Voluntary activation of human elbow flexor muscles during maximal concentric contractions

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Abstract
1. To measure voluntary activation of human elbow flexor muscles during maximal concentric contractions, the twitch interpolation method was modified to enable detection of torque increments evoked by single stimuli during contractions of up to 300 deg s-1. Subjects flexed the elbow to rotate a loaded beam 'as fast as possible' (load typically 23-58 N m) from 70 deg below to 70 deg above the horizontal. Electrical stimuli were delivered to biceps brachii when the beam passed through the horizontal. Voluntary activation was estimated from the amplitude of the interpolated twitch, which was expressed as a percentage of the twitch produced by relaxed muscles shortening at the same velocity. 2. In eleven subjects, the level of voluntary activation during repeated maximal concentric contractions (median 99.4%) did not differ significantly from that during maximal isometric contractions (98.0%). Voluntary activation during maximal contractions did not depend on shortening velocity and was the same when tested at two angles 30 deg apart. 3. To induce fatigue, five subjects repeatedly lifted and lowered a heavy load at about 30 deg s-1, and continued for ten to twelve contractions after they needed assistance to continue lifting. All maintained the capacity to attain maximal levels of activation. 4. It is concluded that voluntary drive to elbow flexor muscles during maximal concentric contractions is usually maximal or near-maximal, and that this level of drive can be maintained during development of peripheral fatigue.