Search

Background: In general, it has been suggested that females are more fatigue-resistant than males, with the magnitude of difference being most pronounced during low-intensity sustained contractions. However, the mechanisms for the apparent sex difference have not yet been fully elucidated in the literature. This study aimed to examine sex-related differences in fatigability and patterns of neuromuscular responses for surface electromyographic (sEMG) and mechanomyographic (sMMG) amplitude and frequency (MPF) characteristics during a sustained submaximal bilateral, isometric leg extension muscle action. Methods: A sample of 20 young recreationally active males and females with previous resistance training experience performed a sustained, submaximal, bilateral isometric leg extension until task failure. Time-to-task failure was compared using a nonparametric bootstrap of the 95% confidence interval for the mean difference between males and females. Additionally, patterns of response for sEMG and sMMG amplitude and MPF of the dominant limb were examined using linear mixed effect models. Results: There were no differences in time-to-task failure between males and females. Additionally, neuromuscular responses revealed similar patterns of responses between males and females. Interestingly, sEMG amplitude and sMMG amplitude and MPF all revealed non-linear responses, while sEMG MPF demonstrated linear responses. Conclusion: These data revealed that time-to-task failure was not different between males and females during sustained submaximal bilateral, isometric leg extension. Interestingly, the parallel, non-linear, increases in sEMG and sMMG amplitude may indicate fatigue induced increases in motor unit recruitment, while the parallel decreases in sMMG MPF may be explained by the intrinsic properties of later recruited motor units, which may have inherently lower firing rates than those recruited earlier. [ABSTRACT FROM AUTHOR]

Purpose: This study aimed to examine if peripheral fatigue is adjusted during knee extensor (KE) exercise in order not to surpass a critical threshold patient with type 1 diabetes (T1D) and the consequences of this mechanism on the force–duration relationship. Methods: Eleven T1D individuals randomly performed two different sessions in which they performed 60 maximum voluntary contractions (MVC; 3 s contraction, 2 s relaxation). One trial was performed in the non-fatigued state (CTRL) and another after fatiguing neuromuscular stimulation of the KE (FNMES). Peripheral and central fatigue were quantified by the difference between pre and post exercise in quadriceps voluntary activation (ΔVA) and potentiated twitch (ΔPtw). Critical torque (CT) was determined as the average force of the last 12 contractions, whereas W' was calculated as the area above the CT. Results: Although FNMES led to a significant decrease in potentiated twitch (Ptw) before performing the 60-MVCs protocol (p < 0.05), ΔVA (∼ −7.5%), ΔPtw (∼ −39%), and CT (∼816 N) post-MVCs were similar between the two conditions. The difference in W' between CTRL and FNMES was correlated with the level of pre-fatigue induced in FNMES (r2 = 0.60). In addition, W' was correlated with ΔPtw (r2 = 0.62) in the CTRL session. Conclusion: Correlative results in the present study indicate that regulating peripheral fatigue mechanisms at a critical threshold limit W'. Additionally, peripheral fatigue during KE exercise is limited to an individual threshold in T1D patients. [ABSTRACT FROM AUTHOR]

Background: Recovery is a key factor to promote adaptations and enhance performance. Sprint Interval Training (SIT) is known to be an effective approach to improve overall physical function and health. Although a 2-day rest period is given between SIT sessions, the time-course of recovery after SIT is unknown. Purpose: The aim of this study was to determine whether the neuromuscular and autonomic nervous systems would be impaired 24 and 48 h after an SIT session. Methods: Twenty-five healthy subjects performed an 8 × 15 s all-out session on a braked cycle ergometer with 2 min of rest between repetitions. Isometric maximal voluntary contraction (iMVC) and evoked forces to electrical nerve stimulation during iMVC and at rest were used to assess muscle contractile properties and voluntary activation before (Pre), 1 (Post24h), and 2 (Post48h) days after the session. Two maximal 7 s sprints with two different loads were performed at those same time-points to evaluate the maximal theoretical force (F0), velocity (V0) and maximal power (Pmax) production during a dynamic exercise. Additionally, nocturnal heart rate variability (HRV) was assessed the previous and the three subsequent nights to the exercise bout. Results: No significant impairments were observed for the iMVC or for the force evoked by electrical stimulation 1 day after the session. Similarly, F0, V0, and Pmax were unchanged at Post24h and Post48h. Furthermore, HRV did not reveal any temporal or frequential significant difference the nights following SIT compared to Pre. Conclusion: The results of this study show a full recovery of neuromuscular and autonomic functions a day after an all-out SIT session. [ABSTRACT FROM AUTHOR]

Purpose: The 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. Methods: Fifteen 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. Results: Early-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). Conclusion: These 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. [ABSTRACT FROM AUTHOR]

Diabetes alters numerous physiological functions and can lead to disastrous consequences in the long term. Neuromuscular function is particularly affected and is impacted early, offering an opportunity to detect the onset of diabetes-related dysfunctions and follow the advancement of the disease. The role of physical training for counteracting the deleterious effects of diabetes is well accepted but at the same time, it appears difficult to reliably assess the effects of exercise on functional capacity in patients with diabetic peripheral neuropathy (DPN). In this paper, we will review the specific characteristics of various neuromuscular dysfunctions associated with diabetes according to the DPN presence or not, and their changes over time. We present several propositions regarding the onset of neuromuscular alterations in people with diabetes compared to people with DPN. It appears that motor unit loss and neuromuscular transmission impairment are among the main mechanisms explaining the considerable degradation of neuromuscular function in the transition from a diabetic to neuropathic state. Rate of force development and contractile properties could start to decrease with the onset of preferential type II fiber atrophy, commonly reported in people with DPN. Finally, Mmax amplitude could decrease with neuromuscular fatigue only in people with DPN, reflecting the fatigue-related neuromuscular transmission impairment reported in people with DPN. In this review, we show that the different neuromuscular parameters are altered at different stages of diabetes, according to the presence of DPN or not. The precise evaluation of these parameters might participate in adapting the physical training prescription. [ABSTRACT FROM AUTHOR]

