Tendon-driven soft exosuits have gained much attention in recent years due to their ability to support people with mobility problems during daily life activities. A task that is particularly assisted by such robotic systems, due to its challenging nature, is stair climbing. However, assisting foot clearance during the swing phase of stair climbing with a tendon-driven system that actively supports hip and knee flexion has not been investigated.
In the current study, we built a tendon-driven system to experimentally identify the foot clearance, the joint moments, the tendon forces, and the tendon velocities, that the system should provide to lift the foot above a stair-step. We experimented with able-bodied subjects that, while standing still, were instructed to keep their right leg relaxed to mimic muscle weakness. With this experimental approach, we showed how to systematically identify the basic requirements for developing an assistive device.
The system provided up to 40.1 Nm at the hip and up to 23.2 Nm at the knee when simultaneously flexing both joints, while not being constrained by any space limitations. When modified to mimic a wearable, portable exosuit, that does not protrude more than 10 cm from the body, the setup provided an average foot clearance of 17 cm with thigh tendon forces up to 455 N and shank tendon forces up to 50 N. Consequently, exosuits that support hip and knee flexion should exploit the low required shank tendon forces and the possibility of simultaneously flexing the hip and knee joints to more efficiently assist foot clearance during stair climbing.