Abstract
Due to the limitations of standard wet Silver/Silver Chloride (Ag/AgCl) hydrogel electrodes and the growing demand for long-term high fidelity surface electromyography (EMG) recording, dry epidermal electrodes are of great interest. Evaluating the usability and signal fidelity of dry epidermal electrodes could help determine the extent of potential applications using EMG electrodes. We collected EMG signals over eight days from the right rectus femoris of seven subjects using single-use dry epidermal electrodes and traditional Ag/AgCl electrodes while covered and uncovered during dynamic movements (leg extension, sit-to-stand, and treadmill walking at 0.75 m/s and 1.30 m/s). We quantified signal fidelity using signal-to-noise ratio (SNR); signal-to-motion ratio (SMR); and a metric we previously developed, the Signal Quality Index, which considers that better EMG signal quality requires both good signal-to-noise ratio and good signal-to-motion ratio. Wear patterns over the eight days degraded EMG signal quality. Uncovered epidermal electrodes that remained intact and maintained good adhesion to the skin had signal-to-noise ratios, signal-to-motion ratios, and Signal Quality Index values that were above the acceptable thresholds for limited dynamic lower limb movements (leg extension and sit-to-stand). This indicated that dry epidermal electrodes could provide good signal quality across all subjects for five days for these movements. For walking, the signal-to-noise ratios of the uncovered epidermal electrodes were still above the acceptable threshold, but signal-to-motion ratios and the Signal Quality Index values were far below the acceptable thresholds. The signal quality of the epidermal electrodes that showed no visible wear was stable over five days. As expected, covering the epidermal electrodes improved signal quality, but only for limited dynamic lower limb movements. Overall, single-use dry epidermal electrodes were able to maintain high signal quality for long-term EMG recording during limited dynamic lower limb movements, but further improvement is needed to reduce motion artifacts for whole body dynamic movements such as walking.
Highlights
Myoelectrical signals measured noninvasively from superficial muscles using surface electromyography (EMG) electrodes are often used to control exoskeletons and prostheses where long-term EMG recordings would be beneficial [1,2,3]
We found that the averaged signal-to-motion ratio across days of the covered epidermal electrodes was significantly higher compared to the uncovered epidermal electrodes during post leg extension (p = 0.043)
We found that the averaged signal-to-motion ratios across days of the covered epidermal electrodes were higher compared to the uncovered epidermal electrodes during baseline leg extension and sit-to-stand, but these were not significant (p’s = 0.08)
Summary
Myoelectrical signals measured noninvasively from superficial muscles using surface electromyography (EMG) electrodes are often used to control exoskeletons and prostheses where long-term EMG recordings would be beneficial [1,2,3]. Hybrid Assistive Limb (HAL) exoskeleton, Ferris et al.’s pneumatically powered ankle–foot orthosis, and Au et al.’s powered ankle–foot prosthesis record EMG from at least one lower limb muscle to adjust the magnitude and timing of assistance [4,5,6,7,8,9]. How well these myoelectric controlled assistive devices function during locomotion depends on the quality of the EMG recording. The current gold standard for EMG recording is conventional Silver/Silver Chloride (Ag/AgCl) hydrogel (wet) electrodes [13,14,15], but the contact between the electrode and skin becomes inconsistent as the gel dries out over time, producing impedance variation, more motion artifacts and noise, and degraded signal quality [13]
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