Purpose: To evaluate perceived fatigue (PF) and neuromuscular fatigue (NMF) in patients with COPD and chronic respiratory failure (CRF) on long-term oxygen therapy (CRF-COPD group), and the relationships between PF, NMF, patient's characteristics, comparing severe patients with COPD to patients without CRF (COPD group).Methods: This cross-sectional study compared 19 CRF-COPD patients with 10 COPD patients attending a rehabilitation program. PF was determined by Fatigue Severity Scale (FSS), while dyspnea by the Barthel Dyspnea Index (BDI). We assessed quadriceps NMF via electrical nerve stimulation during and following a Maximal Voluntary Contraction (MVC) detecting changes after a Constant Workload Cycling Test (CWCT) at 80% of the peak power output at exhaustion.Results: CRF-COPD patients showed higher PF (+ 1.79 of FSS score, p = 0.0052) and dyspnea (+ 21.03 of BDI score, p = 0.0023) than COPD patients. After the fatiguing task and normalization for the total work, there was a similar decrease in the MVC (CRF-COPD -1.5 ± 2.4 vs COPD -1.1 ± 1.2% baseline kJ-1, p = 0.5819), in the potentiated resting twitch force (CRF-COPD -2.8 ± 4.7 vs COPD -2.0 ± 3.3% baseline kJ-1, p = 0.7481) and in the maximal voluntary activation (CRF-COPD -0.1 ± 3.9 vs COPD -0.9 ± 1.2 -2.0 ± 3.3% baseline kJ-1, p = 0.4354). FSS and BDI were closely related (R = 0.5735, p = 0.0011), while no correlation between PF and NMF was found.Conclusion: Patients with CRF-COPD develop higher levels of perceived fatigue and dyspnea than patients with COPD; while neuromuscular fatigue is similar, suggesting a mismatch between symptoms and neuromuscular dysfunction. [ABSTRACT FROM AUTHOR]

Purpose: Muscle pain can impair exercise performance but the mechanisms for this are unknown. This study examined the effects of muscle pain on neuromuscular fatigue during an endurance task. Methods: On separate visits, twelve participants completed an isometric time-to-task failure (TTF) exercise of the right knee extensors at ~ 20% of maximum force following an intramuscular injection of isotonic saline (CTRL) or hypertonic saline (HYP) into the vastus lateralis. Measures of neuromuscular fatigue were taken before, during and after the TTF using transcranial magnetic stimulation (TMS) and peripheral nerve stimulation. Results: The mean pain intensity was 57 ± 10 in HYP compared to 38 ± 18 in CTRL (P < 0.001). TTF was reduced in HYP (4.36 ± 0.88 min) compared to CTRL (5.20 ± 0.39 min) (P = 0.003). Maximum voluntary force was 12% lower at minute 1 (P = 0.003) and 11% lower at minute 2 in HYP (P = 0.013) compared to CTRL. Voluntary activation was 4% lower at minute 1 in HYP compared to CTRL (P = 0.006) but not at any other time point (all P > 0.05). The TMS silent period was 9% longer at 100 s during the TTF in HYP compared to CTRL (P = 0.026). Conclusion: Muscle pain reduces exercise performance through the excacerbation of neuromuscular fatigue that is central in origin. This appears to be from inhibitory feedback from group III/IV nociceptors which acts to reduce central motor output. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Purpose: To compare the effects of different hypoxia severities on exercise capacity, cardio-respiratory, tissue oxygenation and neuromuscular fatigue characteristics in response to exhaustive intermittent cycling. Methods: Eleven well-trained cyclists, repeated supra-maximal cycling efforts of 15 s (30% of anaerobic power reserve, 609 ± 23 W), interspersed with 45 s of passive rest until task failure. The exercise was performed on separate days in normoxia (SL; simulated altitude/end-exercise arterial oxygen saturation = 0 m/~ 96%), moderate (MH; 2200 m/~ 90%) and severe (SH; 4200 m/~ 79%) hypoxia in a cross-over design. Neuromuscular tests, including brief (5 s) and sustained (30 s) maximal isometric voluntary contractions of the knee extensors, were performed at baseline and exhaustion. Results: Exercise capacity decreased with hypoxia severity (23 ± 9, 16 ± 6 and 9 ± 3 cycle efforts in SL, MH and SH, respectively; P < 0.001; η2 = 0.72). Both cerebral (P < 0.001; η2 = 0.86) and muscle (P < 0.01; η2 = 0.54) oxygenation decreased throughout the exercise, independent of condition (P ≥ 0.45; η2 ≥ 0.14). Compared to SL, muscle oxygenation was globally lower in MH and SH (P = 0.011; η2 = 0.36). Cardiovascular solicitation neared maximal values at exhaustion in all conditions. Peak twitch amplitude with single and paired electrical stimuli (P < 0.001; η2 ≥ 0.87), maximal torque (P < 0.001; η2 ≥ 0.48) and voluntary activation measured using transcranial magnetic stimulation (P ≤ 0.034; η2 ≥ 0.31) during brief and sustained MVCs were all reduced at exhaustion, independent of condition (P ≥ 0.196; η2 ≥ 0.15). Conclusion: Despite reduced exercise capacity with increasing severity of hypoxia during exhaustive intermittent cycling, neuromuscular fatigue characteristics were not different at task failure and cardiovascular solicitation neared maximum values. [ABSTRACT FROM AUTHOR]

