Maturation-related changes in the development and etiology of neuromuscular fatigue

International audiencePurpose: The aim of the present study was to investigate the role of maturation on the etiology of neuromuscular fatigue induced by repeated maximal voluntary isometric contractions (MVIC).Methods: Nine prepubertal boys (9.9 ± 1.3 years), eight male adolescents (13.6 ± 1.3 years) and eleven men (23.4 ± 3.0 years) performed a series of repeated isometric MVICs of the knee extensors until the MVIC torque reached 60% of its initial value. Magnetic stimulations were delivered to the femoral nerve every five MVICs to follow the course of voluntary activation level (VA) and the potentiated twitch torque (Qtwpot).Results: Task failure was reached after 52.9 ± 12.7, 42.6 ± 12.5, and 26.6 ± 6.3 repetitions in boys, adolescents and men, respectively. VA remained unchanged in men whereas it decreased significantly and similarly in boys and adolescents (p < 0.001). In contrast, Qtwpot remained unchanged in boys and decreased significantly less in adolescents than adults (p < 0.05).Conclusions: Children and adolescents experience less peripheral and more central fatigue than adults. However, adolescents experience more peripheral fatigue than children for a comparable amount of central fatigue. This finding supports the idea that the tolerance of the central nervous system to peripheral fatigue could increase during maturation

Appropriateness of indirect markers of muscle damage following lower limbs eccentric-biased exercises: A systematic review with meta-analysis

Purpose The aim of this review was to (1) characterize the time-course of markers of exercise-induced muscle damage (EIMD) based on the level of maximal voluntary contraction torque loss at 24-48h post-exercise (MVCloss24-48h), (2) identify factors (e.g., exercise and population characteristics) affecting the level of MVCloss24-48h, and (3) evaluate the appropriateness of EIMD markers as indicators of MVCloss24-48h. Methods Magnitude of change of each EIMD markers was normalized using the standardized mean differences method to compare the results from different studies. Time-course of EIMD markers were characterized according to three levels of MVCloss24-48h based on a clustering analysis of the 141 studies included. Association between MVCloss24-48h levels and participant´s characteristics or exercise type/modalities were assessed. Meta-regressions were performed to investigate the associations between MVCloss24-48h and EIMD markers changes at \u3c6h, 24h, 48h, 72h and \u3e 96h after exercise. Results Time-course of EIMD markers recovery differs between levels of MVCloss24-48h. Training status and exercise type/modality were associated with MVCloss24-48h level (p \u3c 0.05). MVCloss24-48h was correlated to changes in myoglobin concentration ( \u3c 6h), jump height (24h) and range of motion (48h) (p \u3c 0.001). Conclusion As the exercise could differently affect markers as function of the EIMD severity (i.e., MVCloss24-48h levels), different markers should be used as function of the timing of measurement. Mb concentration should be used during the first hours after the exercise ( \u3c 6h), whereas jump height (24h) and range of motion (48h) could be used as surrogate for maximal voluntary contraction later. Moreover, training status and exercise type/modality could influence the magnitude of MVCloss24-48

Resting Muscle Shear Modulus Measured With Ultrasound Shear-Wave Elastography as an Alternative Tool to Assess Muscle Fatigue in Humans

The aim of this study was to investigate the time course of the resting vastus lateralis (VL) muscle shear elastic modulus (μ) measured with ultrasound shear-wave elastography during repetition of isometric maximal voluntary contractions (MVCs) of the knee extensors (KE). Fifteen well-trained young males repeated 60 5-s isometric MVCs. Evoked electrical stimulations and the VLμ were measured every ten MVCs at rest. The resting VLμ significantly decreased (−34.7 ± 6.7%; P &lt; 0.001) by the end of the fatigue protocol. There was also a 38.4 ± 12.6 % decrease in MVC after exercise (P &lt; 0.001). The potentiated doublet and single twitch torque amplitudes and properties were significantly modified by the end of exercise (P &lt; 0.001). This study shows the time course of the resting VLμ during the repetition of maximal voluntary fatiguing exercise of the KE muscles. The decrease of the resting VLμ could directly affect the force transmission capabilities accounting for peripheral fatigue

Designing perturbative metamaterials from discrete models

Identifying material geometries that lead to metamaterials with desired functionalities presents a challenge for the field. Discrete, or reduced-order, models provide a concise description of complex phenomena, such as negative refraction, or topological surface states; therefore, the combination of geometric building blocks to replicate discrete models presenting the desired features represents a promising approach. However, there is no reliable way to solve such an inverse problem. Here, we introduce ‘perturbative metamaterials’, a class of metamaterials consisting of weakly interacting unit cells. The weak interaction allows us to associate each element of the discrete model with individual geometric features of the metamaterial, thereby enabling a systematic design process. We demonstrate our approach by designing two-dimensional elastic metamaterials that realize Veselago lenses, zero-dispersion bands and topological surface phonons. While our selected examples are within the mechanical domain, the same design principle can be applied to acoustic, thermal and photonic metamaterials composed of weakly interacting unit cells

The effect of knee joint angle on neuromuscular changes after an exercise-induced muscle damage.

