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Ageing is accompanied by numerous changes within the sensory and motor components of the muscle spindle pathway. To further document these age-related changes, this study compared the characteristics of the Hoffmann (H) reflex and M wave, evoked with several pulse durations, between young and old adults. The H-reflex and M-wave recruitment curves were recorded at rest in the flexor carpi radialis of 12 young (21-36 years) and 12 older adults (62-80 years). For each pulse duration (0.05, 0.2 and 1 ms), the maximal M-wave (MMAX ) and H-reflex (HMAX ) amplitude, the M-wave amplitude associated with HMAX (MHmax ) and the H-reflex amplitude for a stimulus intensity evoking an M-wave of 5% MMAX (HM5% ) were measured. The strength-duration time constant and response threshold were estimated from the charge/stimulus-duration relation for the H reflex and M wave. Results indicate that varying pulse duration mainly induces a similar effect on H-reflex and M-wave recruitment curves between young and older adults. Regardless of pulse duration, old adults had lesser HMAX (p = 0.029) and HM5% (p < 0.001) but greater MHmax (p < 0.001). The H-reflex and M-wave response thresholds were greater in old than young adults (p = 0.003), but the strength-duration time constant was lesser in old than young adults for the H reflex (p = 0.048) but not the M wave (p = 0.21). These results suggest greater age-related changes in the sensory than the motor component of the H-reflex pathway, which may be indicative of a greater loss of sensory than motor axons or alterations of synapses between Ia afferents and motor neurones., info:eu-repo/semantics/published

This study investigated the effects of postactivation potentiation (PAP) on the force output and muscle architecture in plantar flexor muscles. The mechanical response to a single electrical stimulus (twitch) and to two (doublet) and three (triplet) stimuli (10-ms interpulse interval) was measured before and after a 6-s maximal voluntary contraction (MVC). Ultrasound imaging was used to measure fascicle length and pennation angle of the gastrocnemius medialis at rest and during the electrically induced contractions. Immediately after the conditioning MVC, twitch peak force (+ 40%) and its maximal rate of force development (+ 57%) and relaxation (+ 62%) were greater than before the MVC (P < 0.001). The PAP extent was less for the doublet than for the twitch and for the triplet than for the doublet (P < 0.05). Whereas none of the architectural parameters changed at rest, fascicle shortening and increase in pennation angle during contractions were greater after than before the conditioning MVC, with a greater extent (P < 0.001) during the twitch (+ 28% and + 58%, respectively) compared with the doublet (+ 16% and + 36%) and the triplet (+ 12% and + 14%). Overall, our results indicate that the effect of the conditioning MVC on mechanical output and muscle architecture decreased from the twitch to the triplet in plantar flexor muscles. The decreased PAP observed during doublet and triplet compared with twitch indicates that the benefit of this mechanism to the enhancement of muscle performance becomes progressively less effective during successive muscle activation. NEW & NOTEWORTHY Postactivation potentiation (PAP), a form of activity-dependent potentiation, is defined as the acute enhancement of muscle contractile properties induced by a conditioning contraction, usually a maximal voluntary contraction. The purpose of our study was to investigate the effects of PAP on force production and muscle architecture in plantar flexor (PF) muscles. The results indicate that the, SCOPUS: ar.j, info:eu-repo/semantics/published

Research in movement science aims at unravelling mechanisms and designing methods for restoring and maximizing human functional capacity, and many techniques provide access to neural adjustments (acute changes) or long-term adaptations (chronic changes) underlying changes in movement capabilities. First described by Paul Hoffmann over a century ago, when an electrical stimulus is applied to a peripheral nerve, this causes action potentials in afferent axons, primarily the Ia afferents of the muscle spindles, which recruit homonymous motor neurons, thereby causing an electromyographic response known as the Hoffmann (H) reflex. This technique is a valuable tool in the study of the neuromuscular function in humans and has provided relevant information in the neural control of movement. The large use of the H reflex in motor control research on humans relies in part to its relative simplicity. However, such simplicity masks subtleties that require rigorous experimental protocols and careful data interpretation. After highlighting basic properties and methodological aspects that should be considered for the correct use of the H-reflex technique, this brief narrative review discusses the purpose of the H reflex and emphasizes its use as a tool to assess the effectiveness of Ia afferents in discharging motor neurones. The review also aims to reconsider the link between H-reflex modulation and Ia presynaptic inhibition, the use of the H-reflex technique in motor control studies, and the effects of ageing. These aspects are summarized as recommendations for the use of the H reflex in motor control research on humans., SCOPUS: re.j, info:eu-repo/semantics/published

