How to choose where to place the fingers when grasping a small bar: Effects of object weight and movement distance on grasp point selection

摘要

Choosing appropriate grasp points is necessary for successfully interacting with objects in our environment. Humans place their grasp axis near the center of mass (Lederman & Wing, 2003) to avoid large torques when lifting the object. Humans also generally attempt to minimize energetic costs (Alexander, 1997). We brought these two possible determinants of grasp point selection into conflict by asking subjects (N=21) to grasp elongated objects (10x3x1cm) that were approached from the side, and subsequently lift them. Minimizing energetic costs would result in a bias of the grasping axis towards the side at which the hand started. We used two clearly distinguishable objects with different mass (42.3 or 0.8g). The cost of grasping off-center will be higher for the heavy object, so the deviations from the gravicenter can be expected to be smaller. We indeed found a bias that was smaller for the heavy object. However, our right-handed subjects tended to grasp the objects to the right of the center of mass, irrespective of the starting position. We further investigated this effect in a second experiment (N=19). The rightward bias persisted when vision was removed once the hand was half way to the object, but grasp points may have been selected before then. The bias was reduced when subjects were instructed to place the object onto a small cylinder, as well as when subjects started their movement above the object. Letting our right-handed subjects grasp the objects with their left hand resulted in a leftward bias, so the bias appears to be related to the moving hand rather than to the movement direction. Thus, the selected grasping points seem to reflect a compromise between maximizing stability by grasping near the center of mass and grasping on the side of the acting hand, perhaps to increase visibility of the object.