Purpose: Lower limbs' neuromuscular force capabilities can only be determined during single sprints if the test provides a good fit of the data in the torque-velocity (T–V) and power-velocity (P–V) relationships. This study compared the goodness of fit of single sprints performed against traditional (7.5% of the body mass) vs. optimal load (calculated based on the force production capacity and ergometer specificities), and examined if reducing the load in fatigued state enhances T–V and P–V relationship goodness of fit. Methods: Thirteen individuals performed sprints before (PRE) and after (POST) a fatiguing task against different loads: (1) TRAD: traditional, (2) OPT: optimal, and (3) LOW-OPT: optimal load reduced according to fatigue levels. Results: At PRE, OPT sprints presented a higher R2 of the T–V relationship (0.92 ± 0.06) and lower time to reach maximal power (Pmax) (48 ± 9%) when compared with TRAD sprints (0.89 ± 0.06 and 66 ± 22%, respectively, p < 0.01). At POST, the range of velocity spectrum was greater in the LOW-OPT (33 ± 4%) vs. TRAD (24 ± 3%) and OPT (26 ± 8%, p < 0.007). Similarly, the time to reach Pmax was lower in the LOW-OPT (46 ± 12%) vs. TRAD (76 ± 24%) and OPT (70 ± 24%, p < 0.006). Conclusion: Sprints performed against an OPT load and reducing the OPT load after fatigue improve the fit of data in the T–V and P–V curves. Sprints load assignment should consider force production capacities rather than body mass. [ABSTRACT FROM AUTHOR]

Purpose: To examine corticospinal excitability and neuromuscular function following the completion of eccentric (ECC) or concentric (CON) maximal exercises of same mechanical work.Methods: Ten males (29.9 ± 11.8 years) performed maximal isokinetic knee extensor contractions in four experimental sessions. The two first sessions (one in ECC and one in CON) ended with a dynamic peak torque loss of 20%. The work completed in each contraction type was then achieved in the other contraction type. Neuromuscular function- maximal voluntary isometric contraction (MVIC), voluntary activation level (VAL), potentiated doublet (Dt), M-wave- and corticospinal excitability- motor evoked potential (MEP) amplitude and silent period (SP)-were assessed in the vastus lateralis (VL) and rectus femoris (RF) muscles at 20% MVIC before and immediately after exercise.Results: To lose 20% of dynamic peak torque subjects performed 1.8 times more work in ECC than CON (P = 0.03), inducing a non-different decline in MVIC (P = 0.15). VAL dropped after the ECC sessions only (- 8.5 ± 6.7%; all P < 0.027). Only, the CON session featuring the greatest work affected Dt amplitude (- 9.4 ± 23.8%; P = 0.047). In both muscles, MEP amplitude decreased (all P < 0.001) and MEP SP stayed constant (all P > 0.45), irrespective of contraction type (all P > 0.15).Conclusion: Same-work maximal ECC and CON exercises induced similar fatigue level but from different origins (preferentially central for ECC vs peripheral for CON). Yet, net corticospinal excitability did not depend on contraction type. [ABSTRACT FROM AUTHOR]

Purpose: To investigate the relationship between the recovery of neuromuscular fatigue and the recovery of amount of work done above critical power (W´).Methods: Ten healthy men performed, on different days, constant work rate exercises until task failure to determine critical power (CP) and W´. In the three following visits, participants performed two exhausting constant work rate exercises estimated to induce task failure within 6 min (P61 and P62), interspaced by 3, 6 or 15 min of recovery. Neuromuscular function was assessed before and periodically after the P61 using percutaneous electrical femoral nerve stimulation. The W´ recovery was measured from the total work performed above CP during the P62.Results: The P61 induced a full use of W´ and a reduction in maximal voluntary contraction (MVC, - 19 ± 4%), voluntary activation (VA, - 6 ± 2%) and twitch force stimulated at 1 Hz (- 37 ± 11%), 10 Hz (- 50 ± 16%) and 100 Hz (- 32 ± 11%), when compared to baseline (P < 0.05). The time constant of VA recovery was significantly faster than the time constant of W´ recovery (P < 0.05), but there was no significant difference between the time constant of W´ recovery and the time constant of recovery of MVC or twitch force stimulated at 1, 10 and 100 Hz (P > 0.05). However, the time constant of W´ recovery was only associated to the time constant of MVC recovery (r = 0.73, P < 0.05).Conclusion: The W´ recovery is not associated to the recovery of peripheral or central fatigue alone. Rather, W´ seems to be associated to the recovery of the overall capacity to generate force. [ABSTRACT FROM AUTHOR]

Purpose: We investigated the consequence of varying hypoxia severity during an initial set of repeated cycling sprints on performance, neuromuscular fatigability, and exercise-related sensations during a subsequent set of repeated sprints in normoxia.Methods: Nine active males performed ten 4-s sprints (recovery = 30 s) at sea level (SL; FiO2 ~ 0.21), moderate (MH; FiO2 ~ 0.17) or severe normobaric hypoxia (SH; FiO2 ~ 0.13). This was followed, after 8 min of passive recovery, by five 4-s sprints (recovery = 30 s) in normoxia.Results: Mean power decrement during Sprint 10 was exacerbated in SH compared to SL and MH (- 34 ± 12%, - 22 ± 13%, - 25 ± 14%, respectively, p < 0.05). Sprint performance during Sprint 11 recovered to that of Sprint 1 in all conditions (p = 0.267). All exercise-related sensations at Sprint 11 recovered significantly compared to Sprint 1, with no difference for Set 2 (p > 0.05). Ratings of overall perceived discomfort, difficulty breathing, and limb discomfort were exacerbated during Set 1 in SH versus SL (p < 0.05). Compared to SL, the averaged MPO value for Set 2 was 5.5 ± 3.0% (p = 0.003) lower in SH. Maximal voluntary force and twitch torque decreased similarly in all conditions immediately after Set 1 (p < 0.05), without further alterations after Set 2. Peripheral and cortical voluntary activation values did not change (p > 0.05).Conclusion: Exercise-related sensations, rather than neuromuscular function integrity, may play a pivotal role in influencing performance of repeated sprints and its recovery. [ABSTRACT FROM AUTHOR]

