Development and testing of acoustically-matched hydrogel-based electrodes for simultaneous EMG-ultrasound detection

In this manuscript we describe the development and testing of a bipolar electrode for the simultaneous acquisition of ultrasound (US) images and surface electromyograms (EMGs) from the same muscle region. The developed electrode (bEMG-US) consists of two circular sensing regions (20 mm diameter) with fixed inter-electrode distance (3.5 cm, center-to-center). Both the sensing regions and the external structure of the electrode are made of hydrogel layers separated by insulating materials. The electrical properties (i.e., impedance and noise of the electrode-skin interface) and the quality of EMGs detected with the developed electrodes during electrically elicited contractions were assessed and compared with those provided by commercially available EMG electrodes. The effect of the bEMG-US electrode on US images was evaluated by comparing images detected from the same muscle region with and without the electrode interposed between the US probe and the skin. Tests on five subjects showed that the electrode-skin impedance of bEMG-US electrodes was higher than that of conventional EMG electrodes (mean (range): 15.6 (8.5-21.1) kΩ vs. 8.2 (4.9-16.5) kΩ). Despite higher impedance values, both electrode systems provided comparable, electrode-skin noise levels (1.4 (1.1-1.7) µV vs. 1.3 (1.0-1.5) µV) and M waves (normalized mean square error: 2.6 (0.6-6.8)%). The quality of US images detected with and without the bEMG-US electrode between the US probe and the skin was comparable, as demonstrated by the low errors in the estimation of anatomical variables in the two experimental conditions (range: (0.37-2.35) deg for pennation angle and (-0.31-0.1) cm for muscle thickness). Results demonstrate that bEMG-US can be used to acquire concurrently EMGs and US images from the same muscle region with a negligible effect on the quality of the two detected signals, thus allowing for a simultaneous, multimodal evaluation of muscle activation.