Fig. 2. Center of mass trajectories (A) and leg kinematics (B,C) for spring-mass running with and without retraction (C, _R=50 deg s^-1; B, _R=0 deg s^-1). The bars in A indicate the change in centre of mass height between touch-down and take-off. B and C are expanded views of plot A from 0 to 6 m (boxed). For each simulated run, the same initial apex height was used (y₀=1.25 m), and for the simulation with retraction, a retraction angle of _R=60° was assumed (C). Here the model with retraction reached a steady state condition after two steps in contrast to approximately 8 steps for the model without retraction (A,B). The red dotted lines in B and C denote the steady state landing angle ₀^*.