Characterizing the comfort limits of forces applied to the shoulders, thigh and shank to inform exosuit design

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.

现有研究均强调，在设计外骨骼服（exosuits）及其他人体物理增强型辅助设备时，佩戴舒适性的重要性不言而喻，但目前仍缺乏定量数据以帮助设计者明确何种强度的外力会令使用者产生不适。为填补这一研究空白，我们针对肩部、大腿及小腿部位受外力作用时的人体舒适极限展开了量化表征。本研究的目标包括：（1）对多名健康受试者的舒适极限进行量化表征；（2）分析不同测试天数下的舒适极限变化；（3）探究外力施加速率的高低是否会对舒适极限产生影响。我们开展了一项共10名受试者参与的实验，量化肩部受向下拉力、大腿及小腿受轴向拉力时的最大可耐受外力，并将该临界外力定义为舒适极限。我们借助机器人执行器，向受试者大腿和小腿处的柔性套筒以及肩部的穿戴式束带施加一系列幅值递增的外力。受试者被要求，当感到不适且不愿再承受更高强度的外力时，立即按下切断开关以终止外力施加。整体而言，受试者各部位的平均舒适极限约为体重的0.9~1.3倍：肩部为621±245牛（N）、大腿为867±296牛（N）、小腿为702±220牛（N），该数值高于此前文献中外骨骼服所施加的外力水平。但不同受试者的个体舒适极限差异显著，范围约为250~1200牛（N）。随着受试者逐渐适应测试，平均舒适极限在多轮测试中显著提升（p<3×10^-5）：首日的平均舒适极限约为550~700牛，至第四日时提升至650~950牛。具体而言，肩部、大腿及小腿的舒适极限分别提升了20%、35%及22%。最后，受试者通常能够耐受更快施加的更高强度外力。本研究结果为外骨骼服设计者与终端用户提供了首批基准参考数据，同时为后续在更大样本量、更长时间跨度以及不同人群中开展舒适极限研究铺平了道路。