Fig. 4.

Overview over the 3 force variability modes. The grasp force mode (I) is the dimension of force variability that is associated with simultaneous, or in-phase, increases and decreases by all 3 finger normal forces. Graphically (bottom), this can be expressed as motion along a line in 3-dimensional (3D) normal force space, which has components in every dimension. The compensation force mode (II) explains out-of-phase variation of index and middle finger force, with no contribution by the thumb. In 3D normal force space, this corresponds to motion at a constant thumb force level, between index and middle finger axes, along a slightly curved line. Finally, the hinge force mode (III) explains that force variability whereby middle and index finger vary their normal force in phase while varying out of phase with the thumb normal force. This will lead to accelerations of the object.