Dynamic Hand Space Envelope during Reaching and Grasping

The objective of this paper is to investigate the dynamic space envelope during reaching and grasping tasks. Some amount of space is required for the hand and arm to move without interference in reach-to-grasp tasks. The required space (‘dynamic space envelope’) has not been examined in spite of its importance. In this paper, we tested the hypothesis that the dynamic space envelope is a function of object size, hand size, grip type and distance. Six subjects (5 males, 1 female) participated in an experiment, in which they reached for and grasped three differently sized cylindrical objects (D: 26 mm, 60 mm, 114 mm) placed 40 cm in front of the subjects. Twenty-three markers were attached to the dorsal side of the hand and a 3-D motion capture system recorded the positions of the markers during reaching and grasping. The total distance from start to end positions is evenly divided into 10 ranges, and the areas of the required spaces were calculated for each range. It was observed that dynamic space envelope increased as the hand size increased (on average, 155 mm² increase per 1 mm increase of hand length). Increasing object size required more space envelope (on average, 15 mm² increase per 1 mm increase of object diameter). Power grip required 10% more space than pinch grip on average. It was found that the largest space was required when the hand reached 70% of total distance. The information of a dynamic space envelope enables the design of a workspace or automobile interior which minimizes interference of hand movement.