Neuromuscular responses to fatiguing locomotor exercise

Supraspinal, spinal, and motor unit adjustments to fatiguing isometric contractions of the knee extensors at low and high submaximal intensities in males

Contraction intensity is a key factor determining the development of muscle fatigue, and it has been shown to induce distinct changes along the motor pathway. The role of cortical and spinal inputs that regulate motor unit (MU) behavior during fatiguing contractions is poorly understood. We studied the cortical, spinal, and neuromuscular response to sustained fatiguing isometric tasks performed at 20% and 70% of the maximum isometric voluntary contraction (MVC), together with MU behavior of knee extensors in healthy active males. Neuromuscular function was assessed before and after performance of both tasks. Cortical and spinal responses during exercise were measured via stimulation of the motor cortex and spinal cord. High-density electromyography was used to record individual MUs from the vastus lateralis (VL). Exercise at 70%MVC induced greater decline in MVC (P = 0.023) and potentiated twitch force compared with 20%MVC (P < 0.001), with no difference in voluntary activation (P = 0.514). Throughout exercise, corticospinal responses were greater during the 20%MVC task (P < 0.001), and spinal responses increased over time in both tasks (P ≤ 0.042). MU discharge rate increased similarly after both tasks (P ≤ 0.043), whereas recruitment and derecruitment thresholds were unaffected (P ≥ 0.295). These results suggest that increased excitability of cortical and spinal inputs might be responsible for the increase in MU discharge rate. The increase in evoked responses together with the higher MU discharge rate might be required to compensate for peripheral adjustments to sustain fatiguing contractions at different intensities.NEW & NOTEWORTHY Changes in central nervous system and muscle function occur in response to fatiguing exercise and are specific to exercise intensity. This study measured corticospinal, neuromuscular, and motor unit behavior to fatiguing isometric tasks performed at different intensities. Both tasks increased corticospinal excitability and motor unit discharge rate. Our findings suggest that these acute adjustments are required to compensate for the exercise-induced decrements in neuromuscular function caused by fatiguing tasks.

Exacerbated impairments in neuromuscular function when two bouts of team sport match simulations are separated by 48 h

Intermittent team sports, involving high metabolic and mechanical demands, elicit prolonged impairments in neuromuscular function which persist for ∼48-72 h. Whether impairments in neuromuscular function are exacerbated when such exercise is repeated with incomplete recovery is unknown. This study assessed the neuromuscular, heart rate and metabolic responses to two bouts of ∼90 min modified team sport match simulations separated by 48 h in 12 competitive football players. Before and 2 min after both bouts, knee extensor isometric maximal voluntary contraction (MVC), contractile function (Qtw,pot ) and voluntary activation (VA) were measured. Heart rate (HR), sprint time, blood lactate and glucose were measured throughout both bouts. MVC was reduced relative to baseline at post-bout 1 (21 ± 12%; P = 0.003) and pre-bout 2 (14 ± 11%; P = 0.009), and was lower post-bout 2 (33 ± 14%; P < 0.001) relative to post-bout 1 (P = 0.036). Qtw,pot was reduced post-bout 1 (30 ± 11%; P < 0.001) and pre-bout 2 (9 ± 6%; P = 0.004), and was not different post-bout 2 (28 ± 8%; P < 0.001) relative to post-bout 1 (P = 0.872). VA was reduced post-bout 1 (8 ± 7%; P = 0.023), recovered pre-bout 2 (P = 0.133) and was lower post-bout 2 (16 ± 7%; P < 0.001) relative to post-bout 1 (P = 0.029). Total sprint time was longer, and HR, blood lactate and glucose were lower during bout 2 than bout 1 (P ≤ 0.021). Thus, impairments in neuromuscular function are exacerbated when high-intensity intermittent exercise is performed with incomplete recovery concurrent with accentuated reductions in VA. The lower blood lactate and glucose during the second bout might be due, at least in part, to reduced glycogen availability upon commencing exercise and consequently a greater reliance on glucose extraction. NEW FINDINGS: What is the central question of this study? There is limited evidence on whether impairments in neuromuscular function are exacerbated when prolonged high-intensity intermittent exercise is repeated with incomplete recovery: what are the neuromuscular consequences of performing two bouts of a modified team sport match simulations separated by 48 h? What is the main finding and its importance? Impairments in knee extensor force generating capacity are exacerbated concurrent with accentuated reductions in nervous system activation of muscle when prolonged high-intensity intermittent exercise is repeated with 48 h recovery.

