People and animals adapt their gait to the environment as they perform activities in a variety of environments. However, there are cases where the parts of the body necessary for walking are damaged in some way, resulting in walking difficulties. An example is paralysis caused by a stroke. A split-belt treadmill is occasionally used for the investigation to analyze how the stroke effects on the motion. However, the mechanical properties of the split-belt treadmill on the body have not been clarified. It is also unknown how the mechanical closed-loop between the body and the environment, generated by synchronizing the movements of the two belts, affects the gait. In this study, we investigated that the effect of the mechanical closed-loop structure between the body and the environment on walking using the robot and the mechanical effect of the floor reaction force on the body. Further, we conducted walking experiments using the developed robot, obtained body and environmental information, and analyzed the results. As the result, it was observed that the motion data differed based on the coupling of the treadmill. In other words, it was suggested that the mechanical closed-loop structure certainly influenced the physical balances on walking motion. Furthermore, it is confirmed that the coupling of treadmills increases the body’s sway. Although our results are given from a robotic experiment, it is expected that these measures would be one of the important index in human rehabilitations.

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