The purpose of this study was to assess the effects of contraction intensity, gender, and muscle on median frequency of the three superficial portions of the quadriceps femoris muscle. Thirty healthy volunteers were assessed for isometric electromyogram activity of the vastus medialis (VM), vastus lateralis (VL), and rectus femoris (RF) muscles with the knee at 60° flexion. Subjects performed 5-s isometric contractions at 10, 20, 30, 40, 50, 60, 70, 80, and 90% of the average of three maximal voluntary contractions. Median frequency (f_med) of the three muscles was assessed through a power spectral analysis performed over 11 consecutive 512-ms epochs overlapping each other by one-half their length. The f_med for each of the 11 epochs was then determined, followed by calculation of the mean and SD. The major findings of this study demonstrated that overall f_med was significantly highest for the VL and lowest for the VM, whereas RF f_med was between that of the other two muscles. Similar findings were observed for f_med variability as the VL was significantly higher than the VM and RF, with no gender differences or differences between the latter two muscles. The results demonstrate that the largest change in f_med as a function of contraction intensity occurred for the VL in men (18.6%) and women (7.6%). These findings suggest that muscle fiber-type homogeneity may exist in the VM and RF, which displayed negligible changes in f_med, whereas the VL may possess greater morphological variability.