Figure 4
Phase plane of spine joint angle \(\phi \) and snapshots of six types of solutions. G and E indicate gathered and extended flights, respectively. \(\Delta _{{\dot{\phi }}}\) is the difference between \({\dot{\phi }}^+\) and \({\dot{\phi }}^-\), which indicate angular velocities immediately prior to and following foot contacts, respectively. \(\omega c_1\) is the amplitude of angular velocity. \(\varepsilon \) is ratio of the angular velocity change (\(\varepsilon = \Delta _{{\dot{\phi }}}/\omega c_1\)). When \(\varepsilon >1\), solutions have one type of flight (solutions of type G, E, GG, and EE). Solutions of type GE and EG satisfy \(\varepsilon <1\).