Purpose: This study aimed to determine the effects of hypoxia and/or blood flow restriction (BFR) on an arm-cycling repeated sprint ability test (aRSA) and its impact on elbow flexor neuromuscular function.Methods: Fourteen volunteers performed an aRSA (10 s sprint/20 s recovery) to exhaustion in four randomized conditions: normoxia (NOR), normoxia plus BFR (NBFR), hypoxia (FiO2 = 0.13, HYP) and hypoxia plus BFR (HBFR). Maximal voluntary contraction (MVC), resting twitch force (Db10), and electromyographic responses from the elbow flexors [biceps brachii (BB)] to electrical and transcranial magnetic stimulation were obtained to assess neuromuscular function. Main effects of hypoxia, BFR, and interaction were analyzed on delta values from pre- to post-exercise.Results: BFR and hypoxia decreased the number of sprints during aRSA with no significant cumulative effect (NOR 16 ± 8; NBFR 12 ± 4; HYP 10 ± 3 and HBFR 8 ± 3; P < 0.01). MVC decrease from pre- to post-exercise was comparable whatever the condition. M-wave amplitude (- 9.4 ± 1.9% vs. + 0.8 ± 2.0%, P < 0.01) and Db10 force (- 41.8 ± 4.7% vs. - 27.9 ± 4.5%, P < 0.01) were more altered after aRSA with BFR compared to without BFR. The exercise-induced increase in corticospinal excitability was significantly lower in hypoxic vs. normoxic conditions (e.g., BB motor evoked potential at 75% of MVC: - 2.4 ± 4.2% vs. + 16.0 ± 5.9%, respectively, P = 0.03).Conclusion: BFR and hypoxia led to comparable aRSA performance impairments but with distinct fatigue etiology. BFR impaired the muscle excitation-contraction coupling whereas hypoxia predominantly affected corticospinal excitability indicating incapacity of the corticospinal pathway to adapt to fatigue as in normoxia. [ABSTRACT FROM AUTHOR]

Purpose: The assessment of voluntary activation of the knee extensors using transcranial magnetic stimulation (VATMS) is routinely performed to assess the supraspinal function. Yet methodological scrutiny of the technique is scarce. The aim of the present study was to examine face validity and reliability of VATMS and its two main determinants (superimposed twitch during a maximal voluntary contraction [SIT100%] and estimated resting twitch [ERT]).Methods: SIT100%, ERT, and VATMS were measured on ten healthy males (age 24 ± 5 years) before and following intermittent isometric fatiguing exercise on two separate occasions.Results: The findings indicated issues regarding the accuracy of ERT and suggested a three-point relationship should not be used to determine ERT. Reliabilities for VATMS, SIT100%, and ERT were acceptable pre- but much weaker post-exercise (especially for SIT100%). Despite statistically significant changes in main neuromuscular variables following the intermittent isometric fatiguing exercise (P < 0.05), when post-exercise reliability was considered, the exercise effect on VATMS was smaller than the smallest detectable change in 18 of the 20 individual tests performed, and for the whole sample for one of two visits. Finally, maximal voluntary contraction was reduced significantly following the neuromuscular assessment (NMA) pre-exercise but recovered during the NMA post-exercise.Conclusion: This is the first study to demonstrate a lack of sensitivity of key neuromuscular measurements to exercise and to evidence both presence of neuromuscular fatigue following the NMA in itself, and recovery of the neuromuscular function during the NMA post-exercise. These results challenge the face validity of this routinely used protocol. [ABSTRACT FROM AUTHOR]

Purpose: The purpose of this study was to investigate the changes in blood lactate concentration (BL) and muscle activity patterns during a 15-m speed climbing competition that consisted of ten consecutive climbing actions on a standardized artificial wall in trained rock climbers.Methods: Twelve trained rock climbers participated in this study. Surface electromyography (sEMG) and video signals were synchronized and recorded during climbing. The blood lactate was also tested 3 min after completing the climb.Results: The average climbing time was 8.1 ± 2.1 s for the 15-m speed climb across all subjects, accompanied by a BL of 7.6 ± 1.9 mmol/L. The climbing speed and power firstly increased and then slightly decreased relative to peak value during the 15-m speed climbing. The results showed there was a positive correlation between the BL and the climbing time, r = 0.59, P = 0.043. The sEMG showed the flexor digitorum superficialis (FDS) electric activity was the highest, followed by the biceps brachii (BB) and latissimus dorsi. The instantaneous median frequency of sEMG of FDS and BB significantly decreased during the 15-m speed climbing. All the participants showed the higher sEMG RMS (%) in the terminal phase than that in the initial phase, especially with a greater increase in the left upper limbs. However, the lower limbs muscles presented no significant changes in the sEMG amplitude during climbing.Conclusions: The FDS and BB play an important role in completing the 15-m speed climbing. The median frequency of arm EMG decreased more than that of legs, suggesting more fatigue. The blood lactate concentration increases in the current study suggest that a certain amount of glycolysis supplies energy in completing 15-m speed rock climbing. Based on the current data, it is suggested that muscular endurance of FDS and BB muscles in upper limbs should be improved for our climbers in this study. [ABSTRACT FROM AUTHOR]

