So as a spring force is linear to displacement (0th derivative), and a dampner force is linear to the velocity (1st derivative), add a thing where its force is linear to the acceleration (2th derivative).
Also I noticed that the vertical upward acceleration in terminal stance is not captured in the model, maybe this could be captured by a velocity dampner in only the Bz direction?
So as a spring force is linear to displacement (0th derivative), and a dampner force is linear to the velocity (1st derivative), add a thing where its force is linear to the acceleration (2th derivative).