Reductions in motoneuron excitability during sustained isometric contractions are dependent on stimulus and contraction intensity

Cervicomedullary stimulation provides a means of assessing motoneuron excitability. Previous studies demonstrated that during low-intensity sustained contractions, small cervicomedullary evoked potentials (CMEPs) conditioned using transcranial magnetic stimulation (TMS-CMEPs) are reduced, whereas large TMS-CMEPs are less affected. As small TMS-CMEPs recruit motoneurons most active during low-intensity contractions whereas large TMS-CMEPs recruit a high proportion of motoneurons inactive during the task, these results suggest that reductions in motoneuron excitability could be dependent on repetitive activation. To further test this hypothesis, this study assessed changes in small and large TMS-CMEPs across low- and high-intensity contractions. Twelve participants performed a sustained isometric contraction of the elbow flexor for 4.5 min at the electromyography (EMG) level associated with 20% maximal voluntary contraction force (MVC; low intensity) and 70% MVC (high intensity). Small and large TMS-CMEPs with amplitudes of ∼15% and ∼50% Mmax at baseline, respectively, were delivered every minute throughout the tasks. Recovery measures were taken at 1-, 2.5- and 4-min postexercise. During the low-intensity trial, small TMS-CMEPs were reduced at 2–4 min (P ≤ 0.049) by up to −10% Mmax, whereas large TMS-CMEPs remained unchanged (P ≥ 0.16). During the high-intensity trial, small and large TMS-CMEPs were reduced at all time points (P < 0.01) by up to −14% and −33% Mmax, respectively, and remained below baseline during all recovery measures (P ≤ 0.02). TMS-CMEPs were unchanged relative to baseline during recovery following the low-intensity trial (P ≥ 0.24). These results provide novel insight into motoneuron excitability during and following sustained contractions at different intensities and suggest that contraction-induced reductions in motoneuron excitability depend on repetitive activation. NEW & NOTEWORTHY This study measured motoneuron excitability using cervicomedullary evoked potentials conditioned using transcranial magnetic stimulation (TMS-CMEPs) of both small and large amplitudes during sustained low- and high-intensity contractions of the elbow flexors. During the low-intensity task, only the small TMS-CMEP was reduced. During the high-intensity task, both small and large TMS-CMEPs were substantially reduced. These results indicate that repetitively active motoneurons are specifically reduced in excitability compared with less active motoneurons in the same pool

Rate of force development and rapid muscle activation characteristics of knee extensors in very old men

International audienceThe age-related decrease in neuromuscular performance is usually accentuated in very old age (> 80 yr) as evidenced by a marked reduction in maximal force production. However, little is known about the ability to rapidly produce force, which limits daily activities and increase the risk of falling. We aimed to assess rapid force production characteristics and rate of muscle activation of the knee extensors in 15 very old (82 +/- 1 yr) vs 12 young (24 +/- 4 yr) men. Maximal force (F-max) and maximal rate of force development (RFDmax) were determined during separated specific isometric contractions. EMG from the vastus lateralis was analyzed to assess the rate of EMG rise (RER). Finally, RFD and RER were examined at time intervals of 0-50, 50-100, 100-200, 0-200 ms and 0-30, 0-50, 0-75 ms, respectively. We reported lower F-max (414 +/- 91 N vs. 661 +/- 139 N) and absolute REDmax (8720 +/- 2056 N*s(-1) vs. 5700 +/- 2474 N*s(-1)) in the very old men compared to young men (P < 0.01). When normalized to F-max, RFD(max )was similar between groups. Normalized RFD at 0-50 and 0-200 ms were lower (- 34% and - 46%, P = 0.04) for the very old men, while no difference was observed at 50-100 ms and 100-200 ms. RER values were higher (similar to 346%, P < 0.01) for the young men at every time interval. These data suggest that the decline in REDmax is associated with the decrease in F-max. Impairments in RFD were accentuated in the first phase of the contraction, which has been associated in the literature with impairments in neural factors associated with aging

Active versus local vibration warm-up effects on knee extensors stiffness and neuromuscular performance of healthy young males

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Task failure during sustained low‐intensity contraction is not associated with a critical amount of central fatigue

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Fatigue, physical activity, and quality of life in people self-reporting symptoms of chronic venous disease

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Effect of race distance on performance fatigability in male trail and ultra-trail runners

The etiology of changes in lower-limb neuromuscular function, especially to the central nervous system, may be affected by exercise duration. Direct evidence is lacking as few studies have directly compared different race distances. This study aimed to investigate the etiology of deficits in neuromuscular function following short versus long trail-running races. Thirty-two male trail runners completed one of five trail-running races as LONG (>100 km) or SHORT (<60 km). Pre- and post-race, maximal voluntary contraction (MVC) torque and evoked responses to electrical nerve stimulation during MVCs and at rest were used to assess voluntary activation and muscle contractile properties of knee-extensor (KE) and plantar-flexor (PF) muscles. Transcranial magnetic stimulation (TMS) was used to assess evoked responses and corticospinal excitability in maximal and submaximal KE contractions. Race distance correlated with KE MVC (ρ = -0.556) and twitch (ρ = -0.521) torque decreases (P ≤ 0.003). KE twitch torque decreased more in LONG (-28 ± 14%) than SHORT (-14 ± 10%, P = 0.005); however, KE MVC time × distance interaction was not significant (P = 0.073). No differences between LONG and SHORT for PF MVC or twitch torque were observed. Maximal voluntary activation decreased similarly in LONG and SHORT in both muscle groups (P ≥ 0.637). TMS-elicited silent period decreased in LONG (P = 0.021) but not SHORT (P = 0.912). Greater muscle-contractile property impairment in longer races, not central perturbations, contributed to the correlation between KE MVC loss and race distance. Conversely, PF fatigability was unaffected by race distance

