Passive knee flexion increases forward impulse of the trailing leg during the step-to-step transition

TL;DR

This study uses a robot to compare passive and active knee flexion during walking, revealing that passive flexion significantly enhances forward momentum during step transitions, which could inform better assistive device design.

Contribution

First investigation of passive versus active knee flexion effects on gait mechanics using a robotic model, highlighting the importance of timing in momentum generation.

Findings

Passive knee flexion increases trailing leg momentum by 87%.

Passive flexion results in 188% larger center of mass momentum change.

Timing of gait events significantly affects momentum transfer.

Abstract

Human walking efficiency relies on the elastic recoil of the Achilles tendon, facilitated by a "catapult mechanism" that stores energy during stance and releases it during push-off. The catapult release mechanism could include the passive flexion of the knee, as the main part of knee flexion was reported to happen passively after leading leg touch-down. This study is the first to investigate the effects of passive versus active knee flexion initiation, using the bipedal EcoWalker-2 robot with passive ankles. By leveraging the precision of robotic measurements, we aimed to elucidate the importance of timing of gait events and its impact on momentum and kinetic energy changes of the robot. The EcoWalker-2 walked successfully with both initiation methods, maintaining toe clearance. Passive knee flexion initiation resulted in a 3% of the gait cycle later onset of ankle plantar flexion, leading to 87% larger increase in the trailing leg horizontal momentum, and 188% larger magnitude increase in the center of mass momentum vector during the step-to-step transition. Our findings highlight the role of knee flexion in the release of the catapult, and timing of gait events, providing insights into human-like walking mechanics and potential applications in rehabilitation, orthosis, and prosthesis development.