Aim: Acetaminophen is a commonly used medicine for pain relief and emerging evidence suggests that it may improve endurance exercise performance. This study investigated some of the physiological mechanisms by which acute acetaminophen ingestion might blunt muscle fatigue development.Methods: Thirteen active males completed 60 × 3 s maximum voluntary contractions (MVC) of the knee extensors with each contraction separated by a 2 s passive recovery period. This protocol was completed 60 min after ingesting 1 g of maltodextrin (placebo) or 1 g of acetaminophen on two separate visits. Peripheral nerve stimulation was administered every 6th contraction for assessment of neuromuscular fatigue development, with the critical torque (CT), which reflects the maximal sustainable rate of oxidative metabolism, taken as the mean torque over the last 12 contractions. Surface electromyography was recorded continuously as a measure of muscle activation.Results: Mean torque (61 ± 11 vs. 58 ± 14% pre-exercise MVC) and CT (44 ± 13 vs. 40 ± 15% pre-exercise MVC) were greater in the acetaminophen trial compared to placebo (both P < 0.05). Voluntary activation and potentiated twitch declined at a similar rate in both conditions (P > 0.05). However, the decline in electromyography amplitude was attenuated in the acetaminophen trial, with electromyography amplitude being greater compared to placebo from 210 s onwards (P < 0.05).Conclusion: These findings indicate that acute acetaminophen ingestion might be ergogenic by increasing CT and preserving muscle activation during high-intensity exercise. [ABSTRACT FROM AUTHOR]

Vibratory stimuli are thought to have the potential to promote neural and/or muscular (re)conditioning. This has been well described for whole-body vibration (WBV), which is commonly used as a training method to improve strength and/or functional abilities. Yet, this technique may present some limitations, especially in clinical settings where patients are unable to maintain an active position during the vibration exposure. Thus, a local vibration (LV) technique, which consists of applying portable vibrators directly over the tendon or muscle belly without active contribution from the participant, may present an alternative to WBV. The purpose of this narrative review is (1) to provide a comprehensive overview of the literature related to the acute and chronic neuromuscular changes associated with LV, and (2) to show that LV training may be an innovative and efficient alternative method to the 'classic' training programs, including in the context of muscle deconditioning prevention or rehabilitation. An acute LV application (one bout of 20-60 min) may be considered as a significant neuromuscular workload, as demonstrated by an impairment of force generating capacity and LV-induced neural changes. Accordingly, it has been reported that a training period of LV is efficient in improving muscular performance over a wide range of training (duration, number of session) and vibration (frequency, amplitude, site of application) parameters. The functional improvements are principally triggered by adaptations within the central nervous system. A model illustrating the current research on LV-induced adaptations is provided. [ABSTRACT FROM AUTHOR]

Purpose: While acute hypoxic exposure enhances exercise-induced central fatigue and can alter corticospinal excitability and inhibition, the effect of prolonged hypoxic exposure on these parameters remains to be clarified. We hypothesized that 5 days of altitude exposure would (i) normalize exercise-induced supraspinal fatigue during isolated muscle exercise to sea level (SL) values and (ii) increase corticospinal excitability and inhibition.Methods: Eleven male subjects performed intermittent isometric elbow flexions at 50% of maximal voluntary contraction to task failure at SL and after 1 (D1) and 5 (D5) days at 4350 m. Transcranial magnetic stimulation and peripheral electrical stimulation were used to assess supraspinal and peripheral fatigues. Pre-frontal cortex and biceps brachii oxygenation was monitored by near-infrared spectroscopy.Results: Exercise duration was not statistically different between SL (1095 ± 562 s), D1 (1132 ± 516 s), and D5 (1440 ± 689 s). No significant differences were found between the three experimental conditions in maximal voluntary activation declines at task failure (SL -16.8 ± 9.5%; D1 -25.5 ± 11.2%; D5 -21.8 ± 7.0%; p > 0.05). Exercise-induced peripheral fatigue was larger at D5 versus SL (100 Hz doublet at task failure: -58.8 ± 16.6 versus -41.8 ± 20.1%; p < 0.05). Corticospinal excitability at 50% maximal voluntary contraction was lower at D5 versus SL (brachioradialis p < 0.05, biceps brachii p = 0.055). Cortical silent periods were shorter at SL versus D1 and D5 (p < 0.05).Conclusions: The present results show similar patterns of supraspinal fatigue development during isometric elbow flexions at SL and after 1 and 5 days at high altitude, despite larger amount of peripheral fatigue at D5, lowered corticospinal excitability and enhanced corticospinal inhibition at altitude. [ABSTRACT FROM AUTHOR]

Purpose: There have been recent attempts to characterize the mechanisms associated with fatigue-induced task failure. We compared the time to failure and the corresponding changes in the surface electromyogram (EMG) during sustained maximal and submaximal isometric force tasks.Methods: EMG activity was measured from the biceps brachii of 18 male participants as they sustained either a maximal or submaximal (60 % MVC) isometric contraction of the dominant elbow flexors until force could not be maintained above 55 % MVC.Results: Intensity-dependent patterns of change were observed for EMG amplitude and mean power frequency (MNF) between the two force tasks. Interestingly, the only significant predictor of failure time was the rate of change in EMG MNF during the submaximal task (r (2) = 0.304). In addition, EMG amplitude at submaximal failure was significantly lower (p < 0.05) than the values obtained during MVC.Conclusions: The patterns of EMG response emphasize the basis of neuromuscular fatigue and task dependency. Additionally, our data suggest that the EMG MNF should be used when monitoring the progression of local muscle fatigue. [ABSTRACT FROM AUTHOR]

