2-min Maximal Voluntary Contraction Research Articles

Modulation of neuromuscular excitability in response to acute noxious heat exposure has no additional effects on central and peripheral fatigability.

Background: Whole-body hyperthermia (WBH) has an adverse effect on the nervous system and neurophysiological performance. In the present study, we examined whether short-duration whole-body immersion in 45°C water (HWI-45°C), which produces a strong neural and temperature flux without inducing WBH, can increase or impair neurophysiological performance in humans. Methods: Fifteen men (aged 25 ± 6 years) were enrolled in this study and participated in three experiments: 1) a brief (5-min) immersion of the whole body in 37°C water (WI-37°C); 2) a brief (5-min) HWI-45°C; and 3) a control trial in a thermoneutral condition at an ambient temperature of 24°C and 60% relative humidity. Before and after the immersions, neuromuscular function (electromyographic activity, reflexes, electrically and voluntary induced torque production, voluntary muscle activation level) were tested. To provoke central inhibition, the participants performed a sustained 2-min maximal voluntary contraction (MVC). Results: Thermophysiological strain was greater after HWI-45°C than after WI-37°C. Electrophysiological modulations of motor drive transmission and peripheral modulations of muscle contractility properties in response to HWI-45°C seemed to have little effect on central activation of the exercising muscles and no effect on MVC production. Conclusion: Although exposure to acute noxious heat was effective in evoking neuromuscular excitability, the increases in core temperature (∼0.2°C) and muscle temperature (∼0.6°C) did not induce moderate or severe WBH. These changes did not seem to affect central structures; that is, there were no additional increases in central and/or peripheral fatigue during a sustained 2-min MVC.

Pre-Exercise Rehydration Attenuates Central Fatigability during 2-Min Maximum Voluntary Contraction in Hyperthermia.

Background and objectives: Hyperthermia with dehydration alters several brain structure volumes, mainly by changing plasma osmolality, thus strongly affecting neural functions (cognitive and motor). Here, we aimed to examine whether the prevention of significant dehydration caused by passively induced whole-body hyperthermia attenuates peripheral and/or central fatigability during a sustained 2-min isometric maximal voluntary contraction (MVC). Materials and Methods: Ten healthy and physically active adult men (21 ± 1 years of age) performed an isometric MVC of the knee extensors for 2 min (2-min MVC) under control (CON) conditions, after passive lower-body heating that induced severe whole-body hyperthermia (HT, Tre > 39 °C) with dehydration (HT-D) and after HT with rehydration (HT-RH). Results: In the HT-D trial, the subjects lost 0.94 ± 0.15 kg (1.33% ± 0.13%) of their body weight; in the HT-RH trial, their body weight increased by 0.1 ± 0.42 kg (0.1% ± 0.58%). After lower-body heating, the HT-RH trial (vs. HT-D trial) was accompanied by a significantly lower physiological stress index (6.77 ± 0.98 vs. 7.40 ± 1.46, respectively), heart rate (47.8 ± 9.8 vs. 60.8 ± 13.2 b min−1, respectively), and systolic blood pressure (−12.52 ± 5.1 vs. +2.3 ± 6.4, respectively). During 2-min MVC, hyperthermia (HT-D; HT-RH) resulted in greater central fatigability compared with the CON trial. The voluntary activation of exercising muscles was less depressed in the HT-RH trial compared with the HT-D trial. Over the exercise period, electrically (involuntary) induced torque decreased less in the HT-D trial than in the CON and HT-RH trials. Conclusions: Our results suggest that pre-exercise rehydration might have the immediate positive effect of reducing physiological thermal strain, thus attenuating central fatigability even when exercise is performed during severe (Tre > 39 °C) HT, induced by passive warming of the lower body.

Do aerobic characteristics explain isometric exercise-induced neuromuscular fatigue and recovery in upper and lower limbs?

