The alignment of the limb trajectory relative to the v-nullcline depends on phase. (A), (B), (C), and (D) display structures in (q, v) phase space in the eStance, fStance, fSwing, and eSwing phase, respectively, with panel ordering selected to reflect the order in which the phases occur, progressing clockwise. In each, the q-nullcline, which is the q-axis, is shown in black. A portion of the trajectory is plotted in blue, with different colored time points highlighted, and the same color is used for each corresponding v-nullcline, since the v-nullcline changes position over time as the output of the active motoneuron changes. Note that the v-nullcline positions differ drastically between phases, due to dominance switching in the CPG and the influence of ground contact.