Contribution of visually perceived size to the scaling of fingertip forceswhen lifting a 'small' object

The effects of visually perceived size of an object on the scaling of finge rtip forces during lifting tasks were investigated using a small and lightw eight object. A grip apparatus was attached to the top surface of three dif ferent size boxes of equal weight. 15 healthy adults were asked to grasp th e grip apparatus with the thumb and index finger, lift it to a height of 5 cm, hold it for 8 sec., and then put it down. Force transducers embedded in the grip apparatus measured grip and load forces. When subjects lifted the same size object repeatedly, there were no size effects on the grip and lo ad forces used by each subject. When the size was pseudorandomly varied, ho wever, the grip and load forces exerted by 7 subjects significantly increas ed with increased size, while there were no size effects for the remaining subjects (n = 8). These results suggest a smaller contribution of informati on on size to the force programming when lifting a small object than when l ifting a larger and heavier object as used in previous studies.