Many studies have described the dynamic modulation of corticospinal excitability of the prime movers during motor preparation. However although anticipatory postural adjustments (APA) are an inherent part of most voluntary movements, investigation of trunk muscle corticospinal excitability during motor preparation has been neglected in the literature. In the present study, the corticospinal excitability of the superficial multifidus (sMF) and rectus abdominis (RA) muscle has been assessed during the preparation of rapid arm flexions and extensions in fifteen participants. A Warning signal informed participants to prepare to move prior to a Go signal. Transcranial magnetic stimulation was applied during baseline and at 6 time intervals before (Delay period) or after (Motor execution period) the Go signal. Results revealed a significant inhibition of the amplitude of sMF motor-evoked potentials in both flexion and extension movements within the Delay period compared to baseline, while no significant modulation was observed for RA. During the Motor Execution period for arm extension, sMF displayed even more inhibition, along with a large and significant facilitation of RA. During the Motor execution period for arm flexion, sMF presented a trend toward larger motor-evoked potential amplitude compared to Delay period. These results suggest the existence of two concurrent mechanisms underlying motor preparation for APA: (i) before the Go signal, a nonspecific inhibitory mechanism for sMF, likely to preclude motor program release; (ii) after the Go signal, a task-specific modulation of corticospinal excitability consistent with the EMG pattern during the early phase of movement.