The current dissertation addresses strategies used by human performers to control orientation of the hand-held objects. It has the following objectives: to test Hierarchical Organization Hypothesis, Principle of Superposition Hypothesis, and Mechanical Advantage Hypothesis in multi-digit human prehension, to investigate the central nervous system (CNS) strategies to solve kinematic and kinetic redundancies of multi-digit prehension, to study the CNS strategies to reduce variability to stabilize the hand-held object, and to investigate age-related changes of multi-digit synergies. The series of experiments on multi-digit prehension during moment production revealed the following conclusions. (1) The changes of virtual finger force direction and magnitude were more systematic than individual finger counterparts during multi-finger prehension. (2) Individual finger forces and moments showed error compensations, which resulted in small variability of virtual finger forces and moments. The facts stated in (1) and (2) support the hypothesis on hierarchical organization of prehension synergies. (3) The sets of force and moment variables associated with the moment production about the vertical axis in the grasp plane and the axis orthogonal to the grasp plane consisted of two non-correlated subsets each; one subset of variables related to the control of grasping forces ( grasp control) and the other subset associated with the control of the orientation of the hand-held object (torque control). Among all variables in 3D, the grasping forces were decoupled from the other variables. The data support the principle of superposition in human prehension. (4) The fingers with longer moment arms with respect to the moment axis produced larger forces during moment production. The data support the mechanical advantage hypothesis. (5) The observed high correlations among forces and moments associated with the control of object orientation were explained by 'chain effects', the sequences of cause-effect relations necessitated by mechanical constraints. (6) Aging is associated with an impaired ability to produce high and accurate grasping forces and pronation-supination moments during multi-finger prehension: elderly persons showed more strength deficit in pronation-supination moment production than grasping force production, and they also revealed lower error compensations among finger forces and moments during multi-finger prehension.
展开▼
