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

During walking, the brain and nervous system coordinate muscle activity to efficiently regulate body movement. Simultaneously, passive structures in the legs interact with the ground, generating reaction forces that contribute to leg and body motion. A well-known example of this active-passive coordination is the human ankle, which plays a crucial role in propelling both the leg and the entire body forward with each step. 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, this study 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 delayed the onset of ankle plantar flexion by 3% of the gait cycle compared to active knee flexion initiation, 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. The findings highlight the role of knee flexion in the release of the catapult and timing of gait events. These insights contribute to improving the control and mechanics of human-centered robotic and assistive devices. Specifically, enabling passive knee flexion initiation could be beneficial in humanoid robots with passive ankles, and in ankle-knee prostheses and orthoses with passive ankles for saving on control effort, and reducing hardware complexity otherwise required for active knee flexion before the step-to-step transition. Additionally, this approach enhances horizontal momentum gain in the trailing leg during the step-to-step transition, with the potential to improve locomotion efficiency.