Human-Interface Dynamics of Knee Exoskeletons with Lateral and Anteroposterior Attachment

Assistive lower-body exoskeletons aim to improve quality of life for broad populations including older adults and people in physically exhausting manual jobs. By applying torque to augment human motion with backdrivable actuators, these devices can reduce human joint effort without restricting volitional motion. However, these backdrivable actuators are coupled by mechanical interfaces to soft tissues of the human body that together introduce resonator dynamics that can delay or diminish the torque assistance. Low interface stiffness and uncompensated dynamics can cause inefficient power delivery to the user, alter their perceived assistance and comfort, and destabilize feedback controllers. We hypothesize that the low stiffness in lateral strap interfaces, like those in the opensource M-BLUE exoskeleton, can be improved by mechanical redesign. Building on the open-source M-BLUE exoskeleton, this paper introduces an alternative interface design that loads the leg through anterior and posterior pads (normal loading) and straps, in which the pads provide extension assistance and the straps provide flexion assistance. We compare the interface dynamics of lateral and normal loading interfaces on N = 10 human subjects using both quasi-static spring measurements and frequency response methods, finding the new design to be 85.7% stiffer $(p<0.01)$ for a range of leg poses and in both flexion and extension loading.