Differences in time to task failure and fatigability between children and young adults: A systematic review and meta-analysis

The transition from childhood to adulthood is characterized by many physiological processes impacting exercise performance. Performance fatigability and time to task failure are commonly used to capture exercise performance. This review aimed to determine the differences in fatigability and TTF between youth (including both children and adolescents) and young adults, and to evaluate the influence of exercise modalities (i.e., exercise duration and type of exercise) on these differences. Medline, SPORTDiscus and Cochrane Library were searched. Thirty-four studies were included. The meta-analyses revealed that both children (SMD −1.15; p &lt; 0.001) and adolescents (SMD −1.26; p = 0.022) were less fatigable than adults. Additional analysis revealed that children were less fatigable during dynamic exercises (SMD −1.58; p &lt; 0.001) with no differences during isometric ones (SMD –0.46; p = 0.22). Children (SMD 0.89; p = 0.018) but not adolescents (SMD 0.75; p = 0.090) had longer TTF than adults. Additional analyses revealed 1) that children had longer TTF for isometric (SMD 1.25; p &lt; 0.001) but not dynamic exercises (SMD −0.27; p = 0.83), and 2) that TTF differences between children and adults were larger for short- (SMD 1.46; p = 0.028) than long-duration exercises (SMD 0.20; p = 0.64). Children have higher endurance and are less fatigable than adults. These differences are influenced by the exercise modality, suggesting distinct physiological functioning during exercise between children and adults. The low number of studies comparing these outcomes between adolescents versus children and adults prevents robust conclusions and warrants further investigations in adolescent individuals.

Global trends and hot topics in electrical stimulation of skeletal muscle research over the past decade: A bibliometric analysis

Background

Over the past decade, numerous advances have been made in the research on electrical stimulation of skeletal muscle. However, the developing status and future direction of this field remain unclear. This study aims to visualize the evolution and summarize global research hot topics and trends based on quantitative and qualitative evidence from bibliometrics.

Methods

Literature search was based on the Web of Science Core Collection (WoSCC) database from 2011 to 2021. CiteSpace and VOSviewer, typical bibliometric tools, were used to perform analysis and visualization.

Results

A total of 3,059 documents were identified. The number of literature is on the rise in general. Worldwide, researchers come primarily from North America and Europe, represented by the USA, France, Switzerland, and Canada. The Udice French Research Universities is the most published affiliation. Millet GY and Maffiuletti NA are the most prolific and the most co-cited authors, respectively. Plos One is the most popular journal, and the Journal of Applied Physiology is the top co-cited journal. The main keywords are muscle fatigue, neuromuscular electrical stimulation, spinal cord injury, tissue engineering, and atrophy. Moreover, this study systematically described the hotspots in this field.

Conclusion

As the first bibliometric analysis of electrical stimulation of skeletal muscle research over the past decade, this study can help scholars recognize hot topics and trends and provide a reference for further exploration in this field.

Predicting individual muscle fatigue tolerance by resting-state EEG brain network

OBJECTIVE

Exercise-induced muscle fatigue is a complex physiological phenomenon involving the central and peripheral nervous systems, and fatigue tolerance varies across individuals. Various studies have emphasized the close relationships between muscle fatigue and the brain. However, the relationships between the resting-state electroencephalogram (rsEEG) brain network and individual muscle fatigue tolerance remain unexplored.

APPROACH

Eighteen elite water polo athletes took part in our experiment. Five-minute before- and after-fatigue-exercise rsEEG and fatiguing task (i.e., elbow flexion and extension) electromyography (EMG) data were recorded. Based on the graph theory, we constructed the before- and after-task rsEEG coherence network and compared the network differences between them. Then, the correlation between the before-fatigue rsEEG network properties and the EMG fatigue indexes when a subject cannot keep on exercising anymore was profiled. Finally, a prediction model based on the before-fatigue rsEEG network properties was established to predict fatigue tolerance.

