OBJECTIVE The purpose of the present study was twofold: (a) to examine the influence of the innervation zone (IZ) on EMG signals from bipolar electrode arrangements that have their center point directly over the IZ, 10 mm distal to the IZ, 10 mm proximal to the IZ, and 20 mm distal to the IZ and (b) to investigate the effects of normalization on EMG amplitude and mean power frequency (MPF) values over the IZ. METHODS Ten men (mean +/- SD age = 23.6 +/- 3.0 yr) performed submaximal to maximal isometric muscle actions of the dominant leg extensors, and four separate bipolar surface EMG signals were detected simultaneously from the vastus lateralis. One bipolar electrode arrangement had its center point located directly over the IZ for the vastus lateralis, whereas the other electrode arrangements had their center points 10 mm proximal, 10 mm distal, and 20 mm distal to the IZ. RESULTS The results showed that there were no consistent patterns among the four electrode arrangements for the absolute and the normalized EMG amplitude and MPF versus isometric torque relationships. The IZ had the greatest influence on absolute EMG amplitude and MPF values when the center point of the electrode arrangement was directly over the IZ and a decreased effect when the center point was shifted away from the IZ. In addition, normalization reduced the influence of the IZ on EMG amplitude and MPF values. CONCLUSIONS Thus, these findings suggested that in situations where it is difficult to identify the precise location and/or avoid the IZ, the EMG signals can provide useful information regarding isometric torque-related changes in normalized EMG amplitude and center frequency values.