Fig. 5. The influence of retraction speed _R on the robustness of running is shown with respect to a permanent change in leg stiffness k_LEG. For each run (v_x,0=5 m s^-1, y₀=1 m), the maximum and minimum leg stiffness (k_MIN, k_MAX) required to keep the system in a periodic running movement are depicted (A,C,E). The retraction angle _R (denoted in A,C,E) is chosen according to the mean retraction angle for stable fixed points in Fig. 3 at k_LEG=20 kN m^-1. (B,D,F) The adaptation of the leg angle to the changed leg stiffness.