MAIN RESULTS

Results of this study revealed the significant differences between the before- and after-exercise rsEEG brain network and found significant high correlations between before-exercise rsEEG network properties in the beta band and individual muscle fatigue tolerance. Finally, an efficient support vector regression (SVR) model based on the before-exercise rsEEG network properties in the beta band was constructed and achieved the accurate prediction of individual fatigue tolerance. Similar results were also revealed on another thirty-subject swimmer data set further demonstrating the reliability of predicting fatigue tolerance based on the rsEEG network.

SIGNIFICANCE

Our study investigates the relationship between the rsEEG brain network and individual muscle fatigue tolerance and provides a potential objective physiological biomarker for tolerance prediction and the regulation of muscle fatigue.

Neuromuscular and perceptual mechanisms of fatigue accompanying task failure in response to moderate-, heavy-, severe-, and extreme-intensity cycling

A comprehensive characterization of neuromuscular and perceptual mechanisms of fatigue at task failure following exercise across the entire intensity spectrum is lacking. This study evaluated the extent of peripheral and central fatigue, and corresponding perceptual attributes, at task failure following cycling within the moderate-(MOD), heavy-(HVY), severe-(SVR), and extreme-(EXT) intensity domains. After a ramp-incremental test, eleven young males performed four constant-power output trials to the limit of tolerance (T_lim) at four distinct domain-specific workloads. These trials were preceded and followed by 5-s knee-extension maximal voluntary contractions (MVC) and femoral nerve electrical stimuli to quantify peripheral and central fatigue. Additionally, perceptual measures including ratings of global fatigue, legs pain, dyspnea and perceived effort (RPE) were also collected. At T_lim, reductions in MVC were independent of intensity (P>0.05). However, peripheral fatigue was greater following EXT and SVR and progressively, but distinctively, lower following HVY and MOD (P<0.05). Central fatigue was similar after SVR, HVY, and MOD, but absent after EXT (P<0.05). At T_lim, subjective ratings of global fatigue were progressively higher with lower exercise intensities, while ratings of legs pain and dyspnea were progressively higher with higher exercise intensities. On the other hand, RPE was maximal following HVY, SVR, and EXT, but not MOD. The findings demonstrate that at T_lim the extent of peripheral fatigue is highly domain-specific whereas the extent of central fatigue is not. Sensations such as fatigue, pain, and dyspnea may integrate with mechanisms of sense of effort to determine task failure in a manner specific to each intensity domain.

The macamide relieves fatigue by acting as inhibitor of inflammatory response in exercising mice: From central to peripheral

Macamides are the major and unique bioactive compounds of Lepidium meyenii (Walp.) or Maca. N-benzyl-(9Z, 12Z)-octadecadienamide (N-benzyl-linoleamide) is one of the most biologically active macamides with various pharmacological activities - anti-fatigue, neuroprotective, antioxidant, anti-tumoral activities, anti-inflammatory, and analgesic. In this study, the anti-fatigue properties of N-benzyl-(9Z, 12Z)-octadecadienamide were further evaluated by a weight-loaded forced swimming test. Results indicated N-benzyl-(9Z, 12Z)-octadecadienamide supplementation increased the forelimb grip strength of mice and exercising time remaining on the Rota-rod test. Furthermore, significant decreases in pro-inflammatory factors and reactive oxygen species (ROS) contents were observed in mice receiving N-benzyl-(9Z, 12Z)-octadecadienamide treatment after a 30 min swimming test, which was equivalent to that of caffeine. Histological analysis also indicated that N-benzyl-(9Z, 12Z)-octadecadienamide attenuated damage to the liver in mice by up-regulating the expression of heme oxygenase-1 (HO-1) and inhibiting the expression of Interleukin (IL)-1β during exercise. Pearson correlation analysis suggested peripheral fatigue indexes, including energy sources, metabolites were significantly correlated with inflammatory factors and ROS levels. Likewise, central fatigue parameters are also associated, including hippocampal inflammatory response and hypothalamic neurotransmitters. Hence, macamides can be considered to have great potential as a natural drug with high efficiency and low side effects for fatigue management.

