Sidney Grosprêtre

H reflex and spinal excitability: methodological considerations

The Hoffmann reflex has been the tool most commonly used in exercise studies to investigate modulations in spinal excitability. However, the evolution of electromyographic responses with the increase in stimulation intensity has rarely been assessed when the muscle is active. The purpose of this study was thus to identify that part of the recruitment curve at which the investigation of the Hoffmann reflex is the most reliable in assessing spinal excitability during muscle contraction. Two recruitment curves were determined from the soleus and the medialis gastrocnemius, in passive and active (50% of maximal isometric voluntary contraction) conditions. No differences were found between the H reflexes in the two conditions in the ascending part of the recruitment curves, while the intensity necessary to elicit the same percentage of maximal H wave was different in the descending part of the curve, up to the maximal M wave. We concluded that during motor tasks, changes in spinal excitability should be assessed by recording H responses in the ascending part of the curve, where modulations do not depend either on the background electrical activity of the muscle tested or on methodological considerations.

Modulation of spinal excitability by a sub-threshold stimulation of M1 area during muscle lengthening

It is well known that the H-reflex amplitude decreases during passive muscle lengthening in comparison with passive shortening. However, this decrease in spinal synaptic efficacy observed during passive lengthening seems to be lesser during eccentric voluntary contraction. The aim of the present study was to examine whether spinal excitability during lengthening condition could be modulated by magnetic brain stimulation. H reflexes of the triceps surae muscles were elicited on 10 young healthy subjects, and conditioned by a sub-threshold transcranial magnetic stimulation (TMS). The conditioning stimulation was applied over the M1 area of triceps surae muscles at an intensity below motor threshold with a conditioning-test interval of 5ms. Conditioned and non-conditioned H-reflexes were elicited at rest, during passive lengthening and shortening, and during submaximal contractions (concentric, eccentric and isometric). During passive and active lengthening, H reflexes conditioned by a sub-threshold TMS pulse increased on average by 50% compared with non-conditioned responses. No significant effect was found during isometric and concentric conditions. Activation of the corticospinal pathway would partially cancel inhibitions caused by muscle stretch, and according to the time-delayed effect, this result suggested the existence of a specific polysynaptic pathway. In additional experiments, H responses were conditioned by cervico-medullary stimulations, showing that the modulation described by the previous results involves subcortical mechanisms. This study provides further evidences that the modulation of the final cortico-spinal command reaching the muscle depends on a central mechanism that controls peripheral input, such as Ia afference discharge during lengthening.

New evidence of corticospinal network modulation induced by motor imagery.

Motor imagery (MI) is the mental simulation of movement, without the corresponding muscle contraction. Whereas the activation of cortical motor areas during MI is established, the involvement of spinal structures is still under debate. We used original and complementary techniques to probe the influence of MI on spinal structures. Amplitude of motor-evoked potentials (MEPs), cervico-medullary-evoked potentials (CMEPs), and Hoffmann (H)-reflexes of the flexor carpi radialis (FCR) muscle and of the triceps surae muscles was measured in young, healthy subjects at rest and during MI. Participants were asked to imagine maximal voluntary contraction of the wrist and ankle, while the targeted limb was fixed (static condition). We confirmed previous studies with an increase of FCR MEPs during MI compared with rest. Interestingly, CMEPs, but not H-reflexes, also increased during MI, revealing a possible activation of subcortical structures. Then, to investigate the effect of MI on the spinal network, we used two techniques: 1) passive lengthening of the targeted muscle via an isokinetic dynamometer and 2) conditioning of H-reflexes with stimulation of the antagonistic nerve. Both techniques activate spinal inhibitory presynaptic circuitry, reducing the H-reflex amplitude at rest. In contrast, no reduction of H-reflex amplitude was observed during MI. These findings suggest that MI has modulatory effects on the spinal neuronal network. Specifically, the activation of low-threshold spinal structures during specific conditions (lengthening and H-reflex conditioning) highlights the possible generation of subliminal cortical output during MI.

The Etiology of Muscle Fatigue Differs between Two Electrical Stimulation Protocols.

PURPOSE This study aimed at investigating the mechanisms involved in the force reduction induced by two electrical stimulation (ES) protocols that were designed to activate motor units differently. METHODS The triceps surae of 11 healthy subjects (8 men; age, ~28 yr) was activated using ES applied over the tibial nerve. Two ES protocols (conventional [CONV]: 20 Hz, 0.05 ms vs wide-pulse high-frequency [WPHF]: 80 Hz, 1 ms) were performed and involved 40 trains (6 s on-6 s off) delivered at an intensity (IES) evoking 20% of maximal voluntary contraction. To analyze the mechanical properties of the motor units activated at IES, force-frequency relation was evoked before and after each protocol. H-reflex and M-wave responses evoked by the last stimulation pulse were also assessed during each ES protocol. Electromyographic responses (∑EMG) were recorded after each train to analyze the behavior of the motor units activated at IES. Metabolic variables, including relative concentrations of phosphocreatine and inorganic phosphate as well as intracellular pH, were assessed using P-MR spectroscopy during each protocol. RESULTS Larger H-reflex amplitudes were observed during WPHF as compared with CONV, whereas opposite findings were observed for M-wave amplitudes. Despite this difference, both the force reduction (-26%) and metabolic changes were similar between the two protocols. The CONV protocol induced a rightward shift of the force-frequency relation, whereas a significant reduction of the ∑EMG evoked at IES was observed only for the WPHF. CONCLUSIONS These results suggest that a decreased number of active motor units mainly contributed to WPHF-induced force decrease, whereas intracellular processes were most likely involved in the force reduction occurring during CONV stimulation.

