Motor training strengthens corticospinal suppression during movement preparation.

Training can improve motor skills and modify neural activity at rest and during movement execution. Learning-related modulations may also concern motor preparation but the neural correlates and the potential behavioral relevance of such adjustments remain unclear. In humans, preparatory processes have been largely investigated using transcranial magnetic stimulation (TMS) with several studies reporting decreased corticospinal excitability (CSE) relative to a baseline measure at rest; a phenomenon called preparatory suppression. Here, we investigated the effect of motor training on such preparatory suppression, in relation to resting CSE, in humans. We trained participants to initiate quick movements in an instructed-delay reaction time (RT) task and used TMS to investigate changes in CSE over the practice blocks. Training on the task speeded up RTs, with no repercussion on error rates. Training also increased resting CSE. Most interestingly, we found that CSE during action preparation did not mirror the training-related increase observed at rest. Rather, compared to the rising baseline, the degree of preparatory suppression strengthened with practice. This training-related change in preparatory suppression (but not the changes in baseline CSE) predicted RT gains: the subjects showing a greater strengthening of preparatory suppression were also those exhibiting larger gains in RTs. Finally, such relationship between RTs and preparatory suppression was also evident at the single-trial level, though only in the non-selected effector: RTs were generally faster in trials where preparatory suppression was deeper. These findings suggest training induces changes in motor preparatory processes, that are linked to an enhanced ability to initiate fast movements