Planar conformity of movements in 3D reaching tasks for persons with Multiple Sclerosis.

Robotic rehabilitation of the upper limb has been proved beneficial for people with Multiple Sclerosis (MS). In order to provide task-specific therapy for MS, given its complex impairing nature, it is desired to take advantage of the robots' ability to move and provide force feedback in generic three-dimensional motions. Previous investigations have shown that hand motion during reaching tasks is piecewise planar, therefore, it is possible to simplify the design of robotic rehabilitation trajectories by confining them on planes oriented in space. To do so, it must first be established whether this natural tendency for tracking a plane during hand movement is affected by the presence of neurological impairment, such as Multiple Sclerosis. This work hypothesizes that in the case of people with MS, planar mapping is not dependent on the resulting impairment. Furthermore, it is investigated whether neurophysiological models such as Fitts' law and Minimum Jerk Model remain in effect when considering the plane-mapped trajectories and also how these are affected by MS in contrast to natural movement. To test the hypotheses a group of healthy people and a group of people with MS at various stages of the disease have performed reaching tasks in the context of a haptic-interface based Nine-Hole-Pegboard-Test (NHPT) which facilitates the generation of trajectories with various distances and durations in the three-dimensional space. The results suggest that, despite the neurological impairments, the planar adaptation of people with MS is almost as persistent as in the case of healthy people. Furthermore it is shown that Fitts' law and MJM apply for plane-mapped trajectories and can be utilized in the design of simplified robotic rehabilitation schemes.