Morgan Levenez

Spinal Mechanisms Contribute to Differences in the Time to Failure of Submaximal Fatiguing Contractions Performed With Different Loads

This study compared the mechanisms that limit the time to failure of a sustained submaximal contraction at 20% of maximum when the elbow flexors either supported an inertial load (position task) or exerted an equivalent constant torque against a rigid restraint (force task). The surface electromyogram (EMG), the motor-evoked potential (MEP) in response to transcranial magnetic stimulation (TMS) of the motor cortex, and the Hoffmann reflex (H-reflex) and maximal M-wave (Mmax) elicited by electrical stimulation of the brachial plexus were recorded in biceps brachii during the two tasks. Although the time to failure for the position task was only 44% of that for the force task, the rate of increase of the average EMG (aEMG; % initial MVC) and MEP area (% Mmax) did not differ significantly during the two tasks. At task failure, however, the increases in normalized aEMG and MEP area were significantly (P < 0.05) greater for the force task (36.4 and 219.9%) than for the position task (22.4 and 141.7%). Furthermore, the superimposed mechanical twitch (% initial MVC), evoked by TMS during a brief MVC of the elbow flexors immediately after task failure, was increased similarly in both tasks. Although the normalized H-reflex area (% Mmax) decreased during the two fatiguing contractions, the reduction was more rapid and greater during the position task (59.8%) compared with the force task (34.7%). Taken together, the results suggest that spinal mechanisms were a major determinant of the briefer time to failure for the position task.

Effects of noradrenaline and dopamine on supraspinal fatigue in well-trained men.

PURPOSE Prolonged exhaustive exercise induces a failure of the nervous system to activate the involved muscles maximally (i.e., central fatigue). Part of central fatigue may reflect insufficient output from the motor cortex (i.e., supraspinal fatigue), but the cause is unresolved. To investigate the potential link between supraspinal fatigue and changes in brain concentration of dopamine and noradrenaline in temperate environment, we combined neurophysiological methods and pharmacological manipulation of these two neurotransmitters. METHODS Changes in performance of a cycling exercise (time trial [TT]) were tested after oral administration of placebo (Pla), dopamine, or noradrenaline reuptake inhibitors (methylphenidate and reboxetine [Rebox], respectively) in well-trained male subjects. Changes in voluntary activation, corticospinal excitability, and muscle contractile properties were tested in the knee extensors using transcranial magnetic stimulation and motor nerve electrical stimulation before and after exercise. A psychomotor vigilance task (PVT) was also performed. RESULTS Compared with Pla, methylphenidate did not affect exercise performance (P = 0.19), but more time was needed to complete the TT after administration of Rebox (approximately 9%, P < 0.05). For the latter condition, the reduced performance was accompanied by a central/supraspinal fatigue (5%-6%, P < 0.05) and worsened PVT performance (7%, P < 0.05). For the three conditions, corticospinal excitability was unchanged, and peripheral fatigue was similar. Because the ingestion of Rebox induced a greater decrease in voluntary activation and PVT performance after the TT than Pla, with no modification in corticospinal excitability, the noradrenaline reuptake inhibitor likely affected supraspinal circuits located before the motor cortex. CONCLUSION These results suggest that noradrenaline, but not dopamine reuptake inhibition, contributes to the development of central/supraspinal fatigue after a prolonged cycling exercise performed in temperate conditions.

Effet du crochetage myo-aponévrotique du triceps sural sur la tension passive et l’architecture musculaire à l’étirement

Resume Introduction Le crochetage myo-aponevrotique est une methode instrumentale de traitement, non-invasive, d’algies traumatiques ou inflammatoires de l’appareil locomoteur. Nous avons entrepris cette etude afin d’objectiver un eventuel effet de 10 min de crochetage du triceps sural sur l’architecture musculaire du gastrocnemius medialis. Methode L’imagerie echographique nous a permis de quantifier les modifications de l’architecture musculaire et de les associer a la mesure de la tension passive developpee par l’unite myo-tendineuse a l’etirement. L’angle de pennation (forme par un faisceau de fibres musculaires et l’aponevrose profonde), ainsi que l’allongement fasciculaire du gastrocnemius medialis lors d’un etirement passif, ont ete analyses chez 16 sujets humains repartis en un groupe traite par crochetage et un groupe controle. Resultats Nos resultats montrent qu’a 35 de dorsiflexion, la longueur des fascicules musculaires etait augmentee de 7,0 ± 5,0% (P Niveau de preuve : 2 (etude therapeutique).

Changes in muscle resting tension, architecture and spinal reflex after hook treatment in healthy subjects

The diacutaneous myo-aponeurosis technique is an instrument-based technique (hook treatment) used on patients suffering from pains of inflammatory or traumatic origin affecting the locomotor system (Ekman 1972). The purpose of this study was to investigate a possible effect of hook treatment of the triceps surae. Mechanical and neural parameters were recorded to try to identify the underlying mechanisms of the muscular relaxation perceived by therapists following the application of this technique (Veszely et al. 2000).

Effects of unipedal standing training on mechanical and neuromuscular systems

The proprioception training is frequently used in the prevention of articular lesions and in the rehabilitation of sports injuries. This training restores the neuromuscular functions and is essential to recover the articulation stability (Fitzgerald et al. 2000; Taube et al. 2007). The therapist frequently uses the Freeman’s board, but other instruments of rehabilitation exist. This study on unipedal standing training, using a soft foam carpet, aims to assess the effects on the neuromuscular adaptations and the articular stability.

Electro-mechanical characteristics of ballistic contractions performed from different initial conditions

Fast contractions, often called ballistic contractions, are frequently used in sports. However, the performance of such contractions is influenced by the conditions under which it is performed. For example, it has been shown that a sustained contraction (,25% maximal voluntary contraction—MVC) prior to a ballistic contraction can have a negative effect on its maximal rate of torque development (Van Cutsem and Duchateau 2005). Therefore, a careful investigation of the electromechanical characteristics of ballistic contractions performed from different initial conditions is interesting to understand the difference in their performance.

Neural adjustments in antagonist muscles during a submaximal fatiguing contraction

The concurrent activation of agonist and antagonist muscles, which is commonly referred as coactivation, is thought to increase joint stability during contractions. The degree of coactivation depends on the task being performed and varies during the course of a fatiguing contraction (Kellis, 1998). This observation suggests that coactivation must be continuously adjusted by the nervous system during a fatiguing contraction so that the performance of the agonist muscle is not impeded. Although a decrease in coactivation during the course of a sustained contraction would extend the time to task failure, this would be accompanied by a decrease in joint stability. Conversely, an increase in coactivation may contribute to the loss of force-generating capacity associated with fatigue (Psek & Cafarelli, 1993). The purpose of the present study was to examine the involvement of spinal mechanisms in the control of coactivation during a submaximal fatiguing contraction sustained until failure.