[Influence of local heating on the electrical and thermal conductivity of the skin and on the derived surface EMG (author's transl)]

Frequency and amplitude analysis of the EMG during exercise on the bicycle ergometer

The surface EMG was recorded from above the quadriceps muscle in 3 male subjects during bicycle ergometry at work loads between 20 and 100% of the VO2 max to measure the EMG amplitude (RMS) and frequency (assessed from the center frequency of the power spectra) during this type of work. During brief (3 min) bouts of work the RMS amplitude of the EMG was linearly related to the work load; the center frequency of the EMG power spectra was the same at all work loads examined. In contrast, during sustained bouts of work maintained for 80 min at 20 and 40% of the VO2 max, the RMS amplitude of the EMG remained constant while the center frequency initially increased for the first 20 min of work and then progressively decreased as the work continued. When work loads of 60, 80, and 100% of the VO2 max were sustained to fatigue, the RMS amplitude continually increased while the EMG frequency decreased from the beginning to the end of the work periods. The results of this study showed that the EMG is a complex waveform, being influenced not only by fatigue, but to even a larger extent in many cases, the temperature of the exercising muscles. Therefore, although muscular fatigue caused an increase in the RMS amplitude and decrease in the center frequency, the increase in muscle temperature associated with the work opposed these changes by causing a reduction in the RMS amplitude and an increase in the center frequency.

[Temperature dependent alterations of the surface-EMG and ECG: an investigation of the electrical transfer characteristics of the human skin (author's transl)]

The influence of local heating of the skin on the integrated EMG (registered with surface and subcutaneous electrodes), the amplitude of the ECG, the skin blood flow, the electrode impedance, the electrode-to-skin impedance, and the tissue impedance is investigated. Except for the increasing skin blood flow each of the variables exhibits a significant reduction with an increase in skin temperature. From these results the existence of two mechanisms is deduced mediating thermal influence on bioelectric signals picked up by surface electrodes: 1. An alteration of the signal source. 2. An alteration of the electric transfer characteristics of the tissue between signal source and electrode. Especially in quantitative surface electromyography the temperature dependence of the signal can be a source of error.