“…And third, due to its simplicity, the model can be easily extended to its variations for a systematic analysis. iida@csail.mit.edu, russt@mit.edu Previously this simple model showed limit cycles of passive dynamic walking in slopes [9], and the model was enhanced with various motor control to deal with a flat terrain [10], [11], [12], [13] and with the other variations of environment [14], [15], [16], [17], [18]. The investigations of this model, however, were mainly conducted in simulation with a few exceptions [10], [12], and the model has not been tested in the real-world rough terrains previously.…”