  The mechanism behind the generation of human movements is of great interest
in many fields (e.g. robotics and neuroscience) to improve therapies and
technologies. Optimal Feedback Control (OFC) and Passive Motion Paradigm (PMP)
are currently two leading theories capable of effectively producing human-like
motions, but they require solving nonlinear inverse problems to find a
solution. The main benefit of using PMP is the possibility of generating
path-independent movements consistent with the stereotypical behaviour observed
in humans, while the equivalent OFC formulation is path-dependent. Our results
demonstrate how the path-independent behaviour observed for the wrist pointing
task can be explained by spherical projections of the planar tasks. The
combination of the projections with the fractal impedance controller eliminates
the nonlinear inverse problem, which reduces the computational cost compared to
previous methodologies. The motion exploits a recently proposed PMP
architecture that replaces the nonlinear inverse optimisation with a nonlinear
anisotropic stiffness impedance profile generated by the Fractal Impedance
Controller, reducing the computational cost and not requiring a task-dependent
optimisation.
