Introducing rotary force to a template model can explain human compliant slope walking

Like level-ground walking, biological experiments have shown that humans largely maintain compliant leg behavior during slope walking, which greatly reduces the mechanical cost of transportation. Nowadays biped robots are becoming more and more functional on irregular terrains, yet no theoretical model can describe explicitly the cause of the characteristic ground reaction force (GRF) patterns observed in human slope walking. To fill the knowledge gap, this study extended Geyer's template biped level-ground walking model to explaining the slope walking GRF. By comparing the current Geyer's model with the human slope walking data, it was reasoned out that only using radial force from the legs could not account for the shifted anterior-posterior (AP) GRF without breaking the compliant leg behavior in the normal direction. With introducing the leg rotary force, the extended Geyer's model was then able to address the shifted AP GRF effectively. For legged robotics, this study primarily suggests that letting the leg rotary force collaborate with the compliant leg behaviour can improve the agility and energy-efficiency of dynamic walking on irregular terrains.