Inaccuracy of internal models in force fields and complementary use of impedance control

Understanding the mechanisms which the brain uses to control body movements have been important in the both fields of robotics and neuroscience. Recent neuroscience studies have shown that humans can acquire a neural representation of the relation between motor command and movement, i.e., learn an internal model of the environment dynamics. In this paper, we investigate motor adaptation of human arm movements to external dynamics. In an experiment, we tried to determine whether humans can learn an internal model of a mixed force field (V+P) that was the sum of a velocity-dependent force field (V) and a position-dependent force field (P). The experimental results show that the subjects did not learn the internal model of V+P accurately and they compensated for the loads by using impedance control. Our results suggest that humans use impedance control when internal models become inaccurate because of the complexity of the external dynamics.