Figure 2.

Block scheme of the experimental set- up. General scheme of the interaction between a subject and a haptic manipulator. The haptic manipulator imposes a virtual, or external, environment P. C describes the human controller, i.e. impedance of the wrist (inverse of admittance). Torque disturbance, d, together with the human (reaction) torque, T, are the inputs of the external environment, resulting in angle θ. During postural control, the objective of the human subject (grey box) is to 'maintain position' and the internal reference angle will be constant, or zero: θ_ref = 0.