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The root mean square (RMS), the average rectified value (ARV), and the mean frequency (MNF) are indices of muscle fatigue. In this paper, the relationship between the muscle fatigue and these metrics was examined. The correlation among the muscle fatigue indices was also considered by plotting the normalized metrics of surface versus intramuscular EMG. The EMG data was divided into equal segments, and the metrics were calculated in each segment. The calculated metrics were plotted in time domain, and linear regression analysis was performed to find the tendencies and relationships between surface and intramuscular metrics. As the muscle fatigue progressed, the slope of RMS and ARV increased, while that of MNF deceased. For the normalized RMS, ARV, and MNF, the surface versus intramuscular EMG was plotted, and their correlations were examined. Compared with normalized RMS and AR Vplots, the normalized MNF showed a slower change-rate.