Purpose: There is some evidence that muscular activation during exercise is enhanced by higher levels of blood flow restriction (BFR). However, the impact of different relative levels of BFR on the acute neuromuscular response to resistance exercise is not yet fully understood. We examined the acute effects of low-intensity knee extensions [20 % of 1-repetition maximum (1RM)] with BFR on muscle activation, neuromuscular fatigue and torque in the rectus femoris (RF) and vastus medialis (VM) muscles.Methods: Fourteen men (24.8 ± 5.4 years) exercised at 20 % 1RM combined with 40, 60 and 80 % BFR. Restrictive pressures were calculated based on direct blood-flow measurements taken at rest on each participant. Torque was determined during pre- and post-exercise maximal voluntary contractions. Surface electromyographic activity [root mean square (RMS)] was obtained during dynamic and sustained isometric contractions before and after exercise. The median frequency (MF) of the electromyographic power spectrum was computed for isometric contractions.Results: Torque only decreased in the 80 % BFR condition (-5.2 %; p < 0.01). Except for the VM in the 40 % BFR, MF decreased in both muscles post-exercise in all conditions (p < 0.01). MF decrements were of greater magnitude post-exercise at higher levels of BFR. RMS increased within all sets in both muscles (p < 0.01) and attained higher values in the 80 % BFR condition; except for set 1 in the RF muscle (p < 0.01).Conclusion: Muscular activation, as well as neuromuscular fatigue, varies as a function of relative BFR intensity. Therefore, the individual determination of vascular restriction levels is crucial before engaging in BFR exercise. [ABSTRACT FROM AUTHOR]

This study investigated the effects of acute hypoxia on spinal reflexes and soleus muscle function after a sustained contraction of the plantar flexors at 40 % of maximal voluntary isometric contraction (MVC). Fifteen males (age 25.3 ± 0.9 year) performed the fatigue task at two different inspired O fractions (FiO = 0.21/0.11) in a randomized and single-blind fashion. Before, at task failure and after 6, 12 and 18 min of passive recovery, the Hoffman-reflex ( H) and M-wave ( M) were recorded at rest and voluntary activation (VA), surface electromyogram (RMS), M-wave ( M) and V-wave ( V) were recorded during MVC. Normalized H-reflex ( H/ M) was significantly depressed pre-exercise in hypoxia compared with normoxia (0.31 ± 0.08 and 0.36 ± 0.08, respectively, P < 0.05). Hypoxia did not affect time to task failure (mean time of 453.9 ± 32.0 s) and MVC decrease at task failure (−18 % in normoxia vs. −16 % in hypoxia). At task failure, VA (−8 %), RMS/ M (−11 %), H/ M (−27 %) and V/ M (−37 %) decreased ( P < 0.05), but with no FiO effect. H/ M restored significantly throughout recovery in hypoxia but not in normoxia, while V/ M restored significantly during recovery in normoxia but not in hypoxia ( P < 0.05). Collectively, these findings indicate that central adaptations resulting from sustained submaximal fatiguing contraction were not different in hypoxia and normoxia at task failure. However, the FiO-induced differences in spinal loop properties pre-exercise and throughout recovery suggest possible specific mediation by the hypoxic-sensitive group III and IV muscle afferents, supraspinal regulation mechanisms being mainly involved in hypoxia while spinal ones may be predominant in normoxia. [ABSTRACT FROM AUTHOR]

Loading using variable resistance devices, where the external resistance changes in line with the force:angle relationship, has been shown to cause greater acute neuromuscular fatigue and larger serum hormone responses. This may indicate a greater potential for adaptation during long-term training. Twelve (constant resistance group) and 11 (variable resistance group) men completed 20 weeks of resistance training with 10 men as non-training controls. Training-induced adaptations were assessed by bilateral leg press one repetition maximum, a repetition to failure test using 75 % 1RM, lower limb lean mass and vastus lateralis cross-sectional area. Only the variable resistance training group improved the total number of repetitions (41 ± 46 %) and volume load (52 ± 37 %) during the repetition to failure test ( P < 0.05). Similar improvements in maximum strength and hypertrophy of the lower limbs were observed in both training groups. Also, constant and variable resistance 5 × 10RM leg press loadings were performed before and after training in a crossover design. Acute loading-induced responses were assessed by concentric and isometric force, serum hormone concentrations and phosphorylation of intramuscular signalling proteins (0-30 min post-loading). Greater acute decreases in force ( P < 0.05-0.01), and greater increases in serum testosterone and cortisol concentration ( P < 0.05) and ERK 1/2 phosphorylation ( P < 0.05) were observed following variable resistance loadings before and after training. Greater training-induced improvements in fatigue resistance occurred in the variable resistance training group, which may be due to greater acute fatigue and physiological responses during variable versus constant resistance loadings. [ABSTRACT FROM AUTHOR]

