Inertia Effects in Fitts’ Law for Movement Time

Recent researches involving aimed movements of various body components have shown the need for a second component of task ‘difficulty’ in order to take into account the effects of inertia in the limb/load system. This term generally appears linear in the square-root of movement amplitude or as the product of this quantity with the square-root of transported mass. In underwater movements it appears as the mass effects of accelerating fluid particles about the moving limb. It is suggested that the occurrence of the modified form of Fitts’ law arises when the mass effects of a load exceed the capabilityCapabilities of the limb muscles to accommodateAccommodate the forces arising from acceleration of the load during the distance-covering phase of the movements. It appears that, as a general form, the modified model for Fitts’ law needs to be tested in most experimental arrangements. Fitts’ law for aimed movement time needs to be modified from its usual form when there are significant loads to be transferred, with the form dependent on how the inertia loads are produced. These inertia forces generally arise during the high acceleration distance-covering phase of the movement.