Unlike robots, people modulate internal forces during object manipulation

Internal force is a set of contact forces that does not disturb object equilibrium. The mathematical independence of the internal and manipulation forces allows for their independent (decoupled) control realized in robotic manipulators. As this study shows, the central nervous system uses another strategy: the internal forces change with the manipulation forces. During the parallel manipulations, the internal (grip) force increases or decreases in phase with the manipulation force. During the orthogonal manipulations, the internal changed out of phase with the manipulation force; the plots of the internal force vs. object acceleration resembled an inverted V letter.