Purpose: The purpose of this study was to examine the influence of electrode placement over the estimated innervation zone (IZ) for the vastus lateralis, as well as proximal and distal to the estimated IZ, on the isometric torque-related patterns for electromyographic (EMG) amplitude and mean power frequency (MPF). Methods: Eleven men performed submaximal to maximal isometric muscle actions of the dominant leg extensors. Surface EMG signals were recorded simultaneously from the vastus lateralis muscle with bipolar electrode arrangements placed over the estimated IZ, as well as proximally and distally to the estimated IZ. Results: The results indicate that the patterns of response and mean values for absolute and normalized EMG amplitude and MPF versus isometric torque over the estimated IZ were not consistently different from those away from the IZ. There were, however, mean differences among electrode-placement sites for absolute EMG amplitude values (distal site > proximal and/or estimated IZ sites by approximately 76 [mu]Vrms) that were not eliminated with normalization (proximal site > distal and/or estimated IZ sites by approximately 5% max). Conclusion: Although these differences were not attributable to the IZ, they suggest that during isometric muscle actions, normalized EMG amplitude values from different individuals cannot be compared if the EMG signals are detected from different electrode locations over the muscle of interest.
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