Reductions in motoneuron excitability during sustained isometric contractions are dependent on stimulus and contraction intensity

Cervicomedullary stimulation provides a means of assessing motoneuron excitability. Previous studies demonstrated that during low-intensity sustained contractions, small cervicomedullary evoked potentials (CMEPs) conditioned using transcranial magnetic stimulation (TMS-CMEPs) are reduced, whilst large TMS-CMEPs are less affected. Since small TMS-CMEPs recruit motoneurons most active during low-intensity contractions while large TMS-CMEPs recruit a high proportion of motoneurons inactive during the task, these results suggest that reductions in motoneuron excitability could be dependent on repetitive activation. To further test this hypothesis, this study assessed changes in small and large TMS-CMEPs across low- and high-intensity contractions. Twelve participants performed a sustained isometric contraction of the elbow flexor for 4.5 min at the electromyography (EMG) level associated with 20% maximal voluntary contraction force (MVC; low-intensity) and 70% MVC (high-intensity). Small and large TMS-CMEPs with amplitudes of ~15 and ~50% Mmax at baseline, respectively, were delivered every minute throughout the tasks. Recovery measures were taken at 1, 2.5 and 4-min post-exercise. During the low-intensity trial, small TMS-CMEPs were reduced at 2-4 min (p≤0.049) by up to −10% Mmax, while large TMS-CMEPs remained unchanged (p≥0.16). During the high-intensity trial, small and large TMS-CMEPs were reduced at all time-points (p<0.01) by up to −14% and −33% Mmax, respectively, and remained below baseline during all recovery measures (p≤0.02). TMS CMEPs were unchanged relative to baseline during recovery following the low-intensity trial (p≥0.24). These results provide novel insight into motoneuron excitability during and following sustained contractions at different intensities, and suggest that contraction-induced reductions in motoneuron excitability depend on repetitive activation

Local vibration training improves the recovery of quadriceps strength in early rehabilitation after anterior cruciate ligament reconstruction: A feasibility randomised controlled trial

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Impact of running an ultramarathon on neuromuscular fatigue: effects of sex and distance.

International audienceFew studies have assessed sex differences in neuromuscular fatigue after prolonged running. Compared to men, women exhibit less peripheral fatigue in plantar flexor muscles (PF) after running a 110-km-mountain-ultramathon [1]. The purpose of the present study was to further characterize sex differences by examining various running distances. Thirty-six runners completed the study: 18 men (36 ± 8 years, 72.5 ± 9.6 kg) and 18 women (36 ± 8 years, 59.1 ± 5.8 kg). Men and women were matched by relative performance (i.e. percent of winning time of their sex category). Participants completed various races of the Ultra-Trail du Mont-Blanc® ranging from 40 km with 2,300 m of positive elevation change to 170 km with 10,000 m of positive elevation change. One month before the race, participants visited the lab to be familiarized with electrical stimulation on both knee extensor (KE) and PF muscles. Neuromuscular function was tested before and after each race. The testing protocol consisted of a standardized warm-up followed by the assessment of maximal voluntary contractions (MVCs), maximal voluntary activation (superimposed 100 Hz doublet) and contractile properties (potentiated 100 Hz doublet, potentiated 10 Hz doublet and potentiated single twitch (Pt)). For analysis, participants were further subdivided into two groups of 18 runners by distance of race completed (SHORT < 100 km vs LONG ≥ 100 km). A factorial ANOVA with deficit (in percentage of PRE value) as a dependent factor and with sex and distance as between-subject factors was used for the analysis.MVC loss was greater after LONG than SHORT in both KE (p<0.05) and PF (p<0.01) and the decrease in Pt was significantly greater (p<0.05) in LONG compared to SHORT in KE, independent of sex. In PF, the decrease in MVC was greater (p<0.05) in men than women (-35 ± 11% vs -25 ± 13%, respectively), independent of the distance run. However, women exhibited a greater decrease in Pt compared to men in LONG (p<0.05) for PF. Neither sex nor distance effects were observed in maximal voluntary activation for either muscle group.The greater muscle strength loss in LONG compared to SHORT fits the previously reported relationship between strength loss and race duration (2). Surprisingly, the present study displayed greater peripheral fatigue in women compared to men after LONG races. However, women lost less muscle strength in PF than men, suggesting that they were globally more resistant to fatigue in plantar flexor muscles.[1] Temesi J et al. (2015). Med Sci Sports Exerc, 47: 1372-82.[2] Millet GY. (2011). Sports Med, 41(6): 489-506

Sex Differences in Neuromuscular Fatigue and Changes in Cost of Running after Mountain Trail Races of Various Distances

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