Performance characteristics of Parkour practitioners: Who are the traceurs?

Abstract Parkour is a modern physical activity that consists of using the environment, mostly urban, as a playground of obstacles. The aims of this study were (i) to investigate age, anthropometric and training characteristics of Parkour practitioners, called ‘traceurs’ and (ii) to assess jump performances and muscular characteristics of traceurs, compared to those of gymnasts and power athletes. The mean age of the population of traceurs studied (n = 130) was 19.4 ± 4.3 years, women represented 12.4% of the total field and mean training volume was 8.1 ± 0.5 hours/week. Vertical and long jump performances were analysed on smaller samples of participants (four groups, n = 15 per group); and eccentric (−90° s−1, −30° s−1), concentric (30° s−1, 90° s−1) and isometric knee extensors torques were evaluated by means of an isokinetic dynamometer. Traceurs showed greater (P < .01) drop jump performance (64.9 ± 1.5 cm) than gymnasts (60.9 ± 1.1 cm) and greater (P < .001) counter movement jump with arms (59.2 ± 1.5 cm) than power athletes (53.0 ± 1.4 cm). Standing long jump performances were greater (P < .05) for traceurs (282.7 ± 5.2 cm) compared to other athletes (gymnasts: 273.9 ± 7.3 cm; power athletes: 261.3 ± 6.7 cm). Eccentric knee extension torques were greater (P < .05) for traceurs compared to other athletes. This study revealed that Parkour training induces major development of jump and muscular skills. The use of such training has several practical applications as it provides a better resistance to high eccentric load and helps reinforce musculoskeletal structures.

High-frequency neuromuscular electrical stimulation modulates interhemispheric inhibition in healthy Humans.

High-frequency neuromuscular electrical stimulation (HF NMES) induces muscular contractions through neural mechanisms that partially match physiological motor control. Indeed, a portion of the contraction arises from central mechanisms, whereby spinal motoneurons are recruited through the evoked sensory volley. However, the involvement of supraspinal centers of motor control during such stimulation remains poorly understood. Therefore, we tested whether a single HF NMES session applied to the upper limb influences interhemispheric inhibition (IHI) from left to right motor cortex (M1). Using noninvasive electrophysiology and transcranial magnetic stimulation, we evaluated the effects of a 10-min HF NMES session applied to a right wrist flexor on spinal and corticospinal excitability of both arms, as well as IHI, in healthy subjects. HF NMES induced a rapid decline in spinal excitability on the right stimulated side that closely matched the modulation of evoked force during the protocol. More importantly, IHI was significantly increased by HF NMES, and this increase was correlated to the electromyographic activity within the contralateral homologous muscle. Our study highlights a new neurophysiological mechanism, suggesting that HF NMES has an effect on the excitability of the transcallosal pathway probably to regulate the lateralization of the motor output. The data suggest that HF NMES can modify the hemispheric balance between both M1 areas. These findings provide important novel perspectives for the implementation of HF NMES in sport training and neurorehabilitation. NEW & NOTEWORTHY High-frequency neuromuscular electrical stimulation (HF NMES) induces muscular contractions that partially match physiological motor control. Here, we tested whether HF NMES applied to the upper limb influences interhemispheric inhibition. Our results show that interhemispheric inhibition was increased after HF NMES and that this increase was correlated to the electromyographic activity within the contralateral homologous muscle. This opens up original perspectives for the implementation of HF NMES in sport training and neurorehabilitation.

Assessment of Neuromuscular Function Using Percutaneous Electrical Nerve Stimulation

Percutaneous electrical nerve stimulation is a non-invasive method commonly used to evaluate neuromuscular function from brain to muscle (supra-spinal, spinal and peripheral levels). The present protocol describes how this method can be used to stimulate the posterior tibial nerve that activates plantar flexor muscles. Percutaneous electrical nerve stimulation consists of inducing an electrical stimulus to a motor nerve to evoke a muscular response. Direct (M-wave) and/or indirect (H-reflex) electrophysiological responses can be recorded at rest using surface electromyography. Mechanical (twitch torque) responses can be quantified with a force/torque ergometer. M-wave and twitch torque reflect neuromuscular transmission and excitation-contraction coupling, whereas H-reflex provides an index of spinal excitability. EMG activity and mechanical (superimposed twitch) responses can also be recorded during maximal voluntary contractions to evaluate voluntary activation level. Percutaneous nerve stimulation provides an assessment of neuromuscular function in humans, and is highly beneficial especially for studies evaluating neuromuscular plasticity following acute (fatigue) or chronic (training/detraining) exercise.

