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Vastus lateralis surface and single motor unit EMG following submaximal shortening and lengthening contractions

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A single shortening contraction reduces the force capacity of muscle fibers, whereas force capacity is enhanced following lengthening. However, how motor unit recruitment and discharge rate (muscle activation) are adapted to such changes in force capacity during submaximal contractions remains unknown. Additionally, there is limited evidence for force enhancement in larger muscles. We therefore investigated lengthening- and shortening-induced changes in activation of the knee extensors. We hypothesized that when the same submaximal torque had to be generated following shortening, muscle activation had to be increased, whereas a lower activation would suffice to produce the same torque following lengthening. Muscle activation following shortening and lengthening (20° at 10°/s) was determined using rectified surface electromyography (rsEMG) in a 1st session (at 10% and 50% maximal voluntary contraction (MVC)) and additionally with EMG of 42 vastus lateralis motor units recorded in a 2nd session (at 4%-47%MVC). rsEMG and motor unit discharge rates following shortening and lengthening were normalized to isometric reference contractions. As expected, normalized rsEMG (1.15 ± 0.19) and discharge rate (1.11 ± 0.09) were higher following shortening (p < 0.05). Following lengthening, normalized rsEMG (0.91 ± 0.10) was, as expected, lower than 1.0 (p < 0.05), but normalized discharge rate (0.99 ± 0.08) was not (p > 0.05). Thus, muscle activation was increased to compensate for a reduced force capacity following shortening by increasing the discharge rate of the active motor units (rate coding). In contrast, following lengthening, rsEMG decreased while the discharge rates of active motor units remained similar, suggesting that derecruitment of units might have occurred.

On sait très bien que le raccourcissement d’un muscle au cours d’une seule contraction diminue la force des fibres musculaires mais que l’étirement l’augmente. Cependant, on ne sait pas comment les mécanismes de sommation spatiale (recrutement) et temporelle (fréquence de stimulation) s’adaptent à la capacité des fibres musculaires au cours de contractions sous-maximales. De plus, il y a peu d’évidence scientifique quant à l’augmentation de la force musculaire chez les muscles plus volumineux. Nous avons donc vérifié si le raccourcissement et l’étirement des fibres musculaires entraînent des variations au plan de l’activation des extenseurs du genou. Nous testons l’hypothèse selon laquelle il faut une plus grande activation des fibres musculaires pour générer un même moment de force sous-maximal quand le muscle se raccourcit que lorsqu’il s’étire. On évalue l’activation du muscle se raccourcissant ou s’étirant (20° à une vitesse angulaire de 10°/s) par la mesure de l’électromyogramme de surface rectifié (rsEMG) au cours d’une première séance d’exercice d’intensité égale à 10 % et 50 % de la force maximale volontaire (MVC) et, au cours d’une deuxième séance d’exercice d’intensité s’étalant de 4 % à 47 % de la MVC, par la mesure de l’activité de 42 unités motrices du vaste externe. Le rsEMG et la fréquence de stimulation des unités motrices au cours de l’étirement et du raccourcissement sont standardisés en fonction de la force musculaire produite en condition de contraction isométrique. Tel qu’attendu, on enregistre après la standardisation un rsEMG (1,15 ± 0,19) plus important et une fréquence de stimulation (1,11 ± 0,09) plus élevée au cours du raccourcissement (p < 0,05). Au cours de l’étirement, le rsEMG standardisé (0,91 ± 0,10) est, tel qu’attendu, moins de 1,0 (p < 0,05), mais la fréquence de stimulation (0,99 ± 0,08) après la standardisation n’est pas moins élevée (p > 0,05). Pour pallier la diminution de force causée par le raccourcissement, la fréquence de stimulation des unités motrices (sommation temporelle) augmente. Par contre, au cours de l’étirement, on observe une diminution de l’importance du rsEMG sans variation de la fréquence de stimulation, ce qui signifie probablement que le recrutement des unités motrices (sommation spatiale) est moins important.

Document Type: Research Article

Publication date: 2008-12-01

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  • This bimonthly journal has a 30-year history of publishing, first as the Canadian Journal of Sport Sciences, and later as the Canadian Journal of Applied Physiology. It publishes original research articles, reviews, and commentaries, focussing on the application of physiology, nutrition, and metabolism to the study of human health, physical activity, and fitness. The published research, reviews, and symposia will be of interest to exercise physiologists, physical fitness and exercise rehabilitation specialists, public health and health care professionals, as well as basic and applied physiologists, nutritionists, and biochemists.
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