Fig. 4. Return maps y_i+1(y_i) of the apex height y_APEX are shown for different retraction speeds _R (A, _R=0 deg s^-1; B, _R=25 deg s^-1; C, =50 deg s^-1) but for a lower system energy corresponding to a slower running speed of 3 m s^-1 at an apex height y_APEX=1 m. Stable fixed points require non-zero retraction velocities _R>0 (B,C). As in Fig. 3, an increased retraction speed leads to an enlarged attraction of the stable fixed points with respect to a given initial (e.g. disturbed) apex height. Model parameters: m=80 kg, l₀=1 m, k_LEG=20 kN m^-1.