Test-retest reliability of wide-pulse high-frequency neuromuscular electrical stimulation evoked force: WPHF NMES Reliability

Introduction: We aimed to compare forces evoked by wide-pulse high-frequency (WPHF) neuromuscular electrical stimulation (NMES) delivered to a nerve trunk vs. muscle belly and to assess their test-retest intra- and inter-individual reliability. Methods: Forces evoked during two sessions with WPHF NMES delivered over the tibial nerve trunk and two over the triceps surae muscle belly were compared. Ten individuals participated in four sessions involving ten 20-s WPHF NMES contractions interspaced by 40 s recovery. Mean evoked force and force time integral of each contraction were quantified. Results: For both nerve trunk and muscle belly stimulation, intra-individual test-retest reliability was good (intra-class correlation coefficient > 0.9) while inter-individual variability was large (coefficient of variation between 140 and 180%). Nerve trunk and muscle belly stimulation resulted in similar evoked forces. Discussion: WPHF NMES locations might be chosen by individual preference, as intra-individual reliability was relatively good for both locations. This article is protected by copyright. All rights reserved.Introduction: We compare forces evoked by wide‐pulse high‐frequency (WPHF) neuromuscular electrical stimulation (NMES) delivered to a nerve trunk versus muscle belly and assess their test–retest intraindividual and interindividual reliability. Methods: Forces evoked during 2 sessions with WPHF NMES delivered over the tibial nerve trunk and 2 sessions over the triceps surae muscle belly were compared. Ten individuals participated in 4 sessions involving ten 20‐s WPHF NMES contractions interspaced by 40‐s recovery. Mean evoked force and force time integral of each contraction were quantified. Results: For both nerve trunk and muscle belly stimulation, intraindividual test–retest reliability was good (intraclass correlation coefficient > 0.9), and interindividual variability was large (coefficient of variation between 140% and 180%). Nerve trunk and muscle belly stimulation resulted in similar evoked forces. Discussion: WPHF NMES locations might be chosen by individual preference because intraindividual reliability was relatively good for both locations. Muscle Nerve 57: E70–E77, 2018.

Presynaptic inhibition mechanisms may subserve the spinal excitability modulation induced by neuromuscular electrical stimulation

This study aimed at deciphering the origins of spinal excitability modulation that follows neuromuscular electrical stimulation (NMES). Ten participants (age: 24.6 ± 4.2 years) performed 2 randomized NMES sessions on plantar flexors with frequencies of stimulations of 20 or 100 Hz (pulse width: 1 ms) at 20% of maximal voluntary contraction (MVC). Before and after each session, the posterior tibial nerve was stimulated to record H-reflex of soleus (SOL), gastrocnemius medialis (GM) and gastrocnemius lateralis (GL). D1 presynaptic inhibition was assessed by conditioning H reflex with prior common peroneal nerve stimulation. Resting H-reflex of SOL decreased after both protocols, but in a greater extent following the 100 Hz session (100 Hz: -34.6 ± 7.3%, 20 Hz: -17.1 ± 3.8%; P = 0.002), accompanied by an increase of presynaptic inhibition (+22 ± 5.8% at 100 Hz vs. +8 ± 3.7% at 20 Hz, P < 0.001). GM and GL spinal excitability and presynaptic inhibition were also altered after NMES, but in a similarly extent after 20 Hz and 100 Hz protocols. Neuromuscular fatigue following a single session of NMES involves spinal presynaptic circuitry, even at low stimulation frequency. The spinal sensitivity to NMES seems also muscle dependent.

Greater fatigability in knee-flexors vs. knee-extensors after a standardized fatiguing protocol

Abstract The present study aimed to investigate the effects of a standardized fatiguing protocol on central and peripheral fatigue in knee-flexors and knee-extensors. Thirteen healthy men (age: 23 ± 3 years; height: 1.78 ± 0.09 m; body-mass: 73.6 ± 9.2 kg) volunteered for the present study. Maximal voluntary contraction (MVC), Electromyography (EMG) activity, voluntary activation level (VAL) as an index of central fatigue and twitch potentiation as an index of peripheral fatigue were measured before and after the fatiguing protocol. The fatiguing protocol consisted of a 0.6 duty-cycle to exhaustion (6 s isometric contraction, 4 s recovery) at 70% MVC. After the fatiguing protocol, MVC decreased in both (Effect-size (ES) = 1.14) and knee-extensors (ES = 1.14), and EMG activity increased in both knee-flexors (ES = 2.33) and knee-extensors (ES = 1.54). Decreases in VAL occurred in knee-flexors (ES = 0.92) but not in knee-extensors (ES = 0.04). Decreases in potentiation occurred in both knee-flexors (ES = 0.84) and knee-extensors (ES = 0.58). The greater central occurrence of fatigue in knee-flexors than in knee-extensors may depend on the different muscle morphology and coupled with a greater tolerance to fatigue in knee-extensors. The present data add further insight to the complicated knee-flexors-to-knee-extensors strength relationship and the mechanisms behind the different occurrence of fatigue.