[Purpose] This study aimed to examine the difference in the excitability of the primary motor cortex between initiation-predictive and non-predictive tasks, where the onset of muscle relaxation is predicted and not predicted, respectively. [Participants and Methods] Seventeen participants were asked to perform rapid muscle relaxation either through an initiation-predictive or non-predictive task. The baseline was set at 20 percent of the maximum voluntary contraction. Motor-evoked potentials and H-reflexes elicited by transcranial magnetic stimulation and median nerve electrical stimulation, respectively, were measured. The mean stimulation time from the onset of relaxation was calculated, and the motor-evoked potentials and Hoffmann's reflexes elicited during the first (immediately before relaxation) and second half (long before relaxation) were compared. [Results] The amplitude of the motor-evoked potential significantly increased in both initiation-predictive and non-predictive tasks when compared to the baseline, indicating increased excitability of the primary motor cortex. The motor-evoked potential from the initiation-non-predictive task, but not the initiation-predictive task, was associated with increased excitability of the primary motor cortex immediately before relaxation. [Conclusion] Variations in the predictability of motor movements are associated with changes in muscle relaxation control in the central nervous system.