Response to comment re: "Psychophysical and EMG correlates of force exertion in manual work - response," International Journal of Industrial Ergonomics, vol. 13, no. 1, pp. 31-39.
The writer makes a valid point regarding the use of the EMG normalization procedure we used in our study (described by Marras in IJIE 6: 89-93). However, we want to affirm to the writer and to other readers, our belief that the potential problems associated with this procedure had no effect on the results or conclusions of our studies. The "resting" EMG level was indeed subtracted from the task-related EMG level in an attempt to eliminate the influence of noise and crosstalk in the EMG signal. We also used other measures to minimize the introduction of noise; specifically we used small onsite surface electrode-preamplifiers (i.e., active electrodes) for signal pickup. The close proximity of the electrodes and the preamplifier reduce noise at the signal source and minimize cable movement artifact. As a result, the resting RMS EMG signal was generally less than 3% of the mean task-related RMS EMG level. Therefore, we believe it reasonable to conclude (a) the muscles under study were indeed "at rest" during the resting EMG measurement; (b) noise was not a substantial contributor to the strength of the EMG signal; and (c) that the error introduced by the linear subtraction procedure was negligible, although our EMG values tend to be somewhat lower (3% or less) than values calculated using the squared subtraction approach. We would hope that it would be rare for researchers to encounter substantial levels of noise in their EMG signals. Certainly, if noise makes up a large percentage of the overall signal, it may be wise for researchers to search for external means of reducing the noise or to consider alternative methods for reducing crosstalk (e.g., using needle electrodes). However, because it is common for electromyographers to normalize their data, we appreciate the efforts of the writer to bring this point to the attention of other readers who may be doing similar studies.