  Despite decades of study, the mechanisms that determine human locomotion are
still unknown, available models and motor control theories can only partially
capture the phenomenon. This is probably the principal cause of the reduced
efficacy of lower limbs rehabilitation therapies. Recently, it has been
proposed that human locomotion may be planned in the task-space by taking
advantage of the gravitational pull acting on the Centre of Mass (CoM) that we
have used to formulate a task-space planner for straight locomotion at a
constant speed. The proposed model represents the CoM transversal trajectory as
simple harmonic oscillator moving forward at a constant speed. On the other
hand, the vertical trajectory of the CoM is controlled through the ankle
strategies. Our solution is composed of closed-form equations which can plan
human-like trajectories for both the CoM and the foot swing. The model output
can be seen as the optimal trajectory determined based on the average behaviour
of 12 healthy subjects walking at three self-selected speeds. Furthermore, the
planner formulation is compatible with an extended formulation of the Passive
Motion Paradigm which enables us to design a hierarchical architecture of
semi-autonomous controllers. The final architecture can also describe the motor
primitives as a particular case of dynamic primitives, shows strong parallels
with the nervous system organization, and is compatible with the optimal
feedback control theory.