Effects of graded hypoxia during exhaustive intermittent cycling on subsequent exercise performance and neuromuscular responses

PURPOSE

This study examined the effect of graded hypoxia during exhaustive intermittent cycling on subsequent exercise performance and neuromuscular fatigue characteristics in normoxia.

METHODS

Fifteen well-trained cyclists performed an exhaustive intermittent cycling exercise (EICE 1; 15 s at 30% of anaerobic power reserve interspersed with 45 s of passive recovery) at sea level (SL; FiO₂ ~ 0.21), moderate (MH; FiO₂ ~ 0.16) and severe hypoxia (SH; FiO₂ ~ 0.12). This was followed, after 30 min of passive recovery in normoxia, by an identical exercise bout in normoxia (EICE 2). Neuromuscular function of the knee extensors was assessed at baseline, after EICE 1 (post-EICE 1), and EICE 2 (post-EICE 2).

RESULTS

The number of efforts completed decreased with increasing hypoxic severity during EICE 1 (SL: 39 ± 30, MH: 22 ± 13, SH: 13 ± 6; p ≤ 0.02), whereas there was no difference between conditions during EICE 2 (SL: 16 ± 9, MH: 20 ± 14, SH: 24 ± 17; p ≥ 0.09). Maximal torque (p = 0.007), peripheral (p = 0.02) and cortical voluntary activation (p < 0.001), and twitch torque (p < 0.001) decreased from baseline to post-EICE 1. Overall, there were no significant difference in any neuromuscular parameters from post-EICE 1 to post-EICE 2 (p ≥ 0.08).

CONCLUSION

Increasing hypoxia severity during exhaustive intermittent cycling hampered exercise capacity, but did not influence performance and associated neuromuscular responses during a subsequent bout of exercise in normoxia performed after 30 min of rest.

Biomechanical Response of the Lower Extremity to Running-Induced Acute Fatigue: A Systematic Review

Objective: To investigate (i) typical protocols used in research on biomechanical response to running-induced fatigue, (ii) the effect of sport-induced acute fatigue on the biomechanics of running and functional tests, and (iii) the consistency of analyzed parameter trends across different protocols.

Methods: Scopus, Web of Science, Pubmed, and IEEE databases were searched using terms identified with the Population, Interest and Context (PiCo) framework. Studies were screened following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines and appraised using the methodological index for non-randomized studies MINORS scale. Only experimental studies with at least 10 participants, which evaluated fatigue during and immediately after the fatiguing run were included. Each study was summarized to record information about the protocol and parameter trends. Summary trends were computed for each parameter based on the results found in individual studies.

Results: Of the 68 included studies, most were based on in-lab (77.9%) protocols, endpoint measurements (75%), stationary measurement systems (76.5%), and treadmill environment (54.4%) for running. From the 42 parameters identified in response to acute fatigue, flight time, contact time, knee flexion angle at initial contact, trunk flexion angle, peak tibial acceleration, CoP velocity during balance test showed an increasing behavior and cadence, vertical stiffness, knee extension force during MVC, maximum vertical ground reaction forces, and CMJ height showed a decreasing trend across different fatigue protocols.

Conclusion: This review presents evidence that running-induced acute fatigue influences almost all the included biomechanical parameters, with crucial influence from the exercise intensity and the testing environment. Results indicate an important gap in literature caused by the lack of field studies with continuous measurement during outdoor running activities. To address this gap, we propose recommendations for the use of wearable inertial sensors.

Muscle Contractile Characteristics During Exhaustive Dynamic Exercise and Recovery

