Coordination deficits during trunk-assisted reach-to-grasp movements in Parkinson's disease.

The present study investigated how Parkinson's disease (PD) affects temporal coordination among the trunk, arm, and fingers during trunk-assisted reach-to-grasp movements. Seated participants with PD and healthy controls made prehensile movements. During the reach to the object, the involvement of the trunk was altered based on the instruction; the trunk was not involved, moved forward (flexion), or moved backward (extension) in the sagittal plane. Each of the trunk movements was combined with an extension or flexion motion of the arm during the reach. For the transport component, the individuals with PD substantially delayed the onset of trunk motion relative to that of arm motion in conditions where the trunk was moved in the direction opposite from the arm reaching toward the object. At the same time, variability of intervals between the onsets and intervals between the velocity peaks of the trunk and wrist movements was increased. The magnitudes of the variability measures were significantly correlated with the severity of PD. Regarding the grasp component, the individuals with PD delayed the onset of finger movements during reaching. These results imply that PD impairs temporal coordination between the axial and distal body segments during goal-directed skilled actions. When there is a directional discrepancy between the trunk and wrist motions, individuals with PD appear to prioritize wrist motion that is tied to the task goal over the trunk motion. An increase in disease severity magnifies the coordination deficits.