To better understand characteristics of neuromuscular fatigue in supramaximal cycling exercise, this study examined changes in surface electromyography (sEMG) frequency during maximal voluntary isometric contractions (MVC) following a 30-s Wingate anaerobic test (WAnT) using discrete wavelet transform (DWT). The changes in sEMG were also compared between DWT and mean frequency (MNF) obtained by fast Fourier transform (FFT). 17 healthy men performed a WAnT with a 7.5 % of body mass load. Knee extensor MVC torque was measured before and 1, 3, 6, 9, 12 and 15 min following WAnT, and sEMG was recorded from vastus lateralis muscle during the torque measures. sEMG was analysed for (RMS), MNF by FFT and frequency domains of DWT (divided into six domains). MVC torque decreased 21-23 % at 3-15 min, RMS increased 26-34 % at 1-15 min, and MNF decreased 8-10 % from baseline (76.3 ± 3.2 Hz) at 1-3 min post-cycling ( P < 0.05). The DWT frequency domains showed that the changes lasted longer than MNF such that the intensity increased at 12 and 15 min for domain 2 (125-250 Hz), all time points for domain 3 (62.5-125 Hz), and 1-6 min for domains 4 (31.2-62.5 Hz) and 5 (15.6-31.2 Hz). The magnitude of increase in the intensity at 1 min post-exercise (45-60 %) was largest for domains 3 and 5 ( P < 0.05). A significant correlation was evident only between the magnitude of changes in the domain 5 and MNF ( r = −0.56). It is concluded that DWT provides information on neuromuscular fatigue that is not detected by MNF derived from FFT. [ABSTRACT FROM AUTHOR]

This study investigated the effect of hot conditions on repeated sprint cycling performance and post-exercise alterations in isometric knee extension function. Twelve physically active participants performed 10 × 6-s 'all-out' sprints on a cycle ergometer (recovery = 30 s), followed 6 min later by 5 × 6-s sprints (recovery = 30 s) in either a neutral (24 °C/30 %rH) or a hot (35 °C/40 %rH) environment. Neuromuscular tests including voluntary and electrically evoked isometric contractions of the knee extensors were performed before and after exercise. Average core temperature during exercise was higher (38.0 ± 0.1 vs. 37.7 ± 0.1 °C, respectively; P < 0.05) in hot versus neutral environments. Peak power output decreased (−17.9 % from sprint 1 to sprint 10 and −17.0 % from sprint 11 to sprint 15; P < 0.001) across repetitions. Average peak power output during the first ten sprints was higher (+3.1 %; P < 0.01) in the hot ambient temperature condition. Maximal strength (−12 %) and rate of force development (−15 to −26 %, 30-200 ms from the onset of contraction) decreased ( P < 0.001) during brief contractions after exercise, irrespectively of the ambient temperature. During brief maximal contractions, changes in voluntary activation (~80 %) were not affected by exercise or temperature. Voluntary activation declined ( P < 0.01) during the sustained contraction, with these reductions being more pronounced ( P < 0.05) after exercise but not affected by the ambient temperature. Resting twitch amplitude declined ( P < 0.001) by ~42 %, independently of the ambient temperature. In conclusion, heat exposure has no effect on the pattern and the extent of isometric knee extensor fatigue following repeated cycling sprints in the absence of hyperthermia. [ABSTRACT FROM AUTHOR]

The aim of this study was to characterize the effect of a 5 km running time trial on the neuromuscular properties of the plantar flexors. Eleven well-trained triathletes performed a series of neuromuscular tests before and immediately after the run on a 200 m indoor track. Muscle activation (twitch interpolation) and normalized EMG activity were assessed during maximal voluntary contraction (MVC) of plantar flexors. Maximal soleus H-reflexes and M-waves were evoked at rest (i.e. H and M, respectively) and during MVC (i.e. H and M, respectively). MVC significantly declined (−27%; P < 0.001) after the run, due to decrease in muscle activation (−8%; P < 0.05) and M-normalized EMG activity (−13%; P < 0.05). Significant reductions in M-wave amplitudes ( M: −13% and M: −16%; P < 0.05) as well as H/ M (−37%; P < 0.01) and H/ M (−25%; P < 0.05) ratios occurred with fatigue. Following exercise, the single twitch was characterized by lower peak torque (−16%; P < 0.001) as well as shorter contraction (−19%; P < 0.001) and half-relaxation (−24%; P < 0.001) times. In conclusion, the reduction in plantar flexors strength induced by a 5 km running time trial is caused by peripheral adjustments, which are attributable to a failure of the neuromuscular transmission and excitation-contraction coupling. Fatigue also decreased the magnitude of efferent motor outflow from spinal motor neurons to the plantar flexors and part of this suboptimal neural drive is the result of an inhibition of soleus motoneuron pool reflex excitability. [ABSTRACT FROM AUTHOR]

The purpose of this study was to determine the effects of beta-alanine supplementation and high-intensity interval training (HIIT) on electromyographic fatigue threshold (EMG(FT)) and efficiency of electrical activity (EEA). A total of 46 men completed four, 2-min work bouts on a cycle ergometer. Using bipolar surface electrodes, the EMG amplitude was averaged and plotted over the 2-min. The resulting slopes were used to calculate EMG(FT) and EEA. Following initial testing, all participants were randomly assigned to either placebo (PL; n = 18), beta-alanine (BA; n = 18) or control groups (CON; n = 10). Following randomization, participants engaged in 6 weeks of HIIT training. Significant improvements in EMG(FT) and EEA resulted for both training groups. In conclusion, HIIT appeared to be the primary stimulus effecting EMG(FT) or EEA, suggesting adaptations from HIIT may be more influential than increasing skeletal muscle carnosine levels on delaying fatigue in recreationally active men. [ABSTRACT FROM AUTHOR]

