Corticospinal excitability is enhanced while preparing for complex movements.

Movement complexity is known to increase reaction time (RT). More recently, transcranial magnetic stimulation (TMS) of the motor cortex has revealed that movement complexity can alter corticospinal excitability. However, the impact of a sequential addition of movement components on corticospinal excitability during the preparatory phase of a simple RT task is unknown. Thus, the purpose of this study was to examine how motor evoked potentials (MEPs) in the premotor period were affected by the complexity of a movement in a simple RT paradigm. Participants (n = 12) completed ballistic movements with their dominant arm, in which they directed a robotic handle to one, two or three targets (32 trials per condition). TMS was delivered prior to movement at 0, 70, 80 or 90% of each participant's mean premotor RT, at the stimulator intensity which yielded a triceps brachii MEP of ~ 10% the maximal M-wave. As expected, premotor RT slowed with increasing task complexity. Although background electromyographic activity (EMG) of the triceps brachii during the preparation phase did not differ among conditions, MEP amplitude increased with movement complexity (i.e., MEPs were greater for the 2- and 3-movement conditions, compared to the 1-movement condition at 80% of premotor RT). We propose the lengthened RTs could be due in part to less suppression of particular motor circuits, while other circuitry is responsible for the increased MEPs. This study demonstrates that, prior to movement, corticospinal excitability increases as a consequence of movement complexity.