title: Torque decrease during submaximal evoked contractions of the quadriceps muscle is linked not only to muscle fatigue.
abstract: The aim of this study was to analyze the neuromuscular mechanisms involved in the torque decrease induced by submaximal electromyostimulation (EMS) of the quadriceps muscle. It was hypothesized that torque decrease after EMS would reflect the fatigability of the activated motor units (MUs), but also a reduction in the number of MUs recruited as a result of changes in axonal excitability threshold. Two experiments were performed on 20 men to analyze 1) the supramaximal twitch superimposed and evoked at rest during EMS (Experiment 1, n = 9) and 2) the twitch response and torque-frequency relation of the MUs activated by EMS (Experiment 2, n = 11). Torque loss was assessed by 15 EMS-evoked contractions (50 Hz; 6 s on/6 s off), elicited at a constant intensity that evoked 20% of the maximal voluntary contraction (MVC) torque. The same stimulation intensity delivered over the muscles was used to induce the torque-frequency relation and the single electrical pulse evoked after each EMS contraction (Experiment 2). In Experiment 1, supramaximal twitch was induced by femoral nerve stimulation. Torque decreased by ~60% during EMS-evoked contractions and by only ~18% during MVCs. This was accompanied by a rightward shift of the torque-frequency relation of MUs activated and an increase of the ratio between the superimposed and posttetanic maximal twitch evoked during EMS contraction. These findings suggest that the torque decrease observed during submaximal EMS-evoked contractions involved muscular mechanisms but also a reduction in the number of MUs recruited due to changes in axonal excitability.
authors.affiliation: Laboratoire INSERM U1093, Cognition, Action et Plasticité Sensorimotrice, Université de Bourgogne, Faculté des Sciences du Sport, Dijon, France boris.matkowski@u-bourgogne.fr.
authors.forename: Boris
authors.initials: B
authors.lastname: Matkowski
authors.affiliation: Laboratoire INSERM U1093, Cognition, Action et Plasticité Sensorimotrice, Université de Bourgogne, Faculté des Sciences du Sport, Dijon, France.
authors.forename: Romuald
authors.initials: R
authors.lastname: Lepers
authors.affiliation: Laboratoire INSERM U1093, Cognition, Action et Plasticité Sensorimotrice, Université de Bourgogne, Faculté des Sciences du Sport, Dijon, France.
authors.forename: Alain
authors.initials: A
authors.lastname: Martin
mesh_terms: D000328:Adult; D004558:Electric Stimulation / Q000379:methods; D004576:Electromyography / Q000379:methods; D005267:Femoral Nerve / Q000502:physiology; D006801:Humans; D008297:Male; D009046:Motor Neurons / Q000502:physiology; D009119:Muscle Contraction / Q000502:physiology; D018763:Muscle Fatigue / Q000502:physiology*; D052097:Quadriceps Muscle / Q000502:physiology*; D019415:Torque
keywords: electrical stimulation; femoral nerve stimulation; isometric contraction; torque-frequency relation