ABSTRACTThis study investigated the relationships between aerobic characteristics and (i) neuromuscular fatigue induced by 2-min sustained isometric maximal voluntary contractions (MVC) and (ii) subsequent recovery, in the upper and lower limbs. In a pseudo-randomized order, eleven healthy males completed four sessions on different days: maximal incremental cycling test (100 W + 40 W every 2 min); maximal arm-cranking test (50 W + 20 W every 2 min); and 2-min sustained isometric MVCs of the knee extensors (KE) and elbow flexors (EF). Neuromuscular assessment was performed with transcranial magnetic and peripheral nerve stimulation to evaluate central and peripheral neuromuscular factors of fatigue and the subsequent recovery. Peak oxygen uptake, gas exchange threshold and the corresponding power outputs were correlated with recovery of voluntary force after the 2-min KE MVC. Regression analysis showed that power output at the gas exchange threshold alone explained 72% of the variability in ∆recovery of KE voluntary force. No relationships with fatigue or recovery in EF were observed. These results suggest that participants with greater aerobic capacities experience the same amount of fatigue and faster recovery of voluntary force in KE but not EF. The potential reasons behind the relationship in KE but not EF are discussed.

Age- and Sex-Related Differences in Motor Performance During Sustained Maximal Voluntary Contraction of the First Dorsal Interosseous.

Age and sex affect the neuromuscular system including performance fatigability. Data on performance fatigability and underlying mechanisms in hand muscles are scarce. Therefore, we determined the effects of age and sex on force decline, and the mechanisms contributing to force decline, during a sustained isometric maximal voluntary contraction (MVC) with the index finger abductor (first dorsal interosseous, FDI). Subjects (n = 51, age range: 19–77 years, 25 females) performed brief and a 2-min sustained MVC with the right FDI. Abduction force and root mean squared electromyographic activity (rms-EMG) were recorded in both hands. Double-pulse stimulation was applied to the ulnar nerve during (superimposed twitch) and after (doublet-force) the brief and sustained MVCs. Compared to females, males were stronger (134%, p < 0.001) and exhibited a greater decline in voluntary (difference: 8%, p = 0.010) and evoked (doublet) force (difference: 12%, p = 0.010) during and after the sustained MVC. Age did not affect MVC, force decline and superimposed twitch. The ratio between the doublet- and MVC-force was greater in females (0.33, p = 0.007) and in older (0.38, p = 0.06) individuals than in males (0.30) and younger (0.30) individuals; after the sustained MVC this ratio increased with age and the increase was larger for females compared to males (p = 0.04). The inadvertent contralateral, left force and rms-EMG activity increased over time (2.7–13.6% MVC and 5.4–17.7% MVC, respectively). Males had higher contralateral forces than females (p = 0.012) and contralateral force was higher at the start of the contralateral contraction in older compared with young subjects (difference: 29%, p = 0.008). In conclusion, our results suggest that the observed sex-differences in performance fatigability were mainly due to differences in peripheral muscle properties. Yet the reduced amount of contralateral activity and the larger difference in evoked versus voluntary force in female subjects indicate that sex-differences in voluntary activation should not be overlooked. These data obtained in neurological healthy adults provides a framework and help the interpretation and referencing of neurophysiological measures in patients suffering from neuromuscular diseases, who often present with symptoms of performance fatigability.

The effect of ischaemic preconditioning on central and peripheral fatiguing mechanisms in humans following sustained maximal isometric exercise.

