Figure 3.

Failure modes for powered walking with knees. (a) With WP, the stance leg moves too slowly to drive the swing leg through its normal pendulum motion, preventing a forward step from being taken (parameters: $\hat{P}=0.01$, no springs). (b) The model can also stumble (ST), where the swing foot lands while swinging forward with the knee flexed (parameters: $\hat{P}=0.12$, no springs). (c) Another failure is no knee stop (NKS), where the swing knee never reaches extension even if the leg otherwise completes a swing (parameters: $\hat{P}=0.10$; k_kh=100; r_kh=0.95). All steps end at GC, although some simulations (light grey lines in (a) and (b)) are depicted beyond this point to demonstrate the passive motions that would occur if the floor did not intervene.