Parkinson Disease vs Aging Features Impairments in Formation and Release of a Power Grip in Real and Pantomimed String-Pulling

Parkinson disease (PD) impairs skilled reaching tasks but less is known about its effects on rhythmic bimanual actions. We used a bimanual hand-over-hand string-pulling task that requires repeated reach, grasp, pull, push and release movements. We predicted that participants with PD would show impairment in making the rotary skilled hand movement of retrieving the string, analogous to impairments observed in single handed target reaching. Nine participants with PD (mean UPDRS-III: 24.6), 16 older adults, and 10 young adults successfully performed a real string-pulling task and then pantomimed the action. Video analyses combined optical-flow estimation of whole-body movement speed, markerless 2D pose estimation of upper-limb kinematics, and expert frame-by-frame scoring of hand and finger shaping. Machine-learning classification then identified the discriminative information carried by these multilevel movement components. Participants with PD showed lower movement speed, a smaller hand workspace, and reduced hand and finger shaping, with older adults showing intermediate values. All participants grasped the real string with a power grip, but participants with PD showed reduced rotary movements of the arm and the wrists and absent of arpeggio-like sequential finger movement during grasp and release. During pantomime, participants with PD moved faster and made larger hand movements, while grasp-related hand opening and finger sequencing was reduced or absent. The findings suggest that PD is expressed across this multicomponent movement manifold, and that the movement organization changes with task demands: real string pulling requires formation and release of a power grip, whereas pantomime does not place demands on grasping ability.