What is the central question of this study? Does ischaemic preconditioning (IPC) influence central and peripheral fatiguing mechanisms during sustained maximal isometric exercise? What is the main finding and its importance? Voluntary activation and pre- to postexercise reductions in resting twitch torque values were unchanged by IPC. However, an effect on tissue oxygenation was observed within the IPC trials, where greater concentrations of deoxyhaemoglobin were recorded with concurrent upward trends of total haemoglobin concentrations. Using a direct assessment of neural drive, we found that IPC had no influence on either central or peripheral fatiguing pathways after maximal isometric exercise. Ischaemic preconditioning (IPC) is thought to inhibit neural feedback from metabolically sensitive muscle afferents during exercise. It was hypothesized that IPC could affect mechanisms associated with centrally mediated fatigue after a maximally fatiguing protocol. Eleven resistance-trained men completed three 2min maximal voluntary contractions (MVCs) via an isometric leg extension preceded by treatments of IPC (three bouts of 5min bilateral leg occlusions at 220mmHg), SHAM (three bouts of 5min at 20mmHg) or CON (30min passive rest). Femoral nerve stimulation was used to explore central and peripheral fatigue pathways. These pathways were profiled at baseline (BL), before the 2min MVC (Pre) and 10s afterwards (Post). Tissue oxygenation was measured throughout the 2min MVCs via near-infrared spectroscopy. The Pre to Post MVC (-71±56 Nm; d=1.33±0.51, P<0.01) and twitch torque (-51±20Nm; d=3.76±0.84, P<0.01) levels declined without differences between conditions (MVC, P=0.67; twitch torque, P=0.39). Voluntary activation was also unaffected by condition (P=0.80). Peak deoxyhaemoglobin concentrations were elevated in the IPC trials relative to CON (3.7±3.0μmoll-1 ; d=1.02±0.46, P<0.01) and SHAM (3.0±3.7μmoll-1 ; d=0.82±0.57, P<0.05). These findings demonstrate that IPC does not affect central or peripherally mediated mechanisms of fatigue during a sustained 2min maximal effort isometric leg-extension task.

Active recovery affects the recovery of the corticospinal system but not of muscle contractile properties.

PurposeActive recovery is often used by athletes after strenuous exercise or competition but its underlying mechanisms are not well understood. We hypothesized that active recovery speeds-up recovery processes within the muscle and the central nervous system (CNS).MethodsWe assessed muscular and CNS recovery by measuring the voluntary activation (VA) in the vastus lateralis muscle with transcranial magnetic stimulation (VATMS) and peripheral nerve stimulation (VAPNS) during maximal voluntary contractions (MVC) of the knee extensors in 11 subjects. Measurements were performed before and after a fatiguing cycling time-trial, after an active and a passive recovery treatment and after another fatiguing task (1 min MVC). The measurements were performed a second time 24 h after the time-trial.ResultsWe observed a time × group interaction effect for VATMS (p = 0.013). Post-hoc corrected T-tests demonstrated an increased VATMS after active recovery when measured after the 1 min MVC performed 24 h after the time-trial (mean ± SD; 95.2 ± 4.1% vs. 89.2 ± 6.6%, p = 0.026). No significant effects were observed for all other variables.ConclusionsActive recovery increased aspects of central, rather than muscle recovery. However, no effect on MVC was seen, implying that even if active recovery speeds up CNS recovery, without affecting the recovery of muscle contractile properties, this doesn´t translate into increases in overall performance.

No Effect of β-alanine on Muscle Function and Kayak Performance

If β-alanine supplementation counteracts muscular fatigue development or improves athletic performance was investigated. Elite kayak rowers (10 men and 7 women) were supplemented with either 80 mg·kg body mass·d of β-alanine or placebo for 8 wk. Muscular fatigue development was investigated by applying a 2-min elbow flexor maximal voluntary contraction (MVC). EMG was recorded continuously, and voluntary activation was determined 30, 60, 90, and 115 s into the 2-min MVC. In addition, performance was evaluated as 1000-m and 5 × 250-m kayak ergometer rowing. Force reduction during the 2-min MVC was similar before and after supplementation with β-alanine (30.9% ± 10.3% vs 36.0% ± 14.1%) and placebo (35.5% ± 7.7% vs 35.1% ± 8.0%). No time effect was apparent in voluntary activation during the 2-min MVC. In addition, there was no detectable effect of β-alanine supplementation on 1000-m kayak ergometer performance (β-alanine: 0.26% ± 0.02% vs placebo: -0.18% ± 0.02%) or accumulated 5 × 250-m time (β-alanine: -1.0% ± 0.3% vs placebo: -1.0% ± 0.2%). In 5 × 250 m, mean power output was reduced to a similar extent from first to fifth interval before and after supplementation with β-alanine (23% ± 11% vs 22% ± 10%) and placebo (26% ± 13% vs 20% ± 5%). Two-minute MVC characteristics are unaffected by β-alanine supplementation in elite kayakers, and likewise, both a 1000-m kayak ergometer time trial lasting 4-5 min and a 5 × 250-m repeated sprint ability were unaltered by supplementation.