The objective of this study was to verify the following hypothesis: the pedal rate that minimizes root mean square (RMS) slope and the slow component amplitude of oxygen consumption could be close to the freely chosen pedal rate (FCPR) used by well-trained cyclists. Nine male competitive cyclists performed a 21 min submaximal exercise on a cycle ergometer at a workload of 65% of their respective peak aerobic power. For each session, the subject's pedal rate was freely chosen or assigned to 60, 75, 90 or 105 rev min(-1). When pedal rates were imposed, the electromyographic root mean square slope, the oxygen uptake during the third minute and the 20th min, and the slow component amplitude of oxygen consumption were used in the analysis. In order to determine the optimal pedal rate (OPR), a quadratic function was fitted to the data by regression, for each variable measured. The mean values of OPR relative to oxygen uptake during the third min (71+/-9 rev min(-1)) were lower than the mean values of the OPR relative to the slow component amplitude of oxygen consumption (82+/-8 rev min(-1)), the electromyographic root mean square slope (80+/-7 rev min(-1)) and freely chosen pedal rate (86+/-13 rev min(-1)). Freely chosen pedal rate was not significantly different from the OPR in reference to the amplitude of the slow component of oxygen consumption, electromyographic root mean square slope, and oxygen uptake during the 20th min. OPR for RMS slope was correlated (R=0.72) to FCPR. Expert cyclists were likely to use a spontaneous pedal rate that minimizes neuromuscular fatigue. [ABSTRACT FROM AUTHOR]

The aim of the present study was to examine muscle mechanical characteristics before and after a marathon race. Eight elite runners underwent a pre-test 1 week before the marathon and post-tests 30 min, two and five-day-post-marathon. Actual marathon race performance was 2:34:40 +/- 0:04:13. Energy expenditure at marathon pace (EE(Mpace)) was elevated 4% post-marathon (pre: 4,465 +/- 91 vs. post 4,638 +/- 91 J kg bodyweight(-1) km(-1), P < 0.05), but was lowered by 6 and 9.5% two- and five-day-post-marathon compared to EE(Mpace) pre-marathon. Countermovement jump (CMJ) power decreased 13% post-marathon (pre: 21.5 +/- 0.9 vs. post: 18.9 +/- 1.2 W kg(-1); P < 0.05) and remained depressed two- (18%) and five-day (12%) post-marathon. CMJ force was unaltered across all four tests occasions. Knee extensor and plantar flexor maximal voluntary contraction (MVC) decreased from 176.6 +/- 9.5 to 136.7 +/- 16.8 Nm and 144.9 +/- 8.7 to 119.2 +/- 15.1 Nm post-marathon corresponding to 22 and 17%, respectively (P < 0.05). No significant changes were detected in evoked contractile parameters, except a 25% increase in force at 5 Hz, and low frequency fatigue was not observed. In conclusion, leg muscle power decreased acutely post-marathon race and recovered very slowly. The post-marathon increase in EE(Mpace) might be attributed to a reduction in stretch shortening cycle efficiency. Finally, since MVC was reduced after the marathon race without any marked changes in evoked muscle contractile properties, the strength fatigue experienced by the subjects in this study seems to be related to central rather than peripheral mechanisms. [ABSTRACT FROM AUTHOR]

Stretch-shortening cycle (SSC)-type fatigue is associated with acute and delayed functional defects, and appears to be a useful model to reveal the flexibility of both central and reflex adjustments to the contractile failure. SSC fatigue was induced in an experimental (EXP) group (n=6) on a sledge ergometer with an exhaustive rebound exercise with submaximal effort. The acute (POST) and 2-day delayed (2D) neuromuscular changes with fatigue were examined in a short submaximal rebound task (REBOUND) and in a maximal isometric plantarflexion test (ISOM). The EXP group results were compared to those of a control group (n=6) who did not perform the exhaustive SSC exercise and did not present any change in the tests. In the EXP group, the ISOM test revealed mostly a large decrease in maximal plantarflexion force at 2D that was correlated with the reduced mean soleus muscle (SOL) activation. Indicating "task-dependent" fatigue effects on the neural changes, the REBOUND test revealed both acute and delayed increases in SOL activation. Supporting central neural changes, SOL preactivation increased in POST and 2D. The neural flexibility along time and across muscles was demonstrated by the shifted increase in SOL activation from the braking phase in POST to the push-off phase in 2D, and associated increased gastrocnemius medialis preactivation in 2D. In contrast, activation during the stretch-reflex period was constant in POST, and decreased in 2D. These results would support the influence of musculotendinous afferents on the flexible neural adjustments to the SSC-induced contractile failure. [ABSTRACT FROM AUTHOR]

Central fatigue was investigated under an isolated active condition whereby the possible effects of supraspinal fatigue were minimized. Therefore, ten subjects were fatigued by simultaneously and repeatedly mechanically stretching and electrically stimulating their calf muscles for 1 h. This was performed using an ankle ergometer. The active fatigue task included a total of 2400 muscle stretches with an intensity of 10% of the maximal voluntary contraction (MVC). This protocol clearly impaired neuromuscular function, as revealed by a significant reduction in MVC (P<0.01) and the neural input to the muscle (average EMG) (P<0.01–0.001). The interpolated nerve stimulation compensated for this force loss by 4.28% (P<0.05). Stretch-reflex recordings revealed a notable post-fatigue reduction in the peak-to-peak amplitude (59.1%, P<0.01) and stretch-resisting force of the muscle (14.1%, P<0.01). The maximal H-reflex declined by 50.5% (P<0.001) and did not recover while the leg was kept ischemic. It is suggested that the existing protocol with minor metabolic loading can induce central fatigue, which seems to be of reflex origin from the fatigued muscle. Although the role of presynaptic inhibition of Ia terminals is possibly reinforced, disfacilitation via reduced spindle sensitivity cannot be excluded. [ABSTRACT FROM AUTHOR]