Coordination and control of forces during multifingered grasping in Parkinson's disease.

In this study, we focused on how subjects with Parkinson's disease (PD) grasp and lift with five-digits of the hand. This task provided the opportunity to simultaneously examine (a) the coordination of multiple segments (i.e., digits), (b) the sequencing of multiple tasks (i.e., force development, object lift, and hold), and (c) the control of force output. We found that PD patients coordinated and controlled five-digit forces comparable to that of age-matched controls. Specifically, these groups developed and maintained similar force amplitudes and force sharing patterns across all grasping phases. In addition, PD patients demonstrated similar levels of variability both within and across trials. In the frequency domain, however, some differences were observed across groups, especially in PD patients exhibiting obvious action tremor (AT) at a single modal frequency. In these subjects (four of nine PD patients), there was a systematic disruption, i.e., a phase-shifting away from approximately 0 degrees, in-phase force synchronization patterns normally observed between digits. This disruption typically occurred at and around the AT frequency, while at many other frequencies synchronization patterns were maintained. The composite of these findings implies that although global features observed in five-digit grasping in PD patients are preserved, more subtle aspects of the coordination between digits, as revealed by frequency domain analysis, are not. These results are discussed in relation to the neural mechanisms that might underlie physiological synchronization of forces and its pathological disruption.