Robot force in Z dimension between the experimental group and the control group (non-impaired arm of stroke and healthy participants). Change of robotic assistance force in the z (vertical) direction for stroke participants using their paretic arm ("Stroke-P"), stroke participants using their non-paretic arm ("Stroke-N"), and control participants without stroke ("Control"). Task A: Baseline tracking without distractor or sound feedback. Task B: with visual distractor. Task C: with visual distractor and sound feedback. Task D: with sound feedback and no distractor. (* = significant difference in the change of robotic assistance compare to zero assistance: in particular Task B -Task A has p = 0.0004, the Task C -Task B has p = 0.0085 and the Task A - Task D has p = 0.0023).