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Assessment of the H-reflex using two synchronized magnetic stimulators in order to increase stimulus durations: a comparison with electrical stimulation

International audienceMagnetic nerve stimulation (MNS) may be a less painful alternative to electrical nerve stimulation (ENS) for Hoffmann reflex (H-reflex) measurement, however standard MNS (sMNS) techniques utilize a short stimulus duration, thereby limiting its use for H-reflex assessment. This limitation may be partly overcome by coupling two magnetic stimulators to increase the pulse duration (coupled MNS: cMNS). The aim of this study was to test this assumption by comparing the H-reflex characteristics evoked by ENS, sMNS and cMNS.Thirteen healthy volunteers were tested with ENS and both MNS in the prone position. Maximal soleus H-reflex (Hmax) and M-wave (Mmax) amplitudes were measured to compute the Hmax/Mmax ratio. Hmax was evoked at rest and during both isometric submaximal (10%MVC) and maximal plantar-flexions (MVC).At rest, MNS techniques underestimated Hmax (ENS: 8.32 ± 2.73 mV; sMNS: 6.85 ± 2.29 mV; cMNS: 7.48 ± 2.23 mV; p < 0.05). In contrast, no difference was observed for Hmax/Mmax (ENS: 0.59 ± 0.17; sMNS: 0.45 ± 0.28; cMNS: 0.47 ± 0.29; p = 0.11). sMNS, cMNS and ENS similarly detected Hmax facilitation during MVC (ENS: +120 ± 248%; sMNS: +228 ± 350%; cMNS: +162 ± 180% of the rest value; p = 0.344).Owing to their shorter stimulus duration, both MNS techniques underestimated the Hmax compared to ENS. However, when the gold standard ENS technique cannot be used, coupled MNS may be recommended since it provides better H-reflex characteristic assessment than standard MNS due to its longer stimulus duration

Repeated bout effect: Neuromuscular and cardiorespiratory responses during downhill walking in relation to muscle function loss.

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Muscle Shear Elastic Modulus Provides an Indication of the Protection Conferred by the Repeated Bout Effect

International audienceBackground: The neuromuscular system is able to quickly adapt to exercise-induced muscle damage (EIMD), such that it is less affected by subsequent damaging exercise, a phenomenon known as the repeated bout effect (RBE). The objective was to determine whether the mechanical properties of the quadriceps, as evaluated by shear wave elastography (SWE), were less affected when a second bout of eccentric-biased exercise was performed 2 weeks later. It was hypothesized that the first bout would confer protection against extensive muscle damage through an adaptation of the muscle stiffness before the second bout (i.e., higher muscle stiffness). Methods: Sixteen males performed two identical bouts of downhill walking separated by 2 weeks (45 min at 4.5 km.h −1 ; gradient: 25%; load: 30% of the body mass). Rectus femoris (RF) and vastus lateralis (VL) resting shear elastic modulus (µ) and EIMD symptoms were measured before and up to 7 days following the exercise bouts. Changes in neuromuscular function was evaluated by maximal voluntary contraction torque, voluntary activation level, evoked mechanical response to single and double (10 and 100 Hz doublets) electrical stimulation. An index of protection (IP) was calculated for EIMD symptoms to assess magnitude the RBE. Results: EIMD symptoms were less affected after the second than the first exercise bout. RF and VL-µ increased (p < 0.001) only after the first exercise. RF µ was elevated up to 2 weeks after the end of the first exercise (p < 0.001) whereas VL µ was only increased up to 24 h. The increase in µ observed 2 weeks after the end of the first exercise was correlated with the IP; i.e., attenuation of alterations in muscle µ, 10 Hz-doublet amplitude and rate of torque development after the second exercise bout (p < 0.05)

Impact of combined (strength + endurance) training versus endurance training on neuromuscular characteristics in obese adolescent girls.

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