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Motor unit firing rates during isometric voluntary contractions performed at different muscle lengths

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Firing rates of motor units and surface EMG were measured from the triceps brachii muscles of able-bodied subjects during brief submaximal and maximal isometric voluntary contractions made at 5 elbow joint angles that covered the entire physiological range of muscle lengths. Muscle activation at the longest, midlength, and shortest muscle lengths, measured by twitch occlusion, averaged 98%, 97%, and 93% respectively, with each subject able to achieve complete activation during some contractions. As expected, the strongest contractions were recorded at 90° of elbow flexion. Mean motor unit firing rates and surface EMG increased with contraction intensity at each muscle length. For any given absolute contraction intensity, motor unit firing rates varied when muscle length was changed. However, mean motor unit firing rates were independent of muscle length when contractions were compared with the intensity of the maximal voluntary contraction (MVC) achieved at each joint angle.Key words: muscle activation, length–tension relationships, force–frequency relationships.

On a mesuré la fréquence de décharge des unités motrices et l'EMG de surface des triceps brachiaux de sujets en santé durant de brèves contractions isométriques sous-maximales et maximales volontaires, à 5 angles articulaires du coude couvrant la plage physiologique complète des longueurs musculaires. L'activation musculaire aux longueurs maximales, moyennes et minimales, mesurée en bloquant les secousses, a été en moyenne de 98 %, 97 % et 93 % respectivement, chaque sujet ayant pu atteindre une activation complète durant certaines contractions. Comme prévu, les contractions les plus fortes ont été enregistrées à une flexion du coude de 90o. La fréquence de décharge moyenne des unités motrices et l'EMG de surface ont augmenté avec l'intensité des contractions à chaque longueur musculaire. Pour une intensité absolue donnée, la fréquence de décharge a varié lorsque la longueur musculaire a été modifiée. Toutefois, la fréquence de décharge moyenne des unités motrices a été indépendante de la longueur musculaire lorsque les contractions ont été comparées à l'intensité de la contraction volontaire maximale (CVM) atteinte à chaque angle articulaire.Mots clés : activation musculaire, relation longueur–tension, relations force–fréquence.[Traduit par la Rédaction]

Document Type: Research Article

Publication date: 2004-07-01

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