Worst-case prediction strategy in force programming when visual information is obstructed

A person's strategy for applying force while lifting an object is dependent upon visual cues. This study investigated the alteration of strategy in fo rce programming when visual information about an object's size was obstruct ed at the moment of lifting. Seven subjects were instructed to use a precis ion grip for repeated lifts of a cube-like grip apparatus attached to a box . The grip apparatus was a special device designed to measure grip and load forces. Three different-sized plastic boxes of equal weight were pseudoran domly presented by attaching them beneath the grip apparatus to the subject s in two visual conditions, In the Full-vision condition, subjects could vi ew the box's size prior to lifting. In the Obstructed-vision condition, a s creen prevented subjects from seeing the box size prior to lifting. In the Full-vision condition, the grip force and load force used by subjects on th e grip apparatus increased with box size. In contrast, the subjects in the Obstructed-vision condition used forces appropriate for the largest box reg ardless of box size. The present results suggest that absence of size infor mation may cause an alteration of strategy used to determine force output i n that subjects may apply a maximum force adequate for the largest box, whi ch could be called a "worst-case" prediction strategy, i.e., when there is doubt, the most secure lift may be selected for all possible cases.