of clinical evaluation, biochemical serum profile, and patient satisfaction with the procedure. This is retrospective non-randomized and non-controlled study collected in a private clinic; all data from healthy participants (n=25), aged between 18 and 55 years, were compiled and analyzed. All received eight 30 min sessions of electromagnetic field ONIX HI-SMS (intensity of the 90%-100%) located in abdominal, twice a week with intervals of 2-3 days. The results show that BMI, fat thickness, and waist circumference improved the body contour after the treatment. There was no statistical difference in the data referring to the values of AST, ALT, ALP, creatinine, cholesterol, LDL-C, VLDL-C, HDL-C, glycemia, LDH, CK, and IL-6. However, there was a reduction of "non-esterified" free fatty acids when compared to baseline. This treatment provided high levels of tolerance, comfort, and high level of satisfaction. Thus, it can be suggested that the treatment with HI-SMS in abdominal muscles proves to be a safe technology with potential for non-invasive therapy for aesthetic purposes.