The joint coordination in reach-to-grasp movements

Reach-to-grasp movements are widely observed in activities of daily living, particularly in tool manipulations. In order to reduce the complexity in redundancy resolution and facilitate upper-limb exoskeleton control in reach-to-grasp tasks, we studied joint coordination in the human arm during such movements. Experimental data were collected on reach-to-grasp movements in a 3-dimensional (3D) workspace for cylinder targets of different positions and grasping orientations. For comparison, reaching movements toward the same targets are also recorded. In the kinematic analysis, the redundant degree of freedom in human arm is represented by the swivel angle. The four grasping-relevant degrees of freedom (GR-DOFs), including the swivel angle and the three wrist joints, behave differently in reach-to-grasp movements comparing to how they behave in reaching movements. The ratio of active motion range (R-AMR) is proposed for quantitatively comparison the task-relevance of the GR-DOFs. Analysis on the R-AMR values shows that the task-relevant GR-DOFs are more actively used, while the task-irrelevant joints are left uncontrolled and maintain their neutral positions. Among the task-relevant GR-DOFs, the smaller joints (micro-structure) are more actively used than the larger joints (macro-structure). The coordination of the task-relevant GR-DOFs is shown to be synergistic. Analysis of the acceleration/deceleration at the GR-DOFs indicates different levels of voluntary control in three phases of the movements. The study of the characteristics of the joint coordination in reach-to-grasp movements provides guide-lines for simplifying the control of the upper limb exoskeleton.