Leg joint synergy emerged from the body dynamics during walking

Although leg joint trajectories during human walking show variance in each step it has been reported that the variance of the toe position is suppressed by joint synergy at some critical points to realize stable walking, e.g., at the moment when the toe passes through the nearest position from the ground during leg swing. Is such joint synergy generated by the central nervous system (CNS) or by the body dynamics given by the physical structure of our body? In this study, we analyzed the leg joint synergy in a passive walker by the Uncontrolled Manifold analysis, and compared the result with the synergy in human walking. The results show that in both of the passive walker and humans the joint synergy that suppresses the variance of the horizontal ankle position becomes large just after the take off of the ankle, and the joint synergy that suppresses the variance of the ankle height becomes large when the ankle passes just under the knee position. These results suggest that the above mentioned joint synergies observed in human walking might be due to the body dynamics. On the other hand, large joint synergy that suppresses the variance of the hip height at the end of the stance phase was observed only in human walking, which implies that this joint synergy is due to the CNS control. These results suggest that human utilizes joint synergy emerged from the body dynamics and the CNS adds control to form synergy at some critical points to adjust the foot position and stabilize walking.