Robotic devices hold potential to improve hand rehabilitation outcome by providing consistent sensorimotor training. However, most robotic devices focus on simply reproducing 'predefined' kinematics of manual tasks without properly considering how human adapts to external assistance, while inherent impedance of the actuator could have a significant impact on the neural adaptation of human subjects. We thus examined the effects of the impedance characteristics of the actuators on human motor adaptation under external assistance. Four male subjects with no known impairment of hand function participated in an experiment, in which subjects performed hand open tasks (against resistance) while actuators of different impedance characteristics (pneumatic vs. motor) were used to provide assistance. It was found that the joint coordination pattern under pneumatic assistance was more similar to that of voluntary movements. More importantly, the pneumatic actuators improved agonist-antagonist ratio during movements. They also induced sustained contraction of task-related muscles during hold phase, while the activation of all muscles during hold phase decreased under motor assistance, possibly due to its poor backdrivability. Our results suggest that pneumatic-type actuators with low inherent impedance could provide many benefits compared to conventional electric motors, as it could reduce cocontraction of antagonist muscles of patients while effectively promoting active participation during training.