Abstract
Recent literature emphasizes the importance of comfort in the design of exosuits and other assistive devices that physically augment humans; however, there is little quantitative data to aid designers in determining what level of force makes users uncomfortable. To help close this knowledge gap, we characterized human comfort limits when applying forces to the shoulders, thigh and shank. Our objectives were: (i) characterize the comfort limits for multiple healthy participants, (ii) characterize comfort limits across days, and (iii) determine if comfort limits change when forces are applied at higher vs. lower rates. We performed an experiment (N = 10) to quantify maximum tolerable force pulling down on the shoulders, and axially along the thigh and shank; we termed this force the comfort limit. We applied a series of forces of increasing magnitude, using a robotic actuator, to soft sleeves around their thigh and shank, and to a harness on their shoulders. Participants were instructed to press an off-switch, immediately removing the force, when they felt uncomfortable such that they did not want to feel a higher level of force. On average, participants exhibited comfort limits of ~0.9–1.3 times body weight on each segment: 621±245 N (shoulders), 867±296 N (thigh), 702±220 N (shank), which were above force levels applied by exosuits in prior literature. However, individual participant comfort limits varied greatly (~250–1200 N). Average comfort limits increased over multiple days (p<3e-5), as users habituated, from ~550–700 N on the first day to ~650–950 N on the fourth. Specifically, comfort limits increased 20%, 35% and 22% for the shoulders, thigh and shank, respectively. Finally, participants generally tolerated higher force when it was applied more rapidly. These results provide initial benchmarks for exosuit designers and end-users, and pave the way for exploring comfort limits over larger time scales, within larger samples and in different populations.
Highlights
Exosuits are wearable devices made primarily from soft/flexible structures that physically augment, assist, or enhance human movement or posture
The purpose of this study was to quantify these comfort limits for exosuit interfaces when pulling forces were applied to three key segments of the body, which are commonly used in exosuit designs (e.g., [1], [3,4,5])
On average, our healthy participants exhibited comfort limits of about 620–870 N ( 0.9–1.3 times the average participant body weight, Fig 2) on each body segment
Summary
Exosuits (soft exoskeletons) are wearable devices made primarily from soft/flexible structures that physically augment, assist, or enhance human movement or posture. One of the most challenging aspects of exosuit design–as well as design for many other wearable assistive devices, from prostheses to rigid exoskeletons–involves the physical interfaces that connect the device to the person [1], [6,7,8]. When exosuit forces are applied to the user, the synthetic and biological interface components can physically displace, deform, and/or shift relative to each other. These interface dynamics affect how force and power are transmitted to the user [7]. We refer to the maximum force which users tolerate as their comfort limit (formally defined in Methods)
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