Passive Dynamic Walking with Arms: Sagittal Plane Bifurcation and Stability

Human locomotion is very efficient and adaptable, with many passive and active musculoskeletal elements responsible for the generated motion. Passive oscillation of the arms may be an important contributing factor to gait efficiency and stability as it has been associated with better energetic performance and three-dimensional stability by lowering the ground reaction moment. However, its influence on sagittal plane gait characteristics is often overlooked and requires a detailed analysis to effectively utilize arm swinging in bioin-spired robotics, musculoskeletal modeling, and assistive systems design. In this study, we develop an anthropomorphic planar passive dynamic walker with arms and detect different modes of arm swinging patterns as the natural dynamic response of the system. Two types of 1-periodic stable gaits with parallel and opposing swinging patterns are generated which are producible independently on a range of slope angles. Stability with respect to slope angles is analyzed for a range of damping values in the arms (0.01, 0.055, and 0.07 Nms/rad) and swing knees (0.00, 0.015, and 0.15 Nms/rad) and compared with a similar model without arms. It is demonstrated that the addition of opposing arm oscillations can increase the range of slope angles with 1-periodic stable gaits by up to 8% without noticeably affecting the kinematics of the lower limbs. The range of slope angles with 1-periodic stable gaits was noticeably wider (up to 98%) for opposing arm swinging than parallel arm swinging. This work shows that two-dimensional musculoskeletal models and bipedal walkers can benefit from the inclusion of passive arms for higher stability and increased anthropomorphism.