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Malgré la simplicité apparente et l’emploi fréquent du terme fatigue dans la vie courante, celui-ci peut revêtir différentes significations. La notion de fatigue musculaire est définie comme étant une diminution transitoire et réversible de la capacité à produire une performance donnée (Enoka & Duchateau, 2008). Elle peut trouver son origine au niveau des systèmes nerveux et musculaire. Par exemple, il a été montré que la diminution de la performance (temps de maintien) repose probablement en partie sur des altérations localisées au niveau du système nerveux central qui limiteraient la durée de l’effort (Gandevia, 2001 ;Hunter et al. 2004). Aussi, la cinétique de récupération des facteurs nerveux après un exercice sous-maximal fatigant pourrait contribuer à réduire la performance lors de la réitération d’un effort (Cogiamanian et al. 2007). Dès lors, le développement de nouvelles stratégies visant à faciliter la récupération nerveuse s’avère particulièrement pertinent et il convient de s’interroger plus spécifiquement sur les effets potentiels de l’utilisation de la stimulation électrique transcrânienne par un courant continu de faible intensité (tDCS) pour contrecarrer le déficit nerveux observé. La tDCS est une méthode non-invasive qui modifie l’excitabilité neuronale pendant et après la période de stimulation (Nitsche & Paulus, 2000 ;Nitsche et al. 2003). Cette technique permet, par exemple, d’augmenter l’excitabilité des neurones corticaux des aires motrices (Nitsche et al. 2005) et de modifier l’excitabilité de réseaux neuronaux spinaux (Roche et al. 2009, 2011). Par ailleurs, la tDCS anodique semblerait améliorer les performances motrices chez des sujets sains (Bastani & Jaberzadeh, 2012) et augmenter le temps d'endurance (durée maximale de l'effort) lors de la réitération d’une contraction isométrique maintenue des fléchisseurs du coude après son application pendant 10 minutes (Cogimanian et al. 2007). Si ces premiers résultats suggèrent l’influence de la tDC, Doctorat en Sciences de la motricité, info:eu-repo/semantics/nonPublished

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Maneuvering a motorcycle in racing conditions or for prolonged time is sufficiently demanding that on many occasions forearm muscles reach a state of functional failure when riders cannot properly brake or operate the throttle. This study intends to discriminate which ones of the several dynamometric parameters used in the literature to characterize the Forcetime (F-t) curve during voluntary contractions are more sensitive to neuromuscular fatigue in simulated motorcycle-riding conditions. Thirty-three adults performed an intermittent fatiguing protocol (IFP) that simulated the brake-pulling and throttle-twisting actions, by using a hydraulic system equipped with a pressure sensor. Sixty pressure-time (P-t) curve parameters, including the rate of pressure development (RPD) and area under the curve were measured to characterize the time course of the braking maximal voluntary contraction (MVC). Two types of variables were used to analyze the P-t curve: 1) Times interval (from 0 to 30- 50-100-500-1000 and 2000 ms); 2) Percentages of MVC (10-30-60-90%MVC). Overall significant (p ≤ 0.05) fatigue-related declines were observed only at time intervals longer than 100 ms and contraction intensities higher than 30%MVC. Strong and significant linear declines (p < 0.001) were observed at 500 ms and 1 s for normalized pressures, as well as for the ratio RPD60%MVC/MVC (p < 0.003) throughout the IFP. Our results suggest considering RPD at time windows of 0-500 ms and 0-1 s, and contraction intensities comprised between 30% and 60% of MVC, as more suitable criteria to study fatigue-related decrements in performance rather than the classical MVC force., SCOPUS: ar.j, info:eu-repo/semantics/published

Changes in corticospinal excitability prior to a contraction may depend on its characteristics, including the rate of torque development. This study compared the specific modulation of cortical and spinal excitability during the preparation phase (last 500 ms before contraction) of fast (ballistic; BAL) and ramp (RAMP) contractions of ankle dorsiflexors, using transcranial magnetic stimulation and peripheral nerve stimulation. The results indicate earlier changes at cortical than spinal level during the preparation phase of both contraction types. However, these adjustments are delayed prior to BAL relative to RAMP contractions. This study suggests that the time course of change in cortical and spinal excitability during the preparation phase of a voluntary action is specific to the intended rate of torque development of the upcoming contraction., SCOPUS: ar.j, info:eu-repo/semantics/published

An increase in myofibrillar protein accretion can occur in the very early post-exercise period and can be potentiated by dietary protein intakes. The positive effect of nutrition on net muscle protein balance is credited to essential amino acids (EAAs), and particularly to the branched-chain amino acid leucine. Thus, leucine plays a key role in feeding-induced muscle protein synthesis by its unique ability to activate signalling pathways modulating translational initiation (Dreyer et al. 2008). Furthermore, strength exercise induces important disturbances in protein turnover, especially in novice athletes, and unaccustomed physical exercise, particularly repeated eccentric muscle contractions, induces muscle soreness and alterations in muscle cellular structure. Still, muscle strength gains induced by resistance training are mainly attributed to neural adaptations in the initial part of training, and subsequently to changes in the force capacity of muscle fibres due to hypertrophy (Duchateau et al. 2006; Sale, 1988). Since acute post-exercise muscle protein accretion does not inform on chronic muscle adaptations, the main purpose of this work is to test the effectiveness of an EAA supplementation on muscle hypertrophy and strength gains in response to resistance training, and on repair-oriented remodelling of disrupted muscle fibres in the early recovery period of an unaccustomed exercise. The first investigations aim at evaluating the effects of an EAA supplementation on muscle mass, architecture, and strength in the early stages of a heavy-load training programme, and at establishing if the enrichment of the supplement with leucine induces greater improvements in these muscle functional adaptations. In a second part, central and peripheral adaptations are examined following training to compare whether EAA supplementation modifies the relative contribution of neural and muscular factors to the increase in muscle torque, and promotes adaptations in muscle mechanical, Doctorat en Sciences de la motricité, info:eu-repo/semantics/nonPublished