Effects of fatigue on corticospinal excitability of the human knee extensors.

What is the central question of this study? Do group III and IV muscle afferents act at the spinal or cortical level to affect the ability of the central nervous system to drive quadriceps muscles during fatiguing exercise? What is the main finding and its importance? The excitability of the motoneurone pool of vastus lateralis was unchanged by feedback from group III and IV muscle afferents. In contrast, feedback from these afferents may contribute to inhibition at the cortex. However, the excitability of the corticospinal pathway was not directly affected by feedback from these afferents. These findings are important for understanding neural processes during fatiguing exercise. In upper limb muscles, changes in afferent feedback, motoneurone excitability, and motor cortical output can contribute to failure of the central nervous system to recruit muscles fully during fatigue. It is not known whether similar changes occur with fatigue of muscles in the lower limb. We assessed the corticospinal pathway to vastus lateralis during fatiguing sustained maximal voluntary contractions (MVCs) of the knee extensors and during firing of fatigue-sensitive group III/IV muscle afferents maintained by postexercise ischaemia after fatiguing MVCs of the knee extensors and, separately, the flexors. In two experiments, subjects (n=9) performed brief knee extensor MVCs before and after 2-min sustained MVCs of the knee extensors (experiment 1) or knee flexors (experiment 2). During MVCs, motor evoked potentials (MEPs) were elicited by transcranial magnetic stimulation over the motor cortex and thoracic motor evoked potentials (TMEPs) by electrical stimulation over the thoracic spine. During the 2-min extensor contraction, the size of vastus lateralis MEPs normalized to the maximal M-wave increased (P<0.05), but normalized TMEPs were unchanged (P=0.16). After the 2-min MVC, maintained firing of group III/IV muscle afferents had no effect on vastus lateralis MEPs or TMEPs (P=0.18 and P=0.50, respectively). Likewise, after the 2-min knee flexor MVC, maintained firing of these afferents showed no effect on vastus lateralisMEPs or TMEPs (P=0.69 and P=0.34, respectively). Motoneurones of vastus lateralis do not become less excitable during fatiguing isometric MVCs. Moreover, fatigue-sensitive group III/IV muscle afferents fail to affect the overall excitability of vastus lateralis motoneurones during MVCs.

Comparison of the effects of whole-body cooling during fatiguing exercise in males and females

The effects of cold stress on exercise performance and fatigue have been thoroughly investigated only in males, and thus the general understanding of these effects relates only to males. The aim of this study was to determine whether whole-body cooling has different effects on performance during fatiguing exercise in males and females. Thirty-two subjects (18 males and 14 females) were exposed to acute cold stress by intermittent immersion in 14°C water until their rectal temperature reached 35.5°C or for a maximum of 170min. Thermal responses and motor performance were monitored before and after whole-body cooling. Whole-body cooling decreased rectal, muscle and mean skin temperatures in all subjects (p<0.05), and these changes did not differ between males and females. Cold stress decreased the fatigue index (FI) of a sustained 2-min maximal voluntary contraction (MVC) only in males (p<0.05). There were no sex differences in central and peripheral fatigability, or muscle electromyographic activity. This observed sex difference (i.e., body cooling-induced decrease in the FI of a sustained MVC in males but not in females) supports the view of sex effects on performance during fatiguing exercise after whole-body cooling.

The effect of metabolic alkalosis on central and peripheral mechanisms associated with exercise-induced muscle fatigue in humans.

