The surface electromyography is widely used in laboratories of movement analysis due to its easy manipulation and its noninvasive character. However, this extended use generates a variation in the protocols and produces difficult to compare results [1]. In the present study, an analysis of sensitivity for the positioning of bipolar electrodes is made on three muscles of the quadriceps: vastus lateralis (VL), vastus medialis (VM) and rectus femoris (RF). The three main factors of the study are: a. the longitudinal position of electrode, b. the orientation of electrode and c. the angle of extension of the knee. Additionally, the response factors are the maxima force and the RMS of the EMG signal. The tests were made for maxima voluntary contraction (MVC) during an isometric force. We conclude that there are differences in the EMG signal when the electrodes are in different position and orientation. The best distance for the electrodes is 22.59 ± 2.84% for the VL, 29.44 ± 1.88% for the VM and 66.28 ± 0.38% for the RF from the apex of the patella. The best orientation of the electrodes is 0° for the VL, 70° for the VM and -20° for the RF.