Our aim was to provide an in vivo assessment of human muscle twitch characteristics during and following an exhaustive dynamic exercise to explore temporal alterations of the rate of force development (RFD) and relaxation (RFR). Eleven healthy participants (mean age ± SD: 24 ± 3 years) completed a dynamic knee-extensor exercise in randomized order at three different intensities, eliciting exhaustion after ∼9 min (56 ± 10 W), ∼6 min (60 ± 10 W), and ∼4 min (63 ± 10 W), in addition to a low-intensity (28 ± 5 W) bout. In a novel setup, an electrical doublet stimulation of m. vastus lateralis was applied during exercise (every 30 s) and recovery for frequent evaluation of key contractile properties (maximal force, RFD, RFR, and electromechanical delay) in addition to M-wave characteristics. RFD and RFR remained stable throughout the low-intensity trial but declined in all exhaustive trials to reach a similar level of ∼40% of pre-exercise values at task failure but with the exponential decay augmented by intensity. Following exhaustion, there was a fast initial recovery of RFD and RFR to ∼80% of pre-exercise values within 1 min, followed by a longer suppression at this level. The M-wave characteristics remained unchanged during all trials. In conclusion, this is the first study to quantify the intensity-dependent alterations of RFD and RFR during and after exhaustive dynamic exercise in humans. A hypothesized reduction and fast reversion of RFD was confirmed, and a surprising compromised RFR is reported. The present unique experimental approach allows for novel insight to exercise-induced alterations in human muscle contractile properties which is relevant in health and disease.

Do Sex Differences in Physiology Confer a Female Advantage in Ultra-Endurance Sport?

Ultra-endurance has been defined as any exercise bout that exceeds 6 h. A number of exceptional, record-breaking performances by female athletes in ultra-endurance sport have roused speculation that they might be predisposed to success in such events. Indeed, while the male-to-female performance gap in traditional endurance sport (e.g., marathon) remains at ~ 10%, the disparity in ultra-endurance competition has been reported as low as 4% despite the markedly lower number of female participants. Moreover, females generally outperform males in extreme-distance swimming. The issue is complex, however, with many sports-specific considerations and caveats. This review summarizes the sex-based differences in physiological functions and draws attention to those which likely determine success in extreme exercise endeavors. The aim is to provide a balanced discussion of the female versus male predisposition to ultra-endurance sport. Herein, we discuss sex-based differences in muscle morphology and fatigability, respiratory-neuromechanical function, substrate utilization, oxygen utilization, gastrointestinal structure and function, and hormonal control. The literature indicates that while females exhibit numerous phenotypes that would be expected to confer an advantage in ultra-endurance competition (e.g., greater fatigue resistance, greater substrate efficiency, and lower energetic demands), they also exhibit several characteristics that unequivocally impinge on performance (e.g., lower O₂-carrying capacity, increased prevalence of GI distress, and sex-hormone effects on cellular function/injury risk). Crucially, the advantageous traits may only manifest as ergogenic in the extreme endurance events which, paradoxically, are those that females less often contest. The title question should be revisited in the coming years, when/if the number of female participants increases.

A single bout of downhill running attenuates subsequent level running-induced fatigue

Fatigue can be defined as exercise-induced strength loss. During running, fatigue can be partially explained by repetitive low-intensity eccentric contractions-induced muscle damage (EIMD). Previous studies showed that a bout of downhill running (DR) attenuated subsequent EIMD. Thus, we tested if a 30-min DR bout would attenuate fatigue induced by subsequent 60-min level running (LR). Twenty-seven male college students were randomly allocated to an experimental (EXP) or a control (CON) group. All participants performed LR on a treadmill at 70% of the velocity (vVO₂peak) corresponding to peak oxygen uptake (VO₂peak). Only EXP performed a 30-min DR (− 15%) on a treadmill at 70% vVO₂peak fourteen days before LR. Indirect EIMD markers and neuromuscular function were assessed before, immediately and 48 h after DR and LR. Knee extension isometric peak torque (IPT) decreased (− 36.3 ± 26%, p < 0.05) immediately following DR with full recovery reached 48 h post-DR. Muscle soreness developed (p < 0.05) immediately (37 ± 25 mm) and 48 h (45 ± 26 mm) post-DR. IPT and rate of torque development (RTD) at late phases (> 150 ms) from the onset of muscle contraction decreased significantly (− 10.7 ± 6.1% and from − 15.4 to − 18.7%, respectively) immediately after LR for the CON group and remained below baseline values (− 5.6 ± 8.5% and from − 13.8 to − 14.9%, respectively) 48 h post-LR. However, IPT and late RTD were not significantly affected by LR for the EXP group, showing a group x time interaction effect. We concluded that a single DR bout can be used to attenuate fatigue induced by a LR performed fourteen days after.