What is the central question of this study? Does metabolic alkalosis affect central and peripheral mechanisms associated with exercise-induced muscle fatigue in humans? What is the main finding and its importance? Inducing metabolic alkalosis before exercise preserved voluntary activation, but not muscle excitation, after a 2min maximal voluntary contraction (MVC) followed by ischaemia. An effect of pH was also observed in maximal rates of torque development, where alkalosis mitigated the reduction in maximal rates of torque development after the initial 2min MVC. For the first time, these results demonstrate a differential effect of pH on voluntary activation as well as maximal rates of torque development after sustained, maximal voluntary knee extension in humans. The increased concentration of protons during fatiguing exercise may contribute to increased activation of group III and IV afferents and subsequently reduced central drive, but this has yet to be confirmed in exercising humans. Here, we determined whether inducing metabolic alkalosis differentially affects descending central drive after fatiguing exercise and whether this effect may, in part, be explained by attenuating group III and IV afferent firing. Eleven men performed a maximal 2min voluntary knee extension (MVC) followed by a 2min rest and subsequent 1min MVC with an occlusive cuff either in placebo [PLA; 0.3g(kg body weight)(-1) calcium carbonate] or alkalosis conditions [ALK; 0.3g(kg body weight)(-1) sodium bicarbonate]. Femoral nerve stimulation was applied before exercise, after the 2min MVC and at 40-60s intervals throughout the remainder of the protocol to explore central and peripheral mechanisms associated with reductions in maximal force and rate of torque development. Although voluntary activation declined to a similar extent after the 2min MVC, during the ischaemic period voluntary activation was higher during ALK (PLA, 57±8%; ALK, 76±5%). Maximal voluntary torque declined at similar rates during the task (203±19Nm), but maximal rate of torque development was significantly higher in the ALK conditions after the 2min MVC (mean difference of 177±60Nms(-1) ). These results demonstrate the effect of pH on voluntary activation as well as maximal rates of torque development after sustained, maximal voluntary knee extension in humans.

Effect of the time of day on central and peripheral fatigue during 2-min maximal voluntary contractions in persons with multiple sclerosis: gender differences

There is a lack of data on fatigue changes within 24h among patients with multiple sclerosis. The purpose of this study was to evaluate the effect of time of day on central and peripheral fatigue during a continuous 2-min maximal voluntary contraction of the quadriceps muscle in women and men with multiple sclerosis (MS). We studied age-matched MS patients (range, 40–50years). The inclusion criteria for patients were: a Kurtzke Expanded Disability Status score and a Fatigue Severity Scale score. We found a significant gender difference in central activation ratio (CAR) in the evening. At the end of the 2-min maximal voluntary contraction (MVC), the voluntary torque decreased by about 65% in men and women with MS in both the morning and evening. We also observed that, in women, CAR decreased markedly during the first 30s in the evening test. The most interesting finding of our study is that central fatigue increased, whereas peripheral fatigue decreased markedly in the evening only in women. It remains unclear why women’s central fatigue is greater in the evening than in the morning.

Fatigue-related firing of distal muscle nociceptors reduces voluntary activation of proximal muscles of the same limb

With fatiguing exercise, firing of group III/IV muscle afferents reduces voluntary activation and force of the exercised muscles. These afferents can also act across agonist/antagonist pairs, reducing voluntary activation and force in nonfatigued muscles. We hypothesized that maintained firing of group III/IV muscle afferents after a fatiguing adductor pollicis (AP) contraction would decrease voluntary activation and force of AP and ipsilateral elbow flexors. In two experiments (n = 10) we examined voluntary activation of AP and elbow flexors by measuring changes in superimposed twitches evoked by ulnar nerve stimulation and transcranial magnetic stimulation of the motor cortex, respectively. Inflation of a sphygmomanometer cuff after a 2-min AP maximal voluntary contraction (MVC) blocked circulation of the hand for 2 min and maintained firing of group III/IV muscle afferents. After a 2-min AP MVC, maximal AP voluntary activation was lower with than without ischemia (56.2 ± 17.7% vs. 76.3 ± 14.6%; mean ± SD; P < 0.05) as was force (40.3 ± 12.8% vs. 57.1 ± 13.8% peak MVC; P < 0.05). Likewise, after a 2-min AP MVC, elbow flexion voluntary activation was lower with than without ischemia (88.3 ± 7.5% vs. 93.6 ± 3.9%; P < 0.05) as was torque (80.2 ± 4.6% vs. 86.6 ± 1.0% peak MVC; P < 0.05). Pain during ischemia was reported as Moderate to Very Strong. Postfatigue firing of group III/IV muscle afferents from the hand decreased voluntary drive and force of AP. Moreover, this effect decreased voluntary drive and torque of proximal unfatigued muscles, the elbow flexors. Fatigue-sensitive group III/IV muscle nociceptors act to limit voluntary drive not only to fatigued muscles but also to unfatigued muscles within the same limb.

