Leaning in Passive Bipeds Changes the Game: How Including Leaning 3D Biped Affects Gait and Stability

Abstract Despite considerable advancements, passive bipedal locomotion in 3D space remains a significant technical challenge. This study develops the natural dynamics of a general 3D passive walking model with curved feet, allowing both rolling and pivoting motions. Unlike planar models, this model explicitly integrates the interplane coupling effects for the first time. Simulations are performed for two distinct gait styles (roll and anti-roll modes) to investigate the effects of model parameters and initial conditions on the resulting gait. The results reveal that the curved-foot model, unlike its point-foot counterpart, exhibits a longer step period and slower walking speed. This implies a smoother transition and a softer foot-ground impact than the point-foot, enhancing the overall stability of the gait cycle. Most notably, the incorporation of the leaning motion introduces greater energy dissipation than that predicted by planar or decoupled models, especially as the hip width increases. The proposed model indicates that the leaning motion increases foot clearance, yet at the same time introduces instability resulting from the frontal-plane effects of ground contact at the foot strike.