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Basic exercises of repetitive movement forearm in the long term can form a pattern of muscle activity. This exercise is often used as part of rehabilitation therapy for people with lost basic forearm movement due to paralysis. In many cases of rehabilitation exercise, a specific procedure must be performed which is depend on the people condition. One of problems in repetitive exercise is the emergence of muscle fatigue. This condition must be prediction before a decrease in the capacity of the muscles to perform motor tasks on an ongoing basis. Surface Electromyography (sEMG) is a technique for measuring muscle activity that can be used to predict muscle fatigue. This study aims to determine the pattern of biceps brachii muscle activity and to find an indication of muscle fatigue in repetitive movements. The open device wireless sEMG was configured to support forearm rehabilitation exercise without limitation movement due to sEMG wiring problem. In this study three healthy narcotics were asked to take arm extension flexion exercises. The exercise consists of three sessions where each session is 4-minutes long with a 1-minute break between sessions. The sEMG signal is taken at the biceps brachii portion and customized signal processing scheme was used to determine the mean power frequency (MPF) of the signal as one of sEMG feature for the indication of muscle fatigue. The three volunteers show different patterns of muscle activity. Volunteer-1 shows an increasingly clear and accurate pattern of muscle contraction-relaxation, while Volunteer- 2 shows a pattern of contraction-relaxation that gets faster along with repetition of movement. Conversely, the pattern of contraction-relaxation in Volunteer-3 cannot be clearly distinguished. The analysis showed a tendency for MPF to decrease at the beginning and end of the exercise, as well as at the beginning and end of each session. Based on statistical tests, the decrease in MPF values that occurred at the beginning and end of each session of the three narcotics was not significantly different (p> 0.05). The decrease in MPF values that occurred at the beginning and end of the training of Volunteer-1 and 3 was significantly different (p <0.05), but in Volunteer-2 the decrease was not significantly different (p> 0.05). A decrease in MPF values indicates muscle fatigue which can be caused by several possible mechanisms of decreasing muscle cell sensitivity. From this study, it can be concluded that the pattern of muscle activity of each individual is different and the flexion extension of the arm extension repetitively indicates the presence of muscle fatigue.