Firing of antagonist small‐diameter muscle afferents reduces voluntary activation and torque of elbow flexors

During muscle fatigue, firing of small-diameter muscle afferents can decrease voluntary activation of the fatigued muscle. However, these afferents may have a more widespread effect on other muscles in the exercising limb. We examined if the firing of fatigue-sensitive afferents from elbow extensor muscles in the same arm reduces torque production and voluntary activation of elbow flexors. In nine subjects we examined voluntary activation of elbow flexors by measuring changes in superimposed twitches evoked by transcranial magnetic stimulation of the motor cortex during brief (2-3 s) maximal voluntary contractions (MVC). Inflation of a blood pressure cuff following a 2-min sustained MVC blocked blood flow to the fatigued muscle and maintained firing of small-diameter afferents. After a fatiguing elbow flexion contraction, maximal flexion torque was lower (26.0 ± 4.4% versus 67.9 ± 5.2% of initial maximal torque; means ± s.d.; P < 0.001) and superimposed twitches were larger (4.1 ± 1.1% versus 1.8 ± 0.2% ongoing MVC, P = 0.01) with than without ischaemia. After a fatiguing elbow extensor contraction, maximal flexion torque was also reduced (82.2 ± 4.9% versus 91.4 ± 2.3% of initial maximal torque; P = 0.007), superimposed twitches were larger (2.7 ± 0.7% versus 1.3 ± 0.2% ongoing MVC; P = 0.02) and voluntary activation lower (81.6 ± 8.2% versus 95.5 ± 6.9%; P = 0.04) with than without ischaemia. After a fatiguing contraction, voluntary drive to the fatigued muscles is reduced with continued input from small-diameter muscle afferents. Furthermore, fatigue of the elbow extensor muscles decreases voluntary drive to unfatigued elbow flexors of the same arm. Therefore, firing of small-diameter muscle afferents from one muscle can affect voluntary activation and hence torque generation of another muscle in the same limb.

Activity‐dependent depression of the recurrent discharge of human motoneurones after maximal voluntary contractions

Despite maximal voluntary effort, the output of human motoneurone pools diminishes during fatigue. To assess motoneurone behaviour, we measured recurrent discharges evoked antidromically by supramaximal nerve stimulation after isometric maximal voluntary contractions (MVCs).They were measured as F-waves in the electromyographic activity (EMG). Supramaximal stimuli to the common peroneal and ulnar nerves evoked F-waves at rest before and after MVCs in tibialis anterior (TA) and abductor digit minimi (ADM), respectively. F-waves were depressed immediately after a sustained MVC. For TA, the size and time course of depression of the F-wave area (26 ± 13%; mean ± SD; P =0.007) and persistence (∼20%) were similar after a 10-s or 1-min MVC. For ADM, the decline in F-wave area (39.8 ± 19.6%; P <0.01) was similar after the two contractions but the decline in persistence (probability of occurrence) of the F-wave differed (14.6 ± 10.5% and 32.5 ± 17.1% after 10-s and 1-min MVCs respectively). Comparison of a very long (2-min) with a very short (2-s)MVC in ADM showed that the depression of F-wave area, as well as persistence, was greater after the longer contraction. This suggests, at least for ADM, that the depression is related to the duration of voluntary activity and that the decrease in F-waves could contribute to central fatigue. To examine whether changes in motor axon excitability caused the depression, we measured compound muscle action potentials (M-waves) to submaximal stimulation of the ulnar nerve after a 2-s and 2-min MVC. Submaximal M-waves were not depressed after a 2-s MVC. They were depressed by a 2-min MVC, but the time course of depression of the F- and M-waves differed. Thus, depression of F-waves does not simply reflect reduced excitability of peripheral motor axons.Hence, we propose that activity-dependent changes at the soma or the initial segment depress the recurrent discharge of human motoneurones and that this may contribute to central fatigue.

