Abstract

Specific facilitation of the agonist motoneuron pool (pretibial muscle; TA) was investigated under the two different reaction time conditions, i.e. simple (S) and choice (C) reaction time tasks. The motor task was bilateral ankle dorsiflexion, and two different strengths of voluntary ankle dorsiflexion (25, (S) and 50 (L) of maximum contraction force) were controlled by visually guided tracking. The excitability of the agonist motoneuron pool was investigated by the H-reflex method. As the differences between the EMG reaction times of TA on both sides were extremely small under both the S and C tasks, it might be reasonable to use the non-stimulated side as the onset of voluntary movement. Not only the onset of H-reflex facilitation with respect to EMG onset, but also the reaction times were significantly prolonged under the conditions of task C compared with those of task S. In addition, movement times were also prolonged and peak dF/dt values were significantly decreased under the conditions of task C. These results allow us to conclude that the modulation of pre-motor facilitation of the agonist H-reflex is controlled by descending command from the brain, end would be caused by slower EMG reaction times. In addition, we suggest that the prolonged motor times and decreased dF/dt values under the conditions of task C are attributable to slower recruitment and a lower firing rate of motor unit. Verf.-Referat