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Changes in impedance at the electrode-skin interface of surface EMG electrodes during long-term EMG recordingsChanges in the impedance at the electrode-skin interface of SEMG electrodes on the tibialis anterior were assessed in nine subjects. SEMG signals were recorded using a bipolar electrode configuration that conformed to the SENIAM recommendations for SEMG data collection. Impedance measurements were made between a pair of bipolar electrodes using a custom-built device consisting of a PC and an impedance conversion circuit. The impedance device enabled the simultaneous application and recording of a waveform constructed of a known combination of sinusoids passed between the two electrodes. SEMG recordings at 10% of each subject's maximal voluntary force during ankle dorsiflexion were made for a 30-s period every 15-min over a two hour period. Impedance was measured immediately before and after each SEMG recording. All subjects gave their written informed consent.
Evolution in impedance at the electrode-skin interface of two types of surface EMG electrodes during long-term recordingsThe evolution in impedance at the electrode-skin interface of Beckman and Red Dot electrodes was assessed during long-term recordings. Impedance was measured between each pair of electrodes, arranged in a bipolar configuration on tibialis anterior (n=13). A waveform constructed of sinusoids of known frequencies, evenly distributed on a log scale from 1–16,384 Hz, was applied through the electrodes, and the result recorded by a DAQ system. SEMG signals were recorded at 1000 Hz during isometric dorsiflexion contractions of 30 s, performed every 15 min for 2 h. Impedance data were acquired at 65,536 Hz immediately before and after SEMG recordings. Large individual differences in impedance levels were observed at low frequencies. At high frequencies, impedance values depended only on the electrode type. Impedance decreased steadily with time for Beckman electrodes (p < 0.05), but did not decrease significantly for Red Dot electrodes. The magnitude of the reduction over time varied widely between individuals, and was related to the initial impedance values. The impedance-bandwidth product remained constant for each electrode type (95% confidence intervals 146.2–148.2 and 126.1–127.8 for Beckman and Red Dot electrodes respectively). When skin impedance is electrically modelled with a simple network containing a resistor and a capacitor, the capacitance varies with the properties of the electrode used, whereas resistance is dependent on the subject. Furthermore, the EMG spectrum is unaffected by impedance provided skin preparation is sufficient to reduce the impedance below 55 kΩ.