The reduction in human motoneurone responsiveness during muscle fatigue is not prevented by increased muscle spindle discharge

Motoneurone excitability is rapidly and profoundly reduced during a sustained maximal voluntary contraction (MVC) when tested in the transient silent period which follows transcranial magnetic stimulation (TMS) of the motor cortex. One possible cause of this reduction in excitability is a fatigue-induced withdrawal of excitatory input to motoneurones from muscle spindle afferents. We aimed to test if muscle spindle input produced by tendon vibration would ameliorate suppression of the cervicomedullary motor-evoked potential (CMEP) in the silent period during a sustained MVC. Seven subjects performed a 2 min MVC of the elbow flexors. Stimulation of the corticospinal tract at the level of the mastoids was preceded 100 ms earlier by TMS. These stimulus pairs were delivered every 10 s during the 2 min MVC. Stimulus pairs at 30, 50, 70, 90 and 110 s were delivered while vibration (-80 Hz) was applied to the distal tendon of biceps. On a separate day, the protocol was repeated with both stimuli delivered to the motor cortex. The CMEP in the silent period decreased rapidly with fatigue (to -9% of control) and was not affected by tendon vibration (P = 0.766). The motor-evoked potential in the silent period also declined rapidly (to -5% of control) and was similarly unaffected by tendon vibration (P = 0.075). These data suggest motoneurone disfacilitation due to a fatigue-related decrease of muscle spindle discharge does not contribute significantly to the profound suppression of motoneurone excitability during the silent period. Therefore, a change to intrinsic motoneurone properties caused by repetitive discharge is most probably responsible.

Plyometric Training Does Not Affect Central and Peripheral Muscle Fatigue Differently in Prepubertal Girls and Boys

The aim of the study was to evaluate the effect of plyometric training (PT) on central and peripheral (muscle) fatigue in prepubertal girls and boys. The boys (n = 13, age 10.3 ± 0.3 years) and girls (n = 13, age, 10.2 ± 0.3 years) performed continuous 2-min maximal voluntary contractions (MVCs) before and after 16 high-intensity PT sessions. PT comprised two training sessions per week of 30 jumps in each session with 20 s between jumps. The greatest effect of PT was on excitation-contraction coupling, (twitch force increased by 323% in boys and 21% in girls) and height of a counter-movement jump (increased by 37% in boys and 38% in girls). In contrast, the quadriceps voluntary activation index, central activation ratio, and MVC did not change significantly after PT. The thickness of the quadriceps muscle increased by 9% in boys and 14% in girls after PT. In conclusion, boys and girls demonstrated similar changes in indicators of central fatigue (50-60% decrease) and peripheral fatigue (45-55% decrease) after MVC before and after PT.

The effect of multiple sclerosis and gender on central and peripheral fatigue during 2-min MVC

Objective The aim of this study was to evaluate the effect of secondary progressive MS disease on central and peripheral fatigue in women and men during continuous 2-min maximal voluntary contractions (MVC) of the quadriceps muscle. Methods We studied age-matched (40–50-years of age) healthy individuals (men, n = 9; women, n = 10) and MS patients (men, n = 9; women, n = 9). The inclusion criteria for MS patients were a Kurtzke Expanded Disability Status Score <4 and a Fatigue Severity Scale Score >5. Results The electrically-induced torque, MVC, and voluntary activation (VA) values were significantly greater in healthy persons than in MS patients. At the end of exercise, the fatigue index of MVC and VA was greater in MS patients than in the healthy cohort, whereas the electrically-induced torque was significantly decreased in healthy individuals. Peripheral fatigue was directly correlated with VA in MS patients; in contrast, it was inversely correlated with VA in healthy men. The variability of torque increased significantly during exercise and did not depend on MS and gender. Conclusions MS increased central fatigue and diminished peripheral fatigue significantly during 2-min MVC; however, only peripheral fatigue was significantly greater in healthy men than in healthy women. Significance These results suggest the specified pattern of central and peripheral fatigue in MS patients and could be further exploited for potential therapeutic uses, for instance, to model exercise-induced less central fatigue, and this may not be differentiated between men and women.

