Stabilizing unstable object by means of kinematic redundancy

The paper aims to investigate how humans deal with unstable objects under the possibility of choosing different strategy of interaction. The presented task consisted in balancing a 1 degree of freedom (DoF) elastic inverted pendulum by means of 2 DoF of the wrist (fexion/extension and pronation/supination). The pendulum was simulated using a virtual environment and the haptic feedback was generated by a robotic wrist device. The task is a redundant because the subject can choose how to use the 2 DoF in order to move and stabilize a 1 DoF simulated mechanical system: the inverted pendulum. Six subjects volunteered to participate and were tested in four different days performing the same task but experiencing different mechanical systems (pendulum) characterized by lower or higher dynamics due to the possibility to tune the stiffness of the pendulum. Subjects were asked to balance the inverted pendulum maintaining it in a vertical position for a required amount of time. It was found the adopted stabilization strategy was mainly characterized by using only one of the 2 available DoFs of their wrist when the pendulum was stiffer, while in case of lower stiffness of the pendulum (slower dynamic) wrist input redundancy was a more suitable strategy to perform the balancing task.