The influence of electrode shift over the innervation zone and normalization on the electromyographic amplitude and mean power frequency versus isometric torque relationships for the vastus medialis muscle

The purpose of the present study was to: (a) examine the influence of the innervation zone (IZ) for the vastus medialis on EMG signals from bipolar electrode arrangements that have their center point directly over the IZ, 10mm distal to the IZ, 10mm proximal to the IZ, and 20mm distal to the IZ, and (b) investigate the effects of normalization on EMG amplitude and center frequency values over the IZ. Ten men (mean+/-S.D. age=23.6+/-3.0 years) 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 medialis. One bipolar electrode arrangement had its center point located directly over the IZ, while the other electrode arrangements had their center points 10mm proximal, 10mm distal, and 20mm distal to the IZ. The results showed that there were no consistent patterns among the four electrode arrangements for the absolute and normalized EMG amplitude and mean power frequency (MPF) versus isometric torque relationships. Generally speaking, the IZ had the greatest effect on the EMG signal when the center point of the bipolar electrode arrangement was directly over it or 10mm proximal to it. In addition, normalization reduced the influence of the IZ on the absolute EMG amplitude and MPF values. Thus, these findings supported the practice of normalization, and indicated that it is a useful technique for reducing the influence of electrode location on EMG amplitude and MPF data. Future studies should examine the potential for movement of the IZ during isometric muscle actions.