Figure 9.

Hip joint angles vs. time during stepping with an external perturbation torque applied to the joint during ‘air-walking’ in which the leg does not touch the ground. The torque is applied on the femur at the start of the second stance phase at 0.5 s and ends at 1 s, with a duration of one step cycle. The blue 2 Hz D line indicates that the CPG is operating at 2 Hz and driven by the direct stimulus $D$. The red 2 Hz G_w line indicates that the CPG is operating at 2 Hz by neuromodulation of the weak excitatory synapse conductivity $G_w$. Dashed lines indicate when the transition from extension to flexion would occur if there was no perturbation. (A) Given a small torque perturbation (−0.02 Nm), both models recover after the perturbation ends. (B) A large torque perturbation (−0.1 Nm) causes the deletion of an entire step cycle.