Fitts' law in human standing: the effect of scaling

Fitts' law states that the movement time (MT) of an aiming movement is a li near function of the index of difficulty (ID), where ID = log2(2A/W), A is the movement amplitude, and W is the target width. This law implies that MT should remain unchanged as long as A/W remains constant (i.e. the absence of a sealing effect). The goal of this study was to investigate whether, du ring upright posture, reciprocal-pointing movements with the center of pres sure location follow Fitts' law. Six subjects performed the task with six I Ds factorially combined with four As. The results showed that for each A, M T was a linear function of ID. However, the slopes of the linear-regression lines increased with decreases in A. These findings indicate the presence of a scaling effect which violates Fitts' law. (C) 1999 Published by Elsevi er Science Ireland Ltd. All rights reserved.