The effects of digital anaesthesia on predictive grip force adjustments during vertical movements of a grasped object

Grip force adjustments to fluctuations of inertial loads induced by vertica l arm movements with a grasped object were analysed during normal and impai red finger sensibility. Normally grip force is modulated in a highly econom ical way in parallel with fluctuations of load force. Two subjects performe d vertical up and down movements of a grasped object, both with normal fing er sensibility and then cutaneously anaesthetized finger sensibility. Short breaks were taken in between single movements, during which the object was held stationary. After digital anaesthesia was applied to the grasping fin gers, both subjects substantially increased the grip force. The grip force amplitude and timing still anticipated changes in load force, although the established grip force had already overcome movement-induced load force pea ks. This implies that the increase of grip force and consequently the eleva ted force ratio between maximum grip and maximum load force are not process ed to alter the feedforward system of grip force control. Cutaneous afferen t information from the grasping digits appears to be necessary for economic scaling of the grip force level, but it plays a subordinate role in the pr ecise anticipatory temporal coupling of grip and load forces during volunta ry object manipulation.