Little is known about if and how humans can adapt to small magnitude perturbations experienced on a stride-by-stride basis during walking. Here, we show that even small perturbations disrupted gait stability and that subjects could still adapt their reactive balance control. Depending on the perturbation direction, subjects might prefer adjusting their foot placement over their center of mass and vice versa. These findings could help potentially tune balance training to target specific aspects of balance.

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.

Alternatives to conventional wet Silver/Silver Chloride (Ag/AgCl) hydrogel electrodes could greatly improve the ability to record long-term surface electromyography (sEMG). This paper presents preliminary evaluation of dry epidermal electronic system (EES) electrodes over 8 days measurements. We tested 3 subjects and recorded sEMG signals of right rectus femoris during static and dynamic tasks from epidermal electrodes and traditional Ag/AgCl electrodes. For epidermal electrodes that remained intact and maintained adhesion to the skin, signal-to-noise ratio (SNR) values were greater than 16.9 dB and signal-to-motion ratio (SMR) values were greater than 13.1 dB, which were both above the accepted thresholds for good signal quality (SNR > 15 dB and SMR > 12dB). We also developed a new metric, the Signal Quality Index, SQI to help determine whether higher SNR or higher SMR were necessarily indicative of better EMG signal quality. Our results indicated that the single use dry epidermal electrodes were able to maintain high signal quality over 8 days and suggested that dry epidermal electrodes have the potential ability for long-term sEMG recording.

Mobile Electroencephalography (EEG) measures human brain activity during locomotion, extending brain dynamics research into real-world scenarios. However, EEG is highly susceptible to artifacts, and more reliable approaches are needed to attenuate motion artifacts in mobile EEG recordings. Dual-layer EEG, which records scalp EEG simultaneously with isolated motion artifact signals, presents a novel and promising approach. This method assumes that noise-biased EEG data captures the common noise as the isolated motion artifact data. The purpose of this study was to investigate the relationship between signals from both sides of the electrode and their relationship to movement. We developed a benchtop test platform where the top and bottom sides of a dual-sided electrode interfaced with conductive fabric. Using a robotic arm, we moved the dual-sided electrode with different directions, magnitudes, frequencies, and randomness. We quantified correlations between signals from the top and bottom sides to understand the relationship of the dual-sided signals. We also quantified correlations between movement and signals from both the top and bottom sides to examine the extent to which the signals are related to movement . Movement direction affected the correlation signs, and all correlation metrics scaled with movement magnitude rather than frequency. Increased movement randomness reduced top and bottom signal correlation. These findings contribute new insights into signal correlations of the dual-sided EEG electrode and could inform the development of improved dual-layer EEG algorithms and hardware innovations for real-world applications.

Introducing unexpected perturbations to challenge gait stability is an effective approach to investigate balance control strategies. Little is known about the extent to which people can respond to small perturbations during walking. This study aimed to determine how subjects adapted gait stability to multidirectional perturbations with small magnitudes applied on a stride-by-stride basis. Ten healthy young subjects walked on a treadmill that either briefly decelerated belt speed (“stick”), accelerated belt speed (“slip”), or shifted the platform medial-laterally at right leg mid-stance. We quantified gait stability adaptation in both anterior-posterior and medial-lateral directions using margin of stability and its components, base of support and extrapolated center of mass. Gait stability was disrupted upon initially experiencing the small perturbations as margin of stability decreased in the stick, slip, and medial shift perturbations and increased in the lateral shift perturbation. Gait stability metrics were generally disrupted more for perturbations in the coincident direction. Subjects employed both feedback and feedforward strategies in response to the small perturbations, but mostly use feedback strategies during adaptation. Subjects primarily used base of support (foot placement) control in the lateral shift perturbation and extrapolated center of mass control in the slip and medial shift perturbations. These findings provide new knowledge about the extent of gait stability adaptation to small magnitude perturbations applied on a stride-by-stride basis and reveal potential new approaches for balance training interventions to target foot placement and center of mass control. NEW & NOTEWORTHY: Little is known about if and how humans can adapt to small magnitude perturbations experienced on a stride-by-stride basis during walking. Here, we show that even small perturbations disrupted gait stability and that subjects could still adapt their reactive balance control. Depending on the perturbation direction, subjects might prefer adjusting their foot placement over their center of mass and vice versa. These findings could help potentially tune balance training to target specific aspects of balance.