  Supernumerary robotic limbs (SRLs) gained increasing interest in the last
years for their applicability as healthcare and assistive technologies. These
devices can either support or augment human sensorimotor capabilities, allowing
users to complete tasks that are more complex than those feasible for their
natural limbs. However, for a successful coordination between natural and
artificial limbs, intuitiveness of interaction and perception of autonomy are
key enabling features, especially for people suffering from motor disorders and
impairments. The development of suitable human-robot interfaces is thus
fundamental to foster the adoption of SRLs.
  With this work, we describe how to control an extra degree of freedom by
taking advantage of what we defined the Intrinsic Kinematic Null Space, i.e.
the redundancy of the human kinematic chain involved in the ongoing task.
Obtained results demonstrated that the proposed control strategy is effective
for performing complex tasks with a supernumerary robotic finger, and that
practice improves users' control ability.
