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 offswitch, 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.

It has long been held that hip abduction compensates for reduced swing-phase knee flexion angle, especially in those after stroke. However, there are other compensatory motions such as pelvic obliquity (hip hiking) that could also be used to facilitate foot clearance with greater energy efficiency. Our previous work suggested that hip abduction may not be compensation for reduced knee flexion after stroke. Previous study applied robotic knee flexion assistance in people with post-stroke Stiff-Knee Gait (SKG) during pre-swing, finding increased abduction despite improved knee flexion and toe clearance. Thus, our hypothesis was that hip abduction is not a compensation for reduced knee flexion. We simulated the kinematics of post-stroke SKG on unimpaired individuals with three factors: a knee orthosis to reduce knee flexion, an ankle-foot orthosis commonly worn by those post-stroke, and matching gait speeds. We compared spatiotemporal measures and kinematics between experimental factors within healthy controls and with a previously recorded cohort of people with post-stroke SKG. We focused on frontal plane motions of hip and pelvis as possible compensatory mechanisms. We observed that regardless of gait speed, knee flexion restriction significantly increased pelvic obliquity (2.79 deg, p<0.01) compared to unrestricted walking (1.5 deg, p<0.01), but similar to post-stroke SKG (3.4 deg). However, those with post-stroke SKG had significantly greater hip abduction (8.2 deg) compared to unimpaired individuals with restricted knee flexion (4.2 deg, p<0.05). These results show that pelvic obliquity, not hip abduction, compensates for reduced knee flexion angle. Thus, other factors, possibly neural, facilitate exaggerated hip abduction observed in post-stroke SKG.

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In individuals with transtibial limb loss, a contributing factor to mobility-related challenges is the disruption of biological calf muscle function due to transection of the soleus and gastrocnemius. Powered prosthetic ankles can restore primary function of the mono-articular soleus muscle, which contributes to ankle plantarflexion. In effect, a powered ankle acts like an artificial soleus. However, the biarticular gastrocnemius connection that simultaneously contributes to ankle plantarflexion and knee flexion torques remains missing, and there are currently no commercially-available prosthetic ankles that incorporate an artificial gastrocnemius. The goal of this work is to describe the design of a novel emulator capable of independently controlling artificial soleus and gastrocnemius behaviors for transtibial prosthesis users during walking. To evaluate the emulator's efficacy in controlling the artificial gastrocnemius behaviors, a case series walking study was conducted with 4 transtibial prosthesis users. Data from this case series showed that the emulator exhibits low resistances to the user's leg swing, low hysteresis during passive spring emulation, and accurate force tracking for a range of artificial soleus and gastrocnemius behaviors. The emulator presented in this paper is versatile and can facilitate experiments studying the effects of various artificial soleus and gastrocnemius dynamics on gait or other movement tasks. Using this system, it is possible to address existing knowledge gaps and explore a wide range of artificial soleus and gastrocnemius behaviors during gait and potentially other activities of daily living.

Walking is more difficult for transtibial prosthesis users, partly due to a lack of calf muscle function. Powered ankle prostheses can partially restore calf muscle function, specifically push-off power from the soleus. But one limitation of a powered ankle is that emulating the soleus does not restore the multi-articular function of the gastrocnemius. This missing function may explain elevated hip and knee muscle demands observed in individuals walking on powered ankles. These elevated demands can make walking more fatiguing and impact mobility. Adding an Artificial Gastrocnemius to a powered ankle might improve gait for prosthesis users by reducing the prosthesis-side hip and knee demands. This work investigates if an Artificial Gastrocnemius reduced prosthesis-side hip or knee demands for individuals walking with a powered ankle providing high levels of push-off. We performed two case series studies that examined the effects that a passive elastic Artificial Gastrocnemius has on joint moment-impulses when prosthesis users walked with a powered ankle. We found that hip moment-impulse was reduced during stance when walking with an Artificial Gastrocnemius for six of seven participants. The Artificial Gastrocnemius effects on knee kinetics were variable and subject-specific, but in general, it did not reduce the knee flexor or extensor demands. The Artificial Gastrocnemius should be further explored to determine if reduced hip demands improve mobility or the user's quality of life by increasing the distance they can walk, increasing walking economy, or leading to increased physical activity or community engagement.