The purpose of this study is to obtain distinguishing features of the surface EMG (sEMG) generated by the rectus femoris (RF) and vastus medialis (VM) under maximum voluntary contractions by using the fractal analyses method and compare their dynamic behavior. 15 healthy subjects were requested to produce 5 consecutive maximum voluntary isometric contractions of quadriceps muscle, each lasting 5 seconds with a resting period of 10 seconds between contractions at a knee flexion angle of 15o. Force produced by quadriceps muscles and sEMG from RF and VM were recorded. Correlation dimension and root mean square (rms) values of stationary part of each sEMG were computed. Two-way ANOVA analysis for repeated measures reveled that the mean of the correlation dimension was 1.388 +/- 0.004 for RF and 1.376 +/- 0.007 for VM. There was a significant main effect for muscle (p = 0.027) whereas there was not a significant effect for time (p = 0.918). The means of the rms of the sEMG signals from RF and VM were not significantly different. It was concluded that the processes in producing sEMG in RF are more complex than that in VM and the correlation dimension can be used as a distinguishing feature of RF and VM.