Critical Index of Difficulty for Different Body Motions: A Review

K.-C. Gan and E. R. Hoffmann (1988) demonstrated that there is a clear boundary between values of Fitts' index of difficulty (ID) below which movements may be made without ongoing path corrections; above this value it was necessary to use corrections to hit a target of high ID. It is shown that this critical ID (IDcrit) value is dependent on both the body component making the movement and on the form of feedback available to the person. The arm system, having the highest ratio of muscle strength to mass moment of inertia, has the highest value of IDcrit, whereas control of the body center of mass location has the lowest value of IDcrit. It is suggested that the use of visual control to lower ID values with the larger body components is due to the longer movement times available because of the physical characteristics of these components-the time is available, and therefore it is used to make path corrections.