Figure 2.

Multiple paths for transitioning from the control of fingertip motion to fingertip force. There are multiple possible paths in three-dimensional space of joint torques to transition from the torques for motion (start) to the production of well-directed forces upon contact (end). Because the neurophysiological system can transition neither instantaneously nor exactly, it must be done by interpolating in one of several ways in an open-loop manner (because the transition is faster than the delays in sensory feedback). Some possible strategies are: (1) first rotating the vector and then growing its magnitude (as in humans; figure 1) or (2) simultaneously changing both vector direction and magnitude. In this work, we find the optimal strategy so that the fingertip force vector is closest to vertical upon contact.