Force variability depends on core and muscle temperature

The aim of this study was to investigate if voluntary activation and force variability during maximal voluntary contraction (MVC) depends more on muscle (local) or body (core) temperature. Ten volunteers performed a 2-min MVC of the knee extensors under the control (CON) conditions (ambient temperature (21 °C), relative humidity (30%), and air velocity (∼0.1 m/s)) as well as after heating (HT) and cooling (CL) of the lower body. During water manipulation procedure lower body was immersed up to the waist in a water bath at ∼44 °C for 45 min for HT experiment, and ∼15 °C for 30 min for CL experiment. Peak torque, torque variability, muscle voluntary activation and half-relaxation time were assessed during the exercise. HT increased muscle (2.8±0.2 °C) and rectal (1.9±0.1 °C) temperatures while CL lowered muscle (2.2±0.2 °C) temperature, but did not affect rectal temperature. During 2-min MVC, peak torque decreased ( P<0.05; SP>90%) and to a lower level in HT compared to CON and CL experiments (52.6±2.3% versus 69.0±2.3% and 65.6±1.9% MVC, respectively, P<0.05; SP>90%). Torque variability increased significantly during exercise and was significantly larger in HT and lower in CL compared to CON experiment. Voluntary activation of exercising muscle was more depressed in HT (i.e. greater central fatigue) and the smallest effect was found in CL compared to CON. In conclusion increased core and muscle temperature impairs voluntary activation and increases force variability of the exercising muscles while a local muscle cooling decrease force variability but has a small effect on central fatigue.

The Effect of Plyometric Training on Central and Peripheral Fatigue in Boys

The aim of this study was to investigate the effect of high-intensity plyometric training (PT) on central and peripheral fatigue during exercise performed at maximal intensity in prepubertal boys. The boys (n=13, age 10.3+/-0.3 years) performed continuous 2-min maximal voluntary contractions (MVC) before and after 16 high-intensity PT sessions (two training sessions per week, 30 jumps in each session, 20 s between jumps). The greatest effect of PT was on excitation-contraction coupling: twitch force increased by 323.2+/-210.8% and the height of a counter-movement jump increased by 36.7+/-11.7%, whereas quadriceps femoris (QF) muscle voluntary activation index, central activation ratio and MVC did not change significantly after PT. The thickness of QF increased by 8.8+/-7.9% after PT. Central fatigue increased significantly by about 15-20% after PT, whereas peripheral fatigue decreased significantly by about 10% during the 2-min MVC. Central fatigue and peripheral fatigue during the 2-min MVC were inversely related before PT, but this relationship disappeared after PT.

The response to paired motor cortical stimuli is abolished at a spinal level during human muscle fatigue

During maximal exercise, supraspinal fatigue contributes significantly to the decline in muscle performance but little is known about intracortical inhibition during such contractions. Long-interval inhibition is produced by a conditioning motor cortical stimulus delivered via transcranial magnetic stimulation (TMS) 50-200 ms prior to a second test stimulus. We aimed to delineate changes in this inhibition during a sustained maximal voluntary contraction (MVC). Eight subjects performed a 2 min MVC of elbow flexors. Single test and paired (conditioning-test interval of 100 ms) stimuli were delivered via TMS over the motor cortex every 7-8 s throughout the effort and during intermittent MVCs in the recovery period. To determine the role of spinal mechanisms, the protocol was repeated but the TMS test stimulus was replaced by cervicomedullary stimulation which activates the corticospinal tract. TMS motor evoked potentials (MEPs) and cervicomedullary motor evoked potentials (CMEPs) were recorded from biceps brachii. Unconditioned MEPs increased progressively with fatigue, whereas CMEPs increased initially but returned to the control value in the final 40 s of contraction. In contrast, both conditioned MEPs and CMEPs decreased rapidly with fatigue and were virtually abolished within 30 s. In recovery, unconditioned responses required <30 s but conditioned MEPs and CMEPs required 90 s to return to control levels. Thus, long-interval inhibition increased markedly as fatigue progressed. Contrary to expectations, subcortically evoked CMEPs were inhibited as much as MEPs. This new phenomenon was also observed in the first dorsal interosseous muscle. Tested with a high intensity conditioning stimulus during a fatiguing maximal effort, long-interval inhibition of MEPs was increased primarily by spinal rather than motor cortical mechanisms. The spinal mechanisms exposed here may contribute to the development of central fatigue in human muscles.