Pointing beyond reach: The slope of Fitts's law increases with the introduction of new effectors independently of kinetic constraints

Fitts's law states that movement time (MT) is related to precision (index of difficulty, ID). The introduction of a new effector in a Fitts's task induces an increase of the MT/ID slopes (Langolf et al., 1976). It is unclear, however, whether kinetics or pure coordinative constraints based on the introduction of new degrees of freedom (at the joint or muscular level) are responsible for this effect. To determine the influence of body kinetics on the MT/ID slope when pointing beyond reach, we compared pointing movements executed within (control) or beyond reach, and in this latter case with (loaded) and without (distant) an additional mass applied to the trunk. Eight subjects were required to point as fast and as accurately as possible to a target (width: 0.5, 1.0, or 2.5 cm; amplitude: 30 cm). The MT/ID slope increased when pointing beyond reach (control versus distant or loaded). This slope did not increase with an additional load applied to the trunk (distant versus loaded). Therefore, we conclude that the MT/ID slope is more likely a function of the number of degrees of freedom introduced in the task rather than a function of the kinetics constraints.