Inherent structure of manipulative hand movements and its discriminative power

Understanding the inherent structure of manipulative hand movements is a critical step toward automatic recognition of manipulative hand movements and toward automatic planning of natural movements for dexterous robots. Manipulative hand movements involve coordinated movements of the digits to manipulate an object within the hand, and are classified as either simultaneous or sequential. Simultaneous hand movements involve a single pattern of coordination, while sequential hand movements involve a sequence of such patterns. Assuming that each coordinated pattern reflects a stable limit cycle of an internal dynamical system, we have previously suggested that stable pattems of coordination are mostly restricted to in-phase or anti-phase coordination. Formulating the coordination underlying simultaneous hand movements reveals that under these conditions the joint-space trajectories are straight lines. In this paper, we demonstrate the linear nature of the joint-space trajectories projected by the nine basic classes of simultaneous manipulations. The results support our working assumption that in-phase or anti-phase pattems of coordination are prevailing. We conclude that the direction of the trajectory line in the joint space captures the coordination structure underlying a simultaneous hand movement. The vector of direction numbers is extracted and its discriminative power is demonstrated.