To evaluate modulation of spinal cord by central motor commands prior to the onset of a force producing task, we recorded motoneuronal activity by means of H-reflex technique. Subjects were required to grasp an object lain on a table and increase or decrease force production according to two different go-signals. A preparatory signal was delivered 1 second before the go-signals. Subjects performed one set of blocked trials for each go-signal while a third set of trials was characterized by a randomization of the two different go-signals. To elicit the H-reflex we applied a transcutaneal stimulation to median nerve at wrist level while muscle activity of flexor pollicis brevii was registered. A first stimulation was delivered 10 s before the preparatory signal as reference response and a second one during the preparatory period. In the last case the stimulation was randomly distributed in the 60% of trials at two different times: 750 ms and 250 ms prior to the go-signal. In the blocked trials we observed an H-reflex modulation characterized by a facilitation in the early phase of preparatory period (750 ms) and by a strong reduction immediately prior to the task (250 ms); conversely, no modulation was observed in the randomized set of trials. The direction of force production did not affect the reflex response. Overall, these results indicated that the central motor commands might influence the spinal activity during the preparatory period of a force producing task according to the time to go-signal. Moreover, predictability of upcoming task was critical to elicit this influence.
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