When performing synchronous hand and foot movements, the way the limbs are synchronized differs depending on the mode of control. When performed in a reaction time (RT) paradigm (reactive control), EMG onsets become synchronized resulting in asynchronous displacement onset, owing to differences in inertial limb properties. However, when the same synchronous movement is performed as an anticipation-timing task (predictive control), displacement onset is synchronized by unconsciously introducing a small delay between EMG onsets. The present experiment investigated whether switching the “mode” of control for a synchronous task would incur a reaction time cost. Participants (n=12) were asked to simultaneously lift both their right heel and right hand in a simple RT paradigm, and in an anticipating-timing task when a rotating clock hand reached a specified target. On a small subset of anticipation-timing trials (16%), an auditory stimulus was presented either 250ms or 500ms before the target and participants were instructed to switch to reactive control and execute the synchronous movement as quickly as possible after the signal. Results showed that when the auditory stimulus was presented 500ms before the target, participants were able to switch to a reactive control mode, although RT was significantly longer compared to when performing the task in a simple RT paradigm. However, when the auditory stimulus was delivered 250ms before the target, participants were unable to switch to a reactive control mode. These results indicate that even when the task is the essentially the same, unconsciously switching between control modes incurs a reaction time cost.