Effects of induced motor fatigue on walking mechanics and energetics.

Lower-body robotic exoskeletons can be used to reduce the energy demand of locomotion and increase the endurance of wearers. Understanding how motor fatigue affects walking performance may lead to better exoskeleton designs to support the changing physical capacity of an individual due to motor fatigue. The purpose of this study was to investigate the effects of motor fatigue on walking mechanics and energetics. Treadmill walking with progressively increased incline gradient was used to induce motor fatigue. Twenty healthy young participants walked on an instrumented treadmill at 1.25 m/s and 0° of incline for 5 min before (PRE) and after (POST) motor fatigue. We examined lower-limb joint mechanics, metabolic cost, and the efficiency of positive mechanical work (η+work). Compared to PRE, participants had increased net metabolic power by ∼14% (p < 0.001) during POST. Participants also had increased total-limb positive mechanical power (Total P+mech) by ∼4% during POST (p < 0.001), resulting in a reduced η+work by ∼8% (p < 0.001). In addition, the positive mechanical work contribution of the lower-limb joints during POST was shifted from the ankle to the knee while the negative mechanical work contribution was shifted from the knee to the ankle (all p < 0.017). Although greater knee positive mechanical power was generated to compensate for the reduction in ankle positive power after motor fatigue, the disproportionate increase in metabolic cost resulted in a reduced walking efficiency. The findings of this study suggest that powering the ankle joint may help delay the onset of the lower-limb joint work redistribution observed during motor fatigue.

Defining Allowable Stimulus Ranges for Position and Force Controlled Cutaneous Cues.

Haptic cues delivered via wearable devices have great potential to enhance a user's experience by transmitting task information and touch sensations in domains such as virtual reality, teleoperation, and prosthetics. Much is still unknown on how haptic perception, and consequently optimal haptic cue design, varies between individuals. In this work we present three contributions. First, we propose a new metric, the Allowable Stimulus Range (ASR), as a way to capture subject-specific magnitudes for a given cue, using the method of adjustments and the staircase method. Second, we present a modular, grounded, 2-DOF, haptic testbed designed to conduct psychophysical experiments in multiple control schemes and with rapidly-interchangeable haptic interfaces. Third, we demonstrate an application of the testbed and our ASR metric, together with just noticeable differences (JND) measurements, to compare perception of haptic cues delivered via position or force control schemes. Our findings show that users demonstrate higher perceptual resolution in the position-control case, though survey results suggest that force-controlled haptic cues are more comfortable. The results of this work outline a framework to define perceptible and comfortable cue magnitudes for an individual, providing the groundwork to understand haptic variability, and compare the effectiveness of different types of haptic cues.

Improving short-term retention after robotic training by leveraging fixed-gain controllers.

INTRODUCTION: When developing control strategies for robotic rehabilitation, it is important that end-users who train with those strategies retain what they learn. Within the current state-of-the-art, however, it remains unclear what types of robotic controllers are best suited for promoting retention. In this work, we experimentally compare short-term retention in able-bodied end-users after training with two common types of robotic control strategies: fixed- and variable-gain controllers. METHODS: Our approach is based on recent motor learning research, where reward signals are employed to reinforce the learning process. We extend this approach to now include robotic controllers, so that participants are trained with a robotic control strategy and auditory reward-based reinforcement on tasks of different difficulty. We then explore retention after the robotic feedback is removed. RESULTS: Overall, our results indicate that fixed-gain control strategies better stabilize able-bodied users' motor adaptation than either a no controller baseline or variable-gain strategy. When breaking these results down by task difficulty, we find that assistive and resistive fixed-gain controllers lead to better short-term retention on less challenging tasks but have opposite effects on the learning and forgetting rates. CONCLUSIONS: This suggests that we can improve short-term retention after robotic training with consistent controllers that match the task difficulty.

Skin Stretch Haptic Feedback to Convey Closure Information in Anthropomorphic, Under-Actuated Upper Limb Soft Prostheses.

Restoring hand function in individuals with upper limb loss is a challenging task, made difficult by the complexity of human hands from both a functional and sensory point of view. Users of commercial prostheses, even sophisticated devices, must visually attend to the hand to know its state, since in most cases they are not provided with any direct sensory information. Among the different types of haptic feedback that can be delivered, particularly information on hand opening is likely to reduce the requirement of constant visual attention. In recent years, there has been a trend of using underactuated, compliant multi-fingered hands as upper limb prostheses, in part due to their simplicity and ease of use attributed to low degree-of-freedom (d.o.f.) actuation. The trend toward underactuation encourages the design of one d.o.f. haptic devices to provide intuitive sensory feedback from the prosthesis. However, mapping the closure of a multi-d.o.f. prosthetic hand to a simple and intuitive haptic cue is not a trivial task. In this paper, we explore the use of a one d.o.f. skin stretch haptic device, the rice haptic rocker, to provide intuitive proprioceptive feedback indicating overall hand closure of an underactuated prosthesis. The benefits and challenges of the system are assessed in multi-tasking and reduced vision scenarios for an object-size discrimination task, in an effort to simulate challenges in daily life, and are compared against the haptic resolution of the device using the just noticeable difference. Finally, an evaluation done with a prosthesis user, in the form of a truncated version of the Activities Measure for Upper Limb Amputees (AM-ULA), shows possible benefits of the addition of haptic feedback in tasks with reduced visual attention.