In a simple reaction time (RT) pradigm, delays in RTs are thought to result from delays in motor output process and not decision processes. Particuraly, movement pattern is a variable relating to the output process; electromyographic RTs (EMG-RTs) of the biceps brachii muscles are different for elbow flexion (F) and for supination (S). The present study was to analyze the influence of the preparatory period (PP) on motor output process by measuring the EMG latencies of synergic muscles for F and S : that is, biceps brachii (BB), tricep brachii (TB), forearm flexor (FF), and forearm extensor (FE). The optimum PP was 2.0 sec for F and 2.2sec for S. Amount of reduction of EMG-RTs of BB was larger in S than in F. Interesting evidence was that synchronization of synergic muscles of BB for S and F was observed in subjects whose EMG-RT of BB in F was shorter than that in S. Those findings confirmed recent reports related to the movement complexity effect and to the response readiness (foreperiod amd position changes) effect on RT (Furubayashi & Kasai, 1990; Guadagnoli & Reeve, 1992).