Reaction time (RT) has been used as a measure to evaluate the speed of voluntary movement initiation in physical activity and motor behavior research. Since Weiss (1965) introduced the measurement of RT components with electromyography (EMG), pre-motor time (PMT) and motor time (MT) have been used to identify the central (cognitive) and peripheral (neuromuscular) processing in human performance research (Adeyanju, 1982; Clarkson & Kroll, 1978; Ito, 1991). Psychological refractory period (PRP) refers to the delay of a second response to two closely placed stimuli. The effects of practice on the PRP effect were observed by many researchers, and the results indicate that practice reduces the PRP effect (Ruthruff, Johnston & Van Selst, 2001; Van Selst, Ruthruff & Johnston, 1999; Hazeltine, Teague & Ivry, 2002). However, there is not a unanimous answer to the question of whether the PRP effect can be totally eliminated by extended practice. PURPOSE: The purpose of this study was to examine the changes in fractionated reaction times (PMT and MT) for supination and pronation hand/forearm movements in both simple and PRP situations across four consecutive days. METHODS: Sixteen right hand dominant college students volunteered for the study (8 male, 25.13 ± 3.64 yrs; 8 female, 25.88 ± 2.47 yrs). Based on the EMG activity of the pronator teres and biceps brachii, fractionated RTs for pronation and supination were measured for each hand on four consecutive days. A four-way ANOVA (gender x day x hand x movement) with three repeated measures factorial design was used for data analysis. RESULTS: Significant RT and PMT decreases were found across days for both the simple and the PRP tasks (p<0.05); no significant MT change was found across days. In the simple reaction time condition, RT and MT differences between supination and pronation were found, but not for PMT. The reaction times during the PRP task decreased across four days for both the first and second responses. However, the PRP effect remained significant after practice. The reaction time of the first response during the PRP task was significantly longer than the reaction time during the simple task across days. As a result, the PRP effect was a function of inter-stimulus interval/stimulus onset asynchrony (SOA), but was not eliminated with practice. CONCLUSION: Practice reduces reaction time in both simple and PRP conditions; however, the PRP effect was found to be robust after practice and was a function of inter-stimulus interval or SOA.Keyword(s): exercise/fitness, measurement/evaluation, performance